4. 4 Bonding in Alcohols and Alkyl halides Lone-pair orbitals FIGURE 4. I hybrid- used in bonding are the 1s orbitals of hydrogen hybridized orbitals and oxygen. (b)Th gles at carbon and re close to tetrahedral, and the carbon-oxygen o bond is about 10 pm shorter than a arbon-carbon single bond 8.5° -o bond distance 142 pm 4. 4 BONDING IN ALCOHOLS AND ALKYL HALIDES The carbon that bears the functional group is sp-hybridized in alcohols and alkyl halides Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C-O-H angle. A similar orbital hybridization model applies to alkyl halides, with the halogen substituent connected to sp-hybridized carbon by a o bond Carbon-halogen bond distances in alkyl halides increase in the order C-F(140 pm)<C-Cl(179 pm)<C-Br (197 pm)<C-I(216 pm) Carbon-oxygen and carbon-halogen bonds are polar covalent bonds, and carbon bears a partial positive charge in alcohols (*c-o%)and in alkyl halides (*c-X) The presence of these polar bonds makes alcohols and alkyl halides polar molecules. The dipole moments of methanol and chloromethane are very similar to each other and to water. > O、xCH3 H H H2C Chloromethane PROBLEM 4.4 Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. Why Figure 4.2 shows the distribution of electron density methanol and chloromethane. Both are similar in that the sites of highest electrostatic potential (red) are near the electronegative atoms-oxygen and chlorine. The polarization of the bond FIGURE 4.2 Electro methanol and chloro- methane. The most pos charged ones red. The elec- trostatic potential is most methanol and near chlori Methanol(CH3OH) Chloromethane(CHCl) in chloromethane Back Forward Main Menu Study Guide ToC Student OLC MHHE Website4.4 BONDING IN ALCOHOLS AND ALKYL HALIDES The carbon that bears the functional group is sp3 -hybridized in alcohols and alkyl halides. Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C±O±H angle. A similar orbital hybridization model applies to alkyl halides, with the halogen substituent connected to sp3 -hybridized carbon by a bond. Carbon–halogen bond distances in alkyl halides increase in the order C±F (140 pm) C±Cl (179 pm) C±Br (197 pm) C±I (216 pm). Carbon–oxygen and carbon–halogen bonds are polar covalent bonds, and carbon bears a partial positive charge in alcohols (C±O) and in alkyl halides (C±X). The presence of these polar bonds makes alcohols and alkyl halides polar molecules. The dipole moments of methanol and chloromethane are very similar to each other and to water. PROBLEM 4.4 Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. Why? Figure 4.2 shows the distribution of electron density in methanol and chloromethane. Both are similar in that the sites of highest electrostatic potential (red) are near the electronegative atoms—oxygen and chlorine. The polarization of the bonds Water (  1.8 D) H O H Chloromethane (  1.9 D) CH3 Cl Methanol (  1.7 D) O H3C H 4.4 Bonding in Alcohols and Alkyl Halides 129 C H H H C H O O H H H H Lone-pair orbitals (a) (b) σ bond C±O±H angle
108.5 C±O bond distance
142 pm FIGURE 4.1 Orbital hybrid￾ization model of bonding in methanol. (a) The orbitals used in bonding are the 1s orbitals of hydrogen and sp3 - hybridized orbitals of carbon and oxygen. (b) The bond an￾gles at carbon and oxygen are close to tetrahedral, and the carbon–oxygen bond is about 10 pm shorter than a carbon–carbon single bond. Methanol (CH3OH) Chloromethane (CH3Cl) FIGURE 4.2 Electro￾static potential maps of methanol and chloro￾methane. The most posi￾tively charged regions are blue, the most negatively charged ones red. The elec￾trostatic potential is most negative near oxygen in methanol and near chlorine in chloromethane. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website