NEET Chemistry Notes Haloalkanes and Haloarenes – Preparation of Aryl Halides
Preparation of Aryl Halides
Preparation of Aryl Halides
These are prepared by the following methods:
- By Electrophilic Substitution of Arenes
It is carried out in the presence of a halogen carrier (i.e. Lewis acid) likeFeCl3,FeBr3 in dark and at low temperature.
In this reaction, benzene diazonium chloride is warmed with cuprous chloride/bromide in HCl to obtain chloro/bromo arenes.
Borodine-Hunsdiecker reaction (used to produce alkyl bromide) proceeds through free radical mechanism and is used to reduce the length of carbon chain.
Physical Properties of Haloalkanes and Haloarenes
The physical properties of alkyl halides and aryl halides are given below:
Boiling and Melting Point
- Alkyl halides and haloarenes are polar compounds and have higher boiling point due to dipole-dipole interaction. Alkyl/aryl halides have higher boiling point than hydrocarbons of comparable molecular mass.
- Decreasing order of boiling point of alkyl halides is as, Rl> RBT> RCA> RF
This is because with the increase in size and mass of halogen atom, the magnitude of van der Waals’ forces increases.
- Straight chain alkyl halides have higher boiling point as compared with branched chain alkyl halide of similar molecular weight, hence boiling points of isomeric haloalkanes decrease with increase in branching due to decrease in surface area, e.g.
- The para isomers of dihalobenzenes has high melting point as compared to their ortho and meta isomers. It is due to symmetry of para isomers that fits in crystal lattice as compared to ortho and meta isomers.
Density and Solubility
Bromo and iodo derivatives of hydrocarbons are heavier than water.
Lower haloalkanes are very slightly soluble in water but others are insoluble in water because haloalkanes/ haloarenes do not form hydrogen bond with water.
Chemical Properties of Haloalkanes
Haloalkanes are highly reactive compounds due to the presence of polar R—X bond. The reactions of haloalkanes may be of two types. These are nucleophilic substitution reactions and elimination reactions.
Nucleophilic Substitution Reactions
In haloalkanes, these reactions proceed either by SN 1 or by SN 2 type of mechanism.
Nucleophilic Substitution Unimolecular Reaction (SN1)
- It follows first order kinetics, rate = k [substrate],
- Reaction is completed in two steps.
- Carbocation is formed and gets rearranged, if possible.
- Nucleophile can attack from front and back side. Therefore, racemic mixture can be formed.
- Tertiary alkyl group and polar solvent favour SN 1
Nucleophilic Substitution Bimolecular Reaction (SN 2)
- It follows second order kinetics, rate = k [substrate] [nucleophile]
- Reaction is completed in one step.
- Transition is formed.
- It involves complete inversion in configuration as the attack of the nucleophile occurs from the back side of reactant, e.g.
- Primary alkyl group and non-polar solvent favour SN 2
- Reactivity of halides towards SN 2mechanism is 1° > 2° > 3° > neo-pentyl halides.
- In SN 2 mechanism, rate of reaction does not depend upon the strength of attacking nucleophile.
The strength of different nucleophiles is.