Alcohol hybrid orbitals

Abstraction of an acetylenic proton gives a carbanion that has the lone pair of electrons in the sp hybrid orbital. Electrons in this orbital are close to the nucleus, and there is less charge separation than in carbanions with the lone pair in sp2 or sp3 hybrid orbitals. Ammonia and alcohols are included for comparison note that acetylene can be deprotonated by the amide ( NH2) ion, but not by an alkoxide ion ( OR). 

The C-O single bond of a carboxylic acid is shorter than the C—O single bond of an alcohol. This can be explained by looking at the hybridization of the respective carbon atoms. In the alcohol, the carbon is sp hybridized, whereas in the carboxylic acid the carbon is sp hybridized. As a result, the higher percent -character in the sp hybrid orbital shortens the C-O bond in the carboxylic acid. 

Alcohols have the —OH functional group attached to a tetrahedral carbon atom, that is, a carbon atom with single bonds to four other atoms. The carbon atom in the functional group is sp hybridized. The oxygen atom is likewise sp hybridized two of the hybrid orbitals form a bonds, whereas the other two hold lone pairs of electrons. 

Like the carbon atom in methane and the nitrogen atom in methylamine, the oxygen atom in methanol (methyl alcohol) and many other organic mole-cnles can he described as sp -hybridized. The C-O-H bond angle in methanol is 108.5°, very close to the 109.5° tetrahedral angle. Two of the fonr sp hybrid orbitals on oxygen are occnpied by nonhonding electron lone pairs, and two are used to form bonds. 

In both alcohols and ethers, the C—O O bond is formed by the overlap of two sp hybrid orbitals, one on the oxygen atom and one on the carbon atom. The C—O bond is slightly shorter, 142 pm, than the C—C bond, 143 pm. The lone pair electrons in both alcohols and ethers are directed toward the corners of a tetrahedron. 

The bond polarities of aryl-halogen and aryl-oxygen bonds compared to those in alkyl hahdes and alcohols provide a measure of the effect of the sp hybrid orbital on the O bond and of the resonance interactions between the nonbonded electrons of the substituent and the aromatic ring. The dipole moment of chlorobenzene is smaller than the dipole moment of chlorocyclohexane. 

The carbon that bears the functional group is sp hybridized m alcohols and alkyl halides Figure 4 1 illustrates bonding m 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 sp hybridized carbon by a ct bond Carbon-halogen bond distances m alkyl halides increase m the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm)... 

The triaLkoxy(aryloxy)boranes are typically monomeric, soluble in most organic solvents, and dissolve in water with hydrolysis to form boric acid and the corresponding alcohol and phenol. Although the rate of hydrolysis is usually very fast, it is dependent on the bulk of the alkyl or aryl substituent groups bonded to the boron atom. Secondary and tertiary alkyl esters are generally more stable than the primary alkyl esters. The boron atom in these compounds is in a trigonal coplanar state with bond hybridization. A vacantp orbital exists along the threefold axis perpendicular to the BO plane. 

The capture of carbocations by alcohols involves a similar donation of a lone pair of electrons on oxygen to the vacant 2p atomic orbital of the sp2-hybridized, sextet carbocation. Note that charge must be conserved so the first formed product is a positively charged oxonium ion. 

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