OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 21

9 marks · Medium difficulty · Structured Questions

Explain optical isomerism, name and draw 3D optical isomers of valine, determine the number of optical isomers for a tripeptide, and draw the hydrolysis products of the tripeptide in acid.

Practise this question

Question

A multi-part chemistry question about alpha-amino acids. Part (a) asks to explain optical isomerism for RCH(NH2)COOH. Part (b)(i) asks for the systematic name of valine, and (b)(ii) provides two boxes to draw 3D structures of its optical isomers. Part (c) shows the chemical structure of compound E formed from three alpha-amino acids, asks for the number of possible optical isomers for compound E in (c)(i), and asks to draw the organic products formed from its hydrolysis with dilute hydrochloric acid in (c)(ii).
Question text

21 This question is about α-amino acids.

(a) The general formula of an α-amino acid is RCH(NH2)COOH.

Most α-amino acids show optical isomerism.

Explain the term optical isomerism.

… [1]

(b) The α-amino acid valine has the R group of –CH(CH3)2.

(i) What is the systematic name of valine?

… [1]

(ii) Draw diagrams to show 3D structures of the optical isomers of valine.

[2]

(c) Three α-amino acids can react together to form compound E, shown below.

OH

OH

C

H2C O

H

H2C N CH

CH C NH

C O C NH

HO O O CH

CH

HO CH3

Compound E

(i) How many optical isomers are possible for compound E?

… [1]

(ii) A student hydrolyses compound E with dilute hydrochloric acid, HCl(aq).

Draw the structures of the organic products formed by this hydrolysis.

[4]

Mark scheme

Show the mark scheme The mark scheme provides answers for question 21. Part (a) accepts non-superimposable mirror images about a chiral centre. Part (b)(i) accepts 2-amino-3-methylbutanoic acid. Part (b)(ii) awards marks for correct groups attached to the chiral carbon and 3D mirror image structures with wedges and dashes. Part (c)(i) gives 16 as the answer. Part (c)(ii) shows the three protonated amino acid structures resulting from acid hydrolysis, awarding marks for correct structures with NH3+ or NH2 groups.

AO

Question Answer Marks Guidance

element

21 (a) Non-superimposable mirror images (about a chiral 1 AO1.1 IGNORE definition of stereoisomers

centre) ✓

(b) (i) 2-amino-3-methylbutanoic acid 1 AO1.2 IGNORE lack of hyphens, extra hyphens, or addition of

OR commas

3-methyl-2-aminobutanoic acid ✓

DO NOT ALLOW the following for methyl: methy,

meth, methly

DO NOT ALLOW the following for amino: amine, amin

(b) (ii) Correct groups attached to chiral C of valine seen once 2 AO1.1 ALLOW any combination of skeletal OR structural OR

e.g. displayed formula as long as unambiguous

IGNORE connectivity for the first marking point but must be

correct for the second mark.

ALLOW bond to any part of the CH of the CH(CH3)2 group

e.g. ALLOW

✓

Each structure must have four central bonds with at least

Two 3D structures of valine that are mirror images with two wedges.

correct connectivity in both ✓ For bond into paper accept:

AO1.2

ALLOW two 3D structures with 2 groups swapped

e.g.

ALLOW R or C3H7 to be shown for CH(CH3)2 for second

mark only.

ALLOW ECF for second mark for small slips such as

missing H e.g. C(CH3)2

27 AO

element

21 (c) (i) 16 ✓ 1 AO2.6

(c) (ii) 4

AO2.5 ALLOW any combination of skeletal OR structural OR

4 displayed formula as long as unambiguous

IGNORE connectivity

ALLOW + charge on H of NH group, i.e. NH +

If structures are shown with NH3 groups (without the +

charge) OR as NH + groups allow ECF for subsequent use.

ALLOW structures shown as correctly balanced salts, e.g

NH Cl OR NH +Cl– all marks can be awarded.

1 mark for each correct structure with

• Either NH + OR NH ✓✓✓

1 mark for

• all 3 correct structures with NH + ✓

How to answer it

Study Guide: Alpha-Amino Acids and Proteins

📌 What this question tests

This question assesses your understanding of stereoisomerism in organic chemistry, specifically optical isomerism and chiral centres. It tests IUPAC naming conventions for amino acids, 3D tetrahedral drawing skills using wedges and dashes, calculating total possible stereoisomers (2ⁿ rule), and predicting organic products formed via acid hydrolysis of peptide bonds in acidic conditions (HCl).

Question (a)

Explaining Optical Isomerism

✅ Correct Answer

Non-superimposable mirror images (about a chiral centre).

Marks: 1 / 1

💡 Key Knowledge

  • A chiral centre is a carbon atom bonded to four different groups or atoms.
  • Optical isomers are a type of stereoisomerism.

❌ Common Errors

General definitions of stereoisomerism (like "same structural formula, different spatial arrangement") do not score the mark here. You must specifically mention mirror images and that they are non-superimposable.

Question (b)

Valine: IUPAC Naming and 3D Structures

✅ Correct Answers

(i) Systematic Name: 2-amino-3-methylbutanoic acid (or 3-methyl-2-aminobutanoic acid )

(ii) 3D Structures: Two tetrahedral representations around the central chiral carbon showing two bonds in the plane, one wedge, one dash, and pointing to mirror-image arrangements with correct groups attached.

Marks: (i) 1/1 | (ii) 2/2

🧠 Exam Technique for 3D Drawings

  • Always use standard conventions: plain/solid lines for bonds in the plane of the paper, a wedged bond coming out, and a dashed/hatched bond going into the paper.
  • Ensure the four groups ( -H , -NH₂ , -COOH , and -CH(CH₃)₂ ) are clearly attached to the central carbon.

❌ Common Errors

In naming, writing "methy" instead of "methyl", or missing hyphens/commas can be penalised depending on clarity. For 3D drawings, failing to use wedges and dashes or drawing identical superimposable structures loses marks.

Question (c)

Compound E: Isomers and Acid Hydrolysis

✅ Correct Answers

(i) Number of optical isomers: 16

(ii) Hydrolysis products: Three separate amino acid molecules resulting from breaking the peptide (amide) bonds. In dilute HCl, the amino groups are protonated as -NH₃⁺ (or acceptable variants with ECF).

Marks: (i) 1/1 | (ii) 4/4

📐 Calculation & Logic Step-by-Step (c)(i)

  1. Identify chiral centres (n): Count the number of carbon atoms in Compound E bonded to 4 different groups. Compound E has 4 chiral centres.
  2. Apply the formula: Maximum number of optical isomers = 2ⁿ.
  3. Calculate: 2⁴ = 16.

🧠 Exam Technique for Hydrolysis Products

  • Recognise that peptide links ( -CO-NH- ) are broken down by water, catalyzed by acid.
  • Remember that under acidic conditions (using HCl), basic amine groups accept a proton to form ammonium ions ( -NH₃⁺ ). Carboxylic acid groups ( -COOH ) and phenol groups (on the tyrosine residue) remain protonated/unchanged.

Topics

Module 6: Organic chemistry and analysis · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.