CHAPTER 2 Nitriles, Isonitriles, and Dialkyl Cyanamides Nitriles 27 Isonitriles 30 Dialkyl Cyanamides 31 Organothiocyanates 31 References 31 Figures Tables Figure 2-1 32 (28) Table 2-1 39 (27) Figure 2-2 32 (29) Table 2-2 40 (29) Figure 2-3 33 (29) Table 2-3 40 (29) Figure 2-4 34 (29) Table 2-4 41 (29) Figure 2-5 35 (29) Table 2-5 42 (30) Figure 2-6 36 (30) Table 2-6 42 (30) Figure 2-7 37 (30) Table 2-7 43 (31) Figure 2-8 38 (30) * Numbers in parentheses indicate in-text page reference. NITRILES Table 2.1 lists IR and=or Raman data for nitriles in different physical phases. In the vapor phase, compounds of form R-CN or NC(CH2)4CN exhibit the CN stretching vibration in the region 2250±2280 cm1, while in the neat phase the CN stretching vibration for the corresponding compound occurs at a frequency 5 to 22 cm1 lower than in the vapor phase. Conjugated organonitriles such as benzonitrile, 2-X-, 3-X- or 4-X-substituted benzonitriles, and acrylonitrile exhibit CN stretching in the region 2222±2240 in the vapor phase, and at a frequency 8±19 cm1 lower in the neat phase. Conjugation with the CN group causes the CN stretching mode to vibrate at a lower frequency compared to those for alkylnitriles. Compounds such as 3-halo-propynonitrile (or 1-cyano-2-haloacetylene) have the following empirical structure: X-CCCN. It is apparent that the CN group is joined to the CC group in a linear manner. Thus, the CN and CC groups are conjugated, and one might expect that CN stretching frequencies would occur at lower frequencies than those for alkanonitriles, benzo- nitriles, and methyacrylonitrile. However, it is noted that the CN stretching vibration occurs in 1 1 the region 2263±2298 cm (CCl4 solution) and 2270±2293 cm (vapor phase). These CN stretching frequencies are higher than expected. In CCl4 solution, the CC stretching vibration 27 28 Nitriles, Isonitriles, and Dialkyl Cyanamides occurs at 2195, 2122, and 2128 cm1 for the Cl, Br and I analogs of 3-halopropynonitrile, respectively (6). It is likely that there is coupling between the CCCN stretching modes, which causes the CN stretching mode to occur at higher frequency and the CC stretching vibration to occur at lower frequency than expected. Figure 2.1 is a plot of the Raman data (CN stretching) for acetonitrile, proprionitrile, isobutyronitrile, pivalonitrile vs the number of protons on the a-carbon atom and this plot shows that the CN stretching vibration decreases in frequency as the number of a-hydrogen atoms decreases from 3 to 0 (7). Tafts s* values for CH3,CH3CH2, (CH3)2CH, and (CH3)3C are 0, 0:100, 0:190, and 0:300, respectively (7). Thus, the CN stretching vibration decreases in frequency as the inductive electron release to the nitrile group is increased. This effect would tend to lengthen the CN bond, causing it to vibrate at a lower frequency. In all cases, in Table 2.1 the CN stretching frequency occurs at a higher frequency in the vapor phase than in the neat or solution phase by 5±47 cm1. It is of interest to note that the frequency difference between the CN stretching mode in the vapor- and neat phases decreases by 31, 20, 17, and 14 cm1 for acetonitrile, propronitrile, isobutyronitrile, and pivalonitrile, respectively, and this is in decreasing frequency order for both vapor- and neat phases. It is of interest to consider why the frequency difference for CN stretching decreases between the two physical phases progressing in the series from acetonitrile through privalonitrile. It is suggested that this difference is caused by steric factors of the alkyl groups, which alter the amount of dipolar interaction between nitrile groups in the neat phase, while in the vapor phase the dipolar interaction between the nitrile groups is negligible. The steric constant Es for CH3,C2H5, (CH3)2CH, and (CH3)3C is 0.00, 0:07, 0:47, and 1:54, respectively (9). Thus, as Es becomes larger the CN groups in the neat phase are spaced farther apart, which weakens the dipolar interaction between nitrile groups. The inductive effect of the alkyl group most likely contributes to some extent to the amount of dipolar interaction, but this is probably a smaller effect because the inductive effect is independent of physical phase. R-CN NC-R dipolar interaction The cyanogen halides exhibit the CN stretching vibration at 2290, 2201, 2187, and 2158 cm1 in the neat phase for the F, Cl, Br and I analogs, respectively (4). In the vapor phase the CN stretching for the Cl and Br analogs occurs at frequencies higher by 47 and 13 cm1, respectively (4, 5). In this cyanogen halide series the CN stretching vibration decreases as the CN bond length increases. For example, in the solid state the CN bond length in the solid state is 1.26, 1.58, 1.77, and 2.03 AÊ for FCN, ClCN, BrCN, and ICN, respectively (10). In the vapor phase the CN bond is less restricted, and the CN bond length is 1.67 and 1.79 AÊ for ClCN and BrCN, respectively (10). The relative steric factor of F, Cl, Br, CH3, and I (based on F as zero) is (0.00), 0:31, 0:49, 0:49, and 0:69, respectively. The inductive value s* for F, Cl, Br, and I is 1.10, 1.05, 1.00, and 0.85, respectively (11). Thus, the steric factor of the halogen atom increases as the CN bond length increases, while the inductive effect of the halogen atom decreases progressing in the series FCN through ICN. Combination of the preceding factors is most likely the cause for Factors Affecting Molecular Vibrations and Chemical Shifts 29 the CN stretching frequency for compounds of form R-CN to occur at intermediate frequencies between FCN and ClCN (see Table 2.1). The dipolar interaction between these linear XCN molecules could also be different than for R-CN molecules, which are not linear. For example, XCN NCCX The larger inductive effect of Cl vs Br, and the large steric effect of Br vs Cl could account for the fact that the frequency difference between the vapor and neat phases for the CN stretching frequency is 47 cm1 for ClCN and 13 cm1 for BrCN. Table 2.2 lists IR data for acetonitrile 1% wt.=vol. in various solvents (12). The solvents are numbered 1±15 in Table 2.2. The CN stretching vibration and the combination tone CC stretching plus symmetric CH3 bending are in Fermi resonance (FR). The CN stretching vibration has been corrected for FR in each of the solvents. Figure 2.2 shows a plot for unperturbed nCN for acetonitrile vs AN, where AN is the solvent acceptor number (12). This plot shows that the nCN frequency does not correlate well with AN, especially the AN value for dimethyl sulfoxide (DMSO). Figure 2.3 shows plots of nCN (uncorrected and corrected for FR) vs (nCN in methyl alcohol) minus (nCN in solvent). Both plots are linear, and any set of data plotted in this manner yields a linear mathematical relationship (12). The plots clearly show that FR causes nCN to occur at lower frequency due to resonance with the nCC d sym. CH3 combination tone (CT). It should be noted that the numbering sequence is different in both plots. The extent of FR interaction between nCN and CT (nCC d sym. CH3) is altered in each solvent system because nCCandd sym. CH3, as well as nCN are affected differently. Table 2.3 compares IR nCN stretching frequencies for benzonitrile 10 wt.=vol.% and 1 wt.=vol.% vs those for 1% wt.=vol. acetonitrile nCN frequencies corrected for FR in different solvents (12). This table shows that nCN for benzonitrile occurs at lower frequency than nCN for acetonitrile (corrected for FR), from 27 to 34 cm1 in these solvents. The shift to lower frequency in the case of benzonitrile is due to resonance of the CN group with the p system of the phenyl group. Table 2.3 also shows that nCN for benzonitrile occurs at lower frequency in solution at 10 wt.=vol.% than at 1 wt.=vol.%. These data are plotted in Figure 2.4. These plots indicate that at higher wt.=vol.% solute there is some dipolar interaction between solute molecules, which lowers the nCN frequency. Figure 2.5 is a plot of unperturbed nCN, (1% wt.=vol. acetonitrile), cm1 vs nCN for benzonitrile (1% wt.=vol. benzonitrile), cm1 where each compound has been recorded individually in the same solvent. Point 1 for hexane does not ®t the essentially linear relation- ship. Table 2.4 lists IR data for the nCN frequency for 4-cyanobenzaldehyde in 0 to 100 mol% CHCl3=CCl4 solutions (1 wt.=vol.% solutions). Figure 2.6 shows a plot of nCN for 4- cyanobenzaldehyde vs mol% CHCl3=CCl4 (13). The nCN mode increases in frequency as the mole % CHCl3=CCl4 increases to 45%. It then decreases in frequency to 75%, after which nCN frequency is relatively constant. The nCN frequency for 4-cyanobenzaldehyde is higher in frequency in CHCl3 solution than in CCl4 solution, and this same observation has been noted for 30 Nitriles, Isonitriles, and Dialkyl Cyanamides benzonitrile (12, 13). The behavior of nCO for 4-cyanobenzonitrile has already been discussed; furthermore, its frequency increases as the reaction ®eld is increased (14). Figure 2.6 then shows that nCN as well as nCO are affected, although not in the same manner.