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.
They can be classified on the basis of:
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,
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 CnH2n+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.
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.
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.
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:
The common and IUPAC names of some representative haloarenes are given below:
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.
(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:
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.
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