Infrared spectroscopy

The very broad absorption from 2500 to 3000 cm⁻¹ is the O-H of a carboxylic acid, and the strong sharp dip at about 1715 cm⁻¹ is C=O (1670-1740). Together, plus C-O near 1240 cm⁻¹, they show COOH: A is propanoic acid, CH₃CH₂COOH. An alcohol O-H absorbs higher, at 3200-3600 cm⁻¹, and an alcohol has no C=O. An aldehyde has C=O but no broad O-H band. An ester (such as methyl ethanoate, also C₃H₆O₂) has C=O and C-O but no O-H absorption at all.

C=O absorbs at 1670-1740 cm⁻¹, so 1720 cm⁻¹ fits an aldehyde, ketone, carboxylic acid or ester. Alcohol O-H absorbs much higher, 3200-3600 cm⁻¹, and is broad, not sharp. C-O is a single bond, weaker, so it absorbs lower, at 1040-1300 cm⁻¹. The triple C≡N bond is strong and absorbs at 2200-2250 cm⁻¹.

Infrared photons carry about the right energy to make bonds stretch and bend more vigorously. A bond absorbs at its own natural frequency, which depends on the bond strength and the masses of the atoms, and only if the vibration changes the molecule's dipole. Promoting electrons needs the higher energies of visible or UV light. Ionisation and fragmentation happen in a mass spectrometer, using high-energy electrons. Infrared does not have enough energy to break bonds; the molecule is unchanged after absorbing.

Below 1500 cm⁻¹ many bending and skeletal vibrations overlap, giving a complex pattern that is unique to each compound, like a fingerprint. It cannot be read bond by bond, but matching it to a database spectrum confirms an identity, even for isomers with the same functional groups. The O-H region above 3000 cm⁻¹ is a functional-group region, not the fingerprint region. IR does not give M_r or the number of carbon atoms; those come from the M⁺ and M+1 peaks of a mass spectrum.

N₂ is made of two identical atoms; stretching the N≡N bond never creates a dipole, so it cannot absorb infrared. CO₂ is non-polar overall, but its bending and asymmetric stretching vibrations do change the dipole, so it absorbs infrared emitted by the Earth and is a greenhouse gas. H₂O has polar O-H bonds and absorbs strongly. CH₄ absorbs through its C-H vibrations and is a more powerful greenhouse gas per molecule than CO₂.

The only absorption above 2000 cm⁻¹ is the C-H near 2950 cm⁻¹, so there is no O-H. There is a strong C=O at about 1740 cm⁻¹ and strong C-O absorptions at 1040-1300 cm⁻¹. C=O plus C-O with no O-H is an ester, which fits C₄H₈O₂ and the fruity smell. Butanoic acid has the same formula but would show a very broad O-H at 2500-3000 cm⁻¹. Butan-1-ol (C₄H₁₀O) would show a broad O-H at 3200-3600 cm⁻¹ and no C=O. Butanal (C₄H₈O) has C=O but no strong C-O band, and the wrong formula.

Propan-2-ol is oxidised to propanone, CH₃COCH₃. The alcohol O-H band (3200-3600 cm⁻¹, broad) disappears and the ketone C=O (1670-1740 cm⁻¹) appears; when no O-H band remains, no alcohol is left. A very broad band at 2500-3000 cm⁻¹ would mean a carboxylic acid, but a ketone is not oxidised further by dichromate. C-H bonds are still present in propanone, so the 2850-2950 cm⁻¹ absorption stays. 1640-1680 cm⁻¹ is C=C, which would come from dehydration, not oxidation.

Addition across C=O turns it into C-OH and adds a C≡N group. The product therefore gains an alcohol O-H (broad, 3200-3600 cm⁻¹) and a nitrile C≡N (sharp, 2200-2250 cm⁻¹), and it loses the C=O absorption at 1670-1740 cm⁻¹ that propanone shows; so a C=O band is not new. The very broad 2500-3000 cm⁻¹ band belongs to a carboxylic acid, which forms only if the nitrile is later hydrolysed. The product has no N-H bond (the N is in C≡N) and no C=C.