A LEVEL CHEMISTRY
OCR A PAPER 2 EXAM
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Q1 [info] >>> answerThis 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. answerMechanism:
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] answer- 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] answer- 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] answerC10H18O
Q2 c] >>> Explain why geraniol and citronellal are structural isomers of each other [1 mark] answer-
They both have the same molecular formula [C10H18O] but different structural formulae
Q3 a] >>> Explain the term stereoisomerism [1 mark] answerSame structural and molecular formulae
but different arrangements of atoms in space
OCR A PAPER 2 EXAM
Qs
Save
Q1 [info] >>> answerThis 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. answerMechanism:
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] answer- 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] answer- 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] answerC10H18O
Q2 c] >>> Explain why geraniol and citronellal are structural isomers of each other [1 mark] answer-
They both have the same molecular formula [C10H18O] but different structural formulae
Q3 a] >>> Explain the term stereoisomerism [1 mark] answerSame structural and molecular formulae
but different arrangements of atoms in space
