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

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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

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