Ester

What are esters?

Esters are usually derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group, and most commonly from carboxylic acids and alcohols. That is, esters are formed by condensing an acid with an alcohol.Esters are ubiquitous. Most naturally occurring fats and oils are the fatty acid esters of glycerol. Esters with low molecular weight are commonly used as fragrances and found in essential oils and pheromones. Phosphoesters form the backbone of DNA molecules. Nitrate esters, such as nitroglycerin, are known for their explosive properties, while polyesters are important plastics, with monomers linked by ester moieties.

Type of ester

Ester Name	Odor or occurrence
Allyl hexanoate	pineapple
Benzyl acetate	pear, strawberry, jasmine
Bornyl acetate	pine
Butyl butyrate	pineapple
Ethyl acetate	nail polish remover, model paint, model airplane glue
Ethyl butyrate	banana, pineapple, strawberry
Ethyl hexanoate	pineapple, waxy-green banana
Ethyl cinnamate	cinnamon
Ethyl formate	lemon, rum, strawberry
Ethyl heptanoate	apricot, cherry, grape, raspberry
Ethyl isovalerate	apple
Ethyl lactate	butter, cream
Ethyl nonanoate	grape
Ethyl pentanoate	apple
Geranyl acetate	geranium
Geranyl butyrate	cherry
Geranyl pentanoate	apple
Isobutyl acetate	cherry, raspberry, strawberry
Isobutyl formate		raspberry
Isoamyl acetate		pear, banana (flavoring in Pear drops)
Isopropyl acetate		fruity
Linalyl acetate		lavender, sage
Linalyl butyrate		peach
Linalyl formate		apple, peach
Methyl acetate		glue
Methyl anthranilate		grape, jasmine
Methyl benzoate		fruity, ylang ylang, feijoa
Methyl butyrate		pineapple, apple, strawberry
Methyl cinnamate		strawberry
Methyl pentanoate		flowery
Methyl phenylacetate		honey
Methyl salicylate		Modern root beer, wintergreen, Germolene and Ralgex ointments (UK)
Nonyl caprylate		orange
Octyl acetate		fruity-orange
Octyl butyrate		parsnip
Amyl acetate		apple, banana
Pentyl butyrate		apricot, pear, pineapple
Pentyl hexanoate		apple, pineapple
Pentyl pentanoate		apple
Propyl acetate		pear
Propyl hexanoate		blackberry, pineapple, cheese, wine
Propyl isobutyrate		rum
Terpenyl butyrate		cherry

Naming of ester

Carboxylic acid esters, formula RCOOR′ (R and R′ are any organic combining groups), are commonly prepared by reaction of carboxylic acids and alcohols in the presence of hydrochloric acid or sulfuric acid, a process called esterification. In the reaction the hydroxyl group (OH) of the carboxylic acid is replaced by the alkoxy group of the alcohol.
The reverse of the esterification reaction is an example of hydrolysis. Esters may also be obtained by reaction of acid halides or acid anhydrides with alcohols or by reaction of salts of carboxylic acids with alkyl halides. One ester may be converted to another ester by reaction (transesterified) with an alcohol, a carboxylic acid, or a third ester in the presence of a catalyst.

The hydrolysis of esters in the presence of alkalies, a reaction called saponification, is utilized in the preparation of soaps from fats and oils and is also used for the quantitative estimation of esters.

Ester got a lot of different name, so its hard for us to remember each of them. But there is a way to give them a name.

• ·          The easiest way to deal with naming esters is to recognise the carboxylic acid and the alcohol that they can be prepared from.
• ·          The general ester, RCO2R' can be derived from the carboxylic acid RCO2H and the alcohol HOR'
• ·          The first component of an ester name, the alkyl is derived from the alcohol, R'OH.
• ·          The second component of an ester name, the -oate is derived from the carboxylic acid, RCO2H.

• ·          Alcohol component

1.     the root name is based on the longest chain containing the -OH group.

2.     The chain is numbered so as to give the -OH the lowest possible number.

• ·          Carboxylic acid component

1.     the root name is based on the longest chain including the carbonyl group.

2.     Since the carboxylic acid group is at the end of the chain, it must be C1.

3.     The ester suffix for the acid component is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -oate = -anoate etc.

• ·          The complete ester name is the alkyl alkanoate

Properties of ester

Structure and bonding

Esters contain a carbonyl center, which gives rise to 120°C-C-O and O-C-O angles. Unlike amides, esters are structurally flexible functional groups because rotation about the C-O-C bonds has a low barrier. Their flexibility and low polarity is manifested in their physical properties; they tend to be less rigid (lower melting point) and more volatile (lower boiling point) than the corresponding amides.[3] The pKa of the alpha-hydrogens on esters is around 25.

Physical properties and characterization

Esters are more polar than ethers but less polar than alcohols. They 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 confers some water-solubility. Because of their lack of hydrogen-bond-donating ability, esters do not self-associate. Consequently esters are more volatile than carboxylic acids of similar molecular weight.

Characterization and analysis

Esters are usually identified by gas chromatography, taking advantage of their volatility. IR spectra for esters feature an intense sharp band in the range 1730–1750 cm−1 assigned to νC=O. This peak changes depending on the functional groups attached to the carbonyl. For example, a benzene ring or double bond in conjugation with the carbonyl will bring the wavenumber down about 30 cm−1.

Applications and occurrence

Esters are widespread in nature and are widely used in industry. In nature, fats are, in general, triesters derived from glycerol and fatty acids.[5] Esters are responsible for the aroma of many fruits, including apples, durians, pears, bananas, pineapples, and strawberries.[6] Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.

Advantages of esters

Of all the chemical compounds approved by the Food and Drug Administration (FDA) for use as flavorings in food, esters make up the largest single portion of this group. The flavoring esters are a major constituent of essential oils which are used in foods as a natural flavoring. (An essential oil is a solution obtained from liquefying the vapors formed from heating the organic matter of a flavorful plant.) The American Indians used the dried

leaves of a fern that commonly grows in the eastern United States, especially in North Carolina and Pennsylvania, to make wintergreen tea. When the essential oil of this plant was analyzed by scientists, it was made up almost entirely of the ester, methyl salicylate. This ester contains a benzene ring or phenyl group in its molecular structure and will produce methanol when heated with water. Methyl salicylate, or "sarsaparilla" flavor is also the major ingredient in "birch beer" or root beer. It also gives that wintergreen taste to toothpaste, chewing gum, and candy. Methyl salicylate can also be used to relieve muscular aches and pains and rheumatic conditions. Other aromatic esters are used as flavors or perfumes. Chocolate flavoring uses isoamyl phenylacetate, and strawberry is a mix of ethylmethylphenylglycinate and methyl cinnamate. The aromatic ester, benzyl acetate, is used largely in jasmine and gardenia fragrances, and benzyl benzoate, an ester with practically no odor, is used as a base in many perfumes. Some aromatic esters, such as Bitrex, are very bitter tasting, but are added to household products such as cleaning fluids, nail polish removers, and detergents to keep children from wanting to drink these poisonous substances

Salicylic acid acetate, or aspirin, is only one of many esters used as medicines. Phenyl salicylate, a similar aromatic ester, is used in the treatment of rheumatic arthritis. Methyl phenidate, an ester which produces methanol when it is heated with water, is used to stimulate the central nervous system. The pharmaceutical industry has discovered that certain undesirable properties of drugs, such as bad taste or swelling of the skin at the spot of an injection, can be avoided by converting the original drug into an ester. The antibiotic dalactine (clindamycin), a bitter tasting drug, was converted to its palmitate ester in order to make its flavor less harsh.

The macrolides are large ring lactones or cyclic esters. Most of these unusual esters are isolated from microorganisms that grow in the soil and are being used as antibiotics in human and veterinary medicine. The well known antibiotic erythromycin is an example of a macrocylic lactone consisting of 12 carbon atoms and one oxygen atom bonded to a carbonyl group and the more potent roxithromycin contains 14 carbon atoms, an oxygen atom and a carbonyl group bonded as a cyclic ester.

Disadvantages of ester

Esters are hard to dispose due to their chemical properties in industries. Besides, esters can cause allergies for people who can't withstand it. Other than that, ester can be consider as hazardous if overused ,example like a person will fainted or hard to breathe when they facing some smell they can't withstand like durian smell for  foreigner .Sometimes those artificial esters will also affect the environment by polluting it.