Solved Examples on Alkyl and Aryl Halides

Question 1: Identify A,B,C,D,E and F in the following series of reaction.

Solution:

Question 2: Give the major product (with proper explanation) when following halogen compounds are treated with sodium ethoxide

Solution:

 

Note: There can be elimination from 2° and 3° carbonium to give alkene. 

 

Question 3: What are the products of the following reactions?

 

OCO₃ (nucleophile) can't attack 3° carbon having  high electron - density hence elimination takes place giving.

Nucleophilic attack on methyl carbon is possible giving ether (Williamson synthesis).

Question 3:  Heating many alkyl chlorides or bromides in water effects their conversion into alcohol through S_N1 reaction. Order each of the following sets compounds with respect to solvolytic reactivity.

 

Solution:       

(a)  I < II < III

(b) III < II < I

                    

(c) III < I < II (allylic) 

Order is based on stability of carbocation, formed as the intermediate, which is

1° < 2° < 3° < allylic 

Question 5:   When alkyl halides are treated with aqueous AgNO3, silver halide precipitate and an alcohol is formed. From what you know about the SN1 reaction, propose a mechanism for the following conversion.

               

Solution:  

 

Question 6:  2-Bromopentane, when treated with alcoholic KOH yields a mixture of three alkenes A,B and C. Identify A,B and C. Which is predominant?

(Assume reaction proceeds through E2 mechanism)

Solution:

By Saytzeff rule, substituted alkenes are more stable, hence B or C is predominant than A. Out of cis and trans, trans is more stable. Hence C is predominant.

Question 7: Vinyl  chloride does not give S_N reaction but allyl chloride gives. Explain

Solution:  In vinyl  chloride C—Cl bond is stable due to resonance  (as in chlorobenzene).

Hence S_N reaction in which Cl is replaced by nucleophile is not possible. In addition to this, sp² hybridised carbon is more acidic than sp³ carbon, hence removal of proton (H⁺) is easier than removal of halide (Cl^–).

In allyl chloride, S_N reaction is easier since allyl carbonium ion formed after removal of Cl^– is tabilised by resonance.

Question 8:  When CH₃—CH=CH–CH₂Cl reacts with alcoholic KCN, a mixture of isomeric products is obtained. Explain.

Solution:  It can undergo S_N1 and S_N2 reaction. By S_N2 reaction only one product is formed . But by S_N1 reaction, intermediate is carbonium ion.

Thus we get two isomeric products suing S_N1 reaction.

Question 9:  C₄H₈Cl₂ (A) on hydrolysis forms C₄H₈O (B) which forms an oxime, but does not reduce Fehling solution. B also gives iodoform test. Identify A and B and explain reactions.

Solution:  (A) by hydrolysis forms (C) replacing two —Cl by one O. Thus A has two Cl atoms at same carbon i.e., gem positions.

We assign therefore following structures and compare their properties to match the given properties.

Question 10:  Identify end product of the following

Solution: 

CBr₄ + MeLi → MeBr + LiCBr₃

(halogen-metal exchange)

LiCBr₃  → LiBr +:CBr₂

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