7.4.1.2. From carbonyl compounds

Alcohols are prepared by the reduction of aldehydes and ketones by addition of hydrogen in the presence of catalysts (catalytic hydrogenation). The usual catalyst is a finely divided metal such as platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4). Aldehydes yield primary alcohols whereas ketones give secondary alcohols.

In this section, we learn how alcohols can be synthesized by reducing aldehydes or ketones. When an aldehyde undergoes reduction, it gains hydrogen atoms, resulting in a primary alcohol. For example, if we take formaldehyde (an aldehyde) and add hydrogen, we produce methanol (a primary alcohol). On the other hand, when a ketone is reduced, it forms a secondary alcohol. This is due to the difference in the molecular structure of aldehydes and ketones, where aldehydes have one carbon atom bonded to the functional group, while ketones have two. The resulting alcohols depend on the structure of the original carbonyl compound.

Think of reducing an aldehyde like filling up a balloon. An aldehyde has one end open (the carbonyl group) and by adding hydrogen, you’re closing that end up to form a stable structure (an alcohol). For example, turning a half-filled cup (an aldehyde) into a full cup (the resulting alcohol) through the addition of more liquid (hydrogen).

Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent. However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. Commercially, acids are reduced to alcohols by converting them to the esters, followed by their reduction using hydrogen in the presence of a catalyst (catalytic hydrogenation).

This part discusses how carboxylic acids, which are stronger acids than aldehydes or ketones, can also be transformed into alcohols. The process involves a strong reducing agent, lithium aluminium hydride (LiAlH4). This powerful reagent does a fantastic job of attaching more hydrogen to carboxylic acids, effectively converting them into primary alcohols. However, since LiAlH4 is quite costly, in industrial settings, it's often more economical to first convert the acid to an ester and then reduce that ester using hydrogen, which serves the same purpose at a lower cost.

Consider a car being heavily weighed down, representing a carboxylic acid. To lighten the weight (reduce) and enhance its performance (convert to alcohol), you might first unload it (convert to an ester) before giving it a speed boost (adding hydrogen) so that it can function better on the road.

Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol.

Grignard reagents are a special class of compounds that serve as strong nucleophiles. Here, they play a crucial role in synthesizing alcohols by attacking the carbonyl carbon in aldehydes or ketones. This reaction creates a new intermediate compound (the adduct) that can then undergo hydrolysis, ultimately yielding an alcohol. This method is particularly useful in organic synthesis due to its versatility and effectiveness.

Imagine you’re building a LEGO tower (the aldehyde or ketone). When you bring in a special piece (the Grignard reagent), it locks into your structure (attacks the carbonyl), allowing you to add even more layers (hydrolysis) on top, resulting in a more complex and tall tower (the alcohol).

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