Question

Poly(ethylene terephthalate) (PET) can be prepared by this reaction. Propose a mechanism for the step-growth reaction in this polymerization.

Short answer

Answer: The primary reactants in the formation of PET include ethylene glycol (HO-CH2-CH2-OH) and terephthalic acid (HOOC-C6H4-COOH). These reactants undergo a condensation reaction to form ester linkages (-CO-O-) and release water as a byproduct.

Step-by-step solution

Identify reactants in PET formation

In the formation of PET, the primary reactants include ethylene glycol (HO-CH2-CH2-OH) and terephthalic acid (HOOC-C6H4-COOH). These reactants undergo a condensation reaction to form ester linkages (-CO-O-) and release water as a byproduct.

Initial nucleophilic attack

The oxygen atom in the hydroxyl group (-OH) of ethylene glycol acts as a nucleophile, and attacks the carbonyl carbon (C=O) of one carboxyl group (-COOH) in terephthalic acid. This results in a transfer of electrons from the oxygen in the hydroxyl group to the carbonyl carbon, forming a new bond between the carbonyl carbon and hydroxyl oxygen. The carbonyl's pi bond (C=O) shifts the electrons to its oxygen, giving it a negative charge.

Proton transfer and ester bond formation

The negatively charged oxygen of the initial carbonyl group (-O-) abstracts a proton (H+) from the hydroxyl group that attacked in step 2. This results in the formation of a new ester bond (-CO-O-) between the terephthalic acid and ethylene glycol molecules. The byproduct of this proton transfer is a water molecule (H2O).

Repeat nucleophilic attack and bond formation

The oxygen atom in the remaining hydroxyl group of the ethylene glycol molecule can now act as a nucleophile and attack the carbonyl carbon of another terephthalic acid molecule. This continuation will result in the formation of another ester bond, linking two terephthalic acid molecules with an ethylene glycol molecule in between them.

Polymerization

The steps from 2 to 4 will continue to repeat, leading to the formation of a long chain of ester bonds between terephthalic acid and ethylene glycol, ultimately creating the PET polymer. The reaction releases water as a byproduct, and the growth of the polymer chain is called a step-growth polymerization.