Ester
In
chemistry, esters are
organic compounds in which an
organic group replaces a
hydrogen atom in an
oxygen acid. An oxygen acid is an acid whose
molecule has an -
group from which the hydrogen can dissociate as an H+ ion.
The most common esters are the carboxylate esters, where the acid in question is a
carboxylic acid. For example, if the acid is
acetic acid, the ester is called an acetate. Esters may also be formed with inorganic acids; for example,
dimethyl sulfate is an ester, and sometimes called "
sulfuric acid, dimethyl ester".
Encyclopedia
In
chemistry,
esters are
organic compounds in which an
organic group replaces a
hydrogen atom in an
oxygen acid. An oxygen acid is an acid whose
molecule has an
- group from which the hydrogen can dissociate as an H
+ ion.
The most common esters are the
carboxylate esters, where the acid in question is a
carboxylic acid. For example, if the acid is
acetic acid, the ester is called an acetate. Esters may also be formed with inorganic acids; for example,
dimethyl sulfate is an ester, and sometimes called "
sulfuric acid, dimethyl ester".
Esters are named similarly to
salts; although they don't really have cations and anions, the terminology follows the same pattern: a more electropositive part followed by a more electronegative part.
An ester can be thought of as a product of a
condensation reaction of an acid and an
alcohol , although there are other ways to form esters. Condensation is a type of
chemical reaction in which two
molecules are joined together and eliminate a small molecule, in this case two
-OH groups are joined eliminating a
water molecule. A condensation reaction to form an ester is called esterification. Esterification can be catalysed by the presence of H
+ ions. Sulfuric acid is often used as a
catalyst for this reaction. The name ester is derived from the German
Essig-Ä
ther, an old name for acetic acid ethyl ester .
Naming of esters
Esters can be produced by an equilibrium reaction between an
alcohol and a
carboxylic acid. The ester is named according to the
alkyl group and then the
alkanoate which make it up. For example, the reaction between
methanol and
butyric acid yields the ester
methyl butyrate C
3H
7-COO-CH
3 . The simplest ester is H-COO-CH
3 .
For esters derived from the simplest carboxylic acids, the traditional names are recommended by IUPAC,
viz, formate, acetate, propionate, butyrate, though out of these only acetate may carry further substituents. For esters from higher acids, the alkane name with an
-oate ending is generally preferred, e.g., hexanoate. Common esters of aromatic acids include benzoates such as
methyl benzoate, and
phthalates, with substitution allowed in the name.
Physical properties
Esters participate in
hydrogen bonds as hydrogen-bond acceptors, but cannot act as hydrogen-bond donors, unlike their parent alcohols. This ability to participate in hydrogen bonding makes them more
water-soluble than their parent
hydrocarbons. However, the limitations on their hydrogen bonding also make them more hydrophobic than either their parent alcohols or parent acids. Their lack of hydrogen-bond-donating ability means that ester
molecules cannot hydrogen-bond to each other, which makes esters generally more volatile than an
carboxylic acid of similar molecular weight. This property makes them very useful in organic analytical chemistry: unknown organic acids with low volatility can often be esterified into a volatile ester, which can then be analysed using
gas chromatography,
gas liquid chromatography, or
mass spectrometry.
Many esters have distinctive odors, which has led to their use as artificial flavorings and fragrances. For example:
| allyl hexanoate | pineapple |
| benzyl acetate | pear, strawberry, jasmine |
| bornyl acetate | pine tree flavor |
| butyl butyrate | pineapple |
| ethyl acetate | peach, pineapple, raspberry |
| ethyl butyrate | banana, pineapple, strawberry |
| ethyl hexanoate | strawberry |
| ethyl cinnamate | cinnamon |
| ethyl formate | lemon, rum, strawberry |
| ethyl heptanoate | apricot, cherry, grape, raspberry |
| ethyl isovalerate | apple |
| ethyl lactate | grape |
| ethyl nonanoate | grape |
| ethyl valerate | apple |
| geranyl acetate | geranium |
| geranyl butyrate | cherry |
| geranyl pentanoate | apple |
| isobutyl acetate | cherry, raspberry, strawberry |
| isobutyl formate | raspberries |
| isopentyl acetate | pear,banana |
| linalyl acetate | lavender, sage |
| linalyl butyrate | peach |
| linalyl formate | apple, peach |
| menthyl acetate | peppermint |
| methyl anthranilate | grape, jasmine |
| methyl benzoate | fruity, ylang ylang |
| methyl benzyl acetate | cherry |
| methyl butyrate | pineapple, apple |
| methyl cinnamate | strawberry |
| methyl pentanoate | flowery |
| methyl phenyl acetate | honey |
| methyl salicylate | root beer, wintergreen, Germolene™ and Ralgex™ ointments |
| nonyl caprylate | orange |
| octyl acetate | fruity-orange |
| octyl butyrate | parsnip |
| pentyl acetate | apple, banana |
| pentyl butyrate | apricot, pear, pineapple |
| pentyl hexanoate | apple, pineapple |
| pentyl pentanoate | apple |
| propyl isobutyrate | rum |
| terpenyl butyrate | cherry |
Ester synthesis
Esters can be prepared in the laboratory in several ways:
Ester reactions
Esters react in a number of ways:
- Esters may undergo hydrolysis - the breakdown of an ester by water. This process can be catalyzed both by acids and bases. The base-catalyzed process is called saponification. The hydrolysis yields an alcohol and a carboxylic acid or its carboxylate salt.
- Esters also react if heated with primary or secondary amines, producing amide
...
s.
See also
External links
-
- A simple guide to naming and making Esters, as well as the Chemistry behind it.
References
