Which of the following pairs of monomers are used to synthesize nylon $66$?

• A. $H_{2}N-{\mathrm{(CH}}_{2}2$-NH2 (ethylenediamine) and $O_{2}C-{\mathrm{(CH}}_{2}2$-COOH (succinic acid)
• B. $H_{2}N-{\mathrm{(CH}}_{2}6$-NH2 (hexamethylenediamine) and $O_{2}C-{\mathrm{(CH}}_{2}4$-COOH (adipic acid)
• C. $H_{2}N-{\mathrm{(CH}}_{2}6$-NH2 (hexamethylenediamine) and $\mathrm{HOOC}-\mathrm{COOH}$ (oxalic acid)
• D. $H_{2}N-{\mathrm{(CH}}_{2}12$-NH2 (dodecamethylenediamine) and $O_{2}C-{\mathrm{(CH}}_{2}10$-COOH (sebacic acid)

Answer: (B)

Verified step by step guidance

1. Understand that nylon 66 is a polyamide formed by the condensation polymerization of a diamine and a dicarboxylic acid, each containing 6 carbon atoms in their main chains.
2. Identify the diamine monomer with 6 carbon atoms between the two amine groups, which is hexamethylenediamine, represented as \(\mathrm{H_2N-(CH_2)_6-NH_2}\).
3. Identify the dicarboxylic acid monomer with 6 carbon atoms between the two carboxyl groups, which is adipic acid, represented as \(\mathrm{HOOC-(CH_2)_4-COOH}\) (note that the total carbons including the carboxyl carbons sum to 6).
4. Recall that the polymerization involves the amine groups reacting with the carboxylic acid groups to form amide bonds, releasing water molecules in the process.
5. Conclude that the correct pair of monomers for nylon 66 synthesis is hexamethylenediamine and adipic acid, because their carbon chain lengths correspond to the '66' designation in nylon 66.