OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 20

10 marks · Medium difficulty · Structured Questions

Answer questions on reaction mechanisms involving curly arrows, heterolytic fission, drawing products of elimination or substitution, completing a nucleophilic addition-elimination mechanism for an acyl chloride, and identifying the role of a hydroxide ion.

Practise this question

Question

Question text

20 This question is about reaction mechanisms.

(a) Chemists use curly arrows in reaction mechanisms.

(i) What does a curly arrow show in a reaction mechanism?

… [1]

(ii) Draw structures to show the products in the reaction mechanism below.

H + H

[2]

(iii) Use the mechanism in (ii) to explain what is meant by heterolytic fission.

… [2]

(b) An incomplete reaction mechanism is shown below.

(i) Complete the mechanism by adding curly arrows and any missing species.

–

O O

H3C C H3C C Cl

Cl OH

–

H3C C + …

[4]

(ii) What is the role of OH− in this mechanism?

… [1]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

20 (a) (i) Movement of an electron pair 1 AO1.1 For electron pair,

ALLOW lone pair OR bonding pair OR 2 electrons

(a) (ii) 2 AO3.1 ALLOW any combination of skeletal OR structural

×2 OR displayed formula as long as unambiguous

IGNORE any other products

Correct carbon skeleton

‘+’ charge on correct carbon skeleton

(a) (iii) Heterolytic 2

one (bonded) atom/O receives both/2 electrons AO1.2 ALLOW 2 electrons go to one (bonded) atom/O

Fission IGNORE formation of ions/radicals

Breaking of a covalent bond

OR breaking of C-O bond AO1.1 For O atom,

ALLOW species

DO NOT ALLOW element OR molecule

‘Bond breaking’ is not sufficient

(no reference to covalent)

26 element

(b) (i) 4 AO3.2 IGNORE any dipoles shown

×4

NOTE: curly arrows can be straight, snake-like, etc.

but NOT double headed or half headed arrows

Curly arrow from OH– must

• go to the C of C=O

curly arrow to Cl AND

AND

– • start from, OR be traced back to any point

Cl as product across width of lone pair on O of OH–

• OR start from – charge–OH ion

Curly arrow from C=O bond must start from, OR

be traced back to, any part of C=O bond and go to

Curly arrow from O– must

Question Answer 27 Marks Guidance

element

• go to C=O bond

AND

• start from, OR be traced back to, any point

across width of lone pair

• OR start from ‘–‘ charge of O–

Curly arrow from C–Cl bond must start from, OR be

traced back to, any part of C–Cl bond and go to Cl

(b) (ii) (OH–) donates an electron pair/lone pair 1 AO1.2

(OH– acts as a) nucleophile

Total 10

How to answer it

Reaction Mechanisms & Organic Chemistry

What this question tests

This question assesses your core organic chemistry mechanism skills: defining curly arrows and heterolytic fission, interpreting and completing nucleophilic addition-elimination mechanisms (acyl chloride reactions), drawing organic products with correct charges and skeletal structures, and identifying reagent roles (nucleophiles).

Question 20(a)

Curly Arrows and Bond Fission

✅ Correct Answers

(i): Movement of an electron pair (allow lone pair, bonding pair, or 2 electrons).
(ii): Carbon skeleton showing a carbocation (positive charge on the central carbon attached to the ethyl groups) plus an H₂O molecule.
(iii): The oxygen/bonded atom receives both electrons from the bond, AND it is the breaking of a covalent bond (specifically the C-O bond).

💡 Key Knowledge

Curly arrows always start from a source of electrons (lone pair or covalent bond) and point to where the new bond/ion is formed.
Heterolytic fission results in unequal sharing of electrons, producing ions (unlike homolytic fission which produces free radicals).

🧠 Exam Technique

For drawing organic ions (part a-ii), make sure the positive (+) charge is clearly placed on the correct carbon atom of the carbon skeleton, and neutral leaving molecules like H₂O are included.

❌ Common Errors

Writing vague definitions like "bond breaking" without specifying that it is a covalent bond undergoing heterolytic fission will lose marks in part (iii).

Mark breakdown: (i) 1 mark | (ii) 2 marks | (iii) 2 marks

Question 20(b)

Acyl Chloride Mechanism & Nucleophilic Role

✅ Correct Answers

(i) Mechanism arrows & species:
1. Curly arrow from -OH lone pair/charge to the delta-positive carbon of C=O.
2. Curly arrow from the C=O double bond up to the oxygen atom, forming a -O⁻ intermediate.
3. Curly arrow from the -O⁻ lone pair/charge reforming the C=O double bond.
4. Curly arrow from the C-Cl bond breaking, going directly to the Chlorine atom.
Missing species: Cl⁻ (chloride ion).
(ii) Role of OH⁻ : Donates an electron pair / lone pair, OR acts as a nucleophile.

💡 Key Knowledge

Acyl chlorides react via addition-elimination (or nucleophilic addition-elimination). The initial attack breaks the pi bond of the carbonyl group, and the intermediate subsequently eliminates the chloride ion as the C=O bond reforms.

🧠 Exam Technique

Arrow origins are heavily scrutinized by examiners! Ensure curly arrows start precisely from the lone pair or the bond line, not floating randomly in space. Arrow heads must point directly to atoms or bonds.

❌ Common Errors

Drawing the first curly arrow starting from the hydrogen atom of OH⁻ instead of the oxygen's lone pair/negative charge, or failing to include the Cl⁻ leaving group product.

Mark breakdown: (i) 4 marks | (ii) 1 mark

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.