CHAPTER 11 Alkenes; Infrared Spectroscopy and Mass Spectroscopy

Nuclear Magnetic Resonance of Alkenes11-4 pi electrons exert a deshielding effect on alkenyl hydrogens. The proton NMR spectra of trans-2,2,5,5-tetramethyl-3-hexene shows only two peaks. The methyl protons and alkenyl protons are too far from each other to produce detectable coupling. The resonance of the allenyl protons at 5.30 ppm is typical of hydrogens bound to alkenyl carbons. Terminal alkenyl hydrogens (RR’C=CH 2 ) resonate at 4.6– 5.0 ppm. Internal alkenyl hydrogens (RCH=CHR) resonate at 5.2–5.7 ppm 18

The deshielding for alkenyl hydrogens has two causes. Less important is the electron withdrawing effect of the sp 2 hybridized carbon. More important is the effect of the external magnetic field on the  cloud of electrons. The  electrons are forced to assume a circular motion when the magnetic field is perpendicular to the double bond axis. The circular motion of the  electrons induces a second magnetic field which reinforces the external field.

Cis coupling through the double bond is different from trans. Unsymmetrically substituted double bonds lead to non-equivalent alkenyl hydrogens which leads to spin-spin coupling. Within a set of cis/trans isomers, the coupling constant for the trans isomer, J, is always larger than for the cis isomer. 20

Coupling between hydrogens on adjacent carbons is called vicinal. Coupling between hydrogens on the same carbon is called geminal and is usually small in alkenes. Coupling to neighboring alkyl hydrogens (allylic) and 1,4- or long- range coupling is also possible, which may produce complicated spectral patterns.

Further coupling leads to more complex spectra. In 3,3-dimethyl-1-butene, H a resonates at 5.86 ppm in the form of a doublet with two relatively large coupling constants (J ab =18 Hz, J ac =10.5 Hz). H b and H c also absorb as doubles due to their coupling to H a and their mutual coupling (J bc = 1.5 Hz). 22

In 1-pentene, there is additional coupling to the attached alkyl group. In addition, the double bond causes a slight deshielding of the allylic CH 2 group. The coupling between the allylic hydrogens and the neighboring alkenyl hydrogen is about the same as the coupling with the two CH 2 hydrogens on the other side. As a result, the multiplet for the allylic CH 2 group appears as a quartet.

Alkenyl carbons are deshielded in 13 C NMR. 24 Relative to alkanes, corresponding alkene carbons absorb at about 100 ppm lower field.

Infrared Spectroscopy11-5 IR spectroscopy measures the vibrational excitation of atoms around the bonds that connect them. The positions of the absorption lines are related to the types of functional groups present. The IR spectrum as a whole is unique for each individual substance. 26 Absorption of infrared light causes molecular vibrations. The infrared region is range of the electromagnetic spectrum just below visible light. Absorption of light of this wavelength causes vibrational excitation of the bonds in a molecule. Middle infrared light (λ~ μm, or cm -1) has energies from 1 to 10 kcal mol -1 and is most useful.

Hooke’s law relates the parameters affecting the vibrational frequency of two weights connected by a spring. The vibrational frequency of two atoms connected by a bond is also accurately described by Hooke’s law: 26

The infrared spectrum of a molecule is significantly more complex than the vibrational frequencies of all of the bonds present. Various bending motions, and combinations of stretching and bending are also excited by IR radiation, which leads to complicated patterns.

Functional groups have typical infrared absorptions. Vibrational bands of functional groups appear at characteristic wavenumbers, and entire IR spectrum of a compound is unique and can be distinguished from that of any other substance.

Compare the IR spectra of pentane and hexane: > 1500 cm -1 : C-H stretching absorptions of alkanes can be seen. Since no function groups are present, no absorptions are seen in the region from 2840–3000 cm -1. < 1500 cm -1 (fingerprint region) : C-C stretching and C-C and C-H bending motions absorb to give complicated patterns. Saturated hydrocarbons show peaks at 1460, 1380, and 730 cm -1.

Now compare hexane to 1-hexene: Peak at 3080 cm -1 : due to the stronger C sp2 -H bond. The C=C stretching band should appear between 1620 and 1680 cm -1 and is seen at 1640 cm -1. The two signals at 915 and 995 cm -1 are characteristic of a terminal alkene. 30

Several other strong bending modes are characteristic for the substitution patterns in alkenes: O-H stretching absorption : most characteristic band in the IR spectra of alcohols. This appears as a broad band over the range 3200–3650 cm -1 (due to hydrogen bonding) Dry, dilute alcohols show a sharp narrow band in the range 3620–3650 cm -1. Haloalkane C-X stretching frequencies are too low (<800 cm -1 ) to be useful