Analytical Techniques Problem Solving

This final lesson puts everything together with exam-style compound identification problems. Each problem requires you to combine data from mass spectrometry, infrared spectroscopy, and NMR to identify an unknown organic compound. Work through each problem systematically — molecular formula first, then functional groups, then the carbon and hydrogen framework.

---

Problem 1: Compound A (C₄H₈O₂)

Mass spectrum: M⁺ = 88, base peak at m/z = 43, another peak at m/z = 45.

IR: Strong C=O at 1745 cm⁻¹, strong C–O at 1250 cm⁻¹, no O–H.

¹H NMR:

• δ = 2.0, 3H, singlet
• δ = 3.7, 3H, singlet

Solution

Step 1: C₄H₈O₂, Mr = 88. DoU = (8 + 2 − 8)/2 = 1. One degree of unsaturation.

Step 2: C=O at 1745 cm⁻¹ with C–O and no O–H → ester.

Step 3: Two singlets, both 3H. No splitting → no adjacent H for either CH₃ group.

• δ = 2.0 (3H, singlet) → CH₃CO (acetyl methyl)
• δ = 3.7 (3H, singlet) → OCH₃ (methoxy)