Figure 9.17
Infrared spectrum of n-decane.CH 2 and CH 3 groups. It should be noted that no bands are observed above 3000 cm–^1 , indicating the
absence of an aromatic, alkene or alkyne structure.
Contrast this spectrum with those of 1,2-dimethylbenzene (Figure 9.18) and 1-octene (Figure 9.19). In
both spectra, additional C—H stretching vibrations appear above 3000 cm–^1 , and characteristic C—H
out-of-plane bending vibrations are seen between 650 and 1000 cm–^1. The bands at 910 and 990 cm–^1 in
the spectrum of 1-octene signify the presence of a vinyl group. This is confirmed by a sharp band at
1640 cm–^1 due to stretching of the alkene double bond. The spectrum of 1,2-dimethylbenzene (o-
xylene) is more complex and includes skeletal vibrations of the ring and C—H in-plane bending
between 1000 and 1600 cm–^1 , together with weak overtone bands in the region between 1600 and 2000
cm–^1. The presence of 1,2-disubstitution is apparent from the single strong band at 740 cm–^1 due to C–H
out-of-plane bending.
Compare, also, the spectrum of n-decane with those of n-propyl alcohol, (Figure 9.20) and n-undecanal
(Figure 9.21). The spectrum of the alcohol is characterized by vibrations due to the O—H and C—O
groups. The O—H
Notes to Table 9.8
The charts are arranged in order of increasing element content. Correlations given in one chart (e.g. those for CH 2 and
CH 3 ) are not repeated in subsequent charts. The frequency limits within which the band of a particular grouping is
usually found are indicated by the black strips and extensions of the range to include unusual examples are shown as
thin lines, e.g. relative intensities are given in a very approximate fashion (see below). Both the position and the
intensity of some absorptions are dependent on state, solvent, etc., and the actual frequency quoted is that most
commonly observed.
Abbreviations
ν = stretch (ν is also used for quoting an absorption frequency, e.g. νmax 1700 cm–^1 ); δ = in-plane bend; γ = out-of-plane
bend; ρ = rock; ω = wag; τ = twist; sym = symmetrical; antisym = antisymmetrically (asymmetric); var. = variable (in
frequency); br = broad; w = weak (in absorption); m = medium; s = strong; v.w. = very weak; vs = very strong; w–m =
weak to medium (i.e. typical intensity range for that particular band)