The two compounds shown are heated with a catalyst (transesterification) to give a polyester, taking one terephthaloyl unit and one diol unit as the repeat unit. Which small molecule is eliminated, and how many molecules per repeat unit formed?

AWater, 2 molecules
BMethanol, 4 molecules
CMethanol, 1 molecule
DMethanol, 2 molecules

Answer: D

The diester is dimethyl terephthalate and the diol is ethylene glycol; the product is PET. Each ester interchange expels the methoxy group of the ester as methanol (CH3OH), not water. A repeat unit uses both ester groups of one terephthalate, so two molecules of methanol are expelled per repeat unit.

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