Absorption frequencies alkenes

The absorption frequency of the alkene bond in conjugation with a carbonyl group is lowered by about 30 cm-1 the intensity of absorption is increased. In s-cis structures, the alkene absorption may be as intense as that of the carbonyl group. s-Trans structures absorb more weakly than s-cis structures. 

Conjugated alkenes appear at lower end of range, and absorptions are medium to strong nonconjugated alkenes appear at upper end of range, and absorptions are usually weak. The absorption frequencies of these bands are raised by ring strain but to a lesser extent than noted with carbonyl functions. 

Spectra-structure correlation charts showing the probable positions of the characteristic absorption frequencies of some 1000 aliphatic, aromatic, and heterocyclic compounds [ ], together with various classes of aliphatic and organometallic derivatives in the 15-to 35-/1 region, are shown in Figs. 3 and 4. Some of the important classes of compounds studied and summarized in these correlation charts are alkanes, alkenes, cyclopropanes, cyclopentanes, cyclohexanes, substituted benzenes, polynuclear hydrocarbons, heterocyclics. 

The absorption of the alkene bond in conjugation with the carbonyl group occurs at a lower frequency than that of an isolated C=C bond the intensity of the conjugated double-bond absorption, when in an s-cis system, is greater than that of an isolated double bond. 

The infrared absorption of 1-butene that occurs at 1830 cm 1 (Figure 10-1) falls in the region where stretching vibrations of alkene bonds usually are not observed. However, this band actually arises from an overtone (harmonic) of the =CH2 out-of-plane bending at 915 cm 1. Such overtone absorptions come at exactly twice the frequency of the fundamental frequency, and whenever an absorption like this is observed that does not seem to fit with the normal fundamental vibrations, the possibility of its being an overtone should be checked. 

The weakening of a bonds by negative hyperconjugation with lone electron pairs also reveals itself in IR and NMR spectra. Thus, C-H, N-H, or O-H bonds oriented trans or antiperiplanar to an unshared, vicinal electron pair are weakened and have therefore a significantly reduced IR vibrational frequency [17]. The C-H vibrational frequency in aldehydes is, for example, lower than that in alkenes (Scheme 2.7). Polycyclic amines with at least two hydrogen atoms antiperiplanar to the lone pair on nitrogen have characteristic absorption bands at 2800-2700 cm-1 which have been used to infer the relative configuration of such amines [18]. 

Alkanes, alkenes, and alkynes also have characteristic C — H stretching frequencies. Carbon-hydrogen bonds involving sp3 hybrid carbon atoms generally absorb at frequencies just below (to the right of) 3000 cm-1. Those involving sp2 hybrid carbons absorb just above (to the left of) 3000 cm-1. We explain this difference by the amount of 5 character in the carbon orbital used to form the bond. The s orbital is closer to the nucleus than the p orbitals, and stronger, stiffer bonds result from orbitals with more s character. Even if an alkene s C=C absorption is weak or absent, the unsaturated C—H stretch above 3000 cm-1 reveals the presence of the double bond. 

The spectrum of hex-l-ene, shows additional absorptions characteristic of a double bond. The C—H stretch at 3080 cm-1 corresponds to the alkene =C—H bonds involving sp2 hybrid carbons. The absorption at 1642 cm-1 results from stretching of the C=C double bond. (The small peak at 1820 cm-1 is likely an overtone at double the frequency of the intense peak at 910 cm-1.)... 

Note, too, that the C-H absorptions in alkanes, alkenes, and alkynes have a characteristic appearance and position. The sp hybridized C-H bonds are often seen as a broad, strong absorption at < 3000 cm , whereas sp and sp hybridized C-H bonds absorb at somewhat higher frequency. 

---