OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 18

AO

Question Answer Marks Guidance

element

18 (a) (i) ethyl 3-bromopropanoate 1 AO1.2 ALLOW one word: ethyl3-bromopropanoate

OR more words, e.g. ethyl 3-bromo propanoate

IGNORE lack of hyphens, or addition of commas

(ii) 5 AO2.5 ALLOW any combination of skeletal OR structural

×5 OR displayed formula as long as unambiguous

ALLOW in either order

ALLOW any vertical bond to the OH group

e.g. ALLOW

OR

OH HO

DO NOT ALLOW OH–

ALLOW in either order

For reaction with OH–,

ALLOW one mark for

O O

Br O– OR HO OH

O

OR HO O

AO

element

(iii) hydrolysis 1 AO1.1 IGNORE ‘acid’ and ‘alkaline’’

IGNORE nucleophilic substitution

(b) 4 AO3.1 ALLOW δ values ± 0.2 ppm, as a range or a value

Proton Chemical Splitting × 4 within the range

environment shift pattern

ALLOW integers for δ values

1 3.0–4.3 Triplet e.g. 2 is equivalent to 2.0

2 2.0–3.0 Triplet ALLOW quadruplet for quartet

3 3.0–4.3 Quartet ALLOW diagrams to show splitting pattern

e.g.

4 0.5–1.9 Triplet

for triplet

Mark by column

Chemical shift: all 4 correct

3 correct for quartet

Splitting pattern: all 4 correct ALLOW splitting patterns shown as numbers

3 correct i.e. ‘3’ for triplet, ‘4’ for quartet

AO

15 element

(c) 1 AO3.1

ALLOW any combination of skeletal OR structural

OR displayed formula as long as unambiguous

OR

OR

O

OH

Br

(d) IF answer on answer line = 24018, AWARD 2 marks 2 AO2.2 ALLOW ECF from incorrect Mr

IF answer on answer line = 27600, AWARD 1 mark ×2

------------------------------------------------------------------------ Alternative method based on repeat unit:

Relative mass of 200 molecules = 200 × 138 = 27600 Mr of 200 repeat units = 200 x 120 = 24000

Mr of polyester = 27600 – 199 × 18 = 24018 Mr of polymer = 24000 + 1 + 17 = 24018

(e) (i)* Refer to marking instructions on page 4 of mark scheme 6 AO3.3 Indicative scientific points may include:

AO

element

for guidance on marking this question. ×6

Calculation of mass of (CH3)2CHCHO

Level 3 (5-6 marks) Using moles

Correct calculation of the mass of (CH3)2CHCHO. 12.75

AND • n(ester) =

102.0

Planned synthesis includes oxidation of aldehyde and = 0.125 (mol)

formation of ester C with most of the reagents and 100

conditions identified and equations are mostly correct. • n((CH3)2CHCHO) = 0.125 ×

= 0.3125 (mol)

There is a well-developed line of reasoning which is • Mass of (CH3)2CHCHO = 72.0 × 0.3125

clear and logically structured. The information presented = 22.5 g

is relevant and substantiated.

Using mass

Level 2 (3-4 marks) 100

Calculation of the mass of (CH3)2CHCHO is partly • Theoretical mass of ester = 12.75 ×

correct = 31.875 (g)

AND 31.875

Planned synthesis includes oxidation of aldehyde and • Theoretical n((CH3)2CHCHO) =

formation of ester C with some of the reagents and = 0.3125 (mol)

conditions identified

• Mass of (CH3)2CHCHO = 72.0 × 0.3125

OR

= 22.5 g

Attempts to calculate mass of (CH3)2CHCHO but makes

little progress ALLOW small slip/rounding errors such as errors in

AND Mr e.g. use of 71 instead of 72 for (CH3)2CHCHO

Planned synthesis includes oxidation of aldehyde and

formation of ester C with most of the reagents and -----------------------------------------------------------------

conditions identified and equations for each step are Examples of partly correct calculations

mostly correct 40

Mass = 3.60 g from 0.125 × × 72

There is a line of reasoning presented with some (% yield inverted)

structure. The information presented is relevant and

supported by some evidence. Mass = 9.00 g from 0.125 × 72

(% yield omitted)

17 AO

element

Level 1 (1-2 marks) Synthesis: reagents and conditions

Calculation of the mass of (CH3)2CHCHO is partly

correct Step 1: Oxidation of aldehyde (CH3)2CHCHO

OR • Reagents: Cr O 2–/H+

Planned synthesis includes both steps with some of the • Conditions: reflux

reagents and conditions identified • Equation:

OR (CH3)2CHCHO + [O] → (CH3)2CHCOOH

Attempts equations for both steps but these may contain

errors

OR Step 2: Formation of ester C

Describes one step of the synthesis with reagents, • Reagents: methylpropanoic acid/(CH3)2CHCOOH

conditions and equation mostly correct and methanol/CH3OH

• Conditions: acid (catalyst)

There is an attempt at a logical structure with a line of reflux/heat

reasoning. The information is in the most part relevant. • Equation:

(CH3)2CHCOOH + CH3OH → (CH3)2CHCOOCH3 + H2O

0 marks IGNORE attempts to form methanol in synthesis

No response or no response worthy of credit.

(e) (ii) 2 AO2.7 ALLOW any combination of skeletal OR structural

× 2 OR displayed formula as long as unambiguous

AO

element

Y (43) = (CH ) CH+

ALLOW positive charge to be anywhere on the

Z (71) (CH ) CHCO+ structure

If ‘+’ charge is missing/incorrect but the structures of For Y and Z,

both fragments are correct, award one mark ALLOW structure of a feasible fragment ion formed

from ester C

H O

H3C C C

CH O CH3

Ester C

e.g.

Y (43) = CH OC+

Z (71) =+CCOOCH

ALLOW 1 mark if both correct ions are shown but in

the incorrect columns

ALLOW 1 mark for both correct ions if one or both

have an ‘end bond’

ALLOW 1 mark if both ions are shown using correct

molecular formulae

Total 22