Edexcel IAL Chemistry Topic 15 Carbonyls, Carboxylic Acids and Chirality

Move between structure and evidence: identify chiral centres, choose carbonyl tests and reagents, and use mechanisms or observations to justify organic products.

Exam points

Identify chiral centres, enantiomers and optical activity from displayed or skeletal structures.
Use Tollens, Fehling, 2,4-DNPH, iodoform and dichromate observations for carbonyls.
Apply organic mechanisms and reagent conditions to predict alcohol, cyanohydrin or acid products.

Covered syllabus points

15.1—Optical isomerism is a result of chirality in molecules with a single chiral centre 15.2—Optical isomerism results from chiral centre(s) in a molecule with asymmetric carbon atom(s) and that optical isomers 15.3—Optical activity is the ability of a single optical isomer to rotate the plane of polarisation of plane-polarised 15.4—What is meant by the term ‘racemic mixture’15.5—Data on optical activity of reactants and products as evidence for SN1 and SN2 mechanisms and addition to carbonyl compounds 15.6—The nomenclature of aldehydes and ketones 15.7—Aldehydes and ketones: i do not form intermolecular hydrogen bonds and this affects their physical properties ii can form 15.8—The reactions of carbonyl compounds with: i Fehling’s or Benedict’s solution, Tollens’ reagent and acidified dichromate(VI)15.9—The nomenclature of carboxylic acids 15.10—Hydrogen bonding affects the physical properties of carboxylic acids, in relation to their boiling temperatures 15.11—Carboxylic acids can be prepared by the oxidation of alcohols or aldehydes and the hydrolysis of nitriles 15.12—The reactions of carboxylic acids with: i lithium tetrahydridoaluminate(III) (lithium aluminium hydride) in dry ether 15.13—The nomenclature of acyl chlorides and esters 15.14—The reactions of acyl chlorides with: i water ii alcohols iii concentrated ammonia iv amines 15.15—The hydrolysis reactions of esters, in acidic and alkaline solution 15.16—How polyesters, such as terylene, are formed by condensation polymerisation reactions 15.17—Data from mass spectra to: i suggest possible structures of a simple organic compound given accurate relative molecular 15.18—Carbon-13, (13C) NMR spectroscopy provides information about the positions of 13C atoms in a molecule 15.19—Data from 13C NMR spectroscopy to: i predict the different environments for carbon atoms present in a molecule, given values 15.20—Both low and high resolution proton NMR spectroscopy to: i predict the different types of proton present in a molecule 15.21—Chromatography separates components of a mixture using a mobile phase and a stationary phase 15.22—Rf values from one-way chromatograms in paper and thin-layer chromatography (TLC) and understand reasons for differences 15.23—High-performance liquid chromatography, HPLC, and gas chromatography, GC, are types of column chromatography that separate

[Maximum number: 12]

This question is about carboxylic acids and their derivatives.

(a)

Lactic acid, $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ , is produced in muscles as a result of anaerobic respiration.

[ 2 ]

(i)

The structure of lactic acid is

Figure for Question (i) — Edexcel A-Level Chemistry A2

Give a reason why lactic acid shows optical isomerism.
(1)

[ 1 ]

(ii)

A laboratory sample of lactic acid does not rotate the plane of plane-polarised monochromatic light.

Give a reason for this observation.

[ 1 ]

(b)

A reaction scheme involving butanoic acid is shown.

Figure for Question (b) — Edexcel A-Level Chemistry A2

Identify X, Y and Z by name or formula.
Alcohol X

Reagent Y

Compound Z

[ 3 ]

(c)

The repeat unit of a polyester is shown.

Figure for Question (c) — Edexcel A-Level Chemistry A2

Give the structures of the two monomers that could form this polyester.
(2)

Table for Question (c) — Edexcel A-Level Chemistry A2

[ 2 ]

(d)

An organic compound E contains carbon, hydrogen and oxygen only.

[ 5 ]

(i)

The accurate relative atomic masses, $A_{r}$ , of the three elements in E are shown in the table.

Table for Question (i) — Edexcel A-Level Chemistry A2

E contains five carbon atoms and gives a molecular ion peak at m / z=102.0678 in its mass spectrum.

Deduce the molecular formula of E.

[ 1 ]

$\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}$

Allow symbols in any order

Ignore any working Ignore + charge

(1)

(ii)

Aqueous sodium hydrogencarbonate is added to a sample of E. No effervescence occurs.

State what can be deduced by this observation.

[ 1 ]

(iii)

Data from the high resolution proton NMR spectrum of E is shown.

Table for Question (iii) — Edexcel A-Level Chemistry A2

Deduce the structure of E.
Justify your answer by labelling the protons responsible for each peak.

[ 3 ]

structure of E

- 2 or 3 proton environments correct

- $4{ }^{\text {th }}$ proton environment correct

(1)

(1)

(1)

Figure for Answer to Question (iii) — Edexcel A-Level Chemistry A2

Example of structure:

Protons can be circled and labelled or just labelled Allow labels using data from the table

Only 1 proton from each group needs to be labelled

Allow whole groups to be labelled, including the carbon atom

(3)

[Maximum number: 10]

This question is about carbonyl compounds.

(a)

Three carbonyl compounds, A, B and C, are straight-chain structural isomers, with the formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}$ .
Only isomer A reacts with Tollens' reagent to give a silver mirror.
Only isomer B reacts with iodine in the presence of alkali to produce pale yellow crystals.

Draw the displayed structures of these three isomers.

Figure for Question (a) — Edexcel A-Level Chemistry A2

[ 3 ]

(b)

Another carbonyl compound, propanal, reacts with HCN in the presence of KCN to form a racemic mixture of two optical isomers of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CN}$ .

[ 3 ]

(i)

Describe how you could distinguish between pure samples of the two optical isomers.

[ 1 ]

(ii)

Explain, with reference to the reaction mechanism, why this reaction produces a racemic mixture.

[ 2 ]

(c)

Propanone, an isomer of propanal, also reacts with HCN in the presence of KCN.

Draw the skeletal formula of the product of this reaction.

[ 1 ]

(d)

State why the product formed in (c)(i) does not show optical isomerism.

[ 1 ]

(e)

${ }^{13} \mathrm{C}$ NMR spectroscopy provides information about the structures of propanal and propanone.

[ 2 ]

(i)

Identify the chemical shift range and carbon environment of one peak you would expect to see in both spectra.

[ 1 ]

(ii)

State the number of peaks you would expect to see in each ${ }^{13} \mathrm{C}$ NMR spectrum.

Propanal

Propanone

[ 1 ]

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Covered syllabus points

Topic 15: Organic Chemistry: Carbonyls, Carboxylic Acids and Chirality question 1

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Question (ii)

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Question (iii)

Topic 15: Organic Chemistry: Carbonyls, Carboxylic Acids and Chirality question 2

Question (a)

Question (b)

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Question (e)

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