Esters . They are organic compounds in which an organic alkyl group (symbolized by R ‘) replaces a hydrogen atom (or more than one) of an oxygenated acid. An oxo acid is an inorganic acid whose molecules have a hydroxyl group (OH -1 ) from which hydrogen (H) can dissociate as a hydrogen ion, hydron or commonly proton, (H + ). Etymologically, the word “ester” comes from the German Essig-Äther (vinegar ether), as ethyl acetate was formerly called.
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- 1 Functional groups
- 1 Obtaining
- 2 Naming of esters
- 3 Physical properties
- 4 Chemical properties
- 5 Distribution in nature
- 1 Acid hydrolysis:
- 2 Alkaline hydrolysis – Saponification
- 3 Transesterification
- 4 Reaction with organometallic
- 5 Reduction to alcohols and aldehydes
- 6 Claisen condensation
- 6 uses
- 7 Sources
- 8 External link
The case of esters consists of two chains separated by an oxygen. Each of these chains must be named separately and the name of the esters always consists of two separate words of the alkyl alkanoate type. The alkyl part of the name is given to the chain that does not contain the carbonyl group. The part of the alkanoate is given to the chain that has the carbonyl group. This procedure is used regardless of the size of the string. The position of the carbonyl group is what determines which is the chain of the alkanoate.
Because the carbonyl group in the esters must be at the end of the alkanoate chain, a locator number is not used.
The chain that is on the oxygen side can be linked by any of its carbon atoms, so in this case if it is not linked by the terminal carbon, the locator number must be used.
In the most common esters the acid in question is a carboxylic acid. For example, if the acid is acetic acid, the ester is called acetate. Esters can also be formed with inorganic acids, such as carbonic acid (originates carbonic esters), phosphoric acid (phosphoric esters), or sulfuric acid. For example, dimethyl sulfate is an ester, sometimes called “sulfuric acid dimethyl ester”.
Esters are formed by reaction between an acid and an alcohol. The reaction occurs with loss of water. Water has been determined to form from the OH of acid and the H of alcohol. This process is called esterification.
They can come from aliphatic or aromatic acids.
The nomenclature of the esters derives from the carboxylic acid and the alcohol from which it comes. Thus, in methyl ethanoate (acetate) we find two parts in its name:
- The first part of the name, ethanoate (acetate), comes from ethanoic (acetic) acid.
- The other half, methyl, comes from methyl alcohol (methanol).
Then the general name of a carboxylic acid ester will be “alkyl alkanoate” where:
- alkane = root of the main carbon chain (if it is an alkane), which is named from the number of carbon atoms.
Ex .: Propan- means chain of 3 carbon atoms joined by single bonds.
- oato = suffix indicating that it is derived from a carboxylic acid.
CH 3 -CH 2 -CO- means “derived from propanoic acid”.
Alkyl: Indicates the alcohol of origin.
Ex: -O-CH 2 -CH 3 is “ethyl”
Together CH 3 -CH 2 -CO-O-CH 2 -CH 3 is named ethyl propanoate.
They are named as alkyl alkanoates (methyl methanoate).
Esters give flavor and odor to many fruits and are the major constituents of animal and vegetable waxes.
Esters can participate in hydrogen bonds as acceptors, but cannot participate as donors in this type of bonds, unlike the alcohols from which they are derived. This ability to participate in hydrogen bonds makes them more water soluble than the hydrocarbons from which they are derived. But the limitlessness of their hydrogen bonds makes them more hydrophobic than the alcohols or acids from which they are derived. This lack of ability to act as a hydrogen bond donor causes it to be unable to form hydrogen bonds between ester molecules, making them more volatile than an acid or alcohol of similar molecular weight.
Many esters have a distinctive aroma, which makes them widely used as artificial flavors and fragrances. For example:
- 2 Ethyl Hexyl Acetate: Mild sweetish smell
- methyl butanoate: pineapple smell
- Methyl Salicylate (Evergreen or Peppermint Oil): Germolene ™ and Ralgex ™ Ointment Scent (UK)
- heptyl octanoate: raspberry odor
- isopentyl ethanoate: banana smell
- pentyl pentanoate: apple smell
- pentyl butanoate: pear or apricot odor
- octyl ethanoate: orange odor.
Esters are also involved in steric hydrolysis: the breakdown of an ester by water. Esters can also be decomposed by strong acids or bases. As a result, they are broken down into an alcohol and a carboxylic acid , or a salt of a carboxylic acid:
In ester reactions, the chain always breaks into a single bond, either between oxygen and alcohol or R, or between oxygen and the R-CO- group, thereby removing alcohol or one of its derivatives . The saponification of the esters, so called because of its analogy with the formation of soaps, is the reverse reaction to esterification: Esters are more easily hydrogenated than acids, generally using ethyl ester treated with a mixture of sodium and alcohol, and they condense with each other in the presence of sodium and with ketones
Distribution in nature
Esters are widely distributed in nature and are the substances that impart the pleasant odor to many fruits and vegetables. Animal and vegetable fats (tallow, butter and oil) are esters of glycerin with the higher fatty acids (saturated and unsaturated). Waxes are also esters of higher fatty acids and higher alcohols.
By heating with water it breaks down into the alcohol and acid from which it comes.
ester + water ————- acid + alcohol
With an excess of water the reaction is complete. It is a reverse process to esterification.
CH 3 .CO.O.CH 3 + H 2 O ———— CH 3 .CO.OH + H.CH 2 .OH
Alkaline hydrolysis – Saponification
Hydrolysis of esters is catalyzed by acids or bases and leads to carboxylic acids. In the presence of a hydroxide and with excess water and heat, a reaction occurs that produces alcohol and the salt of the acid it produces. This salt is the soap that gives the name to the reaction.
ester + hydroxide ————– acid salt + alcohol.
The esters react with alcohols with acid or basic catalysis obtaining a new ester without having to go through the free carboxylic acid. This reaction is called transesterification.
Grignard’s reagents transform esters into alcohols. The reaction cannot be stopped and the addition of two equivalents of the organometallic occurs.
Reduction to alcohols and aldehydes
Aluminum and lithium hydride transforms them into alcohols and DIBAL into aldehydes.
In basic media they form enolates that condense generating 3-ketoesters. Reaction called Claisen condensation.
- Ethyl formate: redcurrant essence, rum.
- Ethyl acetate: apple and pear essence. Nitrocellulose solvent.
- Ethyl butyrate: peach essence.
- Butyl acetate: solvent for nitrocellulose. Lacquers; varnishes; plastics; safety glasses; perfumes.
- Amyl acetate: solvent for lacquers and varnishes