Cyanohydrin of which of the following will yield lactic acid? (a) HCHO (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Lactic acid, scientifically known as 2-hydroxypropanoic acid, is one of the simplest alpha-hydroxy acids (AHAs) and plays a significant role in various biological processes and industries. Its synthesis can be derived from the transformation of a cyanohydrin, which occurs when a carbonyl compound reacts with hydrogen cyanide (HCN), followed by hydrolysis.

Synthesis of lactic acid can be achieved through a two-step reaction starting with an appropriate carbonyl compound. First, the formation of cyanohydrin introduces a cyano group adjacent to the carbonyl group. The cyanohydrin is then hydrolyzed to convert the cyano group into a carboxylic acid, meanwhile maintaining the hydroxyl group already present from the cyanohydrin formation. For lactic acid synthesis, the carbonyl compound must have a three-carbon skeleton and the required groups in the correct positions; propionaldehyde (CH₃CH₂CHO) fulfills this requirement.

Alpha-Hydroxy Acids (AHAs) are a group of organic acids characterized by the presence of a hydroxyl group (-OH) on the carbon atom adjacent to the carbonyl group (the alpha position). These acids, including lactic acid, glycolic acid, and mandelic acid, are widely used in skincare for their exfoliant properties, assisting in the removal of dead skin cells to reveal smoother, brighter skin.

In addition to their cosmetic usage, AHAs have significant roles in the medical and pharmaceutical fields. They are involved in producing important intermediates for medications and have therapeutic effects of their own. Lactic acid, a chiral molecule, can exist in two enantiomeric forms (L- and D-lactic acid), each of which may have different properties and applications.

The reactivity of carbonyl compounds is a cornerstone of organic chemistry. Carbonyl groups, found in aldehydes and ketones, are highly reactive due to the polar nature of the carbon-oxygen double bond. This polarized bond can attract nucleophilic species, such as the cyanide ion from hydrogen cyanide (HCN), leading to the formation of cyanohydrins.

The specific reactivity of a carbonyl compound is influenced by its structure. Methyl groups and hydrogen atoms adjacent to the carbonyl carbon can steer the course of the reaction, as seen in the synthesis of lactic acid from propionaldehyde. When an aldehyde like propionaldehyde is treated with HCN, the forming cyanohydrin is a key intermediate. It is crucial to note that propionaldehyde is particular to lactic acid synthesis, due to its suitable arrangement, while other aldehydes and ketones would yield different alpha-hydroxy acids upon hydrolysis of their respective cyanohydrins.