Aldehydes and ketones are the oxygen analogs of alkenes. Figure 1 compares the structures of formaldehyde, the simplest aldehyde, with that of ethene, the simplest alkene, as well as the structure of acetone, the simplest ketone, with 2-methylpropene, its hydrocarbon analog.
The C=O double bond is common to aldehydes and ketones. It is called the carbonyl group. In aldehydes, at least one of the atoms attached to the carbonyl carbon must be a hydrogen atom. The other may be either a hydrogen or a carbon atom. In ketones both atoms attached to the carbonyl carbon must be carbon atoms.
Like alkenes, aldehydes and ketones are planar, which is to say that the oxygen atom and the other two atoms attached to the carbonyl carbon lie in the same plane as that carbon.
Because of the electronegativity difference between carbon and oxygen of the carbonyl group, aldehydes and ketones are more polar than their hydrocarbon analogs. Comparison of the boiling points of the compounds shown in Figure 1 indicates that the intermolecular forces between molecules of formaldehyde are much greater than they are in ethene. The same holds true for acetone and 2-methylpropene.
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The difference in electronegativity between the carbon and oxygen atoms gives rise to a bond dipole. The intermolecular interactions between molecules of formaldehyde are called dipole-dipole interactions. They are shown schematically from two perspectives in Figure 2.
In View 1, the bond dipoles are represented by the d+ and d- symbols. The d+ symbol means that in comparison to the corresponding carbon atom in ethene, the carbonyl carbon in formaldehyde is electron deficient; the oxygen has pulled some electron density away from the carbon.
In View 2 the partial positive and negative charges are omitted and dashed lines are used to indicate the intermolecular Coulombic attractions that hold one molecule of formaldehyde next to another, i.e. the dipole-dipole interactions. Since the intermolecular interactions are stronger for formaldehyde than for ethene, it takes more energy to overcome them. Consequently formaldehyde has a higher boiling point than ethene. The animation below compares the effect of increasing temperature on the intermolecular interactions between two molecules of ethene with its effect on two molecules of formaldehyde. The disappearance of the structures indicates that they have boiled away. N.B. You may stop the animation at any point by clicking and dragging the mouse simultaneously. When you release the mouse, the animation will resume.
Aldehydes and ketones may be prepared from or converted into alcohols. These conversions all involve oxidation-reduction reactions. Consider the reactions shown in Figure 3.
The reagents necessary to effect these transformations are not important for the purposes of this discussion. Acetaldehyde may be reduced to ethanol, or it may be formed from ethanol by an oxdation reaction. In the former case, the oxidation level of the carbonyl carbon is reduced from +1 to -1. In the latter it increases from -1 to +1. When formaldehyde is converted into ethanol, the oxidation level of the carbonyl carbon is reduced from 0 to -1.
In the reduction of acetaldehyde to ethanol, the carbonyl carbon acquires a hydrogen atom. Given the polarization of the carbonyl group, it should not be surprising that reagent which delivers the hydrogen to the electron deficient carbonyl carbon delivers it as a hydride ion, :H-. Chemists have developed a multitude of reagents that serve as hydride ion donors. We will consider two, sodium borohydride, NaBH4, and lithium aluminum hydride, LiAlH4. The latter compound is a highly reactive, extremely powerful reducing agent. It is capable of reducing the carbonyl group in aldehydes, ketones, carboxylic acids, esters, amides, and acid halides. NaBH4 is a less reactive, more selective, reagent. It will convert aldehydes and ketones into alcohols, but it will not reduce carboxylic acids, esters, or amides. Equation 1 depicts the reduction of aldehydes and ketones by NaBH4 in general terms. The reaction involves addition of a hydride ion to the carbonyl carbon and transfer of a proton from the oxygen of the solvent to the
carbonyl oxygen. Note that each molecule of sodium borohydride is capable of delivering four hydride ions.
Equations 2-4 present three examples of the utility of NaBH4 as a reducing agent.
Equation 2 illustrates the reduction of acetophenone, a simple ketone. Since acetophenone is achiral, the product, 1-phenylethanol, is obtained as a racemic mixture: the hydride ion is delivered to the top face of the carbonyl group and the bottom face with equal ease.
Camphor is a chiral molecule. Consequently the top face of the carbonyl group is not identical to the bottom face. In fact, the product mixture from reaction 3 consists of 86% exo-borneol and 14% endo-borneol. Apparently the bottom face of the carbonyl group is more accessible to the reagent than is the top face.
Reaction 4 was carried out as part of an attempted synthesis of taxol. This example illustrates the selectivity of NaBH4. It reduces the aldehyde group, but does not react with the alkene or the carbonyl group of the ester. LiAlH4 would not be suitable for this transformation.
A possible mechanism for the reduction of a carbonyl group by a hydride reducing agent is shown in Figure 4.
This picture suggests that the borohydride anion delivers a hydride ion to the carbonyl carbon at the same time that a proton is transferred from the solvent to the carbonyl oxygen. As the B-H bond breaks, a new bond between the boron and the oxygen atom of the solvent is formed. While the timing of the bond-making and bond-breaking steps is uncertain, the mechanism has similarities to the mechanism for the esterification of alcohols. The important point is that the hydrogen atom that ends up bonded to the carbonyl carbon comes from the NaBH4, while the hydroxyl hydrogen is derived from the solvent.