OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 19

15 marks · Medium difficulty · Structured Questions

Construct combustion and oxidation equations, explain hydrogen bonding and solubility differences of alcohols, and complete an organic reaction flowchart involving carbonyl reduction, substitution, and elimination.

Practise this question

Question

A structured multi-part chemistry question about alcohols. Part (a) asks for an equation for the complete combustion of an unsaturated alcohol with 5 carbon atoms. Part (b)(i) asks to explain with a diagram why ethanol is soluble in water, including dipoles and lone pairs, and (b)(ii) asks to explain the difference in solubility between hexan-1-ol and hexane-1,6-diol using provided data in Table 19.1. Part (c)(i) shows a reaction flowchart to be completed involving a carbonyl compound, reduction with a reagent, compound K, substitution with NaBr/H2SO4, and acid-catalyzed elimination forming two structural isomers, and (c)(ii) asks for the name of compound K. Part (d) asks to describe both oxidation reactions of butan-1-ol including structures of organic products, balanced equations, and essential reaction conditions.
Question text

19 This question is about alcohols.

(a) Construct an equation for the complete combustion of an unsaturated alcohol with 5 carbon

atoms.

… [1]

(b) Many alcohols, including ethanol, are soluble in water.

(i) Explain, with the aid of a diagram, why ethanol is soluble in water.

Include relevant dipoles and lone pairs.

… [2]

(ii) The solubility of hexan-1-ol and hexane-1,6-diol in water is shown below in Table 19.1.

Alcohol Solubility in water / g dm−3

hexan-1-ol 5.9

hexane-1,6-diol 500

Table 19.1

Explain the difference in solubility of hexan-1-ol and hexane-1,6-diol.

… [1]

(c) Alcohols are important in organic synthesis and can be formed by the reduction of carbonyl

compounds.

(i) Complete the flowchart by filling in each box.

carbonyl compound

reduction

reagent(s): …

OH

NaBr/H2SO4

compound K

acid catalyst

heat

+

structural isomers

[5]

(ii) What is the name of compound K?

… [1]

(d) Butan-1-ol can be oxidised to form two different organic products, depending on the reaction

conditions used.

Describe both oxidation reactions of butan-1-ol.

For each reaction include

• the structure of the organic product

• a balanced equation

• the essential reaction conditions.

In your equations you may use [O] to represent the oxidising agent.

… [5]

Mark scheme

Show the mark scheme The official mark scheme providing answers and guidance for all parts of question 19. It details acceptable equations for combustion, scoring points for ethanol-water hydrogen bonding diagrams with correct dipoles, solubility explanations based on number of OH groups, completed structures and reagents for the reaction flowchart in organic synthesis, naming of 3-methylcyclohexanol, and the structures, equations, and conditions (distillation vs reflux) for the oxidation of butan-1-ol.

Question Answer Marks Guidance

19 (a) C5H10O + 7O2 5CO2 + 5H2O 1 ALLOW multiples

e.g. 2C5H10O + 14O2 10CO2 + 10H2O

ALLOW any equation involving an unsaturated

alcohol with correct balancing

e.g.

C5H8O + 6.5O2 5CO2 + 4H2O

C5H6O + 6O2 5CO2 + 3H2O

C5H4O + 5.5O2 5CO2 + 2H2O

C5H2O + 5O2 5CO2 + H2O

IGNORE state symbols

(b) (i) Diagram showing a water molecule and an ethanol 2 ALLOW any combination of skeletal OR structural

molecule with at least one H + and one O – on BOTH OR displayed formula as long as unambiguous

molecules

DO NOT ALLOW + on H atoms of alkyl group

DO NOT ALLOW any marks for a diagram

Hydrogen bond between one lone pair on O atom in one of containing O2H

the molecules and the H atom of another.

If more than one hydrogen bond is shown they

AND must all be correct to award the mark.

Hydrogen bonding stated or labelled on diagram

e.g. Hydrogen bond

H H

- + -

H C C O H O

H + H( +)

H H

Question Answer 22 Marks Guidance

(ii) Hexane-1,6-diol has more OH groups (than hexan-1-ol) 1 Statements MUST be comparative

AND e.g. hexane-1,6-diol has two –OH groups and

hexan-1-ol has one -OH group

(hexane-1,6-diol) forms more hydrogen bonds with water

ALLOW hydroxyl or hydroxy

DO NOT ALLOW hydroxide/OH–

ALLOW ORA

(c) (i) Starting material from reduction reaction 5 ALLOW any combination of skeletal OR structural

OR displayed formula as long as unambiguous

O

Watch for missing methyl groups

Reagent for reduction

IGNORE H+ / acid or H O or ethanol

NaBH4 ALLOW sodium borohydride

OR sodium tetrahydridoborate

Product from reaction with NaBr/H2SO4 ALLOW LiAlH4

Br

Structural isomers

ALLOW in either order

(ii) 3-methylcyclohexanol 1 ALLOW 3-methylcyclohexan-1-ol

ALLOW 1-methylcyclohexan-3-ol

IGNORE lack of hyphens, or addition of commas

(d) Structures of organic products 5 ANNOTATE WITH TICKS AND CROSSES

H H H O Use of any primary alcohol containing 3, 5 or more

carbons can be awarded up to 4 marks.

H C C C C

ALLOW any combination of skeletal OR structural

H OR displayed formula as long as unambiguous

H H H

IGNORE names

H H H O

DO NOT ALLOW CH3CH2CH2COH for the

H C C C C structure of the aldehyde.

H H H OH ALLOW CH3CH2CH2CO2H for the structure of the

carboxylic acid.

Equations

ALLOW marks for structures from equations as

long as unambiguous.

CH3CH2CH2CH2OH + [O] CH3CH2CH2CHO + H2O

ALLOW molecular formulae in equations

e.g. C4H10O + [O] C4H8O + H2O

CH CH CH CH OH + 2[O] CH CH CH COOH + C4H10O + 2[O] C4H8O2 + H2O

32 2 2 3 2 2

H O C4H9OH + [O] C3H7CHO + H2O

C4H9OH + 2[O] C3H7CO2H + H2O

Reaction conditions IGNORE incorrect structures in equations

i.e. C4H10O + [O] C3H7COH + H2O

Distillation to produce aldehyde/CH3CH2CH2CHO scores equation mark

AND

Reflux to produce carboxylic acid/CH3CH2CH2COOH Conditions must be linked to aldehyde/carboxylic

acid or correct products.

Conditions may be written above arrow of

equation.

Total 15

How to answer it

Comprehensive Study Guide: Alcohols & Organic Synthesis

What this question tests

This exam question evaluates your core knowledge of alcohol chemistry, including combustion equations, intermolecular forces (hydrogen bonding), organic synthesis pathways involving carbonyl reduction, substitution reactions, acid-catalysed elimination (dehydration), and the controlled oxidation of primary alcohols using distillation vs. reflux.

Question Part (a)

Combustion of an Unsaturated Alcohol

✅ Correct Answer

C₅H₁₀O + 7O₂ → 5CO₂ + 5H₂O

1 Mark available

💡 Key Knowledge

  • An unsaturated alcohol with 5 carbon atoms has the general molecular formula CₙH₂ₙ₋₂O (one double bond or ring). Thus, for C=5, H = (2×5) - 2 = 8? Wait, standard formula for unsaturated with 1 degree of unsaturation is C₅H₁₀O (like an unsaturated hexenol isomer or cyclic alcohol).
  • Complete combustion always produces carbon dioxide ( CO₂ ) and water ( H₂O ).

❌ Common Errors

Balancing oxygen incorrectly by forgetting to account for the oxygen atom already present within the alcohol molecule itself.

Question Part (b)(i)

Hydrogen Bonding and Solubility

✅ Correct Answer

A clearly labelled diagram showing water and ethanol molecules with correct partial charges ( δ⁺ on H, δ⁻ on O) and a dotted line representing a hydrogen bond connecting a lone pair on an oxygen atom to a hydrogen atom bonded to an oxygen.

2 Marks available
兖

🧠 Exam Technique

  • Draw partial charges accurately: δ⁺ goes on H atoms attached to O, never on carbon alkyl chain hydrogens.
  • Make sure the hydrogen bond (dotted line) clearly bridges from a lone pair on an oxygen to a δ⁺ hydrogen atom.
Question Part (b)(ii)

Comparing Solubility: Hexan-1-ol vs. Hexane-1,6-diol

✅ Correct Answer

Hexane-1,6-diol has more -OH groups than hexan-1-ol and therefore forms more hydrogen bonds with water.

1 Mark available

❌ Common Errors

Using non-comparative statements (e.g., stating "hexane-1,6-diol forms hydrogen bonds" without noting it forms more than hexan-1-ol). Also, penalised for writing hydroxide ( OH⁻ ) instead of hydroxyl / -OH groups.

Question Part (c)

Synthetic Routes & Functional Group Transformations

✅ Correct Answer

Starting Carbonyl: Cyclohexanone (6-membered ring with a C=O group).
Reduction Reagent: NaBH₄ (or LiAlH₄ ).
Substitution Product (with NaBr/H₂SO₄): Bromocyclohexane.
Elimination Isomers: Cyclohexene and 1-methylcyclopentene (or similar positional isomers).

5 Marks total for flowchart (c)(i) + 1 Mark for name in (c)(ii)

💡 Key Knowledge

Compound K is cyclohexanol (named in part c(ii)). Acid-catalysed dehydration of cyclohexanol removes H₂O to form alkenes via elimination.

Question Part (d)

Oxidation of Butan-1-ol

✅ Correct Answer

1. Aldehyde Route:
Product: Butanal ( CH₃CH₂CH₂CHO )
Equation: CH₃CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CHO + H₂O
Conditions: Distillation (with acidified potassium dichromate, H⁺/K₂Cr₂O₇ ).

2. Carboxylic Acid Route:
Product: Butanoic acid ( CH₃CH₂CH₂COOH )
Equation: CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O
Conditions: Reflux (with excess acidified potassium dichromate).

5 Marks available

🧠 Exam Technique

Conditions must be explicitly linked to the correct product. Writing "reflux" without stating it yields the carboxylic acid loses credit if ambiguous. Use [O] safely as requested by the prompt.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.