Question 61: [Organic > Introductory Organic]

The isomers, citric acid and isocitric acid, are intermediates in the Krebs cycle of the oxidation of glucose in living cells.

How many chiral centres does each acid possess?

Reference: Past Exam Paper –June 2003 Paper 1 Q21

Solution 61:

Answer: B.

All carbon atoms in CO₂H group are not chiral. All carbon atoms in CH₂ groups are not chiral. In citric acid the only remaining left-middle carbon atom is also not chiral since the bottom group exactly matches the top, cancelling out options C and D. In isocitric acid the left-middle carbon atom is chiral since the groups attached to it are: H, CO₂H, CH₂CO₂H and C(OH)CO₂H, which are all different. The bottom-left carbon atom is also chiral since the groups attached to it are: H, OH, CO₂H and the remaining top, which are also all different. Thus the correct answer is option B.

Question 37: [Organic > Introductory Organic]:

Which compounds show cis-trans isomerism?

1 but-2-ene

2 but-1-ene

3 2-methylpropene

Reference: Past Exam Paper – November 2002 Paper 1 Q37

Solution 37:

Answer: D.

In but-2-ene the unsaturated carbons have 2 pairs of the same group thus can exhibit cis-trans isomerism. 

In but-1-ene one of the unsaturated carbons have a repeated group thus it cannot exhibit cis-trans isomerism. 

Since 2 is incorrect 3 must also be incorrect and the only valid option is D.

Question 31: [Organic > Introductory Organic]:

Which compounds have the empirical formula CH2O?

1 methanal

2 ethanoic acid

3 methyl methanoate

Reference: Past Exam Paper – November 2002 Paper 1 Q31

Solution 31:

Answer: A.

Methanal: 1^st degree aldehyde: HCHO: CH₂O

Ethanoic acid: 2^nd degree carboxylic acid: CH₃COOH: C₂H₄O₂: CH₂O

Methyl Methanoate: Ester: HCO₂CH₃: C₂H₄O₂: CH₂O

Thus since all correct option A is the correct choice

Question 29: [Organic > Introductory Organic]:

Which compound is both chiral and acidic?

Reference: Past Exam Paper – November 2002 Paper 1 Q29

Solution 29:

Answer: D.

Options A and B have no chiral centre thus are eliminated. In option A all the carbon atoms have double bonds thus impossible to be chiral. 

In option B all carbon atoms have a recursive group. 

Option C is eliminated because it is not an acidic compound leaving option D as the correct answer.

Question 21: [Organic > Introductory Organic]:

Camphor is used for medical purposes the diagram shows its structure.

How many chiral centres are present in one molecule of camphor?

A 0

B 1

C 2

D 3

Reference: Past Exam Paper – November 2002 Paper 1 Q21

Solution 21:

Answer: C.

The carbon atom in the top-centre of the molecule is chiral due to asymmetry of the left and right sides. The bottom-centre carbon of the molecule is chiral due to the same reason. The exact centre carbon has 2 methyl groups attached to it thus is not chiral. The rest of the carbon atoms either have multiple hydrogen atoms bonded to it or a double bonded oxygen atom. Hence 2 chiral carbons.

Question 20: [Organic > Introductory Organic]:

The Russian composer Borodin was also a research chemist who discovered a reaction in which two ethanal molecules combine to form a compound commonly known as aldol (reaction I). Aldol forms another compound on heating (reaction II).

I 2CH₃CHO àCH₃CH(OH)CH₂CHO

II CH₃CH(OH)CH₂CHO à CH₃CH=CHCHO + H₂O

Which of the following describes reactions I and II?

Reference: Past Exam Paper – November 2002 Paper 1 Q20

Solution 20:

Answer: A.

In the first reaction 2 molecules, regardless of the fact that it is the same molecule, combine to form 1 product, which can only be defined as an addition reaction, eliminating options C and D. 

The second reaction is a form of dehydration which in turn categorises under elimination reactions (it eliminates water molecules).