Aldehydes and ketones (collectively: carbonyls) have a very diverse chemistry and yield a large number of transformations.
Reduction of Aldehydes and Ketones
These reaction will convert the carbon-oxygen double bond into a single bond giving an alcohol as a product. Aldehydes make primary alcohols, while ketones give secondary alcohols in reduction reactions.
Reduction of aldehydes and ketones with hydrogen on heterogeneous catalyst
This reaction typically requires a reasonably high pressure of hydrogen and your typical heterogeneous catalyst such as platinum or palladium. Catalytic hydrogenation of carbonyls is rarely used in practice as this reaction will easily reduce the vast majority of other double or triple bonds in the molecule making it not very selective.
Reduction with complex metal hydrides
Sodium borohydride is typically preferred in this reaction since it is selective towards aldehydes and ketones and is not strong enough to reduce other polar double or triple bonds making it chemoselective unlike lithium aluminum hydride. Additionally, NaBH4 can be used in protic solvents like methanol or ethanol while LAH does not tolerate those being a significantly more basic and reactive.
Reduction of carbonyls with complex hydrides can be generally considered as a nucleophilic attack of the hydride anion on the carbon of the carbonyl.
This is a very selective reduction that removes carbonyl completely and only works for aldehydes and ketones. Due to a high nucleophilicity of hydrazine, the reaction may have unexpected side products when other electrophilic functional groups are present. It also requires a harsh “baking” in the second step with temperatures upwards of 250 ℃ and thus can easily cause the decomposition of the molecule. Due to these limitations, Wolff-Kishner reduction is only used for relatively simple molecules.
Clemmensen reduction of aldehydes and ketones is a significantly milder method. However, due to the use of an acid in this method, it is not used with molecules that are acid sensitive.
Reactions of Aldehydes and Ketones with Organometallic Compounds
We typically see organolithium and Grignard reagents in these reactions, however, other versions are also known.
Reaction of organometallic compounds with aldehydes and ketones is an important synthetic method giving a wide variety of alcohols as products. Aldehydes give secondary alcohols and ketones give tertiary alcohols.
Reversible Nucleophilic Addition Reactions of Aldehydes and Ketones
These reaction give a number of functional groups that are important for organic synthesis. The important aspect of all these reactions is that they can be easily reversed using the Le Chatelier’s principle.
Hydration of aldehydes and ketones
The resulting hydrate product in this reaction is generally unstable and spontaneously eliminates water giving the initial carbonyl. This reaction, however, has some biological significance.
Hemiacetal and acetal formation
This reaction is very similar to hydration in both product and mechanism, however, the nucleophile in this reaction is an alcohol.
It is important to remember that hemiacetals are not very stable and tend to revert to the initial carbonyl. The acetals, however, are stable and are often used in organic synthesis as a protecting group for a carbonyl.
Thioacetals are very similar to regular acetals with the exception that there’s sulfur and not oxygen is acting as a nucleophile. Thioacetals are more stable than acetals and do not tend to participate in an equilibrium to the same extent as acetals, thus it is way harder to hydrolyze those.
Imines are somewhat useful in organic synthesis and are usually used as a mid-step in a synthesis of an amine via reductive amination reaction.
Enamines are hugely useful in organic synthesis. They are used as nucleophiles in Stork enamine synthesis, so it is important to know how enamines are produced in the lab.
Formation of cyanohydrins
Cyanohydrins give an easy way of making α-substituted carboxylic acids. Cyanohydrins are also used as a mid-step in the carbohydrate “extension” methods.
Oxidations of Aldehydes and Ketones
These reactions will convert aldehydes and ketones to the corresponding carboxylic acids or carboxylic acid derivatives.
Baeyer-Villiger (also spelled as Bayer-Villiger) reaction is very important for chemical industry. It is also a very nifty way to “insert” an oxygen into a chain making a complex ester that could be challenging to synthesize otherwize.
Oxidation of aldehydes
There’s a large number of methods used to oxidize aldehydes to the corresponding carboxylic acids. The most common methods include the variations of the Jones reagent, Tollens reagent, or Benedict’s reagent. The latter two are also used as a qualitative reaction to identify aldehydes and especially reducing sugars.
Wittig olefination is an incredibly powerful reaction and gives a double bond in a single step. It can also control for the stereochemistry of the double bond based on the nature of the phospho ylide used in this reaction.