Ch13 - Sample IR spectra

Chapter 13: Spectroscopy

Sample IR Spectra : hydrocarbons
By looking at IR spectra that contain known functional groups and comparing and contrasting them with other IR spectra, one can develop the skills required to be able to "interpret" an "unknown" IR spectra. Remember that for an organic chemist, the primary role of IR is to identify the functional groups that are present. A few examples reflecting some of the more important functional groups are provided below.
Compare them to try to appreciate the subtle differences, comparing frequency, intensity and shape.

1. A long chain alkane, dodecane, CH₃(CH₂)₁₀CH₃ where there are only C-C and C-H vibrations. In this case, the number of C-C and C-H bonds present in the system overcomes their low polarity. The sp3 C-H stretching vibrations are just to the right of 3000 cm^-1

IR spectra of dodecane

2a. An trans-alkene, trans-pent-2-ene. Note that the almost symmetrical nature of the trans alkene means is has a minimal dipole and therefore the C=C is very weak. The sp3 C-H stretching vibrations are just to the right of 3000 cm^-1 and the sp2 C-H just to the left of 3000 cm^-1.

IR of trans-pent-2-ene

b. A cis-alkene, cis-pent-2-ene. Note C=C in the cis-alkene has a slighlty stronger dipole and therefore the C=C is more obvious near 1650cm-1 (but still only weak). The sp3 C-H stretching vibrations are just to the right of 3000cm-1 and the sp2 C-H just to the left of 3000cm-1.

Ir of cis-pent-2-ene

3a. An internal alkyne, hex-2-yne CH₃C≡CCH₂CH₂CH_3. Note that the almost symmetrical nature of the internal alkyne means is has a minimal dipole and therefore the C≡C is very weak (near 2100cm-1).. The sp3 C-H stretching vibrations are just to the right of 3000cm-1 and since it is an internal alkyne, there is no sp C-H near 3300cm-1.

IR of hex-2-yne

3b. A terminal alkyne, hex-1-yne, HC≡CCH₂CH₂CH₂CH₃. Note that the terminal alkyne is more polar and the C≡C is obvious (medium intensity, (near 2100cm-1).. The sp3 C-H stretching vibrations are just to the right of 3000cm-1 and the sp C-H (characteristic) near 3300cm-1.

Ir of hex-1-yne

4. An aromatic hydrocarbon, toluene, we can see both the aromatic and aliphatic CH stretches, and two absorptions for the aromatic C=C stretches.