Absorptions for Alkenes

TABLE C-2 C=C Stretching Frequencies in Cyclic and Acyclic Systems (cm 1) 

11 s = strong, m = medium, w = weak, v = variable. h This band also shows a strong overtone band. 

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Alkanes show IR bands characteristic of C-H bonds in the range from 2840 to 3000 cm . The C=C stretching absorption for alkenes is in the range from 1620 to 1680 cm, that for the alkenyl C-H bond is about 3100 cm . Bending modes sometimes give useful peaks below 1500 cm . Alcohols are usually characterized by a broad peak for the O-H stretch between 3200 and 3650 cm . ... 

By comparison with these less interesting reactions, photocyc-loadditions of alkynes to alkenes and related compounds provide a fascinating extension of the analogous reactions for alkenes. The simplest type of cycloaddition yields a cyclobutene (2.99), but depending on the relative absorption characteristics of the substrates and product at the wavelength of irradiation, the cyclobutene may... 

Characteristic bands are observed for the carbonyl group at 1660-1670 cm-1, the alkenic double bond at 1588-1604 citO1, and the sulfoxide function at 1042-1050 cm-1 in 3-arylidenethiochroman-4-one sulfoxides. The corresponding sulfones show similar absorptions for the C=0 and C=C bonds and the SOz bands are seen at 1148-1154 and 1310-1318cm 1 <1994T13113>. 

A key feature of alkene coordination is the loss of planar symmetry. Indeed, it is this loss of symmetry that allows the vc=c IR absorption of alkene complexes to be observed (IR inactive for the free molecule). Consistent with the synergic bonding description, coordination leads to a decrease in the C=C bond strength and accordingly a decrease in the value of vc=c. The case of prochiral alkenes is of particular importance,... 

Even though the metal-substituted, mesoporous solids allow the oxidation of molecules that is not possible with zeolites, there are several issues that still need to be addressed. First, the activity of the metal-loaded catalysts decreases with increased metal loading, e.g. for Ti-MCM-41, the peak activity for alkene epoxidation is attained at 2 wt. % [44aj. Second, metal leaching can occur and care needs to be exercised in concluding that oxidation is taking place at the framework site rather than via metal ions leached into solution [184, 185]. Leaching has been shown to occur for V-substituted mesoporous materials in the oxidation of alkanes [184], X-ray absorption spectroscopy indicates that the inclination of the heteroatoms to remain in the MCM-41 framework after calcination follow the order Ti > Fe > V > Cr [56],... 

The partial reduction of substrates containing triple bonds is of considerable importance not only in research, but also commercially for stereoselectively introducing (Z)-double bonds into molecular frameworks of perfumes, carotenoids, and many natural products. As with catalytic hydrogenation of alkenes, the two hydrogen atoms add syn from the catalyst to the triple bond. The high selectivity for alkene formation is due to the strong absorption of the alkyne on the surface of the catalyst, which displaces the alkene and blocks its re-adsorption. The two principal metals used as catalysts to accomplish semireduction of alkynes are palladium and nickel. 

The ir-bond stretching of enamines is only slightly affected by interaction with the nitrogen lone pair. It might be argued that the relatively greater shift of the absorption for the less substituted isomer as compared with the more substituted isomer (relative to a tri- or tetra-substituted alkene at 1670 cm ) is an indication of greater lone pair delocalization in the former (Scheme 9). 

Alkenes show a Jt jt absorption for the Jt bond at <= 180 nm (158.9 kcal moT, 665.1 kJ mol ).32 Conjugated alkenes show a shift in absorption toward the visible spectrum (lower energy). Both isomerization (of ethene to ethyne) and fragmentation are observed.32 As the size of the alkyl portion of 1-alkenes increases, the yield of alkenyl radicals decreases.33 Photolysis of a terminal alkene generates the radical (29) in a typical photolysis reaction. 

The C—H bonds in alkenes can vibrate by bending both in-plane and out-of-plane when they absorb infrared radiation. The scissoring in-plane vibration for terminal alkenes occurs at about 1415 cm This band appears at this value as a medium to weak absorption for both monosubstituted and 1,1-disubstituted alkenes. 

The most valuable information for alkenes is obtained from analysis of the C—H out-of-plane region of the spectrum, which extends from 1000 to 650 cm These bands are frequently the strongest peaks in the spectrum. The number of absorptions and their positions in the spectrum can be used to indicate the substitution pattern on the double bond. 

Aromatic hydrocarbons have many IR active vibrations, resulting in complex spectra. The C H aromatic absorption occurs above 3000 cm , but in the same region as that for alkenes. The aromatic... 

The hexane spectrum (Figure 14.15) shows that the C-H stretch at 2850-2960 cm-i and the signals at 1350-1470 cm-i correlate with the absorptions in Table 14.3. Benzene derivatives and other aromatic compormds usually show absorption for the C-H units at 3000-3100 cm" but also at 675-870 cm i (in the fingerprint region). Note the subtle shift of the C-H absorption to lower energy for the aromatic compounds. Other compounds that have a C-H absorption are those for alkenes and alkynes. The CsC-H absorption is at lower energy than the C=C-H absorption, which is lower in energy than the C-C-H absorption for alkanes. 

Figure 14.17 shows the infrared spectra for an alkane (A, 3-methylpentane), an alkene (B, 1-methylcyclohexene), and an alkyne (C, 16-methyl-1-hexadecyne). Note that the CH region is essentially the same in all three spectra, but there is a CsCH absorption in Figure 14.17C. The C=C signal in Figure 14.17B is clearly visible, as is the absorption for the C=C unit in Figure 14.17C. As noted.

Using the information in the figure, the UV spectra of 18-20 may be predicted. For 18, the base value is 215 + 12 for a methyl on the P-carbon = 227 nm. For 19, the base value is 215 + 10 for a methyl on the a-carbon = 225 nm. Compound 20 is not conjugated, so it should show an absorption for the C=C at around 170-180 nm and an absorption for the C=0 at around 150-160 nm. It may be difficult to distinguish 18 from 19 by UV spectroscopy, but structure 20 is certainly ruled out. The inability to distinguish compounds that are very close in structure is a limitation of this method, but usually subtle differences will allow one to make a structure determination. Also remember that infrared spectroscopy and proton NMR spectroscopy (see Chapter 14) may be used. The chemical shifts in the NMR spectra and multiplicity of the methyl group on the alkene unit and the alkene protons themselves will be different for 18 when compared to 19 (see Chapter 14, Section 14.4.3) and this information is used to assist in the identification. 

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