2-Ethyl-1-hexanol was needed for the synthesis of the sunscreen octyl \(p\)-methylcinnamate (See Chapter 23, Chemical Connections: "Sunscreens and Sunblocks"). Show how this alcohol could be synthesized (a) by an aldol condensation of butanal and (b) by a malonic ester synthesis starting with diethyl malonate.

First, let's write the structure of 2-Ethyl-1-hexanol, which has a hexane backbone with an alcohol functional group in position 1 and an ethyl substituent in position 2. H H H H | | | | H-C-C-C-C-C-OH | | | | H H H H | C H | H C H | H

The aldol condensation is a two-step reaction, first forming an aldol by nucleophilic addition (enol or enolate ion), and then condensation by dehydration to produce an α,β-unsaturated carbonyl compound. (a) Reacting butanal (1) with itself: 1. Form the enolate ion from butanal (1) by deprotonation in the presence of a suitable base. 2. Perform nucleophilic addition of the enolate ion to the carbonyl group of the second butanal molecule (1) to form a β-hydroxy aldehyde. 3. Dehydrate the β-hydroxy aldehyde to form the α,β-unsaturated aldehyde. (b) Reduction of the α,β-unsaturated aldehyde: 1. Reduce the α,β-unsaturated aldehyde using a suitable reducing agent (e.g., NaBH4) in a suitable solvent to produce 2-Ethyl-1-hexanol.

Malonic ester synthesis is a method to synthesize substituted carboxylic acids through the reaction of malonic esters with suitable alkyl halides followed by hydrolysis and decarboxylation. (a) Alkylation of diethyl malonate: 1. Convert diethyl malonate (2) to its enolate ion by deprotonation in the presence of a suitable base. 2. Perform an S_N2 reaction between the enolate ion of diethyl malonate (2) and the appropriate alkyl halide (e.g., 1-iodopentane) to form 2-ethylhexyl malonate. (b) Hydrolysis and decarboxylation of 2-ethylhexyl malonate: 1. Hydrolyze the ester groups in 2-ethylhexyl malonate with an aqueous acid catalyst, forming a diacid. 2. Perform decarboxylation of the diacid by heating it in the presence of an acid catalyst, resulting in the formation of 2-ethylhexanoic acid. (c) Reduction of 2-ethylhexanoic acid: 1. Reduce the carboxylic acid with a suitable reducing agent (e.g., LiAlH4) to produce 2-Ethyl-1-hexanol.