The purity of the parent substance of (E)-N-(6,6-dimethylhept-2-en-4-ynyl)-N-methylnaphth-1-ylmethylamine (terbinafine) is evaluated via TLC identification of the main impurities appearing during its synthesis via alkylation of N-methylnaphth-1-ylmethylamine with 1-chloro-6,6-dimethyl-2-hept-2-en-4-yne The possible formation of impurities including N-methyl-N,N-di(methylnaphth-1-yl)amine, N-methylnaphth-1-ylmethylamine, N-methylnaphth-2-ylmethylamine, (E)-N-(6,6-dimethylhept-2-en-4-ynyl)-N-methylnaphth-1-ylmethylamine, (Z)-N-(6,6-dimethylhept-2-en-4-ynyl)-N-methylnaphth-1-ylmethylamine, (E)-N-(6,6-dimethylhept-2-en-4-ynyl)-N-methylnaphth-2-ylmethylamine, and (Z)-N-(6,6-dimethylhept-2-en-4-ynyl)-N-methylnaphth-2-ylmethylamine was confirmed by means of countersynthesis.