Classification & Nomenclature of Haloalkanes & Haloarenes Notes | Study Chemistry for JEE - JEE

What are Haloalkanes and Haloarenes?

When a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by halogen atoms then the compounds are termed haloalkanes and haloarenes. If a hydrogen atom is replaced from an aliphatic hydrocarbon by a halogen atom the resulting compound formed is called haloalkane. It is also known as alkyl halide and halogenalkane.

However, if a hydrogen atom is replaced from an aromatic hydrocarbon by a halogen atom the resulting compound formed is known as haloarene. It is also known as aryl halide or halogenoarene. In a haloalkene (R – X), X represents the halogen group. It is attached to an sp₃ hybridized atom of an alkyl group whereas in haloarene (Ar – X) the halogen is attached to an sp₂ hybridized atom of an aryl group.

Classification of Haloalkanes and Haloarenes

They can be classified on the basis of:

• Number of Halogen atoms
• Compounds with sp³ C—X Bond
• Compounds having the sp² C-X Bond

1) Number of Halogen Atoms:

On the basis of the number of hydrogen, they can be divided into mono, di or poly (tri, tetra, and so on) compounds of haloalkanes and haloarenes. It is named depending on the number of halogen atom these compounds contain in their structures. For example,

2) Compounds with sp³ C—X Bond:

These compounds can be further divided into three types. They are:

i) Alkyl Halides/Haloalkanes (R – X)

In this class, the halogen atom is attached to an alkyl group. The general homologous formula followed by this class is C_nH_2n+1X. They are further classified into mainly three types on the basis of the carbon atom to which the carbon-bearing halogen (X) atom is bonded- primary, secondary, and tertiary. This classification is based on the nature of the carbon atom to which the halogen is attached.

ii) Allylic Halides

This classification of compounds is formed by bonding of halogen group having sp₃ hybridized carbon atom present next to a carbon-carbon double bond structure (C=C). The carbon-carbon double bond structure is also known as allylic carbon. Thus, the name allylic halides.

(iii) Benzylic Halides

This type of compounds is formed when a halogen atom is attached to an sp₃ hybridized carbon atom. The sp₃ hybridized carbon atom should be present next to an aromatic ring in order to form benzyl halides.

3) Compounds Having the sp² C-X Bond

This class of compounds includes vinyl halides and aryl halides.

i) Vinyl Halides

These compounds are formed when a halogen atom is attached to an sp₂ hybridized carbon atom present next to a carbon-carbon double bond (C=C).

ii) Aryl Halides

This class of compounds is formed when the halogen group is bonded to an sp₂ -hybridized atom of carbon in an aromatic ring.

Nomenclature Of Haloalkanes And Haloarenes

• Initially, there was no proper system for the naming of compounds. Mostly there were trivial names that were used depending upon the country and region. These trivial names were based on the discoverer or the nature of the compound or its place of discovery.
• The system of trivial names was not standard and led to much confusion, thus raising the need for a standard system for the naming of organic compounds. IUPAC came up with a set of rules that are used universally for the naming of organic compounds.

There are two names associated with every compound:

Common name: It is different from a trivial name in the sense that it also follows a rule for its nomenclature.

IUPAC name: The IUPAC (International Union of Pure and Applied Chemistry) naming system is the standard naming system that chemists generally use.

Rules of Nomenclature

1. Find the longest carbon chain.
2. Number the longest carbon chain such that the carbon atom(s) to which the halogen(s) is/are attached get the lowest number(s).
3. Multiple halogen atoms are labelled with the Greek numerical prefixes such as di, tri, tetra, to denote the number of identical halogen atoms attached to a carbon atom. If more than one halogen atoms attached to the same carbon atom, the numeral is repeated that much time.
4. In case, different types of halogens are attached, they are named alphabetically.
5. The position of the halogen atom is indicated by writing the position and name of the halogen just before the name of the parent hydrocarbon.

Methodology of Writing Name

• First, write the root word for the parent hydrocarbon (depending upon the no. of carbon atoms in the longest carbon chain).
• Secondly, calculate the number of halogen atoms present. If there are multiple halogen atoms present, then arrange the halogens alphabetically in the prefix, labelling them with their respective positions. But, if the same halogen atom is present more than once then use the prefixes di, tri, tetra, etc.

Nomenclature of Haloalkanes

Alkyl halides are named in two ways. In the common system, the alkyl group is named first followed by an appropriate word chloride, bromide, etc. The common name of an alkyl halide is always written as two separate words. In the IUPAC system, alkyl halides are named haloalkanes. The other rules followed in naming compounds is:

• Select the longest chain of carbon atoms containing the halogen atom.
• Number the chain to give the minimum number to the carbon carrying halogen atom.
• If multiple bonds (double or triple bond) is present, then it is given the preference in numbering the carbon chain.
• The IUPAC name of any halogen derivative is always written as one word.

Nomenclature of Haloarenes

• Aryl halides are named by prefixing “halo” to the name of the parent aromatic hydrocarbon.
• If there is more than one substituent on the ring then the relative positions of the substituents are indicated by mathematical numerals.
• In the common system, the relative position of two groups is shown by prefixes ortho, meta or para.

The common and IUPAC names of some representative haloarenes are given below:

Nature of C-X Bond

It is essential to understand the nature of the C-X bond because it determines the reactivity of the compound having this kind of bond. The C-X bond is highly polar in nature because halogen atoms are electronegative and the carbon atom is electropositive. The difference in electronegativity results in the withdrawal of electron density from the sigma bond pair towards the halogen atom.

This result in the polarization of the C-X bond is polarized in a manner that the carbon atom develops a partial positive charge whereas the halogen atom in the bond develops a partial negative charge. Therefore, the carbon-halogen bond of an alkyl halide is polarized. This is represented as:

The electronegativity of the halogen group varies from one another. The size increases as we go down the group so the fluorine atom is the smallest one in the group and the iodine atom is the largest. Thus, fluorine has the highest electronegativity followed by chlorine then bromine and finally iodine.

Electronegativity of X: F(3.98) > Cl(3.16) > Br(2.96) > I(2.66). However, the electronegativity of the carbon atom is 2.55. The electronegativity difference between C-F is maximum. Therefore, C-F is the most polar among all of them.

Parameters Related to Nature of C-X Bond

(i) Bond Length (A₀)

The nature of the C-X bond depends upon the bond length between the carbon atom and halogen group. We have previously mentioned that size of the halogen group increases as we move down the group (F < Cl < Br < I). Consequently, the difference in the C-F bond will be the smallest and the C-I bond will be the largest.

(ii) Bond Enthalpy Order

The nature of the C-X bond depends upon bond enthalpy order. The size of the carbon and fluorine atom is very similar so the orbitals overlap (2p-2p overlap) into one another. This leads to the formation of a very strong bond. In C-I the atomic size of iodine is very large in comparison to carbon atom so the orbital interaction is very weak.

This results in the formation of weak bond strength. We can conclude that less the bond length stronger will be the bond. Hence, the bond length of C-F is 1.39 A⁰. The stronger the bond, the amount of energy required increases to break that bond. Therefore, C-F has the highest bond enthalpy. The bond enthalpy order is: C − F > C − Cl > C − Br > C – I

(iii) Dipole Moment

Dipole moment helps to calculate the polarity of a chemical bond within a molecule. It occurs due to the separation of positive and negative charges. It is the product of both charge and the distance between them. Bond dipole (μ) is given by the formula μ = q × d

The order of dipole moment in C-X is CH₃Cl > CH₃F > CH₃Br > CH₃I.
Dipole moments of haloalkanes are:

1. CH3F − 1.847D, 
2. CH₃Cl − 1.860 D, 
3. CH₃Br − 1.830 D, 
4. CH₃I − 1.636 D

Can you notice the abnormal order of the dipole moment in the case where CH₃Cl > CH₃F? Even though fluorine is more electronegative than Chlorine but the C-F bond (139 pm) is shorter than the C-Cl bond C − Cl (178 pm). Thus, the dipole moment will be lower in the case of CH₃F in comparison to CH₃Cl.