Edexcel A-Level Chemistry A2 15 Organic Chemistry Carbonyls Carboxylic Acids and Chirality Questions

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

(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

[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 ]

[Maximum number: 9]

This question is about carbonyl compounds.

(a)

Ethanal, $\mathrm{CH}_{3} \mathrm{CHO}$ , reacts with hydrogen cyanide in the presence of cyanide ions to form 2-hydroxypropanenitrile.

Draw the mechanism for this reaction.
Include curly arrows, and any relevant lone pairs and dipoles.

[ 4 ]

An answer that makes reference to the following points:

- curly arrow from lone pair on C of $\mathrm{CN}^{-}$ towards C of aldehyde group

- curly arrow from C=O to, or just beyond, O and dipole on C=O

- intermediate

- curly arrow from lone pair on $\mathrm{O}^{-}$ to H and curly arrow from H-CN bond to anywhere on CN and final organic product

(1)

Penalise an incorrect dipole in M1 and M4 once only Penalise curly arrow not starting from lone pair once only in M1 and M4

Penalise half arrow-heads once only

Allow $\mathrm{CN}^{-}$ to attack C=O from any angle Allow CN bond displayed

If M1 lost as curly arrow from N of $\mathrm{CN}^{-}$ , allow CN joined to carbon through N Ignore connectivity for vertical CN groups

Allow curly arrow from lone pair on $\mathrm{O}^{-}$ to $\mathrm{H}^{+}$ Ignore missing dipole in HCN

Example of mechanism:

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

(b)

A carbonyl compound, F, has the molecular formula $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}$ .

[ 5 ]

(i)

F reacts with iodine in an alkaline solution to give a pale yellow precipitate. Give the name or formula of the group in F identified by this test.

[ 1 ]

$\mathrm{CH}_{3} \mathrm{CO}-/-\mathrm{COCH}_{3} /$ methyl ketone

Allow any combination of structural / displayed formula or skeletal formula

Allow methyl next to ketone / methyl and ketone / methylcarbonyl

Allow $\mathrm{CH}_{3} \mathrm{COR} / \mathrm{RCOCH}_{3}$

Ignore missing continuation bond from structures

Do not award ethanal / methyl secondary alcohol / a specific compound

(ii)

Draw the skeletal formulae of the four possible structures of carbonyl compound F.

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

[ 2 ]

(iii)

The carbon-13 ( ^13 C) NMR spectrum of F has four peaks.

Identify F by drawing its displayed formula.
Justify your answer by labelling the carbon atoms or groups of carbon atoms responsible for the four peaks in the spectrum.

[ 2 ]

displayed formula of F

- carbon atoms labelled

Example of displayed formula:

Allow $\mathrm{CH}_{3}$

Allow other unambiguous labels for the carbon atoms

Allow M2 for labels on structural / skeletal formulae, including labels on formulae in (c)(ii)

Ignore reference to singlet / splitting patterns

(2)

19(d)*

This question assesses a student's ability to show a coherent and logically structured answer with linkages and fully-sustained reasoning.

Marks are awarded for indicative content and for how the answer is structured and shows lines of reasoning.

The following table shows how the marks should be awarded for indicative content.

Number of

indicative

marking points

seen in answer

Number of marks

awarded for

indicative

marking points

6

5-4

3-2

1

0

\end{tabular}

The following table shows how the marks should be awarded for structure and lines of reasoning.
\end{tabular}

Guidance on how the mark scheme should be applied:
The mark for indicative content should be added to the
mark for lines of reasoning. For example, an answer with
five indicative marking points that is partially structured
with some linkages and lines of reasoning scores 4 marks
( 3 marks for indicative content and 1 mark for partial
structure and some linkages and lines of reasoning).
If there are no linkages between points, the same five
indicative marking points would yield an overall score of
3 marks (3 marks for indicative content and no marks for
linkages).

(6)

Comment:

Look for the indicative marking points first, then consider the mark for structure of answer and sustained line of reasoning

In general it would be expected that 5 or 6 indicative points would get 2 reasoning marks, and 3 or 4 indicative points would get 1 mark for reasoning, and 0,1 or 2 indicative points would score zero marks for reasoning.

Additional incorrect chemistry loses a structure and lines of reasoning mark (if 1 or 2 have been awarded).

Indicative content
- IP1 London forces
Pentane (only) has London forces / all have London forces
- IP2 Butanal
Butanal (also) has (permanent) dipole-dipole interactions
- IP3 Propanoic acid
Propanoic acid (also) has (dipole-dipole and) hydrogen bonding
- IP4 Intermolecular forces
The London forces have about the same strength as they have a
similar number of electrons / pentane has 42, butanal has 40 (and
propanoic acid has 40 electrons)
or
butanal has dipole-dipole interactions because it is polar / has a
dipole on C=O
propanoic acid has hydrogen bonding because it contains OH /
COOH group
- IP5 Butanal and pentane
Dipole-dipole interactions are stronger than London forces
more energy is needed to overcome the dipole-dipole interactions
than London forces
- IP6 Butanal and propanoic acid
Hydrogen bonding is stronger than dipole-dipole interactions
more energy is needed to overcome hydrogen bonding than dipole-
dipole interactions

Allow dispersion forces / van der Waals’ forces / forces between an instantaneous dipole and an induced dipole for London forces throughout answer Do not award IP1 if any other forces mentioned

Allow dipole-dipole forces / attractions / bonds Do not award IP2 if hydrogen bonding included

Allow similar relative molecular masses (butanal 72, pentane 72, propanoic acid 74)

Do not award incorrect numbers of electrons / relative molecular masses

Allow dipole-dipole interactions linked to C=O

Allow diagram of hydrogen bond between two molecules

Ignore formation of dimer

Allow London forces are the weakest (intermolecular force)

Do not award if covalent bonds broken or explanation is about intermolecular forces with water

Allow hydrogen bonding is the strongest (intermolecular force)

Do not award if covalent bonds broken or explanation is about intermolecular forces with water

(6)

[Maximum number: 1]

This question is about carboxylic acids and their derivatives.

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

The structure of lactic acid is

Figure for Question 4 — Edexcel A-Level Chemistry A2

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

[Maximum number: 1]

This question is about carbonyl compounds.

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

[Maximum number: 1]

This question is about carboxylic acids and their derivatives.

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

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

Give a reason for this observation.

[Maximum number: 2]

This question is about carbonyl compounds.

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

[Maximum number: 2]

This question is about carbonyl compounds.

A carbonyl compound, F, has the molecular formula $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}$ .

Draw the skeletal formulae of the four possible structures of carbonyl compound F.

Table for Question 8 — Edexcel A-Level Chemistry A2

[Maximum number: 1]

Which substance is the least soluble in water?

propanal

propan-1-ol

propanoic acid

sodium propanoate

[Maximum number: 4]

This question concerns six isomers each with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}$ .

Isomers A, B, C and D are structural isomers that all react with aqueous sodium carbonate to produce carbon dioxide.

Isomer A is a straight-chain compound.
Isomer B has only two peaks in its high resolution proton NMR spectrum, both of which are singlets.

Isomer C contains a chiral centre but isomer D does not.
Give the structures of these four isomers.

Table for Question 10 — Edexcel A-Level Chemistry A2

[Maximum number: 3]

This question concerns six isomers each with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}$ .

State which one of the isomers A, E and F has the highest boiling temperature. Justify your answer.

[Maximum number: 1]

This question concerns six isomers each with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}$ .

Isomer F is a neutral compound that smells of pears.

It can be formed by the reaction between compounds V and W in the presence of concentrated sulfuric acid.

Compound V has an absorption in its infrared spectrum at $1720 \mathrm{~cm}^{-1}$ and a broad absorption around $3050 \mathrm{~cm}^{-1}$ .

Compound W can be formed directly from propanal.

State the conditions required and the reagent needed to form compound W from propanal.

[Maximum number: 2]

This question is about reactions of carboxylic acids and their derivatives.

(a)

Molecule A reacts in two ways in the presence of an acid catalyst.

molecule A

Reaction 1 is intramolecular, in which one molecule of A forms a cyclic ester B.
Reaction 2 is intermolecular, in which a condensation polymer C forms.

[ 1 ]

(i)

Draw the structure of the cyclic ester B.

[ 1 ]

(b)

The molecule shown is a triester, used as a food additive.

[ 1 ]

(i)

Give the molecular formula of this triester.

[ 1 ]

[Maximum number: 2]

This question is about reactions of carboxylic acids and their derivatives.

Some reactions of ethanoyl chloride, CH_3 COCl, are shown.

Figure for Question 14 — Edexcel A-Level Chemistry A2

Write a balanced equation for Reaction Q.

State symbols are not required.

[Maximum number: 1]

The synthetic drugs of the 'Caine' family are used as local anaesthetics. Caine drugs prevent nerve activity by binding to sodium channel receptors in the lipid cell membranes of neurons.

benzocaine

procaine

lidocaine

articaine

When benzocaine and procaine are hydrolysed, one of the products formed is the same in both reactions.

Give the structure of this product.

An answer that makes reference to the following point:

- structure of 4-aminobenzoic acid

Figure for Answer to Question 15 — Edexcel A-Level Chemistry A2

Allow any combination of skeletal, structural or displayed formulae

Example of structure:

Allow Kekulé benzene

Allow protonation of $-\mathrm{NH}_{2}$ to $-\mathrm{NH}_{3}{ }^{+}$

Allow deprotonation of -COOH to $-\mathrm{COO}^{-}$ Allow zwitterion

Do not award any other hydrolysis product

Expert

[Maximum number: 2]

This question is about carboxylic acids and their derivatives.

The repeat unit of a polyester is shown.

Figure for Question 16 — Edexcel A-Level Chemistry A2

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

Table for Question 16 — Edexcel A-Level Chemistry A2

[Maximum number: 1]

This question is about carboxylic acids and their derivatives.

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

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

Table for Question 17 — 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.

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

Allow symbols in any order

Ignore any working Ignore + charge

[Maximum number: 1]

This question concerns six isomers each with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}$ .

State the number of peaks in the carbon-13 $\left({ }^{13} \mathrm{C}\right)$ NMR spectrum of E.

[Maximum number: 3]

This question is about carboxylic acids and their derivatives.

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

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

Table for Question 19 — Edexcel A-Level Chemistry A2

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

structure of E

- 2 or 3 proton environments correct

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

(1)

(1)

(1)

Figure for Answer to Question 19 — Edexcel A-Level Chemistry A2

[Maximum number: 1]

This question is about chromatography.

A spot caused by an amino acid has moved 42 mm from the baseline of a paper chromatogram.
The $R_{\mathrm{f}}$ value for the amino acid under these conditions is 0.62 .
What is the distance moved by the solvent?

680 mm

68 mm

42 mm

26 mm

[Maximum number: 2]

High-performance liquid chromatography (HPLC) is used to separate a mixture into its three components.

The resulting chromatogram is shown.

Figure for Question 21 — Edexcel A-Level Chemistry A2

(a)

Which is correct for the labels on the axes?
□ A
□ B
□ C
□ D

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

[ 1 ]

The only correct answer is D (time, absorption)

A is incorrect because the labels are the wrong way round

B is incorrect because $R_{f}$ is not used in HPLC

C is incorrect because $R_{f}$ is not used in HPLC

(b)

Which is correct for the components E, F and G?

E most attracted to stationary phase; E most abundant

G most attracted to stationary phase; E most abundant

E most attracted to stationary phase; F most abundant

G most attracted to stationary phase; F most abundant

[ 1 ]

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)

Question (d)

Question (i)

Question (ii)

Question (iii)

Question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (c)

Question (d)

Question (e)

Question (i)

Question (ii)

Question 3

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question (a)

Question (i)

Question (b)

Question (i)

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

Question 21

Question (a)

Question (b)