Alkyl halide: Short questions and answer

Alkyl halide: Primary, secondary, tertiary alkyl halide, Nucleophilic substitution reaction, elimination reactions, factors which affect the reactivity of alkyl halides

Question: What are primary, secondary and tertiary alkyl halide?

Answer.

Primary alkyl halide: An alkyl halide in which a halogen atom(F, Cl, Br or I) is directly bonded with primary carbon (carbon that is bonded with one or no other carbon atom), is called primary alkyl halide. e.g.,

Secondary alkyl halide: An alkyl halide in which a halogen atom(F, Cl, Br or I) is directly bonded with secondary carbon (carbon that is bonded with two other carbon atoms), is called secondary alkyl halide.e.g.,

Tertiary alkyl halide: An alkyl halide in which a halogen atom(F, Cl, Br or I) is directly bonded with tertiary carbon (carbon that is bonded with three other carbon atoms), is called tertiary alkyl halide e.g.,

Question: What are Nucleophilic substitution reaction or SN reaction?

Answer.

The reaction in which an electron rich specie (neutral or anion) displaces an atom or a group of atoms bonded with central carbon atom of the molecule is called nucleophilc substation reaction. In these reactions usually one functional group present in a molecule is replaced by an other functional group.

Question: Tertiary alkyl halides show SN1 reactions mostly, why?

Answer.

Answer: SN1 reaction proceeds via two steps:

i. formation of carbonation

ii. attack of nucleophile to form product.

First step is a slow step and rate deremining step. It involves formation of carbocation as an intermediate. The stability of this cabocation determines which substrate is favourable for the reaction.

Generally, hyperconjugation effects of adjacent C-C bonds increase electron density to empty p orbital of carbocation and increase its stability.

Tertiary substrates have the largest number of adjacent C-C bonds with greater hyperconjugative effects. Hence, the most stable carbocations are easily formed and thus favour the SN1 reaction.

Question: What are elimination reactions?

Answer.

The chemical reaction in which two atoms or group of atoms are eliminated from two adjacent carbon atoms is called elimination reaction. For example, reaction of 2,3-dibromobutane with alcoholic KOH

Question: Which factor decides the reactivity of alkyl halide?

Answer. There are two factors which affect the reactivity of alkyl halides

These factors are:

i. Bond energy

ii. Bond polarity

Bond energy: Greater the bond energy (stronger bond) and least reactive the alkyl halide and vice versa. Alkyl fluoride will be least reactive due strong C-F bond and alkyl iodide is most reactive due weak C-I bond

Bond polarity: Greater the bond polarity and greater the reactivity of alkyl halide and vice versa. Alkyl fluoride will be most reactive due to higher polarity of C-F bond and alkyl iodide is least reactive due to lesser polarity of C-I bond.

These two factors oppose each other. Experiments have showed that reactivity of alkyl halide depends upon the bond energy of C-X bond rather than bond polarity of C-X bond. Therefore, overall order of reactivity of alkyl halide is as follows.

R-I > R-Br > R-Cl > R-F

Question: How are tertiary alcohols obtained from R-MgBr?

Answer. Tertiary alcohols are obtained by reaction of Grignard reagent with ketones.

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