Question

What is the correct reactivity order of alkyl halides towards ammonolysis ?

• A. $RI>RBr>RCl$
• B. $RBr>RCl>RI$
• C. $RCl>RBr>RI$
• D. $RI>RCl>RBr$

Answer 1

The correct option is A $RI>RBr>RCl$

When alkyl/benzyl halide reacts with ammonia in ethanol at 373 K, amine is formed. This reaction is known as ammonolysis. Ammono means ammonia molecule and lysis means break. Hence, the process of cleavage of C-X bond by ammonia molecule is known as ammonolysis.

General reaction is as follows:
$N{H}_3+R-X\to RN{H}_3^{+}{X}^{-}$

Then, the substituted ammonium salt reacts with strong base to give free amine.

General reaction is as follows:
$RN{H}_3^{+}{X}^{-}+NaOH\to RN{H}_2+NaX+H_{2}O$

As in this reaction bond breaking of $R-X$ bond is involved, $R-I$ bond can be easily breaked followed by $RBr$ and $RCl$. This is because with the increase in the size of the halogen, bond strength decreases and required less energy to break $R-X$ bond. Therefore, the correct answer is option (a).