James Richard Fromm
Esters have a very sweet fruity smell. Naturally occurring esters are found in fruits. An ester is a product of the reaction of an acid (usually organic) and an alcohol (the hydrogen of the acid R-COOH is replaced by an alkyl group R'). Esters mainly result from the condensation (a reaction that produces water) of a carboxylic acid and an alcohol. The process is called esterification. This reaction can be catalyzed by the presence of H+ ions. Sulphuric acid, H2SO4, is often used as a catalyst for this reaction. The name ester is derived from the German Essig-Aether, an old name for acetic acid ethyl ester (ethyl acetate). Esters have the general formula R-COOR',
Esters are named in the same manner as salts (although esters and salts have completely different properties): two-word names are used. Note that in the general formula, R-COOR' (the carbon is double-bonded to one oxygen atom and single-bonded to another), the alkyl group (R') is always attached to an oxygen atom. This alkyl group (R') is named as the first word of the two-word name. The second word is derived by adding the ending -oate to the stem of the acid name (-oic in the acid name is replaced by -oate).
A reversible reaction between an alcohol and a carboxylic acid causes loss of water and the formation of an ester:
Alcohol + Carboxylic Acid Ester + Water
R'OH + RCOOH RCOOR' + H2O.
Esters are named as derivatives of the carboxylic acid from which they are formed. Condensation of ethanoic acid with methanol will produce methyl ethanoate. As stated above the ending of the acid -oic is changed to -oate, much as if the ester were a salt of the acid. The esterification reactions are generally easily reversible by addition of water; the reverse reaction is called the hydrolysis of the ester and proceeds in the presence of aqueous base.
CH3OH + CH3COOH CH3COOCH3 + H2O
methanol + ethanoic acid methyl ethanoate + water
The esterification process will proceed more nearly to completion if a substance which removes water without reacting with the acid or the alcohol is added to the reaction, such as sulfuric acid. For example, the reaction between ethanoic acid and ethanol produces the ester ethyl ethanoate.
CH3COOH + CH3CH2OH H2SO4 CH3COOCH2CH3+ H2O
ethanoic acid + ethanol H2SO4 ethyl ethanoate + water
The concentrated H2SO4 removes water from the products and is a dehydrating agent. Most esters have very pleasant odors (see below). Many flavoring and scenting agents are made from esters. Esters are volatile liquids which are not ionized and they are soluble in organic solvents but not in water.
The simplest ester is H-COO-CH3 (methyl methanoate).
In the laboratory, methyl methanoate can be produced by the condensation reaction of methanol and methanoic acid, as follows:
HCOOH + CH3OH HCOOCH3 + H2O
methanoic acid + methanol methyl methanoate + water
Industrial methyl methanoate, however, is usually produced by the combination of methanol and carbon monoxide in the presence of a strong base:
CH3OH + CO HCOOCH3
methanol + carbon monoxide methyl methanoate
As shown in the diagram above the hydrogen atom on the right can be replaced with a CH3 group or additional CH2 units, producing other methyl esters, such as:
For esters derived from the simplest carboxylic acids, the traditional names are recommended by IUPAC, such as, 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, with substitution allowed in the name.
Common names of esters are derived from the organic acid and the alcohol from which they are derived. For example, when acetic acid reacts with ethyl alcohol, the ester formed is called ethyl acetate. The IUPAC name is different. Acetic acid is called ethanoic acid by the IUPAC rules. Thus the ester formed is called ethyl ethanoate. IUPAC names ester from two words: first from the prefix of the alcohol and the second from the name of the acid.
ethyl alcohol + acetic acid ethyl acetate + water (common names)
ethanol + ethanoic acid ethyl ethanoate + water (IUPAC names)
General formula RCOOR'. In the following example R is H- while R' is a methyl group -CH3. This will produce the simplest of esters. The reaction of methanoic acid HCOOH and methanol CH3OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid, in this case it is methanoate. Thus HCOOCH3 is named methyl methanoate by the rules laid down for IUPAC nomenclature for esters. The common name of this ester is methyl formate.
methyl methanoate, methyl formate HCOOCH3
formic acid + methyl alcohol methyl formate + water (common names)
methanoic acid + methanol methyl methanoate + water (IUPAC names)
General formula RCOOR'. In the following example R is H- while R' is an ethyl group -C2H5. The reaction of methanoic acid HCOOH and ethanol C2H5OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case ethyl, and the second name from the acid, in this case it is methanoate. Thus HCOOC2H5 is named ethyl methanoate by the rules laid down for IUPAC nomenclature for esters. The common name for this ester is ethyl formate.
ethyl methanoate, ethyl formate, HCOOC2H5
formic acid + ethyl alcohol ethyl formate + water (common names)
methanoic acid + ethanol ethyl methanoate + water (IUPAC names)
General formula RCOOR'. In the following example R is CH3- while R' is also a methyl group -CH3. The reaction of ethanoic acid CH3COOH and methanol CH3OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid, in this case it is ethanoate. Thus CH3COOCH3 is named methyl ethanoate by the rules laid down for IUPAC nomenclature for esters. The common name of this ester is methyl acetate.
methyl ethanoate, methyl acetate CH3COOCH3
acetic acid + methyl alcohol methyl acetate + water (common names)
ethanoic acid + methanol methyl ethanoate + water (IUPAC names)
General formula RCOOR'. In the following example R is CH3- while R' is an ethyl group -C2H5. The reaction of ethanoic acid CH3COOH and ethanol C2H5OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case ethyl, and the second name from the acid, in this case it is ethanoate. Thus CH3COOC2H5 is named ethyl ethanoate by the rules laid down for IUPAC nomenclature for esters. The common name of this ester is ethyl acetate.
ethyl ethanoate, ethyl acetate CH3COOC2H5
acetic acid + ethyl alcohol ethyl acetate + water (common names)
ethanoic acid + ethanol ethyl ethanoate + water (IUPAC names)
General formula RCOOR'. In the following example R is CH3-CH2- while R' is a methyl group -CH3. The reaction of propanoic acid CH3CH2COOH and methanol CH3OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid, in this case it is propanoate. Thus CH3CH2COOCH3 is named methyl propanoate by the rules laid down for IUPAC nomenclature for esters.
CH3CH2COOH + CH3OH CH3CH2COOCH3 and H2O
Propanoic Acid + Methanol Methyl Propanoate + Water
If you have become confused at this juncture you may not wish to continue!
In the following example particular exceptions have been made regarding the IUPAC rules. A six membered ring with alternating double and single bonds is 1,3,5-cyclohexatriene. However, this has been accepted simply as benzene. When an alcohol group is attached rather than call it 1,3,5-cyclohexatrienol, it is referred to as 2-hydroxybenzene, or phenol. Further when this molecule becomes a carboxylic acid with the inclusion of the -COOH group it can be referred to as 2-hydroxy-1,3,5-cyclohexatrienoic acid or 2-hydroxybenzenoic acid with the latter generally utilized as its "chemical" name. It also has a "common" of salicylic acid.
|benzene||2-hydroxybenzene||benzoic acid||2-hydroxybenzenoic acid|
When salicylic acid combines with methanol it becomes the ester known as methyl salicylate or oil of wintergreen. Methanol is also known as methyl alcohol and wood alcohol.
General formula RCOOR'. The reaction of salicylic acid C6H4(OH)CO2H and methanol CH3OH forms this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid. Thus C6H4(HO)CO2CH3 is named methyl salicylate by the exceptions laid down for IUPAC nomenclature for esters. The common name of this ester is methyl salicylate (oil of wintergreen).
|salicylic acid||methyl salicylate|
2-hydroxy-1,3,5-cyclohexatrienoic acid + methanol methyl 2-hydroxy-1,3,5-cyclohexatrienoate + water
2-hydroxybenzoic acid + methyl alcohol methyl-2-hydroxybenzoate + water
salicylic acid + methyl alcohol methyl salicylate + water
propyl pentanoate, propyl valerate
pentanoic acid + propanol propyl pentanoate + water
CH3(CH2)3COOH + CH3CH2CH2OH CH3CH2CH2CH2COOCH2CH2CH3 + H2O
valeric acid + propyl alcohol propyl valerate + water
Condensation of methanoic acid with ethanol will produce ethyl methanoate, an ester having the odor of rum.
Regardless which way you elect to write the formula:
H-COO-CH2-CH3 or CH3-CH2-COOH
The name is represented the same: ethyl methanoate.
|ethyl butanoate (apple)||octyl ethanoate (orange)|
CH2=CHCl-CH-COO-CH2-CH3 or CH3-CH2-COO-CH-CHCl=CH2
is named ethyl-2-chloro-3-butenoate.
Among the most important of the natural esters are fats such as lard, tallow, and butter, and oils such as linseed, cottonseed, and olive. These are made up of esters of such complex acids as palmitic, C15H31COOH, stearic, C17H35COOH, and oleic, C17H33COOH, with the trihydroxyl alcohol, glycerol, C3H5(OH)3.
Soaps are made by boiling natural fats and oils with strong bases such as sodium and potassium hydroxides. When animal fat is treated with sodium hydroxide, glycerol and sodium salts of the fatty acids, palmitic, stearic, and oleic acid, are formed. The reaction in the case of glyceryl stearate (the fat) is given by
(C17H35COO)3C3H5+ 3 NaOH 3 C17H35COONa + C3H5(OH)3
glyceryl stearate + sodium hydroxide sodium stearate + glycerol
To obtain the soap (the sodium salt of the fatty acid) free from glycerol and water, sodium chloride is added which causes the soap to precipitate, or "salt out". The soap, being lighter, collects as a crust on the top where it is removed, partially dried, and pressed into cakes for use.
Hydrolysis of esters : Esters break down into their respective organic acid and alcohol from which they are formed. This process is called hydrolysis.
When sodium hydroxide is added to an ester, say for example to ethyl ethanoate, a salt sodium ethanoate is formed along with ethyl alcohol. The reaction is shown below.
Hydrolysis of an ester with an alkaline solution like sodium hydroxide is known as saponification (soap making). This reaction is used in the preparation of soaps.
The above reaction is a test for checking if esters are present in any solution. Few drops of indicator phenolphthalein is added to a solution of ester and NaOH. The solution shows pink coloration. Heat the solution. When the ester has reacted completely the pink color will disappear.
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 a 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:
|Methyl Phenyl Ethanoate||C6H5-CH2-CO2-CH3||Honey|
|Methyl Salicylate||C6H4(OH)-CO2-CH3||Oil of Wintergreen|
|Ethyl Cinnamate||C6H5-CH=CH-CO2-CH2-CH3||Oil of Cinnamon|
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