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Amino Acids and Amide Links

2017 Paper Two

Question Six
Looking at past AQA A-level Chemistry questions, I wanted to go beyond the answers and focus on thought processes, ideas and tips that will help in examinations.

The series will help you to spot weaknesses and help with revision, or it can serve as addenda to your notes.If you haven't gone through the paper, please look up the questions at, or click the direct link below, and have a go...

AQA A-level CHEMISTRY Paper 2 Organic and Physical ChemistryMonday 19 June 2017

The data sheet gives you the structure of phenylalanine. Recall that a zwitterion has separate positively and negatively charged groups.

The loss of hydrogen from the acid group and gain of hydrogen on the amine part through internal transfer gives this structure

High pH therefore serine is in alkaline conditions - the hydrogen ion is removed from the NH3+ group. Make sure you don't also remove the hydrogen from the alcohol part of the molecule!

This is a condensation reaction (removal of water) to form a peptide/amide link...

The two structures of the dipeptide are possible because each peptide has both an amino group and a carboxylic acid group.

We have an acyl chloride - a reactive molecule that will be 'attacked' by nucleophiles.

This is a nucleophilic addition-elimination reaction - reactants are added without the removal of a group from the molecule. In a second step, an elimination follows producing HCl. This two-step reaction is often confused with a substitution.

The organic product is N-ethylpropanamide (not HCl).
N- prefix because all the carbons form an unbranched chain.
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