Bonding in Alcohols and Alkyl Halides

The carbon that bears the functional group is sp -hybridized in alcohols and alkyl halides. 

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 ). Alcohols and alkyl halides are polar molecules. The dipole moments of methanol and chloromethane are very similar to each other and to water. 

Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. Why  

Electrostatic potential maps of methanol and chloromethane. The electrostatic potential is most negative near oxygen in methanol and near chlorine in chloromethane. The most positive region is near the 0—H proton in methanol and near the methyl group in chloromethane. 

Relatively simple notions of attractive forces between opposite charges are sufficient to account for many of the properties of chemical substances. You will find it helpful to keep the polarity of carbon-oxygen and carbon-halogen bonds in mind as we develop the properties of alcohols and alkyl halides in later sections. 

Alcohols and alkyl halides are polar molecules. The dipole moments of methanol and chlo-romethane are very similar to each other and to water. 

6 Physical Properties of Alcohols and Alkyl Halides Intermolecular Forces 

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The carbon that bears the functional group is 5/r -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 connected to 5/r -hybridized carbon by a 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). 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

Solubility in Water. Alkyl halides and alcohols differ markedly from one another in their solubility in water. All alkyl halides are insoluble in water, but low-molecular-weight alcohols (methyl, ethyl, n-propyl, and isopropyl) are soluble in water in all proportions. Their ability to participate in inter-molecular- hydrogen bonding not only affects the boiling points of alcohols, but also enhances their water solubility. Flydrogen-bonded networks of the type shown in Figure 4.5, in which alcohol and water molecules associate with one another, replace the alcohol-alcohol and water-water hydrogen-bonded networks present in the pure substances. 

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