Spectra of Alkenes

The C=C stretching vibration, which appears in the 1680-1580 cm region, can have variable intensity, depending on the compound in which the group is present. For the symmetric compound tetrachloroethylene no C=C vibration is present in the infrared spectrum. Trichloroethylene has a very strong G=C 

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Stretching near 1640 cm The position of the G=C stretching vibration is sensitive to the groups substituted on the double bond, as described in an earlier section. The compound 5-methyl-1-hexene contains the group CH2=CH—. The spectrum of this compound has the double bond vibration near 1640 cm S while in 3-methyl-cw-2-hexene, where the RRC=CH— group is present, the vibration is at 1665 cm  

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Mass Spectra of Alkenes and Arenes. Resonance Stabilized... 

The most characteristic vibrational modes of alkenes are the out-of-plane C—H bending vibrations between 1000 and 650 cm-1. These bands are usually the strongest in the spectra of alkenes. The most reliable bands are those of the vinyl group, the vinylidene group, and the trans-disubstituted alkene. Alkene absorption is summarized in Appendix Tables D-l and D-2. 

In 1975, it was reported that while lanthanide shift reagents could not be used directly to simplify the NMR spectra of alkenes, when coupled with silver salts substantial shifts could be induced.232 Since then, a number of studies have reported the use of both chiral and achiral lanthanide(III)-silver(I) binuclear shift reagents,233-237 where the ligands were generally fluorinated /3-diketones. 

The mass spectra of alkenes and alkynes usually give distinct molecular ions however, the fragmentation is often complex and not easily interpreted. 

The spectra of alkene selenides persists for several minutes after flashing. A spectrum observed in flashed COSe and ethylene mixture is identical to that produced in flashed CSe2 and ethylene mixture. In Ar or N2, flash photolysis of COSe produces Se2 but no CSe. An added alkene inhibits Se2 formation in experiments either with COSe or CSe2. In flashed CSe2, the rate of decay of Se (4 P) is identical to the rate of appearance of the far ultraviolet bands, within an experimental scatter of + 20% in the pseudo first-order rate constant. Therefore the absorption in the far... 

The double bond of an alkene is capable of absorbing substantial energy. As a result, the mass spectra of alkenes generally show a strong molecular ion peak. A characteristic of the mass spectra of alkenes is that the mass of the molecular ion should correspond to a molecular formula with an index of hydrogen deficiency equal to at least one (see Chapter 1). 

Alkenes and alkynes show strong molecular ion peaks because the double and triple bonds are able to absorb energy. Alkenes with more than 4 carbon atoms often show a strong peak at m/z = 41 due to formation of an aUyl ion. Alkynes show strong (M — 1) peaks. The mass spectra of alkene isomers are very similar, especially the mass spectra of cis- and trans-isomers, so it is not usually possible to locate the position of the double bonds in these compounds through mass spectrometry. Figures 10.17 and 10.18 show the mass spectra of 1-hexene and acetylene, respectively. 

The proton spectra of a range of chlorinated norbornenes are reported. The only 1 2 adduct formed from norbornadiene and cyclopentadiene is (21). The completely analysed spectrum shows the C-5 bridge protons at 51.53 because of steric deshielding which overcomes any shielding by the 7t-systems H-lOa appears at 1.17 and H-lOs at 1.28. The spectra of alkenes and ketones of the types (22) and (23) have also been analysed and conclusions drawn about the shielding effects of the double bonds. ... 

Other examples have been given of the preparation of branched-chain and extended-chain alkenes by application of Wittig reagents to sugar aldehydes and ketones,and the mass spectra of alkenes derived from 1,2 5,6-di-O-iso-propylidene-D-r/Z o-hexos-3-ulose have been described.Compound (25) was... 

An important fragment in the mass spectra of alkenes is the allyl cation m/e = 41). This cation is particularly stable due to resonance. 

The NMR spectra of alkenes are quite distinct and this class of compounds has been extensively studied using NMR. 

Working with the Concepts Interpreting NMR Spectra of Alkenes... 

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