Chapter 18 — Nuclear Magnetic Resonance (NMR) Spectroscopy

What determines the number of NMR signals?

Number of chemically nonequivalent hydrogens/carbons.

What makes atoms equivalent in NMR?

Symmetry, mirror planes, or free bond rotation.

Difference between enantiotopic and diastereotopic hydrogens?

Enantiotopic = equivalent → same signal

Diastereotopic = nonequivalent → different signals

General ¹H NMR regions you should recognize?

Alkane H → upfield (~0–2 ppm)

Next to heteroatom → ~2–4.5 ppm

Alkene H → ~4.5–7 ppm

Aromatic H → ~6.5–8 ppm

Aldehyde H → ~9–10 ppm

Carboxylic acid H → very downfield broad peak

What does splitting tell you?

Number of neighboring equivalent hydrogens using n+1 rule.

Common splitting names?

1 = singlet (s)

2 = doublet (d)

3 = triplet (t)

4 = quartet (q)

5 = quintet

6 = sextet

7 = septet

>7 = multiplet (m)

What coupling constants should you recognize for alkenes?

• Trans: 12–16 Hz

• Cis: 8–10 Hz

• Geminal: 1–2 Hz

What does integration tell you?

Relative number of hydrogens causing a signal.

Most important thing in ¹³C NMR?

Number of unique carbon environments.

Since an IR chart is provided, what should you focus on instead of memorizing peaks?

• Broad O–H peaks

• Strong carbonyl peaks

• Combining IR with NMR and molecular formula

Best order to solve spectroscopy problems?

Molecular formula → degrees of unsaturation

IR → functional groups

NMR → signals, integration, splitting

Assemble fragments

Confirm with mass spec

Formula for degrees of unsaturation?

Formula for degrees of unsaturation?