Question

Which reducing agent is required for the following conversion?
$R-COOH\to R-C{{H}_{2}}OH$
$\begin{align}
  & a)LiAl{{H}_{4}} \\
 & b)NaB{{H}_{4}} \\
 & c){{K}_{2}}C{{r}_{2}}{{O}_{7}} \\
 & d)KMn{{O}_{4}} \\
\end{align}$

Answer 1

Hint: A strong reducing agent is required to carry out the given conversion as a weaker reducing agent will convert the given aliphatic carboxylic acid into an aldehyde instead of a primary alcohol. Therefore, we need to analyse each of the given reagents, their actions and the strength of said action to find the strongest reducing agent.

Complete step by step answer:
Let us analyse the behaviour of each of these reagents one at a time in organic chemical reactions before coming up with a suitable enough answer to the given question.
1. Lithium aluminium hydride is a complex metal hydride which is widely used in organic chemistry as a reducing agent and it is a more powerful reducing agent than the reagent sodium borohydride due to the weaker Al-H bond compared to the B-H bond.
2. Mostly used as a solution in diethyl ether and followed by an environment with excess acid and constant protonation; it will convert esters, carboxylic acids, acyl chlorides, aldehydes, and ketones into the corresponding alcohols.
3. Similarly, it converts amides, nitro-compound, nitriles, imines, oximes, and azide compounds into their respective amines. It is also able to reduce quaternary ammonium cations into their corresponding tertiary amines.
4. On the other hand, while Sodium borohydride reduces many organic carbonyls, its action heavily depends on the circumstances in which the reaction takes place. Most commonly, it is used in the laboratory for converting ketones and aldehydes to alcohols. It can also reduce acyl chlorides, anhydrides, α-hydroxylactones, thioesters, and imines at room temperature or below.
5. It reduces esters slowly and inefficiently with excess reagent and/or elevated temperatures and at a pace much much slower than that of the reduction of the same compounds as carried out by Lithium Aluminium Hydride. Carboxylic acids and amides, on the other hand, are not reduced at all.
6. Potassium dichromate and Potassium Permanganate are both oxidising agents, and therefore cause the opposite reaction than the one mentioned in the question.

Thus, we can safely conclude that the answer to the given question is a).

Note:
Lithium aluminium hydride also reduces alkyl halides to alkanes. Alkyl iodides react the fastest, followed by alkyl bromides and then alkyl chlorides. Primary halides are the most reactive followed by secondary halides. Tertiary halides react only in certain cases.