A LEVEL CHEMISTRY OCR A PAPER
2 EXAM Qs
Q1 [info] >>> - Ans>>This question is about unsaturated aldehydes and alcohols.
3-Methylbut-2-enal, shown below, is used as a food flavouring. 3-Methylbut-2-enal is
reacted with hydrogen bromide, forming a mixture of two organic products.
[refer to the A LEVEL CHEMISTRY OCR A PAPER 2 PMT QP for the structure of 3-
Methylbut-2-enal]
Q1 a] >>> One of the organic products forms in a much greater quantity than the
other organic product.
Outline the reaction mechanism for the formation of one of the organic products.
Include curly arrows and relevant dipoles. - Ans>>Mechanism:
Start product: 3-methylbut-2-enal
- curly arrow from centre of the C = C bond to the [negative dipole] Br atom of the
H - Br bond
- curly arrow from the centre of the H - Br bond to the [positive dipole] H atom on it
- carbocation intermediate with a C - Br bond on the adjacent carbon
- Curly arrow from H to the + on the carbocation intermediate
End product: 3-methyl-2-bromobut-2-anal
[tertiary carbocation is the most stable carbocation]
Q1 b] >>> Explain why one of the organic products forms in a much greater
quantity than the other organic product. [2 marks] - Ans>>- The tertiary
carbocation is the most stable carbocation,
- The major product forms from the tertiary carbocation
Q2 a] >>> Geraniol and citronellal, shown below, are isomers present in 'citronella
oil', used as an insect repellent.
Geraniol and citronellal are structural isomers of each other.
• They also show stereoisomerism.
Describe how the observations from a chemical test would distinguish between
geraniol and citronellal [2 marks] - Ans>>- Add Tollen's reagent to test tubes of
citronellal and geraniol
, - The test tube with citronellal in it would have a silver mirror on it cause citronellal
is an aldehyde, while nothing would happen in the test tube with geraniol
Q2 b] >>> What is the molecular formula of geraniol? [1 mark] - Ans>>C10H18O
Q2 c] >>> Explain why geraniol and citronellal are structural isomers of each other
[1 mark] - Ans>>- They both have the same molecular formula [C10H18O] but
different structural formulae
Q3 a] >>> Explain the term stereoisomerism [1 mark] - Ans>>Same structural and
molecular formulae but different arrangements of atoms in space
Q3 b] >>> The structures of geraniol and citronellal are repeated below with the
carbon atoms numbered. [refer to the A LEVEL CHEMISTRY OCR A PAPER 2 PMT QP
for the structures of geraniol and citronellal]
Explain the types of stereoisomerism shown by geraniol and citronellal.
In your answer, refer to the numbered carbon atoms in the structures above
[4 marks] - Ans>>Geraniol stereoisomers:
- on the C = C bond on carbon 2, there is E / Z isomerism
- the highest priority groups are the CH2OH and the CH2CH2CHC[CH3]2 groups
- E isomer has the highest priority groups on opposite sides of the C = C bond
- Z isomer has the highest priority groups on the same side of the C = C bond
Citronellal stereoisomers:
- carbon 3 is a chiral centre with 4 different groups around carbon 3
- the groups are: CH2CHO, H, CH2CH2CHC[CH3]2, CH3
- it has two optical isomers that are non - superimposible mirror images of each
other
Q4 a] >>> This question is about α-amino acids, RCH(NH2)COOH.
Table 17.1 shows the R groups in four amino acids [refer to the A LEVEL CHEMISTRY
OCR A PAPER 2 PMT QP for the Fig. 17.1 and the flowchart]
In the boxes, draw the organic products for the reactions of serine shown below. -
Ans>>The up arrow: the product of the reaction of serine when H+ is added >>>
*HOCH2CH[+NH3]COOH*
The diagonal arrow leading to the top right box: the product of the reaction of
serine when [CH3]2CHOH/H2SO4 is added >>>
*HOCH2CH[NH2]COOCH[CH3]2 [ester]*
The right arrow: the product of the reaction when excess CH3COCl is added >>>
2 EXAM Qs
Q1 [info] >>> - Ans>>This question is about unsaturated aldehydes and alcohols.
3-Methylbut-2-enal, shown below, is used as a food flavouring. 3-Methylbut-2-enal is
reacted with hydrogen bromide, forming a mixture of two organic products.
[refer to the A LEVEL CHEMISTRY OCR A PAPER 2 PMT QP for the structure of 3-
Methylbut-2-enal]
Q1 a] >>> One of the organic products forms in a much greater quantity than the
other organic product.
Outline the reaction mechanism for the formation of one of the organic products.
Include curly arrows and relevant dipoles. - Ans>>Mechanism:
Start product: 3-methylbut-2-enal
- curly arrow from centre of the C = C bond to the [negative dipole] Br atom of the
H - Br bond
- curly arrow from the centre of the H - Br bond to the [positive dipole] H atom on it
- carbocation intermediate with a C - Br bond on the adjacent carbon
- Curly arrow from H to the + on the carbocation intermediate
End product: 3-methyl-2-bromobut-2-anal
[tertiary carbocation is the most stable carbocation]
Q1 b] >>> Explain why one of the organic products forms in a much greater
quantity than the other organic product. [2 marks] - Ans>>- The tertiary
carbocation is the most stable carbocation,
- The major product forms from the tertiary carbocation
Q2 a] >>> Geraniol and citronellal, shown below, are isomers present in 'citronella
oil', used as an insect repellent.
Geraniol and citronellal are structural isomers of each other.
• They also show stereoisomerism.
Describe how the observations from a chemical test would distinguish between
geraniol and citronellal [2 marks] - Ans>>- Add Tollen's reagent to test tubes of
citronellal and geraniol
, - The test tube with citronellal in it would have a silver mirror on it cause citronellal
is an aldehyde, while nothing would happen in the test tube with geraniol
Q2 b] >>> What is the molecular formula of geraniol? [1 mark] - Ans>>C10H18O
Q2 c] >>> Explain why geraniol and citronellal are structural isomers of each other
[1 mark] - Ans>>- They both have the same molecular formula [C10H18O] but
different structural formulae
Q3 a] >>> Explain the term stereoisomerism [1 mark] - Ans>>Same structural and
molecular formulae but different arrangements of atoms in space
Q3 b] >>> The structures of geraniol and citronellal are repeated below with the
carbon atoms numbered. [refer to the A LEVEL CHEMISTRY OCR A PAPER 2 PMT QP
for the structures of geraniol and citronellal]
Explain the types of stereoisomerism shown by geraniol and citronellal.
In your answer, refer to the numbered carbon atoms in the structures above
[4 marks] - Ans>>Geraniol stereoisomers:
- on the C = C bond on carbon 2, there is E / Z isomerism
- the highest priority groups are the CH2OH and the CH2CH2CHC[CH3]2 groups
- E isomer has the highest priority groups on opposite sides of the C = C bond
- Z isomer has the highest priority groups on the same side of the C = C bond
Citronellal stereoisomers:
- carbon 3 is a chiral centre with 4 different groups around carbon 3
- the groups are: CH2CHO, H, CH2CH2CHC[CH3]2, CH3
- it has two optical isomers that are non - superimposible mirror images of each
other
Q4 a] >>> This question is about α-amino acids, RCH(NH2)COOH.
Table 17.1 shows the R groups in four amino acids [refer to the A LEVEL CHEMISTRY
OCR A PAPER 2 PMT QP for the Fig. 17.1 and the flowchart]
In the boxes, draw the organic products for the reactions of serine shown below. -
Ans>>The up arrow: the product of the reaction of serine when H+ is added >>>
*HOCH2CH[+NH3]COOH*
The diagonal arrow leading to the top right box: the product of the reaction of
serine when [CH3]2CHOH/H2SO4 is added >>>
*HOCH2CH[NH2]COOCH[CH3]2 [ester]*
The right arrow: the product of the reaction when excess CH3COCl is added >>>
