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.
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.
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.
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.
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. TheCO2H
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.
Compounds that contain twoCO2H 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.
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.
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 (Kc 
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)
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).
| LiAlH4: | [Li+][AlH4-] | ||
| NaBH4: | [Na+][BH4-] |
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 Ka
for a typical carboxylic acid is about 10-5, whereas
alcohols have values of Ka of only
10-16.
| 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 | ||||||||||||
CO2H
functional group. | HCO2H | Methanoic acid | ||
| CH3CO2H | Ethanoic acid | ||
| CH3CH2CO2H | Propanoic acid |
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. 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 |
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. 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).
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)
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