Question

The increasing order of reduction of alkyl halides with zinc and dilute HCl is:
A.$R-Cl < R-I < R-Br$
B. $R-Cl < R-Br < R-I$
C. $R-I < R-Br < R-Cl$
D. $R-Br < R-I < R-Cl$

Answer 1

The method of converting a haloalkane to the corresponding alkane in an acidic medium is known as a reduction reaction. Metal zinc is used along with an acid to provide a surface area for reaction to occur.

Complete step by step answer:
The rate of reduction of haloalkane depends upon the reactivity of the halogen towards the alkane. With increasing the reactivity of the halogen, the rate of reduction decreases, and vice versa.
The order of reactivity of the halogens is as follows,
$C{l_2} > B{{\mathbf{r}}_2} > {I_2}$
This is because, from top to bottom, the size of the halogen increases and the C-X bond becomes weak, therefore reduction tendency increases.
So, the rate of reduction of the haloalkane is,
$R-Cl < R-Br < R-I$

The correct option is, B.

Additional information:
-The Clemmensen reduction got its name from a Danish chemist, Erik Christian Clemmensen. -The Clemmensen reduction is a reaction that is performed to reduce carbonyl compounds such as aldehydes or ketones to hydrocarbons that are alkanes using concentrated hydrochloric acid and zinc amalgam.
-The mercury in the zinc-amalgam is alloyed with the zinc metal and does not take part in the reaction, rather it serves only to provide a clean active metal surface. The reaction occurs with zinc but not with other metals of comparable reduction potential.
-This method works best for aromatic ketones, but not so effective for non-aromatic ones. -Aliphatic alcohols are not the intermediates of this reduction; however, allylic and benzylic alcohols easily undergo this reduction. Currently, there are two mechanisms proposed but they are contradictory.
-In one of the mechanisms, the rate-determining step involves the attack of zinc and chloride ion on the carbonyl group and the intermediates are carbanions, whereas, in other heterogeneous processes, the formation of a radical intermediate occurs and a zinc carbenoid species is formed.

Note:
Clemmensen reduction is complementary to another type of reduction, Wolf Kishner reaction, which also converts carbonyl compounds such as aldehydes and ketones to hydrocarbons but in basic conditions, while the former takes place in acidic condition.