OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 16

AO

Question Answer Marks Guidance

element

16 (a) (i) ANNOTATE ANSWER WITH TICKS AND CROSSES 4 NOTE: curly arrows can be straight, snake-like,

etc.

but NOT double headed or half headed arrows

1st curly arrow must

• go to the H atom of H–Br

H3C CHO AND

C C • start from, OR be traced back to any

point across width of C=C

H3C H

H δ+

Br δ−

Curly arrow from C=C bond to H of H–Br AO1.2

DO NOT ALLOW partial charge on C=C 2nd curly arrow must

• start from, OR be traced back to any

Correct dipole shown on H–Br part of δ+H–Brδ– bond

AND curly arrow showing breaking of H–Br bond AO1.2 AND

• go to Brδ–

AO

element

Correct carbocation IGNORE connectivity of CHO and CH3 groups in

AND curly arrow from Br– to C+ of carbocation AO2.5 carbocation and product

DO NOT ALLOW δ+ on C of carbocation e.g. ALLOW

CHO CHO

OR

H C CHO CH3 CHO

C C H H3C C C ALLOW COH for CHO (reaction does not

involve this group)

H3C H H

H

Br− Br− 3rd curly arrow must

OR

• go to the C+ of carbocation

---------------------------------------------------------------------- AND

Correct product AO2.5 • start from, OR be traced back to any

point across width of lone pair on :Br–

CH3 CHO CH3 CHO –

• OR start from – charge of Br ion

H3C C C H H3C C C H

Br H OR H Br

(Lone pair NOT needed if curly arrow shown

from – charge of Br– ion)

IF Br2 is used instead of HBr contact your Team

Leader

(a) (ii) (major product forms from) most/more stable 2 For carbocation,

6 AO

element

intermediate/carbocation AO1.1 ALLOW carbonium ion or cation

(major product forms from a) tertiary carbocation IGNORE descriptions of the major/minor product

OR carbocation bonded to more C atoms / more alkyl in terms of Markownikoff’s rule

groups e.g. H atom joins to C with most H

OR carbocation bonded to no H atoms AO1.2

IGNORE references to stability of the product

--------------------------------------------------------------

ALLOW ORA, i.e.

(minor product forms from) least/less stable

intermediate/carbocation

(minor product forms from a) secondary

carbocation

OR carbocation bonded to fewer C atoms /

more alkyl groups

OR carbocation bonded to H atoms

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(b) (i) Tollens’ (reagent) 2 AO1.2 ALLOW ammoniacal silver nitrate OR Ag+/NH

7 AO

element

×2

Silver (mirror/precipitate/ppt/solid)

with citronellal/the aldehyde ALLOW black ppt OR grey ppt

IGNORE references to acidified dichromate

reacting with both compounds

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ALLOW 2,4-DNP/2,4-DNPH

ALLOW Brady's reagent

Yellow/orange/red precipitate with

citronellal/aldehyde/carbonyl group

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IF other reagents are seen, contact your Team

Leader

(b) (ii) C10H18O 1 AO1.2 DO NOT ALLOW C10H17OH

(b) (iii) 1 Same formula is not sufficient

(no reference to molecular)

Same molecular formula Different arrangement of atoms is not sufficient

AND (no reference to structure/structural)

Different structural formulae AO1.1

For structural formulae,

ALLOW structure/displayed/skeletal formulae/

OR functional groups

Both (geraniol and citronellal) have the molecular DO NOT ALLOW any reference to spatial/space

formula C10H18O

AND ALLOW ECF from incorrect molecular formula

Different structural formulae in (b)(ii)

(iv) Same structural formula 1 AO1.1 ALLOW structure/displayed/skeletal formula

AO

8 element

AND

Different arrangement (of atoms) in space DO NOT ALLOW same empirical formula

OR different spatial arrangement (of atoms) OR same general formula

IGNORE same molecular formula

Reference to E/Z isomerism or optical

isomerism is not sufficient

(v) Geraniol: 4 ANNOTATE ANSWER WITH TICKS AND

(Carbon-carbon) double bond at carbon-2(,3) CROSSES ETC

AND

E OR Z AO1.2 CHECK diagrams of citronellal and geraniol for

annotations that may be worthy of credit

DO NOT ALLOW isomerism at C=C at carbon

6(,-7)

Structure of Z geraniol (E isomer is shown in question)

ALLOW identification of carbon-2(,3) from

correct Z geraniol isomer if not stated in text or

diagram

IGNORE cis OR trans isomerism (none of the

substituent groups attached to the C=C are the

OH AO2.5 same)

IGNORE geometric

ALLOW type of isomerism from E/Z labels, even

if incorrectly assigned

In geraniol,

ALLOW C6H11 OR R to represent alkenyl chain

ALLOW CH3O to represent CH2OH

9 AO

element

Citronellal:

chiral/asymmetric C at carbon-3

OR carbon-3 is bonded to 4 different groups ALLOW identification of carbon-3 from 3D

AND structure citronellal if not stated in text or

optical isomerism AO1.2 diagram

Two 3D structures of citronellal that are mirror IGNORE connectivity of groups around chiral C

images AO2.5

e.g. In citronellal,

ALLOW C6H11 OR R to represent alkenyl chain

ALLOW C2H3O to represent CH2CHO

IF structural formula of alkenyl chain is used

IGNORE one small slip in one/both isomers

e.g.(CH3)2CHCH2CH2 (missing carbon-7)

ALLOW two 3D structures with 2 groups

swapped

e.g.

O

H

O

H

Total 13