The following spectra were obtained for an unknown organic compound - HSC - SSCE Chemistry - Question 30 - 2022 - Paper 1

The mass spectrum shows a molecular ion (M+) at m/z = 86, consistent with C5H10O. The base peak at m/z = 43 indicates the presence of a fragment related to the carbonyl group: CH3CO+.

In the IR spectrum, a strong absorption around 1700 cm⁻¹ indicates the presence of a carbonyl group (C=O). The absence of broad O-H stretching from 2500–3000 cm⁻¹ rules out carboxylic acids.

The 13C NMR indicates five carbons but shows only four distinct environments, suggesting that two carbons are equivalent. Signals at 22 ppm (methyl group adjacent to the carbonyl) and at 18 and 40 ppm are consistent with CH and CH3 groups.

Finally, the Proton NMR shows three distinct environments: a singlet integrated to 3H for the CH3 group attached to the carbonyl, a septet for a CH group that connects to two CH3 groups, and a doublet for the methyl groups that share the adjacent hydrogen atom.

The data from the mass spectrum, IR spectrum, and NMR spectra collectively support the identification of the compound as 3-methylbutan-2-one, providing a precise validation of the proposed structure.

The connected analysis confirms that the unknown organic compound is best represented as:

3-methylbutan-2-one.