The Carbonyl Group

Description of the Carbonyl Group

It is somewhat misleading to write the carbonyl group as a covalent C=O double bond. The difference between the electronegativities of carbon and oxygen is large enough to make the C=O bond moderately polar. As a result, the carbonyl group is best described as a hybrid of the following resonance structures.

We can represent the polar nature of this hybrid by indicating the presence of a slight negative charge on the oxygen (d^-) and a slight positive charge (d⁺) on the carbon of the C=O double bond.

Reactions at the Carbonyl Group

Reagents that attack the electron-rich d ^- end of the C=O bond are called electrophiles (literally, "lovers of electrons"). Electrophiles include ions (such as H⁺ and Fe³⁺) and neutral molecules (such as AlCl₃ and BF₃) that are Lewis acids, or electron-pair acceptors. Reagents that attack the electron-poor d⁺ end of this bond are nucleophiles (literally, "lovers of nuclei"). Nucleophiles are Lewis bases (such as NH₃ or the OH^- ion).

The polarity of the C=O double bond can be used to explain the reactions of carbonyl compounds. Aldehydes and ketones react with a source of the hydride (H^-) ion because the H^- ion is a Lewis base, or nucleophile, that attacks the d⁺ end of the C=O bond. When this happens, the two valence electrons on the H^- ion form a covalent bond to the carbon atom. Since carbon is tetravalent, one pair of electrons in the C=O bond is displaced onto the oxygen to form an intermediate with a negative charge on the oxygen atom.

This alkoxide ion can then remove an H⁺ ion from water to form an alcohol.

Common sources of the H^- ion include lithium aluminum hydride (LiAlH₄) and sodium borohydride (NaBH₄). Both compounds are ionic.

The aluminum hydride (AlH₄^-) and borohydride (BH₄^-) ions act as if they were complexes between an H^- ion, acting as a Lewis base, and neutral AlH₃ or BH₃ molecules, acting as a Lewis acid.

LiAlH₄ is such as good source of the H^- ion that it reacts with the H⁺ ions in water or other protic solvents to form H₂ gas. The first step in the reduction of a carbonyl with LiAlH₄ is therefore carried out using an ether as the solvent. The product of the hydride reduction reaction is then allowed to react with water in a second step to form the corresponding alcohol.

NaBH₄ is less reactive toward protic solvents, which means that borohydride reductions are usually done in a single step, using an alcohol as the solvent.

Carboxylic Acids and Carboxylate Ions

When one of the substituents on a carbonyl group is an OH group, the compound is a carboxylic acid with the generic formula RCO₂H. These compounds are acids, as the name suggests, which form carboxylate ions (RCO₂^-) by the loss of an H⁺ ion.

The carboxylate ion formed in this reaction is a hybrid of two resonance structures.

Resonance delocalizes the negative charge in the carboxylate ion, which makes this ion more stable than the alkoxide ion formed when an alcohol loses an H⁺ ion. By increasing the stability of the conjugate base, resonance increases the acidity of the acid that forms this base. Carboxylic acids are therefore much stronger acids than the analogous alcohols. The value of K_a for a typical carboxylic acid is about 10^-5, whereas alcohols have values of K_a of only 10^-16.

Carboxylic acids were among the first organic compounds to be discovered. As a result, they have well-established common names that are often derived from the Latin stems of their sources in nature. Formic acid (Latin formica, "an ant") and acetic acid (Latin acetum, "vinegar") were first obtained by distilling ants and vinegar, respectively. Butyric acid (Latin butyrum, "butter") is found in rancid butter, and caproic, caprylic, and capric acids (Latin caper, "goat") are all obtained from goat fat. A list of common carboxylic acids is given in the table below.

Common Carboxylic Acids

Common Name

Formula

Solubility in H2O (g/100 mL)

Saturated carboxylic acids and fatty acids

Formic acid

HCO2H

Acetic acid

CH3CO2H

Proprionic acid

CH3CH2CO2H

Butyric acid

CH3(CH2)2CO2H

Caproic acid

CH3(CH2)4CO2H

0.968

Caprylic acid

CH3(CH2)6CO2H

0.068

Capric acid

CH3(CH2)8CO2H

0.015

Lauric acid

CH3(CH2)10CO2H

0.0055

Myristic acid

CH3(CH2)12CO2H

0.0020

Palmitic acid

CH3(CH2)14CO2H

0.00072

Stearic acid

CH3(CH2)16CO2H

0.00029

Unsaturated fatty acids

Palmitoleic acid

CH3(CH2)5CH=CH(CH2)7CO2H

Oleic acid

CH3(CH2)7CH=CH(CH2)7CO2H

Linoleic acid

CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H

Linolenic acid

CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H

Naming Carboxylic Acids

The systematic nomenclature of carboxylic acids is easy to understand. The ending -oic acid is added to the name of the parent alkane to indicate the presence of the CO₂H functional group.

Unfortunately, because of the long history of their importance in biology and biochemistry, you are more likely to encounter these compounds by their common names.

Formic acid and acetic acid have a sharp, pungent odor. As the length of the alkyl chain increases, the odor of carboxylic acids becomes more unpleasant. Butyric acid, for example, is found in sweat, and the odor of rancid meat is due to carboxylic acids released as the meat spoils.

The solubility data in the table above show that carboxylic acids also become less soluble in water as the length of the alkyl chain increases. The CO₂H end of this molecule is polar and therefore soluble in water. As the alkyl chain gets longer, the molecule becomes more nonpolar and less soluble in water.

Dicarboxylic and Tricarboxylic Acids

Compounds that contain two CO₂H functional groups are known as dicarboxylic acids. A number of dicarboxylic acids (see table below) can be isolated from natural sources. Tartaric acid, for example, is a by-product of the fermentation of wine, and succinic, fumaric, malic, and oxaloacetic acid are intermediates in the metabolic pathway used to oxidize sugars to CO₂ and H₂O.

Common Dicarboxylic Acids

		oxalic acid
		malonic acid
		malic acid
		succinic acid
		maleic acid
		fumaric acid
		tartaric acid
		oxaloacetic acid

Several tricarboxylic acids also play an important role in the metabolism of sugar. The most important example of this class of compounds is the citric acid that gives so many fruit juices their characteristic acidity.

Esters

Carboxylic acids (-CO₂H) can react with alcohols (ROH) in the presence of either acid or base to form esters (-CO₂R). Acetic acid, for example, reacts with ethanol to form ethyl acetate and water.

This isn't an efficient way of preparing an ester, however, because the equilibrium constant for this reaction is relatively small (K_c 3). Chemists tend to synthesize esters in a two-step process. They start by reacting the acid with a chlorinating agent such as thionyl chloride (SOCl₂) to form the corresponding acyl chloride.

They then react the acyl chloride with an alcohol in the presence of base to form the ester.

The base absorbs the HCl given off in this reaction, thereby driving it to completion.

As might be expected, esters are named as if they were derivatives of a carboxylic acid and an alcohol. The ending -ate or -oate is added to the name of the parent carboxylic acid, and the alcohol is identified using the "alkyl alcohol" convention. The following ester, for example, can be named as a derivative of acetic acid (CH₃CO₂H) and ethyl alcohol (CH₃CH₂OH).

Or it can be named as a derivative of ethanoic acid (CH₃CO₂H) and ethyl alcohol (CH₃CH₂OH).

The term ester is commonly used to describe the product of the reaction of any strong acid with an alcohol. Sulfuric acid, for example, reacts with methanol to form a diester known as dimethyl sulfate.

Phosphoric acid reacts with alcohols to form triesters such as triethyl phosphate.

Compounds that contain the CO₂R functional group might therefore best be called carboxylic acid esters, to indicate the acid from which they are formed.

Carboxylic acid esters with low molecular weights are colorless, volatile liquids that often have a pleasant odor. They are important components of both natural and synthetic flavors (see figure below).

Fats and Oils

Long-chain carboxylic acids such as stearic acid [CH₃(CH₂)₁₆CO₂H] are called fatty acids because they can be isolated from animal fats. These fatty acids are subdivided into the two categories on the basis of whether they contain C=C double bonds: saturated fatty acids and unsaturated fatty acids.

There are four important unsaturated fatty acids. One of them is a derivative of palmitic acid, and is known as palmitoleic acid.

The other three are derivatives of stearic acid. Oleic acid has a single C=C double bond in the center of the fatty acid chain.

Linoleic acid, has another C=C double bond in the nonpolar half of the fatty acid chain.

Linolenic acid has one more C=C double bond in the same half of the fatty acid chain.

There are several regularities in the chemistry of these unsaturated fatty acids. First, they contain cis double bonds. Second, the double bonds are always isolated from each other by a CH₂ group.

So much attention is paid to the structures of the fatty acids in discussions of these compounds that it is easy to miss an important point: Free fatty acids are seldom found in nature. They are usually tied up with alcohols to form esters (RCO₂R). The most abundant of these esters is the triester formed when a molecule of glycerol (HOCH₂CHOHCH₂OH) combines with three fatty acids, as shown in the figure below. These lipids have been known by a variety of names, including fat, neutral fat, glyceride, triglyceride, and triacylglycerol.

Most animal fats are complex mixtures of different triglycerides. As the percentage of unsaturated fatty acids increases, the melting point of these triesters decreases until they eventually become an oil at room temperature. Beef fat, which is roughly one-third unsaturated fatty acids, is a solid. Olive oil, which is roughly 80% unsaturated, is a liquid.

The effect of unsaturated fatty acids on the melting point of a triglyceride can be understood by recognizing that the cis C=C double bond introduces a rigid 30 bend in the hydrocarbon chain. This bend or "kink" increases the average distance between triglyceride molecules, which decreases the van der Waals interactions between neighboring molecules. Thus, the introduction of unsaturated fatty acids into a triglyceride increases the fluidity of the lipid. The table below compares the relative abundance of the common fatty acids in a typical animal fat (butter) and a vegetable oil (olive oil).

Relative Abundance of Fatty Acids in a Typical Fat and a Typical Oil

Fats and oils are used by living cells for only one purposeto store energy. They are a far more efficient storage system than carbohydrates such as glycogen or starch because they give off between two and three times as much energy when they are burned. (Glycogen releases 15.7 kJ per gram of carbohydrate consumed, whereas lipids give approximately 40 kJ/g.) This explains why the seeds of many plants are relatively rich in oils, which provide the energy the seed needs to grow until the leaves can begin to produce energy by photosynthesis.

The average human contains enough fat (21% of the body weight for men, 26% for women) to provide the energy they need to survive for up to 3 months. But there is only enough glycogen stored in the human body at any time to provide enough energy for one day. Thus, glycogen is only used for the short-term storage of food energy. In "times of plenty," the body stores energy in the form of fat to compensate for "times of shortage" to come.