OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 17

7 marks · Medium difficulty · Structured Questions

Draw 3-D optical isomers of cysteine, the salt of lysine formed with excess hydrochloric acid, and the organic products formed from alkaline hydrolysis of a protein section.

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Question

Question text

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

(a) The α-amino acid cysteine (R = CH2SH) shows optical isomerism.

Draw 3-D diagrams to show the optical isomers of cysteine.

[2]

(b) The α-amino acid lysine (R = (CH2)4NH2) reacts with an excess of dilute hydrochloric acid to

form a salt.

Draw the structure of the salt formed in this reaction.

[2]

A short section of a protein chain is shown below.

CH2 H O

CH N C

N C CH

H O CH2

CH2

H2N O

A student hydrolyses the protein with hot NaOH(aq).

Draw the structures of the organic products formed from this section of the protein.

[3]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

17 (a) 2 Each structure must have four central bonds with at

Correct groups attached to chiral C of cysteine seen once e.g. least two wedges.

For bond into paper accept:

CH SH CH2SH

OR H2N C COOH

NH2

HOOC ALLOW bond to any part of the CH2 of the CH2SH

H H

group

e.g. ALLOW

CH2SH CH2SH

Two 3D structures of cysteine that are mirror images with

correct connectivity in both

ALLOW two 3D structures with 2 groups swapped

e.g.

CH2SH CH2SH CH2SH CH2SH

C C C C

NH2 H2N NH2 COOH

HOOC COOH HOOC H2N

H H H H

IF CH2SH is shown as ‘R’ ALLOW 1 mark for two 3D

structures with correct connectivity that are mirror

images

e.g.

COOH COOH

C C

NH2 H2N

R R

H H

Question Answer 10 Marks Guidance

(b) Correct salt of lysine with both amine groups protonated 2 ALLOW any combination of skeletal OR structural OR

displayed formula as long as unambiguous

H O –

Cl - Note: Cl is required (question asks for salt)

H3N C C ALLOW NH3Cl i.e charges not required

OH ALLOW 1 mark for

(CH2)4

H H O

- O

Cl - + NH3 +

H3N C C H2N C C

(CH ) OH (CH ) OH

24 2 4

NH Cl - +NH

2 OR 3

H O

H3N C C

(CH ) OH

+ NH i.e. no Cl–

IF there is a small slip in the structure ALLOW 1 mark

for diammonium salt e.g

H O

Cl -

H3N C C

(CH ) OH

Cl - + NH

3 (incorrect number of CH2 in R

group)

- O

H3N C C

(CH ) OH

Cl - + NH

3 (H missing from C atom)

displayed formula as long as unambiguous

IGNORE NH3 (question asks for organic products)

ALLOW –COO– OR –COONa

HO HO DO NOT ALLOW negative charge on C atom

CH2 CH2 DO NOT ALLOW –COO–Na (covalent bond) BUT

ALLOW ECF if seen in subsequent structures

-(Na+) - +

CH O CH O (Na )

H N C N C DO NOT ALLOW COOH in this structure

H DO NOT ALLOW (sodium) salt of alcohol group

O i.e.

O OR

- O

CH2

-(Na+)

CH O

H2N C

ALLOW COOH groups in this structure

H N C i.e. award 2 marks for

- + O

CH O (Na )

CH2 H2N C

CH OH

CH2

(Na+) - CH2

O O

i.e. one mark for each group hydrolysed C

HO O

IGNORE small slip in carbon chain

Total 7

How to answer it

OCR A-Level Chemistry: Amino Acids, Isomerism & Proteins

What this question tests

This question assesses your understanding of alpha-amino acid chemistry, specifically optical isomerism (stereoisomerism), acid-base reactions of functional groups with excess mineral acid, and alkaline hydrolysis of peptide bonds (amide links) in proteins.

Part (a) — Optical Isomerism of Cysteine [2 Marks]

✅ Correct Answer

Two distinct 3-D structural representations of cysteine ( R = CH₂SH ) drawn as non-superimposable mirror images.

Key features required: Central chiral carbon bonded to four different groups ( -H , -NH₂ , -COOH , -CH₂SH ), depicted using conventional 3D bond notation (at least two wedges/dashes per structure) showing correct mirror-image connectivity.

💡 Key Knowledge

A chiral center (asymmetric carbon) must be bonded to four completely different groups.
Optical isomers (enantiomers) rotate plane-polarized light in opposite directions.
3D representation relies on standard convention: two bonds in the plane (solid lines), one coming out (wedge), and one going back (dash).

🧠 Exam Technique

When drawing optical isomers, draw the first molecule completely, then draw a vertical mirror line directly beside it. Flip the groups horizontally on the other side to guarantee full marks for stereochemistry.

❌ Common Errors

Omitting 3D bonds (using plain straight lines for all four bonds).
Failing to show a true mirror image relationship between the two structures.
Labeling the side-chain incorrectly or attaching groups to the wrong carbon atom.

Mark breakdown: 1 mark for correct groups attached to the central chiral carbon once; 1 mark for two valid 3D structures that are accurate mirror images.

Part (b) — Salt Formation of Lysine with Excess HCl [2 Marks]

✅ Correct Answer

The structure of the salt formed when lysine ( R = (CH₂)₄NH₂ ) reacts with an excess of dilute hydrochloric acid.

Both amine groups ( -NH₂ ) must be fully protonated into ammonium ions ( -NH₃⁺ ), and chloride counter-ions ( Cl⁻ ) must be clearly shown in the equation/structure.

💡 Key Knowledge

Lysine contains two basic amine groups: the alpha-amine group and the terminal amine group on its alkyl side chain.
In the presence of an excess of a strong acid like HCl, both basic sites accept a proton.
Carboxylic acid groups ( -COOH ) do not react with hydrochloric acid.

🧠 Exam Technique

Read the question stem carefully: it asks for the salt formed, meaning chloride ions ( Cl⁻ ) must be present alongside the protonated diamine cation.

❌ Common Errors

Leaving one of the amine groups unprotonated ( -NH₂ instead of -NH₃⁺ ).
Protonating the carboxylic acid group ( -COOH ).
Omitting the required Cl⁻ counter-ions from the drawn product structure.

Mark breakdown: 1 mark for correct protonation of both amine groups (giving -NH₃⁺ ); 1 mark for including the correct Cl⁻ ions and leaving the -COOH group unprotonated/intact.

Part (c) — Alkaline Hydrolysis of a Protein Chain [3 Marks]

✅ Correct Answer

The products formed when the given protein section is hydrolyzed using hot aqueous sodium hydroxide ( NaOH(aq) ).

Hydrolysis breaks the peptide (amide) bonds, yielding the constituent amino acids in their sodium salt forms where carboxylic acid groups are converted to carboxylate ions ( -COO⁻ Na⁺ ) and amine groups remain unprotonated ( -NH₂ ).

💡 Key Knowledge

Proteins are condensation polymers formed from amino acid monomers linked by peptide bonds ( -CONH- ).
Alkaline hydrolysis splits the peptide links, forming sodium carboxylate salts and free amines.
Under strongly alkaline conditions, any acidic protons (such as those on -COOH or -NH₃⁺ ) are neutralized or kept as carboxylate salts.

🧠 Exam Technique

Identify all peptide bonds in the provided backbone snippet. Slice through each -C(=O)-N(H)- link. Convert every -COOH group into -COO⁻ Na⁺ due to the presence of aqueous NaOH .

❌ Common Errors

Leaving carboxylic acid groups as -COOH instead of converting them to sodium carboxylate salts ( -COO⁻ Na⁺ ) in alkaline conditions.
Protonating the amine groups under alkaline conditions (incorrectly writing -NH₃⁺ ).
Cleaving the wrong covalent bonds within the amino acid carbon skeletons.

Mark breakdown: 1 mark for breaking the peptide backbone correctly; 1 mark for converting carboxylic acid groups to carboxylate sodium salts; 1 mark for correct overall structures of all amino acid fragments produced.

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.