Question

Reaction product formed from mixture of 1-bromopropane, diethyl malonate, sodium ethoxide, sodium hydroxide, acid, and heat.

Answer 1

Diethyl malonate is an active methylene compound or a compound with acidic hydrogen due to the presence of electron-withdrawing groups flanking a methylene group or a group which by resonance contributes to acidity of the methylene proton. Such species can be easily deprotonated at the methylene carbon using a strong enough bse to produce a carbanion that attacks electrophilic centers.

In the given reaction conditions, the alkyl halide which can generate a carbocation after cleavage of the good leaving group bromide, attacks the nucleophilic center of diethyl malonate now deprotonated by the strong basic conditions. Due to the presence of very a strong base sodium ethoxide, the active methylene is deprotonated not just once but twice giving a dialkylated product. This dialkylated ester upon treatment with an acid under heat undergoes decarboxylation and hydrolysis at the ester site to yield a doubly propylated acetic acid molecule. This particular reaction is called Malonic Ester Synthesis where the malonate or any such active methylene is used a building block for a synthesis. When a base of high strength is used, dialkylation proceeds without control and to stop at the monoalkylated stage, a base with a pKb comparable only to the first pKa of the active methylene is selected to effect only one deprotonation and alkylation.