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

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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

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