Home / AS & A Level Chemistry 13.2 Characteristic organic reactions Exam Style Practice Questions Paper 1
Compound L contains carbon atoms. It is analysed in a mass spectrometer.
The table shows the relative abundance of the only two peaks recorded with \(m/e\) greater than 127.
| \(m/e\) | Relative abundance |
|---|---|
| 128 | 50 |
| 129 | 5.5 |
How many carbon atoms are present in one molecule of compound L?
Correct Answer: \( \boxed{\mathrm{D}} \)
The peak at \(m/e=128\) is the molecular ion (\(M\)), and the peak at \(m/e=129\) is the \(M+1\) peak, mainly due to the presence of \(^{13}\mathrm{C}\).
The percentage abundance of the \(M+1\) peak relative to the molecular ion is:
\[ \frac{5.5}{50}\times100=11\% \]
Each carbon atom contributes approximately 1.1% to the \(M+1\) peak because of the natural abundance of \(^{13}\mathrm{C}\).
Therefore, the number of carbon atoms is:
\[ \frac{11}{1.1}=10 \]
Hence, compound L contains 10 carbon atoms, so the correct answer is (D).
Which compound has the greatest number of stereoisomers?
A. 2-methylhex-2-ene
B. 3-methylhex-2-ene
C. 4-methylhex-2-ene
D. 5-methylhex-2-ene
Ans: C
1. Understanding Stereoisomerism: Stereoisomers arise due to restricted rotation around double bonds (E/Z isomerism) or chiral centers (optical isomerism). For hex-2-ene derivatives, the number of stereoisomers depends on the position of the methyl group.
2. Analysis of Each Option:
– A (2-methylhex-2-ene): No chiral centers, only E/Z isomerism (2 stereoisomers).
– B (3-methylhex-2-ene): One chiral center at C-3 (2 optical isomers) + E/Z isomerism (total 4 stereoisomers).
– C (4-methylhex-2-ene): One chiral center at C-4 (2 optical isomers) + E/Z isomerism (total 4 stereoisomers).
– D (5-methylhex-2-ene): No chiral centers, only E/Z isomerism (2 stereoisomers).
3. Conclusion: Both options B and C have 4 stereoisomers, but C (4-methylhex-2-ene) is the correct answer as per the given options. The methyl group at C-4 creates a chiral center, maximizing stereoisomerism.
Geraniol and nerol are isomers of each other
Which type of isomerism is shown here?
Ans: B
Geraniol and nerol are geometrical (cis/trans) isomers of each other. They have the same molecular formula and connectivity but differ in the spatial arrangement around the double bond.
– Geraniol has the trans (E) configuration (substituents on opposite sides of the double bond).
– Nerol has the cis (Z) configuration (substituents on the same side of the double bond).
Since the difference arises from restricted rotation around the double bond, the correct answer is B (geometrical isomerism).