Alkenes and Alkynes and Rad on React

0 ALKENES AND ALKYNES AND RAD ON REACT arbon-carbon double and triple bonds undergo a wide variety of addition reactions in which one of the multiple bonds is broken and two new bonds to carbon are formed: A B \ / I I C=C + A-B ---+ -C-C- / \ I I The importance of such reactions to synthetic organic chemistry is para- mount. It is our intention in this and the following chapter to show the great diversity, utility, and specificity of addition reactions of alkenes and alkynes. We will begin with a brief discussion of the physical and spectroscopic properties of alkenes and alkynes. But the major emphasis in the chapter is on two main types of reactions, ionic addition and radical-chain addition. For ionic additions we will make extensive use of the classification of reagents as electrophiles and nucleophiles, as described in Chapter 8. 10-1 Physical and Spectroscopic Properties of Alkenes and Alkynes 351 10-1 PHYSICAL AND SPECTROSCOPIC PROPERTIES OF ALKENES AND ALKYNES 10-1A Physical Properties In general, the physical properties of alkenes are similar to those of alkanes. The data of Table 10-1 allow comparison of the boiling points, melting points, and densities of several alkenes with the corresponding alkanes that have the same carbon skeleton. Like the continuous-chain alkanes, the 1-alkenes form a homologous series of compounds that show regular changes in physical properties with increasing chain length. The boiling points, melting points, and densities of the simple alkynes (also included in Table 10-1) are somewhat higher than those of the corresponding alkanes or alkenes, and these properties also show regular changes as the chain length is increased. 10-1B Spectroscopic Properties of Alkenes The infrared spectra of allcenes are sufficiently different from those of alkanes in most instances to make it possible to recognize when a double bond is present. For example, in the infrared spectrum of 1-butene (Figure 10-1) the absorption band near 1650 cm-I is characteristic of the stretching vibration of the double bond. In general, the intensity and position of this band depends on the structure of the alkene; it varies with the degree of branching at the double bond, with the presence of a second unsaturated group in conjugation \ I1 \ I1 with the first (i.e., C=C--C=C or C=C-c=O), andwiththe / \ / symmetry of the substitution of the double bond (see Section 9-7B). However, in many cases the absorption bands caused by the various modes of vibration of the alkenic C-I3 bonds frequently are more useful for detecting a double bond and identifying its type than is the absorption band caused by C=C stretch. With 1-butene, absorptions arising from the C-H vibrations of the terminal =CH2 group occur near 3 100 cm-l, 1420 cm-l, and 9 15 cm-l, and those of the -CH= grouping near 3020 cm-l, 1420 cm-l, and 1000 cm-l. In general, absorption bands at these frequencies are from the grouping -CH=CH2. The bands near 1420 cm-l are due to in-plane bending, whereas those at 9 15 cm-l to 1000 cm-l arise from out-of-plane bending. The other intense absorptions, near 1460 cm-I and 3000 cm-l, are due to C-H vibrations of the CH,CH2- group (see Section 9-7D). These illustrate a further point- namely, the positions of the infrared absorptions of alkyl C-H bonds are significantly different from those of alkenic C-H bonds. The double bonds of an alkene with no alkenic hydrogens are difficult to detect by infrared spectroscopy and in such cases Raman spectroscopy is helpful (see Section 9-8). 352 10 Alkenes and Alkynes I. Ionic and Radical Addition Reactions Table 10-1 Comparison of Physical Properties of Alkanes, Alkenes, and Alkynes BPl MP, Density, Hydrocarbon Formula "C "C di0 ethane ethene ethyne propane propene ProPYne butane 1-butene cis-2- butene trans-2- butene I-butyne 2-butyne pentane 1-pentene cis-2-pentene trans-2-pentene 1-pentyne 2-pentyne hexane I-hexene cis-2-hexene trans-2-hexene cis-3-hexene trans-3-hexene 1-hexyne 2-hexyne 3-hexyne aAt the triple point (i.e., the temperature at which the solid, liquid, and vapor all are in equilibrium). bUnder pressure. The infrared absorption of 1-butene that occurs at 1830 cm-l (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 (har- monic) of the =CH, out-of-plane bending at 915 cm-l. 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. 10-1 B Spectroscopic Properties of Alkenes -+.- -CH,,-CH, C=C I I I I I I 3600 2800 2000 2000 1800 1600 1400 1200 1000 800 frequency, cm-I Figure 10-1 Infrared spectrum of 1-butene showing the vibrational assignments made to the various absorptions Exercise 10-1 Deduce possible structures of the substances whose infrared spectra are shown in Figure 10-2. Assign as many of the bands as you can to specific stretching and bending vibrations by comparison with Figure 10-1. Be sure your structural assignments fit with the spectrum-that is, think twice about assigning a structure that has -CH,- groups if there are no -CH,- bands in the spectrum, or that has no -CH, groups when there appear to be strong -CH, absorptions. With regard to electronic spectra, a n electron of a simple alkene can be excited to a higher energy (n*) state by light of wavelength 180 nm to 100 nm. However, many other substances absorb in this region of the spectrum, in- cluding air, the quartz sample cell, and most solvents that might be used to dissolve the sample, and as a result the spectra of simple alkenes are not ob- tained easily with the usual ultraviolet spectrometers. When the double bond \III1 is conjugated as in C=C-C=C or C=C-C=O, then the wave- / \ / lengths of maximum absorption shift to longer wavelengths and such absorptions are determined more easily and accurately (also see Section 9-9B). In proton nmr spectra, the chemical shifts of alkenic hydrogens are toward lower fields than those of alkane hydrogens and normally fall in the range 4.6-5.3 ppm relative to TMS (see Section 9-10E and Table 9-4). Spin- spin couplings of alkenic hydrogens are discussed in Section 9-106- and 9-105. 10 Alkenes and Alkynes I, Ionic and Radical Addition Reactions i I 1 I I I I 2000 1800 1600 1400 1200 1000 800 frequency, cm-' Figure 10-2 Infrared spectra for Exercise 10-1 Exercise 10-2 Deduce the structures of the substances whose proton nmr spectra are shown in F~gure10-3. Analyze the spectra in as much detail as you can in terms of chemical shifts and spln-spin spl~tt~ng. 10-1 B Spectroscopic Properties of Al kenes , Figure 10-3 Proton nmr spectra at 60 MHz with TMS as the standard at 0 ppm. See Exercise 10-2. 356 10 Alkenes and Alkynes I. Ionic and Radical Addition Reactions 10-1C Spectroscopic Properties of Al kynes The infrared spectrum of a monosubstituted alkyne such as ethynylbenzene, C,W,C--CH (Figure 10-4), has a strong band near 3300 cm-l, which is characteristic of the carbon-hydrogen stretching vibration in the grouping =C-H. At a lower frequency (longer wavelength) around 2100 cm-l, there is a band associated with the stretching vibration of the triple bond (also see Figure 9-36). Therefore the presence of the grouping -C=CH in a molecule may be detected readily by infrared spectroscopy. However, the triple bond of a disubstituted alkyne, R-C=C-R, is detected less easily because there is no =C- W absorption near 3 300 cm-l, and furthermore the C--C absorption sometimes is of such low intensity that it may be indiscernible. Raman spectroscopy (Section 9-8) or chemical methods must then be used to confirm the presence of a triple bond. Alkynes, like alkenes, undergo electronic absorption strongly only at wavelengths in the relatively inaccessible region below 200 nm. However, when the triple bond is conjugated with one or more unsaturated groups, radiation of longer wavelength is absorbed. To illustrate, ethyne absorbs at 150 nm and 173 nm, whereas 1-buten-3-yne (CI-I,=CH-C--CH) absorbs at 219 nm and 227.5 nm. The effects of such conjugation on spectra is discussed in more detail in Section 9-9B. The proton n~lclearmagnetic resonance spectrum of ethynylbenzene is shown in Figure 10-5. The peaks near 435 Hz and 185 Hz correspond to resonances of the phenyl and --C-H protons, respectively. The difference frequency, cm-' Figure 10-4 Infrared spectrum of ethynylbenzene in carbon tetrachloride solution 10-1C Spectroscopic Properties of Al kynes 357 Figure 10-5 The proton nmr spectrum and integral of ethynylbenzene at 60 MHz relative to TMS as 0.00. This spectrum also illustrates the use of nmr for detection of small amounts of impurities. The almost imper- ceptible peaks around 6 ppm are in the correct locations for alkene hydrogens. The integral indicates that the ratio of alkene to ethyne hydrogens is on the order of 1 : 15.

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