Organic analysis — Question 26

QCAA sample response and mark allocation

26 The peak in the IR spectrum at 1700–1750 corresponds to a C=O bond in

• identifies IR peak at 1700–1750

either an aldehyde or a ketone.

corresponds to a C=O bond in aldehyde or ketone [1 mark]

There is no stretch in the IR peak at 2720–3100, therefore, the molecule is not an aldehyde and must be a ketone

• indicates that X is a ketone [1 mark]

Fragment at m/z = 43 is CH₃CH₂CH₂

⁺ and COCH₃ ⁺

• identifies mass fragment at

Fragment at m/z = 71 is CH₃CH₂CH₂CO⁺

43 m/z as CH₃CH₂CH₂

⁺ and COCH₃ +

Empirical formula = 86. This corresponds to the molecular mass shown on the

OR

mass spectrum. Therefore, the molecular formula is C₅H₁₀O.

71 m/z as CH₃CH₂CH₂CO⁺ [1 mark]

• uses mass spectrum data to show that the molecular formula for X is C₅H₁₀O [1 mark]

• provides correct structural formula for pentan-2-one [1 mark]