Combined Structural Analysis

🟣 This is Higher Level (HL) content.

📋 The 3-Step Workflow

1. MS → Find M_r: The molecular ion peak gives the relative molecular mass. Fragment losses reveal structural units (e.g. Loss of 15 = CH₃).
2. IR → Identify functional groups: Broad O–H + sharp C=O = carboxylic acid. Sharp C=O alone = aldehyde/ketone/ester. Use the Data Booklet absorption table.
3. NMR → Map the backbone: Count H environments (number of peaks), determine ratios (integration), and connect fragments using splitting patterns (n+1 rule).

Data: M⁺ = 74 | IR: strong C=O at 1715 cm⁻¹, no broad O–H | NMR: 3 peaks (singlet 2.1δ : quartet 2.5δ : triplet 1.1δ) ratio 3:2:3

Analysis:

• M_r = 74, C=O present, no O–H → ketone
• Singlet (3H) at 2.1δ → CH₃ next to C=O (no adjacent H)
• Quartet (2H) + Triplet (3H) → CH₂CH₃ group
• Answer: Butanone (CH₃COCH₂CH₃)