Q10-35E - Organic Chemistry (Mcmurry)

Question

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give

products in which bromine substitution has occurred next to the aromatic ring (the benzylic position). Explain, based on the bond dissociation energies in Table 6-3 on page 170.

Step-by-Step Solution

Answer

This is because of low bond dissociation energy. 

Step-by-Step Solution Step 1: Benzylic bromination reaction

Benzylic bromination reaction involves use of NBS for bromination of alkylbenzenes. NBS is N-Bromosuccinimide, which is a bromine source.

Step 2: Bond dissociation energy

Bond dissociation energy is the energy required to break a given bond creating two radicals when the molecule is in the gas phase at 25 °C.

Step 3: Result interpretation

Alkylbenzenes such as toluene react with NBS to give products in which bromine substitution has occurred next to the aromatic ring (the benzylic position). This is because the energy needed to break hydrogen from the ring directly is 472 (kJ/mol) and from the methyl group of toluene is 375 (kJ/mol). So, it is easier to break the hydrogen bond from methyl for a bromination reaction.