In the last few articles, we have studied the methods of preparations of alkyl halides. In this article, we shall study the physical properties of alkyl halides. Some physical properties of alkyl halides are as follows:

State:

Lower members (methyl chloride, methyl bromide, methyl fluoride, ethyl bromide, ethyl chloride and ethyl bromide) are gases and higher members are liquids (Up to C18) and solids (Greater than C18).

Odour:

In the pure state, the haloalkanes up to C18 possess pleasant sweet odour. All higher haloalkanes are odourless.

Colour:

Pure haloalkanes are colourless. However, bromoalkanes and iodoalkanes on storing for long period, when exposed to light develop colour.

Boiling Points:

Haloalkanes have higher boiling points as compared to those compared to corresponding alkanes. This is due to their polarity and strong dipole-dipole attractive interaction between haloalkane molecules and greater magnitude of van der Wall’s forces.

• For the same alkyl group the boiling points of haloalkanes are in the order RCl < RBr < RI, because with the increase in the size of halogen atom the magnitude of van der Wall forces of attraction increases.
• Among isomeric alkyl halides, the boiling point decreases with an increase in branching in the alkyl group, because with branching the molecule attains a spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases. The order of boiling point is Primary  > Secondary >= iso > Tertiary
• For the same halogen, the boiling point increases with the increase in the molecular mass because with the increase in the size of the alkyl group the magnitude of van der Wall forces of attraction increases. i.e R – X < R -CH2-X  < R -CH2-CH2-X
• As the number of halogen in a molecule increases the boiling point of the compound increases because of the increase in the number of halogen atoms the magnitude of van der Wall forces of attraction increases. i.e. CH₃Cl < CH₂Cl₂ < CHCl₃ < CCl₄

Solubility:

Alkyl halides are polar in nature (dipole moment 2.05 to 2.15 D) but they are not able to form hydrogen bonds with water molecules. Hence they are sparingly soluble in water. But they are soluble in organic solvents like alcohols, ethers and benzene.

Density:

Alkyl chlorides are generally lighter than water, while alkyl bromides and alkyl iodides are heavier than water. The order of density is RI > RBr > RCl. Poly chlorides are heavier than water. Thus the density of alkyl halides increases with the increase in the number and atomic mass of the halogen atoms. Methyl iodide is the heaviest of all the haloalkanes.

Scientific Reasons:

Density:

Arrange the following in the order of decreasing density. 1-Chloropropane, 1-Iodopropane, 1-Bromopropane

• For the same alkyl group, the density of alkyl halides increases with the increase in the number and atomic mass of the halogen atoms. The atomic mass of I > Atomic mass of Br >Atomic mass of Cl.
• Hence boiling point of 1-Iodopropane > 1-Bromopropane > 1-Chloropropane.

Arrange each of the following set of compounds in the order of increasing densities:

CHCl₃, CH₂Cl₂, CCl₄, CH₃Cl:

• The order of density is RI > RBr > RCl. Poly chlorides are heavier than water. Thus the density of alkyl halides increases with the increase in the number and atomic mass of the halogen atoms.
• Hence, the order of densities is CH₃Cl. < CH₂Cl₂ < CHCl₃ < CCl₄.

C₂H₅Cl, C₂H₅I, C₂H₅Br:

• The order of density is RI > RBr > RCl. Thus the density of alkyl halides increases with the increase in the number and atomic mass of the halogen atoms.
• Hence, the order of densities is C₂H₅Cl < C₂H₅Br < C₂H₅I.

Which alkyl halide has the highest density and why?

• For the same alkyl group, the order of density is R-I > R-Br > R-Cl. Thus R-I will have the highest density.
• For the same halogen group, with the increase in the branching, the molecule acquires the spherical shape with less surface area. Thus the tertiary butyl group will have the smallest size.
• From the above two points, we can say that tertiary butyl iodide should have the highest density.

Boiling Points:

Which isomer of C₅H₁₁Cl has the highest boiling point? Why?

• Consider the following two isomers of C₅H₁₁Cl 

• Among isomeric alkyl halides, the boiling point decreases with an increase in branching in the alkyl group.
• 1-Chloropentane is a straight-chain isomer. It has the strongest interparticle forces. Hence it has the highest boiling point among all the isomers. While 1-Chloro-2,2-dimethylpropane has the highest number of branches in all possible isomers, hence it has the weakest interparticle forces. Hence it has the lowest boiling point among all the isomers.

Which isomer of C₄H₉Cl has the highest boiling point? Why?

• Among isomeric alkyl halides, the boiling point decreases with an increase in branching in the alkyl group.
• 1-Chlorobutane (n-Butyl chloride) is a straight-chain isomer. It has the strongest interparticle forces. Hence it has the highest boiling point among all the isomers. While 2-Chloro-2-methylpropane (tert-Butyl chloride) has the highest number of branches in all possible isomers, hence it has the weakest interparticle forces. Hence it has the lowest boiling point among all the isomers.

Arrange in the order of increasing boiling points. 

Bromobenzene. chlorobenzene, iodobenzene:

• The boiling points of mono halogen derivatives of benzene follow the order Iodo > Bromo > Chloro.
• Hence boiling point of Chlorobenzene <  Bromobenzene < Iodobenzene .

n-pentyl chloride, iso-pentyl chloride, neo-pentyl chloride:

• Among isomeric alkyl halides, the boiling point decreases with the increase in branching in the alkyl group, because with branching the molecule attains spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases. The order of boiling point is Primary  > Secondary >= iso > Tertiary
• Hence boiling point of neo-pentyl chloride < iso-pentyl chloride < n-pentyl chloride.

Bromomethane, Bromoform, Chloromethane, Dibromomethane:

• For the same alkyl group the boiling points of haloalkanes are in the order RCl < RBr< RI, because with the increase in the size of halogen atom the magnitude of van der Wall forces of attraction increases. For the same halogen, the boiling point increases with the increase in the molecular mass. As the number of halogen in a molecule increases the boiling point of the compound increases.
• Hence boiling point of Chloromethane < Bromomethane <  Dibromomethane < Bromoform.

1-Chloropropane, isopropyl chloride, 1-Chlorobutane:

• As the number of halogen in a molecule increases the boiling point of the compound increases. Among isomeric alkyl halides, the boiling point decreases with the increase in branching in the alkyl group, because with branching the molecule attains a spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases. The order of boiling point is Primary  > Secondary >= iso > Tertiary.
• Hence boiling point of isopropyl chloride < 1-Chloropropane < 1-Chlorobutane.

Methyl chloride, methyl bromide, methyl iodide:

• For the same alkyl group the boiling points of haloalkanes are in the order RCl < RBr < RI, because with the increase in the size of halogen atom the magnitude of van der Wall forces of attraction increases.
• Hence the order of boiling points is Methyl chloride (CH₃Cl) < methyl bromide (CH₃Br) < methyl iodide (CH₃I).

Methyl bromide, methylene bromide, bromoform:

• As the number of halogen in a molecule increases the boiling point of the compound increases.
• Hence the order of boiling points is methyl bromide (CH₃Br) < methylene bromide (CH₂Br₂) < Bromoform (CHBr₃).

Propane, n-propyl bromide, isopropyl bromide:

• Haloalkanes have higher boiling points as compared to those compared to corresponding alkanes. This is due to their polarity and strong dipole-dipole attractive interaction between haloalkane molecules.
• Among isomeric alkyl halides, the boiling point decreases with the increase in branching in the alkyl group, because with branching the molecule attains a spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases. The order of boiling point is Primary  > Secondary >= iso > Tertiary.
• Hence the order of boiling points is propane (alkane) < isopropyl bromide (isoalkyl halide) < n-propyl bromide (primary alkyl halide).

n-butyl chloride, iso-butyl chloride, tert-butyl chloride:

• Among isomeric alkyl halides, the boiling point decreases with the increase in branching in the alkyl group, because with branching the molecule attains a spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases. The order of boiling point is Primary  > Secondary >= iso > Tertiary.
• Hence the order of boiling points is tert-Butyl chloride < iso-butyl chloride < n-butyl chloride.

1-Bromopropane, isopropyl bromide, 1- Bromobutane:

• Among isomeric alkyl halides, the boiling point decreases with the increase in branching in the alkyl group, because with branching the molecule attains spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases. The order of boiling point is Primary  > Secondary >= iso > Tertiary. or the same halogen, the boiling point increases with the increase in the molecular mass.
• Hence the order of boiling points is isopropyl bromide  <  1-Bromopropane <  1- Bromobutane

Explain why 1-Chlorobutane has higher B.P. than 2-Chlorobutane?

Among isomeric alkyl halides, the boiling point decreases with the increase in branching in the alkyl group, because with branching the molecule attains spherical shape with less surface area. As a result, interparticle forces become weaker. Hence the boiling point decreases.

The order of boiling point is Primary  > Secondary >= iso > Tertiary. Hence 1-Chlorobutane (primary alkyl halide) has higher B.P. than 2-Chlorobutane (secondary alkyl halide).

Explain why Bromoethane has a higher boiling point than Chloroethane. OR Out of ethyl bromide and ethyl chloride which has a higher boiling point and why?

For the same alkyl group the boiling points of haloalkanes are in the order RCl < RBr < RI, because with the increase in the size of halogen atom the magnitude of van der Wall forces of attraction increases. Hence Bromoethane has a higher boiling point than Chloroethane.

Solubility:

Explain why choroform is not soluble in water although it is polar. OR alkyl halides though polar, are immiscible with water.

• A substance is soluble in water if its molecules are capable of forming hydrogen bonds with water. Chloroform molecules do not form the hydrogen bond with water.
• The energy required to break the bonds between haloalkane molecules is much larger than the energy released during the formation of the bond between haloalkane molecules and water molecules. Hence chloroform is not soluble in water although it is polar.

Alkyl halides are insoluble in water though they contain polar C-X bond. Explain.

• Alkyl halides are polar in nature but they are not able to form hydrogen bonds with water molecules. Hence they are sparingly soluble in water. But they are soluble in organic solvents like alcohols, ethers and benzene.

Dipole Moment:

Which one of the following has the highest dipole moment?

CH₂Cl₂, CHCl₃, CCl₄.

• The dipole moment of CH₂Cl₂ is the highest while that of CCl₄ is zero. Dipole moment of CH₂Cl₂ is greater than  CHCl₂ because the dipole moment of the third C-Cl bond of CHCl₃ opposes the dipole moment of the remaining two C-Cl bonds.

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Physical Properties of Alkyl Halides

The post Physical Properties of Alkyl Halides appeared first on The Fact Factor.

In the last three articles, we have studied the methods of preparations of alkyl halides from alkanes, alkenes, alcohols. In this article, we shall study the method of the preparation of alkyl halids by halide exchange method. This method is typical for preparations of alkyl iodides/fluorides/bromides.

Preparation of Alkyl iodides by Halide Exchange Method(Finkelstein Reaction)

General Reaction:

When alkyl chlorides or bromides when treated with NaI in presence of dry acetone give alkyl iodides.

R–Cl or R–Br  +  NaI → RI + NaCl  or NaBr

Alkyl chloride / bromide   sodium iodide   →    Alkyl iodide sodium chloride / bromide

Example – 1: Preparation of Ethyl iodide (Iodoethane) from ethyl chloride (Chloroethane):

C₂H₅-Cl    +      NaI   C₂H₅I      +   NaCl

Ethyl chloride    sodium iodide    Ethyl iodide  sodium chloride

Example – 2: Preparation of Ethyl iodide (Iodoethane) from ethyl bromide (Bromoethane):

C₂H₅-Br    +      NaI   C₂H₅I      +   NaBr

Ethyl bromide    sodium iodide   Ethyl iodide       sodium bromide

Preparation of Alkyl fluorides by Halide Exchange Method(Swart’s Reaction):

The direct reaction of alkanes with fluorine is highly explosive in nature, hence it can’t be produced by direct fluorination of alkanes.

General Reaction:

When alkyl halides are treated with salts like AgF, Hg₂F₂, CoF₃, SbF₃ fluoroalkanes can be obtained.

Example – 1: Preparation of Ethyl fluoride (Fluoroethane) from ethyl chloride (Chloroethane):

C₂H₅Cl          +     AgF    →  C₂H₅F           +   AgCl

Ethyl chloride   silver fluoride   Ethyl fluoride       silver chloride

Example – 2: Preparation of Methyl fluoride (Fluoromethane) from methyl bromide (Bromoethane):

2  CH₃Br          +         Hg₂F₂     →       2 CH₃F           +   Hg₂Br₂

Methyl bromide Mercurous fluoride  Methyl fluoride  Mercurous bromide

For replacement of two or three halogens CoF₃, SbF₃ are used

Example – 3

Preparation of Alkyl bromides from silver salt of fatty acid (Borodine Hunsdiecker Reaction)

General Reaction:

When silver salt of fatty acid is refluxed with bromine in CCl4, alkyl bromide is obtained.

RCOOAg   +    Br₂        RBr  +  CO₂ +  AgBr

Silver salt of fatty acid   bromine    alkyl bromide   silver bromide

Example -1: Preparation of Methyl Bromide (Bromomethane) from silver acetate:

CH₃COOAg   +    Br₂        CH₃Br  +  CO₂ +  AgBr

Silver acetate bromine    methyl bromide  silver bromide

Example -2: Preparation of Ethyl Bromide (Brommethane) from silver propionate:

C₂H₅COOAg   +    Br₂        C₂H₅Br  +  CO₂ +  AgBr

Silver propionate   bromine          ethyl bromide    silver bromide

Note:

Chloroalkanes can be obtained by this method but yield is very low. Iodoalkanes can not be obtained by this method because iodine forms easter with  silver salt of fatty acid.

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl Halides by Halide Exchange Method

The post Preparation of Alkyl Halides by Halide Exchange Method appeared first on The Fact Factor.

In the last two articles, we have studied the methods of preparation of alkyl halides from alkanes and alkenes. In this article, we shall study the preparation of alkyl halides from alcohols.

The Action of HX on Alcohols:

• The reaction is nucleophilic substitution.
• Methyl and primary alcohol undergo SN¹ mechanism while secondary and tertiary alcohols undergo SN² mechanism.
• Among halogen halides, HCl is the least reactive in nature. Because the chloride ion is a weaker nucleophile than bromide or iodide ions. Hence hydrogen chloride is mixed with anhydrous ZnCl2.
• In these reactions, anhydrous ZnCl2 not only acts as a dehydrating agent but also helps in the cleavage of C-O bond of the alcohol. ZnCl2 is Lewis acid and coordinates with the oxygen atom and thus weakens C-O bond. This results in the formation of carbocation which combines with Cl- ion to form chloroalkane. Tertiary alcohols are highly reactive hence for tertiary alcohol ZnCl2 is not required.
• This reaction is nucleophilic substitution. The stability of carbocations is of order tertiary > secondary > primary. Hence the order of reactivity also follows the same order tertiary > secondary > primary.
• The bond dissociation energy of H-X bond is of order H-Cl > H-Br > H-I. hence the reactivity of halogen acids follows the order HI > HBr > HCl.
• Unlike alkyl chlorides, the secondary and tertiary bromides and iodides cannot be obtained from their respective alcohols. It is because the secondary and tertiary alcohols on heating with concentrated H₂SO₄ undergo dehydration to form alkenes. Hence for these preparations dilute H₂SO₄ is used.

General Reaction:

R–OH  +            HX          →     R–X       +    H₂O

Alcohol     Halogen acid       alkyl halide

Order of Reactivity:

Tertiary alcohol > Secondary alcohol > Primary alcohol

and HI > HBr > HCl.

By Action of Haloacids: 

Preparation of Alkyl Chlorides:

General Reaction:

When alcohol is treated with Lucas reagent alkyl chloride is obtained. Lucas reagent is a solution of a concentrated hydrochloric acid with zinc chloride. This reaction is known as Groove’s process.

R–OH  +            HCl               R–Cl       +    H₂O

Alcohol     Conc.Hydrochloric acid     Alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

C₂H₅OH  +            HCl                C₂H₅Cl       +    H₂O

Ethyl alcohol   Conc.Hydrochloric acid       Ethyl chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Example – 3: (Preparation of tert- Butyl chloride (2-Chloro-2-methylpropane) from tert- Butyl alcohol (2-Methylpropan-2-ol):

Preparation of Alkyl Bromides:

General Reaction:

When alcohol is treated with concentrated hydrobromic acid (NaBr + H2SO4) alkyl bromide is obtained.

R–OH  +            HBr                R–Br       +    H₂O

Alcohol    Hydrobromic acid               Alkyl chloride

Example – 1: Preparation of ethyl bromide (Bromoethane) from ethyl alcohol (Ethanol):

C₂H₅OH  +            HBr                C₂H₅Br       +    H₂O

Ethyl alcohol    Hydrobromic acid                          Ethyl bromide

Example – 2: (Preparation of isopropyl bromide (2-Bromopropane) from isopropyl alcohol (Propan-2-ol):

Example – 3: (Preparation of tert- Butyl bromide (2-Bromo-2-methylpropane) from tert- Butyl alcohol (2-Methylpropan-2-ol):

Preparation of Alkyl Iodides:

General Reaction:

When alcohol is refluxed with Potassium or sodium iodide with 95% phosphoric acid and heated, alkyl iodide is obtained.

R–OH  +     HI     R–I       +    H₂O

Alcohol    Hydroiodic acid         Alkyl iodide

Example – 1: Preparation of ethyl iodide (Iodoethane) from ethyl alcohol (Ethanol):

C₂H₅OH   +       HI      C₂H₅I   +    H₂O

Ethyl alcohol    Hydroiodic acid           Ethyl iodide

Example – 2: (Preparation of isopropyl iodide (2-Iodopropane) from isopropyl alcohol (Propan-2-ol):

By Action of Phosphorous Halides: 

• This method is suitable for preparation of primary and secondary alkyl halides.
• A good yield of tertiary alkyl halides cannot be obtained by this method.
• The reaction of an alcohol with PX₃ does not involve the formation of carbocation and usually occurs without rearrangement of the carbon skeleton.

Preparation of Alkyl Chlorides Using PCl₃:

General Reaction:

When alcohol is treated with phosphorous trichloride, alkyl chloride and phosphorous acid are obtained.

3 R-OH  +            PCl₃            3 R-Cl       +    H₃PO₃

Alcohol       phosphorous trihalide    alkyl halide    phosphorous acid

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

3C₂H₅OH  +            PCl₃           3C₂H₅Cl       +    H₃PO₃

Ethyl alcohol   Phosphorous trichloride    ethyl chloride    Phosphorus acid

Both the products are in the liquid state and are separated by fractional distillation.

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Preparation of Alkyl Chlorides Using PCl₅:

General Reaction:

When alcohol is treated with phosphorous pentachloride, alkyl chloride and phosphoryl chloride (phosphorous oxychloride) are obtained.

ROH         +     PCl₅         RCl         +     POCl₃           +            HCl

Alcohol   Phosphorous pentachloride       Alkyl chloride    Phosphoryl chloride    Hydrogen chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

C₂H₅OH    +   PCl₅         C₂H₅Cl   +     POCl₃   +  HCl

Ethyl alcohol   Phosphorous pentachloride       Ethyl chloride    Phosphoryl chloride    Hydrogen chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Preparation of Alkyl Bromides:

Unlike PCl₃, the compounds PBr₃ and PI₃ are not stable and hence they are to be prepared when they are to be used by treating bromine and iodine with red phosphorous. Compounds PBr₅ and PI₅ do not exist.

Alkyl bromides are prepared by the action of bromine, in presence of red phosphorus, on alcohols.  Phosphorus bromide PBr₃, which is unstable is formed as intermediates in the reaction.

3 R-OH  +        PBr₃            3 R-Br       +    H₃PO₃

Alcohol       phosphorous tribromide    alkyl bromide    phosphorous acid

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

3C₂H₅OH  +        PBr₃          3C₂H₅Br   +    H₃PO₃

Ethyl alcohol   Phosphorous tribromide  Ethyl bromide  Phosphorus acid

Preparation of Alkyl Iodides:

Alkyl iodides are prepared by the action of iodine, in presence of red phosphorus, on alcohols.  Phosphorus iodide PBr₃, which is unstable is formed as intermediates in the reaction.

3 R-OH  +            PI₃            3 R-I       +    H₃PO₃

Alcohol       phosphorous triiodide    alkyl iodide    phosphorous acid

Example – 1: Preparation of ethyl iodide (Iodoethane) from ethyl alcohol (Ethanol):

3C₂H₅OH  +            PI₃            3C₂H₅I       +    H₃PO₃

Ethyl alcohol   Phosphorous triiodide          ethyl iodide    Phosphorus acid

By Action of Thionyl Chloride: 

This reaction is also known as Darzen’s Procedure.

General Reaction:

When alcohol is refluxed with thionyl chloride, alkyl chloride, sulphur dioxide and hydrogen chloride are obtained. Better yield is obtained if pyridine is added in a small amount.

ROH  +  SOCl₂        RCl  + SO₂ ↑­  + HCl  ­↑

Alcohol    Thionyl chloride              Alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

C₂H₅OH  +  SOCl₂        C₂H₅Cl  + SO₂ ↑­  + HCl  ­↑

Ethyl alcohol  Thionyl chloride                 Ethyl chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

The use of thionyl chloride for preparation of alkyl chlorides is most convenient because the other products of reaction (SO2 and HCI) being gases go off  (or can be expelled during distillation very easily). and hence the method requires no special purification/separation.

C₅H₁₁OH on bromination gives compound C₅H₁₁Br. Suggest all possible structures of alcohol and bromide.

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl halides From Alcohols

The post Preparation of Alkyl halides From Alcohols appeared first on The Fact Factor.

In the last article, we have studied the preparation of alkyl halides from alkanes. In this article, we shall study the preparation of alkyl halides from alkenes (olefins).

The Action of HX on Alkenes:

• It is an electrophilic addition reaction.
• Carbonium ion is formed as an intermediate.
• This method is more useful to prepare secondary and tertiary alkyl halides.
• For unsymmetric alkene, Markownikoff’s rule should be applied.
• Hydrogen chloride and hydrogen iodide add according to Markownikoff’s rule.
• The addition of hydrogen bromide takes place according to Markownikoff’s rule only when the reaction is carried out in the total absence of light or oxygen or peroxides. In the presence of any one of these agents the addition of HBr takes place in exactly the reverse way and is called Peroxide e1ffect or anti-Markownikoff addition or Kharasch effect or Kharasch-Mayo effect.

General Reaction

R –CH=CH₂ +  H– X  → R –CH₂–CH₂X       OR    R –CH₂X–CH₂

Alkene      Hydrogen halide           Alkyl halide

Conversion of -C=C- (Alkenes) into -X (Alkyl halides)

Order of Reactivity for Halogen acids:

HI   > HBr > HCI

Preparation of Alkyl Halides From Symmetrical Alkenes: 

R –CH=CH–R   +        H–X      →  R –CH₂–CHX–R

Symmetric alkene         Hydrogen halide            alkyl halide

Preparation of Alkyl Chlorides / Alkyl Bromides / Alkyl Iodides:

R –CH=CH–R   +        H–Cl      →  R –CH₂–CHCl–R

Symmetric alkene        Hydrogen chloride      alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from Ethylene (Ethene):

H₂C=CH₂  +        H–Cl      →  CH₂–CH₂Cl

Ethylene      hydrogen chloride     Ethyl chloride

Example – 2: Preparation of ethyl bromide (Bromoethane) from Ethylene (Ethane):

H₂C=CH₂  +        H–Br      →  CH₂–CH₂Br

Ethylene        Hydrogen bromide    Ethyl bromide

Example – 3: Preparation of Ethyl iodide (iodoethane) from Ethylene (Ethane):

H₂C=CH₂  +        H–I      →  CH₂–CH₂I

Ethylene      Hydrogen  iodide    Ethyl iodide

Example – 4: Preparation of sec-butyl chloride (2-Chlrobutane) from Butylene (But-2-ene):

CH₃ –CH=CH–CH₃  +        H–Cl        →  CH₃ –CH₂–CH₂Cl–CH₃

butylene                Hydrogen chloride       sec-butyl chloride

Example – 5: Preparation of sec-butyl bromide (2-Bromobutane) from β-butylene (But-2-ene):

CH₃ –CH=CH–CH₃  +        H–Br        →  CH₃ –CH₂–CH₂Br–CH₃

butylene                Hydrogen bromide       sec-butyl bromide

Preparation of Alkyl Halides From Unsymmetrical Alkenes: 

R –CH=CH–R’ +  H–X  →  R –CH₂–CHX–R’  or R –CHX–CH₂–R’

Unsymmetric alkene         Hydrogen halide            alkyl halide

Markownikoff’s Rule:

When an unsymmetrical reagent (like HBr) is added to an unsymmetrical alkene, (in the total absence of oxygen and peroxide and light) then the negative part of the reagent gets attached to that unsaturated carbon atom which carries less number of hydrogen atoms.

This behaviour is explained by 1,2-hydride shift to attain greater stability of cation.

Example – 6: Preparation of isopropyl bromide (2-Bromopropane) from propene:

Example – 7: Preparation of sec-butyl bromide (2-Bromobutane) from α-butylene (But-1-ene):

Example – 8: Preparation of tert-butyl bromide (2-Bromo-2-methylpropane) from iso-butylene (2-Methylpropene):

Example – 9: Preparation of 2-Bromo-2-methylbutane from 3-Methyl-but-ene:

The common name of 3-Methyl-but-1-ene is alpha-isoamylene. The above reaction is an example of the 1,2-hydride shift to attain greater stability of cation.

Anti-Markownikoff’s Rule OR Kharasch Effect OR Kharasch Mayo Effect OR Peroxide Effect:

When an unsymmetrical reagent (like HBr) is added to an unsymmetrical alkene in presence of oxygen or peroxide or light then the negative part of the reagent gets attached to that unsaturated carbon atom which carries more number of hydrogen atoms.

Example – 10: Preparation of n-propyl bromide (1-Bromopropane) from propene:

Example – 11: Preparation of n-butyl bromide (1-Bromobutane) from α-butylene (But-1-ene):

Notes:

• This reaction is a free radical addition and exothermic reaction.
• H-Cl has a bond energy of 103 Kcal/mol which is stronger than H-Br bond energy (87 Kcal/mol). Hence there is no breaking up of the H-Cl bond due to peroxide free radicals.
• H-I has a bond energy of 78 Kcal/mol. It forms free radicals easily but instead of attacking the double bond, the iodine radicals formed combine with each other to form an iodine molecule.

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl halides From Alkenes

The post Preparation of Alkyl halides From Alkenes appeared first on The Fact Factor.

In this article, we shall study methods of preparation of alkyl halides from alkanes. Alkyl halides can be prepared from alkanes by their halogenation.

General Reaction:

R – H  +  X– X  →  R – X     +     H – X

Alkane    Halogen    Alkyl halide  Halogen acid

Conversion of -H (Alkanes) into – X (Alkyl halides)

Order of Reactivity for Halogens: 

F₂ > > Cl₂ > Br₂ > I₂

Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl

Preparation of Alkyl Chlorides From Alkanes:

General Reaction:

When alkane is treated with chlorine in presence of ultraviolet light or diffused sunlight alkyl chlorides or chloroalkanes are obtained. The reaction gives a mixture of all possible chloroalkanes.

R – H  +  Cl– Cl  R – Cl     +     H – Cl

Alkane         Chlorine                Alkyl chloride Hydrogen chloride

As the reaction is taking place in the presence of ultraviolet light or diffused sunlight the reaction is known as photohalogenation of alkanes.

Example –1: Preparation of methyl chloride (Chloromethane) from methane

CH₄  +   Cl₂  CH₃Cl       +      HCl

Methane      Chlorine         Methyl Chloride    Hydrogen Chloride

Example –2: Preparation of ethyl chloride (Chloroethane) from ethane

C₂H₆     +    Cl₂  C₂H₂Cl         +      HCl

Ethane       Chlorine                   Ethyl  Chloride       Hydrogen Chloride

Preparation of Alkyl Bromides From Alkanes:

General Reaction :

When alkane is heated with bromine in presence of anhydrous AlBr3 as catalyst alkyl bromide or bromoalkane is obtained.

R – H  +  Br– Br    R – Br     +     H – Br

Alkane         Bromine                Alkyl bromide    Hydrogen bromide

Example- 1: Preparation of ethyl bromide (Bromoethane) from ethane:

C₂H₆     +    Br₂    C₂H₂Br         +      HBr

Ethane     Bromine               Ethyl  bromide Hydrogen  bromide

Example- 2: Preparation of methyl bromide (Bromomethane) from methane:

CH₄  +   Br₂   CH₃Br       +      HBr

Methane      Chlorine         Methyl Chloride   Hydrogen Chloride

Preparation of Alkyl Iodides From Alkanes:

With iodine reaction is reversible. Thus we can not get a good yield of alkyl iodide. Hence direct iodination is difficult.

R – H      +  I – I     ⇌   R – I     +       H – I

Alkane         Iodine      Alkyl iodine     Hydrogen iodine

If the reaction is carried out in the presence of iodic acid HIO₃, or mercuric oxide HgO, or dilute HNO₃ which can oxidise HI formed and the lodoalkane can be obtained. The iodination reaction stops at mono-iodo state.

General Reaction Using Iodic Acid HIO₃:

5 R-H   +  2 I₂ +   HIO₃  →    5 R-I  +  3 H₂O

Alkane   Iodine  Iodic acid    Alkyl iodide

General Reaction Using Mercuric Oxide HgO:

2R-H  +  2 I₂  +    HgO      →       2 R-I  +  Hg I₂ + H₂O

Alkane   Iodine  mercuric oxide        Alkyl iodide

General Reaction Using Dilute Nitric Acid HNO₃:

8R-H  + 4 I₂    +  HNO₃  →       8R-I   + 3H₂O    +   NH₃

Alkane   Iodine    nitric acid     Alkyl iodide                ammonia

Example-1: Preparation of ethyl iodide (Iodoethane) using HIO₃:

5 C₂H₆   +  2 I₂ +   HIO₃  →    5 C₂H₅I  +  3 H₂O

Ethane   Iodine  Iodic acid         Ethyl iodide

Example-2: Preparation of ethyl iodide (Iodoethane) using HgO:

2C₂H₆ +  2 I₂  +    HgO      →       2 C₂H₅I  +  HgI₂ + H₂O

Ethane    Iodine  mercuric oxide     Ethyl iodide

Example-3: Preparation of ethyl iodide (Iodoethane) using HNO₃:

8C₂H₆ + 4 I₂    +  HNO₃  →       8C₂H5I   + 3H₂O    +   NH₃

Ethane   Iodine  Nitric acid             Ethyl iodide        Ammonia

Drawbacks/Disadvantages of direct halogenation (Photohalogenation):

After forming alkyl halide the reaction proceeds further and poly-substitution takes place and gives a mixture of di, tri, tetra etc. haloalkanes.

CH₄  +   Cl₂ CH₃Cl  (Chloromethane) +  HCl

CH₃Cl   +   Cl₂ → CH₂Cl₂ (Dichloromethane) +  HCl

CH₂Cl₂  +   Cl₂ →  CHCl₃  (Trichloromethane) +  HCl

CHCl₃  +   Cl₂  →  CCl₄  (Tetrachlromethane) +  HCl

This reaction continues till all halogens in the alkane are replaced one by one by chlorine or bromine. Thus we get mixture of di, tri, tetra etc. haloalkanes.

The mixture contains less amount of alkyl chlorides or bromides. Chlorination and bromination reactions are not selective. Hence they may give isomers of the monohalogen derivatives of alkanes. Besides the separation of constituents is difficult.

Iodination of alkanes is reversible reaction it requires additional reagents like mercuric oxide or iodic acid or nitric acid for obtaining alkyl iodes. Hence direct halogenation is not the suitable method for the preparation of alkyl halides.

Notes:

• Thermal chlorination of alkanes is called Hare and Mc Bee reaction. It is carried about at 673K.
• Diffused sunlight causes homolytic fission of halogen, hence the reaction is a free radical substitution.
• If alkanes are used in excess, the major product is monohalogen derivatives of alkanes. i.e. alkyl halides.
• The ease of substitution of different types of a hydrogen atom is Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl
• Thus, an alkane containing a tertiary hydrogen atom would be more reactive towards substitution reaction than with only primary hydrogen atom or primary and secondary hydrogen atoms. Isobutane would undergo substitution more rapidly than propane or ethane.
• Alkyl fluorides are not prepared by fluorination of alkanes, because the reaction is highly explosive in nature.
• Propane on chlorination gives a mixture of 2- Chloropropane (55 %) and 1- Chloropropane (45 %)
• Butane on chlorination gives a mixture of n-butyl chloride (28 %) and sec-butyl chloride (72 %)
• iso-Butane on chlorination gives a mixture of tert-butyl chloride (64 %) and isobutyl chloride (36 %)
• Bromination of alkanes is electrophilic substitution reaction, as it forms AlBr4 ion and Br+ ion. Br+ ion takes part in substitution.

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl Halides From Alkanes

The post Preparation of Alkyl Halides From Alkanes appeared first on The Fact Factor.

In this article, we shall study isomerism in alkyl halides and their nomenclature.

Isomerism in Haloalkanes: 

Isomers are the organic compounds, which have the same molecular formula but different structural formula and properties. The phenomenon is called isomerism. Haloalkanes can exhibit the following type of isomerism.

Chain Isomerism:

The haloalkanes with four or more carbon atoms exhibit this type of isomerism. This isomerism is exhibited due to the difference in the carbon chains.

Position Isomerism:

The haloalkanes with three or more carbon atoms exhibit this type of isomerism. This isomerism is exhibited due to the difference in the position of halogen group.

Optical Isomerism:

The haloalkanes which have the same molecular and structural formula, but have a different arrangement of the atoms or group of atoms in space and have a tendency to rotate the plane of polarised light are called optical isomers and the phenomenon is called optical isomerism.

More Examples:

Isomers of monochloro derivatives of 2,3-Dimethylbutane:

Structure of 2,3-Dimethylbutane is

The isomers of monochloro derivatives of 2,3-Dimethylbutane are

Isomers of C₅H₁₁Br and their classification:

Note: 2 – Bromo-2-methylbutane, 2 – Bromo-3-methylbutane, 1 – Bromo-3-methylbutane are enantiomers. i.e. they are optically active compounds

Isomers of C₄H₉Br and their classification: 

Classification of alkyl halides:

1-Bromopropane 

Bromine atom is attached to a primary carbon i.e. this carbon is attached to only one carbon atom. Hence it is primary alkyl halide.

2-Bromopropane 

Bromine atom is attached to a secondary carbon i.e. this carbon is attached to two other carbon atoms. Hence it is secondary alkyl halide.

2-Bromo-2-methylpropane

Bromine atom is attached to a tertiary carbon i.e. this carbon is attached to three other carbon atoms. Hence it is tertiary alkyl halide.

Nomenclature:

Trivial or Common System of Nomenclature:

In the trivial system, haloalkanes are named as alkyl halides. The name is derived by adding the word halide to the name of the corresponding alkyl group. The trivial name is always written as two separate words.

e.g. CH₃Br (Methyl bromide), CH₃CH₂Br (Ethyl bromide), CH₃CH₂Cl (Ethyl chloride).

Different Alkyl Groups With Examples:

Sr.	Alkyl Group	Name of Group	Example	Compound Name
1	CH3–	Methyl	CH3Cl	Methyl chloride
2	CH3CH2–  or C2H5–	Ethyl	C2H5Br	Ethyl bromide
3	CH3CH2CH2–	n-Propyl	CH3CH2CH2I	n-Propyl iodide
4		iso-Propyl	CH3CHBrCH3	iso-Propyl bromide
5	CH3CH2CH2CH2–	n-Butyl	CH3CH2CH2CH2I	n-Butyl iodide
6		sec-Butyl	CH3CHClCH2CH3	sec-Butyl iodide
7		iso-Butyl		iso-Butyl chloride
8		tert-Butyl		tert-Butyl bromide
9		neo-Pentyl		neo-Pentylbromide

IUPAC Nomenclature:

To Draw Structure From IUPAC Name:

No.	IUPAC Name	Structure
1	2-Iodo-3-methylpentane
2	3-Chlorohexane
3	1-Chloro-2,2-dimethylpropane
4	3-Bromo-2-methylpentane
5	2-Bromo-3-ethyl-2-methylhexane
6	1-Chlorobutane	CH3-CH2-CH2-CH2Cl
7	2-Bromo-2-methylpentane
8	1-chloro-2,2-dimethylpropane

Science > Chemistry > Halogen Derivatives of Alkanes > Nomenclature of Alkyl Halides

The post Nomenclature of Alkyl Halides appeared first on The Fact Factor.

In this article, we shall study halogen derivatives of alkanes or haloalkanes.

Organic Compounds Containing Halogens: 

If one or more hydrogen atom is replaced in hydrocarbon by an equivalent number of halogen, the compounds obtained are called halogen derivatives of hydrocarbons. Halogen derivatives of hydrocarbons are further classified as aliphatic halogen compounds and aromatic halogen compounds. Aliphatic halogen compounds are obtained by replacing one or more hydrogen of aliphatic hydrocarbons (alkanes, alkenes, and alkynes).

Aliphatic Halogen Compounds:

Aliphatic halogen compounds are further classified as haloalkanes, haloalkenes and haloalkynes.

Haloalkanes:

• CH₃Cl (Chloromethane)
• CH₂Cl₂ (Dichloromethane)
• CHCl₃ (Trichloromethane / Chloroform)
• CCl₄ (Tetrachloromethane / Carbon tetrachloride)
• CH₃-CH₂-CH₂I   (1-Iodopropane )
• CH₃-CH₂-CH₂-CH₂-Cl    (1-Chlorobutane)

Haloalkenes:

• CH₂= CH-Cl (Chloroethene)
• CH₂= CH-CH₂-I (3-Iodoprop-1-ene)
• CH₃-CH = CH-CH₂-Br  (4-Bromobut-2-ene)

Haloalkynes:

• CH≡C-Cl  (Chloroethyne)
• CH≡C-CH₂I   ( 3-Iodoprop-1-yne)
• CH₃-C≡C-CH₂-Br  (4-Bromobut-2-yne)

Aromatic Halogen Compounds:

Aromatic hydrocarbons are called arenes. Halogen derivatives of arenes are called aromatic halogen compounds. Aromatic halogen compounds are further classified as nuclear halogen derivatives and side chain halogen derivatives.

When a hydrogen atom is directly attached to an aromatic ring is replaced by a halogen atom, the compound obtained is called nuclear halogen derivative of arene or halo arene or aryl halide.

When a hydrogen atom present in a side chain attached to an aromatic ring is replaced by a halogen atom, the derivative obtained is called side chain derivative. The side chain halogen derivatives are regarded as the aryl derivatives of haloalkane.

Halogen Derivatives of Alkanes:

Saturated aliphatic hydrocarbons are called alkanes. Their general formula is C_nH_{2n + 2}. When one or more hydrogen atoms of saturated aliphatic hydrocarbons or aromatic hydrocarbons are replaced by the corresponding number of halogen atoms, (Cl, Br, I), then the new compounds obtained are called, halogen derivatives of alkanes or of arenes. In haloalkanes, the halogen atom is attached to the sp³ hybridized carbon atom.

Examples of Halogen Derivatives of Alkanes:

• CH₃Cl (Methyl chloride) (Chloromethane),
• C₂H₅Br (Ethyl bromide) (Chloroethane).

Examples of Halogen Derivatives of Arenes:

In haloarenes, halogen atom is attached to the sp² hybridized carbon atom.

Classification Halogen Derivatives of Alkanes:

Classification on the Basis of Number of Halogen Atoms: 

Depending upon the number of halogen atoms in halogen derivatives of alkanes are classified as monohalogen derivatives of alkanes and polyhalogen derivatives of alkanes. Polyhalogen derivatives of alkanes are further classified as dihalogen, trihalogen, tetrahalogen derivatives of alkanes and so on.

Monohalogen Derivatives of Alkanes:

When one hydrogen atom of an alkane is replaced by one halogen atom, then the compound obtained is called a monohalogen derivative of alkanes. They are commonly called as alkyl halides or haloalkenes. Their general formula is C_nH_{2n + 1}X. e.g. CH₃Cl (Methyl chloride),  C₂H₅Br (Ethyl bromide)

Dihalogen Derivatives of Alkanes:

When two hydrogen atoms of alkane are replaced by two halogen atoms, then the compound obtained is called a dihalogen derivative of alkanes. Their general formula is C_nH_2nX₂. e.g. C₂H₄Cl₂ (Dichloroethane).

Trihalogen Derivatives of Alkanes:

When three hydrogen atoms of an alkane are replaced by three halogen atoms, then the compound obtained is called a trihalogen derivative of alkanes. Their general formula is C_nH_2n-1X₃. e.g. CHCl₃ (Chloroform)

Tetrahalogen Derivatives of Alkanes:

When four hydrogen atoms of an alkane are replaced by four halogen atoms, then the compound obtained is called a tetrahalogen derivative of alkanes. Their general formula is C_nH_2n-2X₄. e.g. CCl₄ (Carbon tetrachloride)

Monohalogen Derivatives of Alkanes OR Alkyl Halides:

When one hydrogen atom of an alkane is replaced by one halogen atom, then the compound obtained is called a monohalogen derivative of alkanes. They are commonly called as alkyl halides or haloalkenes. Their general formula is C_nH_{2n + 1}X. Where X is either Cl, Br or I.  They are also represented by a general formula R-X, where R is an alkyl group and X is a halogen. e.g. CH₃Cl (Methyl chloride),  C₂H₅Br (Ethyl bromide)

Classification of Monohalogen Derivatives of Alkanes:

Classification on the Basis of Type of Carbon Atom:

Depending upon the type of carbon atom to which halogen is attached alkyl halides are classified into three types.

Primary Alkyl Halides:

A primary carbon atom is an atom which is bonded to only one other carbon or to none. The primary carbon atom is denoted by 1°.  In primary alkyl halide, the halogen is attached to primary (1°) carbon atom.

CH₃-CH₂-CH₂-I            CH₃-CH₂-CH₂-CH₂-Cl

1-Iodopropane           1-Chlorobutane

Secondary Alkyl Halides:

A secondary carbon atom is an atom which is bonded to two other carbon atoms. The secondary carbon atom is denoted by 2°.  In secondary alkyl halide, the halogen is attached to secondary (2°) carbon atom.

R and R’ may be same or different

Tertiary Alkyl Halides:

A tertiary carbon atom is an atom which is bonded to three other carbon atoms. The tertiary carbon atom is denoted by 3°.  In tertiary alkyl halide, the halogen is attached to tertiary (3°) carbon atom.

R, R’ and R’’ may be same or different

Classification of Monohalo Compounds on the Basis of Hybridization State of the carbon in C-X Bond: 

Compounds containing C_sp3 – X Bond:

In this type of monohalo compounds, the halogen atom is directly bonded to an sp³ hybridized carbon atom. Such compounds are further classified as follows

Haloalkanes or Alkyl Halides:

In this type, the halogen atom is attached to the alkyl group denoted by R. Their general formula is C_nH_{2n + 1}X. They are further classified as primary, secondary and tertiary alkyl halides.

If R is alicyclic, then R-X is called halocycloalkane or cycloalkyl halide. Such compounds are either secondary or tertiary.

Allylic Halides:

The halides in which halogen is attached to an sp³ hybridized carbon atom next to carbon-carbon double bond are called allylic halides. Such carbon is called allylic carbon.

Note: Allylic halides may be primary, secondary or tertiary.

Benzylic Halides:

The halides in which halogen is attached to sp³ hybridized carbon atom next to aromatic ring are called benzylic halides. Such carbon is called benzylic carbon.

Note: Benzylic halides may be primary, secondary or tertiary.

Compounds containing C_sp2 – X Bond:

In this type of monohalo compounds, the halogen atom is directly bonded to an sp² hybridized carbon atom. Such compounds are further classified as

Vinylic Halides:

The halides in which halogen is attached to one of the sp² hybridized carbon atoms of carbon double bond are called vinylic halide. Such carbon is called vinylic carbon. The halogen atom is attached to one of the carbon atoms of carbon-carbon double bond.

Aryl Halides:

In aryl halides, halogen is directly attached to one of the sp2 hybridized carbon atoms of the benzene ring.

Compounds containing C_sp – X Bond:

In this type of monohalo compounds, the halogen atom is directly bonded to an sp hybridized carbon atom.

CH-C≡Cl              CH₃-C≡C-I

Chloroethyne           1-Iodoprop-1-yne

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Introduction

The post Introduction to Halogen Derivatives of Alkanes appeared first on The Fact Factor.