The amino acid tyrosine can be synthesised from phenol by the route shown. OH step 1 CHO step 2 CN HO HO phenol HO step 3 NH2 Cl OH step 5 step 4 CO₂H CO₂H HO HO 1. PCl5 2. H₂O CO₂H HO tyrosine

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. [1] step 1 electrophilic substitution ignore acylation
2. [1] step 2 nucleophilic addition
3. [1] hydrolysis
4. [1] step 1 CICH2CHO (allow Br, I for Cl)
5. [1] AlCl3
6. [1] step 2 HCN + NaCN
7. [1] step 3 heat in H3O⁺ / heat H⁺(aq)
8. [1] step 5 NH3 under pressure (+ heat) or heat NH3 in a sealed tube
9. [1+1] Structure of tyrosine with NaOH(aq) product (deprotonated -OH and -CO2H).
10. [1] Structure of tyrosine with HCl(aq) product (protonated -NH2).
11. [1] Structure of tyrosine with Br2(aq) product (bromination of phenol ring, one Br on each ortho position relative to -OH, and Br in meta position if meta to -OH is unsubstituted, or 2 Br if no meta position occupied by another group). The MS shows 2 Br atoms ortho to -OH.
12. [1] P is tyr
13. [1] tyr is 2- AND it is small / has a small Mr
14. [1] (dipeptide / phe-tyr) 2– is about double the Mr / mass of (phe) 1
15. [1] OR mass / charge ratios are about the same for each (for dipeptide / phe-tyr and phe)