Edexcel A-Level Chemistry Paper 3, June 2017: Question 5

6 marks · Medium difficulty · Short Open Response

Draw the permanent dipole, fully displayed formula, name of product, properties and reactions of a Grignard reagent formed from 2-bromopropane reacting with magnesium.

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Question

Question text

5 Grignard reagents are used in organic synthesis as a way of increasing the length of

the carbon chain in a molecule.

(a) The structure of the Grignard reagent formed by the reaction between

2-bromopropane and magnesium is

CH3

H C MgBr

CH3

On the diagram, draw the permanent dipole involving the central carbon atom.

(1)

(b) The Grignard reagent in part (a) reacts with propanal.

(i) Draw the fully displayed formula of the final organic product of this reaction.

(1)

(ii) Name the organic product in (b)(i).

(1)

describe a Grignard reagent.

(2)

acid

*P48954A01836*electrophile

nucleophile

oxidising agent

reducing agent

(d) The solvent used for Grignard reagents has to be completely dry.

By considering the dipole on the O—H bonds in water, predict the identity of the organic

product that forms if water is added to the Grignard compound in part (a).

(1)

(Total for Question 5 = 6 marks)

Mark scheme

Show the mark scheme

5(a) C atom of C−Mg bond labelled as δ− and Mg labelled as δ+ Do not award full + or – charge (1)

Ignore - on Br

Question

Acceptable Answers Additional Guidance Mark

Number

5(b)(i) Ignore other structures (1)

Allow non-displayed formula

Question

Acceptable Answers Additional Guidance Mark

Number

5(b)(ii) (1)

2-methylpentan-3-ol Allow 2-methyl-3-pentanol

No TE on incorrect formula from 5(b)(i)

Question

Acceptable Answers Additional Guidance Mark

Number

5(c) next to nucleophile (1) If more than two boxes ticked scores (0) (2)

next to reducing agent (1)

Question

Acceptable Answers Additional Guidance Mark

Number

5(d) propane / C3H8 Accept name or formula or structural (1)

/ skeletal / displayed formula

Ignore additional inorganic products

Do not award just ‘alkane’

If name and formula given then they both

must be correct

(Total for Question 5 = 6 marks)

How to answer it

Grignard Reagents and Carbon-Chain Extension

📚 What this question tests

This question assesses your understanding of Grignard reagents, including their preparation, polarity, chemical characteristics (acting as nucleophiles and reducing agents), reactions with carbonyl compounds (aldehydes) to lengthen carbon chains, and their extreme sensitivity to protic solvents like water.

Part (a) — Grignard Reagent Dipole

Polarity of the carbon-magnesium bond

✅ Correct Answer

Label the central carbon atom bonded to Mg with a δ− and the magnesium atom with a δ+.

💡 Key Knowledge

Carbon is more electronegative than magnesium. This causes an unusual reversal of polarity (umpolung) where the carbon atom carries a partial negative charge, making it nucleophilic.

❌ Common Errors

Students often lose marks by writing full ionic charges (+ / −) instead of partial charges (δ+ / δ−), or by incorrectly placing a dipole on the Mg—Br bond.

🎯 Mark breakdown: (1) mark for both correct partial dipoles (δ− on C, δ+ on Mg).

Part (b)(i) — Reaction with Propanal (Product Structure)

Drawing the fully displayed formula

✅ Correct Answer

The product is a branched secondary alcohol: 2-methylpentan-3-ol. Every single bond must be explicitly drawn out to satisfy the "fully displayed formula" requirement (showing every atom and every bond).

🧠 Exam Technique

When drawing fully displayed formulae, ensure all C—H, C—C, C—O, and O—H bonds are clearly visible with lines. Do not condense groups like CH₃ into shorthand.

🎯 Mark breakdown: (1) mark for the fully displayed structure of 2-methylpentan-3-ol.

Part (b)(ii) — Naming the Organic Product

IUPAC Nomenclature

✅ Correct Answer

2-methylpentan-3-ol (Acceptable alternative: 2-methyl-3-pentanol ).

❌ Common Errors

Incorrect numbering of the longest carbon chain or failing to identify the correct principal functional group suffix ( -ol ).

🎯 Mark breakdown: (1) mark for correct IUPAC name. No error carried forward if 5(b)(i) was wrong.

Part (c) — Characteristics of Grignard Reagents

Identifying chemical properties

✅ Correct Answer

Tick the boxes for: nucleophile and reducing agent.

💡 Key Knowledge

Because the alkyl group carries a partial negative charge, it attacks electrophilic carbon centers (acting as a nucleophile). They can also transfer hydride equivalents or reduce carbonyl compounds, functioning as reducing agents.

❌ Common Errors

Ticking more than two boxes automatically results in 0 marks. Be selective and only choose the two correct terms.

🎯 Mark breakdown: (2) marks total (1 mark per correct tick). Zero marks if more than two boxes are ticked.

Part (d) — Reaction with Water (Protonation)

Predicting the organic product in wet conditions

✅ Correct Answer

Propane (or formula C₃H₈ , structural/skeletal/displayed formula).

💡 Key Knowledge

Water is a protic solvent containing polar O—H bonds (δ+ on H). The nucleophilic carbon of the Grignard reagent abstracts a proton from water, converting the alkyl-magnesium species into an alkane (in this case, propane).

❌ Common Errors

Writing just "alkane" is too vague and will be rejected. You must name or provide the formula for the specific alkane corresponding to the Grignard reagent used.

🎯 Mark breakdown: (1) mark for identifying propane or its valid chemical formula/structure.

Topics

Organic Chemistry · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.