OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 16

17 marks · Hard difficulty · Structured Questions

Explain sigma and pi bonds, outline the electrophilic addition mechanism of HBr to but-1-ene, discuss stereoisomerism, and complete Diels-Alder reaction mechanisms and products involving but-1-ene and buta-1,3-diene.

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Question text

16 But-1-ene, H2C=CHCH2CH3, and buta-1,3-diene, H2C=CH–CH=CH2, are unsaturated compounds

used to make many organic products.

(a) But-1-ene and buta-1,3-diene have σ-bonds and π-bonds.

(i) Explain what is meant by the terms σ-bond and π-bond.

σ-bond: …

π-bond: …

[2]

(ii) How many σ- and π-bonds are in one molecule of buta-1,3-diene?

σ-bonds: … π-bonds: … [2]

(b) But-1-ene is reacted with hydrogen bromide, forming a mixture of two saturated organic

products.

One of the organic products is formed in a much greater quantity than the other organic

product.

(i) Outline the reaction mechanism for the formation of this major organic product. The

structure of but-1-ene has been provided.

Include curly arrows and relevant dipoles.

H CH2CH3

C C

H H

[4]

(ii) Explain why one organic product is formed in a much greater quantity than the other

organic product.

… [2]

(iii) Buta-1,3-diene is reacted with an excess of hydrogen bromide, forming a mixture of

saturated organic products.

How many saturated organic products could be present in this mixture?

… [1]

The student drew out skeletal formulae for the stereoisomers of buta-1,3-diene:

(i) Explain the term stereoisomerism.

… [1]

(ii) Explain, with a reason, whether the student is correct or incorrect.

… [1]

(d) ‘Diels-Alder’ reactions are used in the synthesis of many important organic compounds.

The Diels-Alder reaction of buta-1,3-diene with ethene is shown below.

(i) Add curly arrows to the diagram below to complete the mechanism for this Diels-Alder

reaction.

[2]

(ii) Two more Diels-Alder reactions of buta-1,3-diene are shown below.

In the boxes, draw the organic product of each reaction.

OCH3

[2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

16 (a) (i) σ-bond: Overlap of orbitals between (bonding) atoms 2 AO1.1 ALLOW labelled diagrams

×2 IGNORE the type of orbital for σ-bond

π-bond: Sideways overlap of (adjacent) p-orbitals DO NOT ALLOW pi-orbital

(ii) σ-bonds: 9 2 AO1.2

×2

π-bonds: 2

(b) (i) 4 AO1.2 NOTE: curly arrows can be straight, snake

×2 like, etc.

but NOT double headed or half headed

AO2.5 arrows

×2

1st curly arrow must

• go to the H atom of H–Br

AND

• start from, OR be traced back to any

point across width of C=C

Curly arrow from C=C bond to H of H–Br

DO NOT ALLOW partial charge on C=C

Correct dipole shown on H–Br 2nd curly arrow must

AND curly arrow showing breaking of H–Br bond • start from, OR be traced back to any

part of δ+H–Brδ– bond

AND

• go to Brδ–

5 AO

element

Correct carbocation 3rd curly arrow must

AND curly arrow from Br– to C+ of carbocation • go to the C+ of carbocation

DO NOT ALLOW δ+ on C of carbocation AND

• start from, OR be traced back to any

point across width of lone pair on :Br–

• OR start from – charge of Br– ion

(Lone pair NOT needed if curly arrow shown

from – charge of Br– ion)

Correct product ALLOW ECF for product from incorrect

carbocation, i.e.

IF Br2 is used instead of HBr contact your

Team Leader

6 AO

element

(ii) (major product forms from) most/more stable 2 AO1.1 For carbocation,

intermediate/carbocation ALLOW carbonium ion or cation

(major product forms from a) secondary carbocation AO1.2 IGNORE descriptions of the major/minor

OR carbocation bonded to more C atoms / more alkyl groups product in terms of Markownikoff’s rule e.g.

OR carbocation bonded to fewer H atoms H atom joins to C with most H

IGNORE references to stability of the

product

-----------------------------------------------------------

ALLOW ORA, i.e.

(minor product forms from) least/less stable

intermediate/carbocation

(minor product forms from a) primary

carbocation

OR carbocation bonded to less C atoms /

less alkyl groups

OR carbocation bonded to more H atoms

(iii) 3 1 AO1.2

AND

Different arrangement (of atoms) in space DO NOT ALLOW same empirical formula

OR different spatial arrangement (of atoms) OR same general formula

IGNORE same molecular formula

Reference to E/Z isomerism or optical

isomerism is not sufficient

(ii) Student is not correct 1 AO3.1

AND

2 groups on one carbon atom (of C=C) are the same DO NOT ALLOW one side of C=C

C–C bond can rotate

7 AO

element

(d) (i) 2 AO2.5 IGNORE any dipoles shown

×2

NOTE: curly arrows can be straight, snake-

like, etc.

but NOT half headed or double headed

arrows

1 mark for each curly arrow

Curly arrow from C=C bond must

start from, OR be traced back to,

Lower left: any part of C=C bond and go to

C–C

Upper left: any part of C=C bond and go to

gap between C=C and C=C

(ii) 2 AO3.2

×2

Total 17

How to answer it

Alkenes, Reaction Mechanisms & Stereoisomerism Study Guide

What this question tests

This question assesses your understanding of unsaturated hydrocarbons (alkenes and dienes), bonding types (sigma and pi bonds), electrophilic addition reaction mechanisms (including curly arrows and carbocation stability), stereoisomerism requirements, and advanced synthetic applications like Diels-Alder cycloaddition reactions.

Question 16 (a)

Bonding and Counting in Alkenes and Dienes

💡 Key Knowledge: Sigma and Pi Bonds

Sigma (σ) bond: Formed by the direct, linear overlap of orbitals between bonding atoms.
Pi (π) bond: Formed by the sideways overlap of adjacent p-orbitals above and below the bonding axis.

✅ Correct Answers: Part (ii)

σ-bonds: 9
π-bonds: 2

❌ Common Errors

Confusing the definition of a pi bond by calling it a "pi-orbital".
Miscounting single sigma bonds in complex structures (remember every single covalent bond contains one σ-bond, and double bonds contain one σ and one π bond).

Mark breakdown: (a)(i) 2 marks | (a)(ii) 2 marks

Question 16 (b)

Electrophilic Addition of HBr to But-1-ene

🧠 Exam Technique: Mechanism Rules

Curly arrows: Must start from the source of electrons (the C=C double bond or a lone pair/bond) and point directly to the atom accepting them.
Dipoles: Label the H-Br bond with partial charges ( H(δ+) and Br(δ-) ) to show bond polarity.

✅ Correct Answers & Explanation

Mechanism ((i)): 1st curly arrow from C=C to Hδ+; 2nd curly arrow breaking the H-Br bond; 3rd curly arrow from Br− lone pair/charge to the secondary carbocation carbon. Product is 2-bromobutane ( CH₃-CH(Br)-CH₂-CH₃ ).
Major Product Reason ((ii)): Forms via the most stable intermediate carbocation (secondary carbocation), which is bonded to more carbon/alkyl groups.
Diene Addition ((iii)): 3 saturated organic products can be formed when buta-1,3-diene reacts with an excess of HBr.

❌ Common Errors

Placing a partial positive ( δ+ ) charge directly onto the non-polar C=C double bond.
Drawing curly arrows that originate in "empty space" rather than clearly touching the bond or lone pair.

Mark breakdown: (b)(i) 4 marks | (b)(ii) 2 marks | (b)(iii) 1 mark

Question 16 (c)

Stereoisomerism in Buta-1,3-diene

💡 Key Knowledge: Stereoisomerism

Definition: Compounds with the same structural formula but with a different arrangement of atoms in space.

✅ Correct Answer & Rationale

Student is NOT correct.
Reason: One of the carbon atoms at the end of the double bond ( CH₂= ) has two identical groups (hydrogen atoms) attached to it, meaning it cannot satisfy the conditions for E/Z stereoisomerism.

❌ Common Errors

Using vague statements like "one side of the C=C" instead of explicitly stating that a single carbon atom has two identical groups attached to it.

Mark breakdown: (c)(i) 1 mark | (c)(ii) 1 mark

Question 16 (d)

Diels-Alder Cycloaddition Reactions

🧠 Exam Technique: Diels-Alder Arrows

Draw three coordinated curly arrows representing a concerted pericyclic ring-closure. One arrow moves from the diene's first double bond, the second moves from the dienophile's pi bond, and the third shifts the diene's remaining double bond into the newly forming ring position.

✅ Correct Products for Part (ii)

Reaction 1 Product: A cyclohexene ring containing two ester / methyl ester groups ( -COOCH₃ ) oriented cis to each other on adjacent carbons.
Reaction 2 Product: A 1,4-cyclohexadiene ring derivative fused from buta-1,3-diene and an alkyne dienophile.

❌ Common Errors

Failing to preserve the correct ring size (cyclohexene/cyclohexadiene skeletons) or misplacing substituent groups during cycloaddition structure prediction.

Mark breakdown: (d)(i) 2 marks | (d)(ii) 2 marks | Total: 17 marks

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.