Question

What sequence of reactions would best accomplish the following reaction?

A. (i) $LiAl{{H}_{4}}$ in ether; (ii) ${{P}_{2}}{{O}_{5}}$ and heat.
B. (i) $LiAl{{H}_{4}}$ in ether; (ii) $3C{{H}_{3}}I$ followed by heating with AgOH.
C. (i) $20%{{H}_{2}}S{{O}_{4}}$ and heat; (ii) ${{P}_{2}}{{O}_{5}}$ and heat.
D. ${{H}_{2}}$ and Lindlar catalyst.

Answer 1

Hint: Alkyl cyanide can not be converted into alkene by a single step, more than one step is involved. Therefore different reagents are used in this regard. One simple method to solve this problem is to apply the different reagents given in the option, only then we can reach that particular pathway.

Complete step by step solution:
Putting the reagents in the given alkyl cyanide one by one:
(i) Alkyl cyanide compound forms alkyl amine with $LiAl{{H}_{4}}$in ether and presence of ${{P}_{2}}{{O}_{5}}$ heat, alkenes are not formed.

(ii) Here also alkyl amine is formed at the first step. Under the presence of $C{{H}_{3}}I$ AgOH Hoffmann elimination reaction occurs. It is the process of forming tertiary amine and alkenes from the treatment of quaternary ammonium ions with excess $C{{H}_{3}}I$ followed by treating silver oxide, water, and heat. If the alkyl groups contain beta hydrogens, then elimination occurs. Finally, we get the desired product.


(iii) Under the condition $20%{{H}_{2}}S{{O}_{4}}$and heat a carboxylic acid is formed followed by decarboxylation with the removal of carbon dioxide in the next step.


(iv) ${{H}_{2}}$and Lindlar catalyst can reduce only internal alkyne into cis alkenes but not reduce alkyl cyanide. Therefore, option B is a suitable pathway to synthesize the given alkene.
Thus, option (B) is correct.

Note: To approach this type of synthesis reaction, one should remember some important reagents and name reactions that will help to guess the product without any broad mechanism. Also, the basic idea of general organic chemistry is mandatory such as inductive effect, acidity, basicity hyperconjugation, resonance, electrophiles, and nucleophiles.