Spectroscopy

IR Spectroscopy

IR identifies functional groups by bond stretches, read in wavenumbers ($\text{cm}^{-1}$). The essential absorptions: a sharp, strong carbonyl near $1700\ \text{cm}^{-1}$; a broad O-H from alcohols around $3200-3550\ \text{cm}^{-1}$; a very broad carboxylic acid O-H spanning $2500-3300\ \text{cm}^{-1}$ overlapping C-H; N-H as one or two spikes near $3300\ \text{cm}^{-1}$; nitriles near $2250\ \text{cm}^{-1}$. Absence of a peak is as diagnostic as its presence.

Proton NMR

Each unique hydrogen environment gives one signal; integration counts relative protons, and chemical shift reports the electronic environment: alkyl $0.5-2$ ppm, protons alpha to a carbonyl or halide $2-4.5$ ppm, vinyl $4.5-6.5$ ppm, aromatic $6.5-8.5$ ppm, aldehyde $9-10$ ppm, carboxylic acid $10-12$ ppm. Splitting follows the n+1 rule: $\text{multiplicity} = n + 1$ where $n$ is the count of nonequivalent neighboring protons.

Mass Spectrometry and UV-Vis

The molecular ion $M^+$ gives molecular mass; fragment peaks reveal pieces lost (loss of 15 suggests methyl). UV-Vis absorbance grows with conjugation.

Key Takeaways

• IR answers "what functional groups"; NMR answers "what carbon-hydrogen skeleton."
• Count NMR signals by symmetry before anything else.
• A triplet-quartet pattern is the classic ethyl fingerprint.
• Carbonyl at ~1700 plus broad 2500-3300 absorption means carboxylic acid.