The infrared spectrum of an organic compound is shown.
Which compound could give this spectrum?
▶️ Answer/Explanation
Ans: C
The spectrum shows a strong, broad absorption around 1700–1725 cm⁻¹, which is characteristic of the C=O stretch of a carbonyl group. Crucially, there is no broad absorption in the 2500–3300 cm⁻¹ region, which would indicate an O-H bond from a carboxylic acid. There is also no absorption in the 3200–3600 cm⁻¹ region for an O-H bond from an alcohol. Therefore, the compound must contain a carbonyl group but no O-H group. Only option C, propanone (\(CH_3COCH_3\)), is a ketone and fits this description perfectly.
The mass spectrum of compound X has M, M+1 and M+2 peaks. Other peaks are also present.
Peak M is the molecular ion peak, M⁺. Peak M has a relative abundance fifteen times that of peak M+1.
Peaks M and M+2 are of equal height.
What could be compound X?
▶️ Answer/Explanation
Ans: D
Equal M and M+2 peaks indicate a bromine atom (Br-79 and Br-81 isotopes). The M⁺ abundance being 15 times M+1 suggests a low number of carbon atoms. Option D (C₆H₁₃Br) has only 6 carbons, fitting the data. Bromine compounds show the M and M+2 peaks in a 1:1 ratio.
In the mass spectrum of compound J, the ratio of the height of the M+1 ion peak to the height of the M+ ion peak is 4 : 91. Compound J forms a carboxylic acid when heated with acidified K₂Cr₂O₇. What is compound J?
▶️ Answer/Explanation
Ans: A
Step 1: Analyze the M+1 peak ratio (4:91)
The ratio of the M+1 peak to the M+ peak is approximately 4.4% (4/91 × 100). This matches the natural abundance of 13C isotopes in a molecule with 4 carbon atoms (since each carbon contributes ~1.1% to the M+1 peak).
Step 2: Determine functional group reactivity
Compound J oxidizes to a carboxylic acid with K₂Cr₂O₇, indicating it must be an aldehyde (butanal) or a primary alcohol (propan-1-ol). Butanone (ketone) and propanenitrile (nitrile) do not oxidize to carboxylic acids under these conditions.
Step 3: Match carbon count
Propan-1-ol (C₃H₇OH) has only 3 carbons, inconsistent with the M+1 ratio. Butanal (C₄H₈O) fits both the 4-carbon requirement and the oxidation behavior.
Thus, the correct answer is A (butanal).
In the mass spectrum of compound J, the ratio of the height of the M+1 ion peak to the height of the M+ ion peak is 4 : 91. Compound J forms a carboxylic acid when heated with acidified K₂Cr₂O₇. What is compound J?
▶️ Answer/Explanation
Ans: A
1. M+1 Peak Ratio (4:91): – The ratio suggests the presence of 4 carbon atoms (since each \(^{13}\text{C}\) contributes ~1.1% to M+1, and \(4 \times 1.1\% \approx 4.4\%\), matching the 4:91 ratio). – This eliminates options C and D (which have 3 carbons).
2. Oxidation to Carboxylic Acid: – Butanal (A) is an aldehyde and oxidizes to butanoic acid with K₂Cr₂O₇. – Butanone (B) is a ketone and resists further oxidation under these conditions.
3. Conclusion: – Only butanal (A) fits both the mass spectrometry data and the oxidation behavior.