Edexcel A-Level Chemistry AS Paper 2, June 2024: Question 3

Question

Answer Additional Guidance Mark

Number

3(a) An answer that makes reference to the following points: (1)

• molecular formula, C4H8 Do not award CH2 for the molecular formula

and

empirical formula, CH2 Allow a calculation involved C4H8

eg C4H8 ÷ 4 = CH2 as long as CH2 given

Question

Answer Mark

Number

3(b)(i) The only correct answer is A (electrophilic addition) (1)

B is not correct because the reaction involves electrophilic not nucleophilic attack

C is not correct because the reaction is an addition not a substitution

D is not correct because the reaction is an addition reaction resulting from electrophilic attack

Question

Answer Additional Guidance Mark

Number

3(b)(ii) An answer that makes reference to the following points: (4)

All marking points scores 4 marks

7 or 8 marking points scores 3 marks

5 or 6 marking points scores 2 marks

3 or 4 marking points scores 1 mark

+

• correct structure for 2-methylpropene :Br ‒

‒

• dipole on HBr molecule

• curly arrow from H‒Br bond to Br

• curly arrow from double bond to H atom of HBr / to

the space between the double bond and the H atom

of HBr

• correct carbocation intermediate to form 2-bromo-2- Allow TE on an incorrect starting alkene structure

methyl propane with + charge on correct carbon

• charge on bromide ion

• lone pair on bromide ion

• curly arrow from Br to correct carbon / to the space

Allow to the positive carbon of a carbocation even if

between the Br and correct carbon

incorrect

This can be awarded if no lone pair is present on Br, but

must be from the lone pair if a lone pair is present

• structure of product correct from either starting

material or carbocation or 2-bromo-2-methylpropane

Question

Answer Additional Guidance Mark

Number

3(b)(iii) An answer that makes reference to the following points: (3)

• structure of 1-bromo-2-methylpropane (1)

Allow skeletal / condensed / hybrid structures as long as

Either structure is clear

• because it is formed via / the intermediate is a Allow a description of a primary carbocation and/or a

primary carbocation (not tertiary) tertiary carbocation

Or

• the carbocation has only one alkyl group attached Ignore discussions of Markovnikov’s rule without

and is not stabilised by the inductive effect of three explanation in terms of inductive effect or stability of

methyl groups (as in the major product) (1) carbocations

• the minor carbocation is less stable than the major Allow a tertiary carbocation is more likely to form than a

carbocation (1) primary one

Allow a comparison of stability of secondary

carbocations as a TE on M2

Do not award the primary halogenoalkane is less stable

compared to the tertiary

If M2 and M3 are not scored allow (1) for the carbocation

is less stable

Question

Answer Additional Guidance Mark

Number

3(b)(iv) An answer that makes reference to the following point: (1)

• the movement of a pair / two electrons Accept the transfer of a pair / two electrons

Allow movement of a bond pair / lone pair of electrons

Question

Answer Additional Guidance Mark

Number

3(c)(i) An answer that makes reference to the following points: (2)

Isomer 1 Isomer 2

• structure of both isomers correct (1)

• names of both isomers correct (1)

Name: trans-but-2-ene / Name: cis-but-2-ene /

E-but-2-ene Z-but-2-ene

Allow name and structure of one isomer correct for (1)

M2 dependent on M1 or near miss

Allow CH3 for the displayed methyl group

Penalise connectivity of CH3 once only

Question

Answer Additional Guidance Mark

Number

3(c)(ii) An explanation that makes reference to the following points: (2)

• because the C=C double bond has restricted rotation (1) Allow no rotation around C=C

Allow limited rotation around C=C

Allow the double bond in place of C=C

Allow the double / C=C restricts rotation

• and there is a CH3 group / H group on either side of the Allow there are two different groups attached

double bond (1) to each carbon (of the C=C double bond)

Allow priority groups being on the same or

different sides

Question

Answer Mark

Number

3(d)(i) (1)

The only correct answer is B ( )

A is not correct because this is a secondary alcohol

C is not correct because this is a primary alcohol

D is not correct because this is a primary alcohol

Question

Answer Additional Guidance Mark

Number

3(d)(ii) An explanation that makes reference to the following points: The carbon with the -OH group attached to it need be (2)

mentioned in only one of the two marking points, the

(because oxidation requires a hydrogen to be attached to the second can score just for the numbers of hydrogens

carbon with the -OH group attached to it and)

• tertiay alcohols have no hydrogen on the carbon with Allow because a C-C bond would have to be broken

the -OH group attached to it / on the carbon to be Allow the carbon with the OH group attached is

oxidised (1) bonded to 3 alkyl groups / other carbons

Ignore tertiary alcohols are bonded to three alkyl /

methyl groups

• whereas primary and secondary alcohols do have a Allow reference to tertiary alcohols do not have a

hydrogen on the carbon with the -OH group attached to hydrogen but other alcohols do

it / on the carbon to be oxidised (1)

(Total for Question 3 = 17 marks)