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Alkyl halides’ characteristics

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Topic updated on 10/13/2020 12:42pm

Physical state and smell

Haloalkanes are colorless, sweet-smelling liquids. The lower members like methyl chloride, methyl bromide and ethyl chloride are colorless gases while members having very high molecular masses are solids.

Solubility

Haloalkanes are not able to form hydrogen bonds with water and, even though they are polar in nature, they are practically insoluble in water. However, they are soluble in organic solvents like alcohol, ether, benzene, etc.

Density

Chloroalkanes are lighter than water while bromides and alkyl iodides are heavier. With the increase in the size of the alkyl group, the densities go on decreasing in the order of :

fluoride > chloride > bromide > iodide.

Boiling points

The boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl where R is an alkyl group. With the increase in the size of halogen, the magnitude of Van der Waals forces increases and, consequently, the boiling points increase. Also, for the same halogen atom, the boiling points of haloalkanes increase with increase in the size of alkyl groups.

Alkyl halides are named as primary, secondary or tertiary depending on the number of H atoms attached to the carbon atom which carries the halogen atom.
primary
secondary
tertiary
Due to the high electronegativity of the halogen atom relative to the carbon atom, the C – X bond is polarized. As a result, there is a deficiency of electrons in that carbon atom. Therefore, nucleophiles attack this position. Nucleophiles are reagents which show a tendency to attack carbon nuclei and which are basic and rich in electrons. A nucleophile will bring a pair of electrons to form a bond with carbon.
Examples- OH– , CN– , R-C≡C– , R-O–
Nucleophilic substitution reactions are characteristic of alkyl halides. Here, the carbon atom forms a new bond with the nuclephile and the halogen atom leaves as a halide ion.
As a nucleophile possesses a pair of electrons available to form a new bond any nucleophile can also act as a base by forming a bond with H+ . Therefore, when an alkyl halide is reacted with a reagent such as :OH– , it can also undergo an elimination reaction by the mechanism shown below.

Here, :OH– instead of reacting as a nucleophile with carbon, reacts as a base and removes a H+ from the carbon atom adjacent to the carbon atom bearing halogen. The hydrogen atoms attached to the carbon atom adjacent to the carbon atom bearing the halogen atoms, have a low acidity due to the polarization of the C-X bond. Thus substitution and elimination are competing reactions in alkyl halides.

Alkyl halides react with Mg in the medium of dry ether to form the Grignard reagent
When an alkyl halide forms a Grignard reagent the polaritiy of the carbon atom originally joined to halogen, changes as shown below.
Due to the polarization of bonds in RMgX, the carbon attached to magnesium acts as a strong nucleophile and a very strong base.
RMgX + H2O → X-Mg-OH + RH
The strong basic character of the Grignard reagent can be shown by the following reactions.
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