OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 19

Question Answer Marks Guidance

Refer to marking instructions on page 5 of mark scheme for 6 Indicative scientific points may include:

guidance on marking this question.

Reaction of alkene and mechanism

Level 3 (5–6 marks) • Suitable reaction, e.g.

Describes addition reactions including the mechanisms of alkene and Br2 OR X2 OR HX OR H2O OR H2 OR

one alkene AND one carbonyl compound AND some polymerisation

additional details May be shown within mechanism

• Mechanism, e.g.

There is a well-developed line of reasoning which is clear

and logically structured. The information presented is H H H H H H

relevant and substantiated. C C H C C H H C C H

Level 2 (3–4 marks) H H Br Br Br

Brd+

Describes an addition reaction including the mechanism of Br

Brd-

one alkene OR one carbonyl compound AND some

ALLOW mechanism for H2 AND H2O to be shown as

additional details

electrophilic addition even though incorrect - consider impact

OR

on communication statement.

Describes addition reactions including an attempt to give

the mechanisms of one alkene AND one carbonyl

ALLOW suitable non-specification alternative e.g. HCN

compound

Additional details (NOT INCLUSIVE)

There is a line of reasoning presented with some structure.

• Electrophilic addition

The information presented is relevant and supported by

some evidence. • Systematic names of reactants and/or products

• Details of functional group interconversion e.g. alkene to

Level 1 (1–2 marks) dibromo

Selects suitable reagents for addition reactions of one • Details on reagents required e.g.

alkene AND one carbonyl compound. - H2 with Ni Catalyst

OR - H2O(g) with H3PO4 catalyst

Attempts to describe an addition reaction including an • Explanation of major and minor product from electrophilic

attempt to give the mechanism of one alkene OR one addition of HX with unsymmetrical alkene

carbonyl compound. • Explanation of carbocation intermediate stability

• Heterolytic fission

There is an attempt at a logical structure with a line of

reasoning. The information is in the most part relevant.

Reaction of carbonyl compound and mechanism

0 marks No response or no response worthy of credit. Suitable reactions, e.g.

• Aldehyde or ketone and HCN OR H–

e.g. RCHO + HCN → RCH(OH)CN

May be shown within mechanism

• Mechanisms, e.g.

OR H O instead of H+ for 2nd stage

ALLOW suitable non-specification alternative e.g. H2O, NH3,

1o amine

IGNORE reactions with carboxylic acids (or derivatives) i.e.

addition-elimination mechanism (condensation reaction)

Additional details (NOT INCLUSIVE)

• Nucleophilic addition

• Systematic names of reactants and/or products

• Details of functional group interconversion e.g. aldehyde

to hydroxynitrile

• In reduction, aldehydes form 1o alcohols and ketones form

2o alcohols

• Details on reagents required e.g.

- formation of hydroxynitriles with NaCN/H+(aq)

- formation of alcohols with NaBH4

• Heterolytic fission

Aspects of the communication statement being met might

typically include:

• Curly arrows starting from lone pairs / negative charges /

bonds.

• All reactants and intermediates have relevant charges and

dipoles.

• Mechanisms given are chemically feasible for the

reactions.

• No additional incorrect reactants have been included.