ester

 

any one of a group of organic compounds with general formula RCO₂R′ (where R and R′ are alkyl groups or aryl groups) that are formed by the reaction between an alcohol and an acid. For example, when ethanol and acetic acid react, ethyl acetate (an ester) and water are formed; the reaction is called esterification. Ethyl acetate is used as a solvent. Methyl acetate, formed by the reaction between methanol and acetic acid, is a sweet-smelling liquid used in making perfumes, extracts, and lacquers. Esters react with water (hydrolysis) under basic conditions to form an alcohol and an acid. When heated with a hydroxide certain esters decompose to yield soap and glycerin; the process is called saponification. Common fats and oils are mixtures of various esters, such as stearin, palmitin, and linolein, formed from the alcohol glycerol and fatty acids. Naturally occurring esters of organic acids in fruits and flowers give them their distinctive odors. Esters perform important functions in the animal body; e.g., the ester acetylcholine is a chemical transmitter of nerve stimuli.

Esters are a class of chemical compounds and functional groups. Esters consist of an inorganic or organic acid in which at least one -OH (hydroxy) group is replaced by an -O-alkyl (alkoxy) group. The most common type of esters are carboxylic acid esters (R¹-C(=O)-O-R²), other esters include phosphoric acid, sulfuric acid, nitric acid, and boric acid esters. Volatile esters often have a smell and are found in perfumes, essential oils, and pheromones and give many fruits their scent. Ethyl acetate and methyl acetate are important solvents, fatty acid esters form fat and lipids, and polyesters are important plastics. Cyclic esters are called lactones. The name "ester" is derived from the German Essig-Äther (literally:vinegar ether), an old name for ethyl acetate. Esters can be synthesized in a condensation reaction between an acid and an alcohol in a reaction known as esterification.

Nomenclature

An ester is named according to the two parts that make it up: the part from the alcohol and the part from the acid (in that order), for example ethyl sulfuric acid ester.

Since most esters are derived from carboxylic acids, a specific nomenclature is used for them. For esters derived from the simplest carboxylic acids, the traditional name for the acid constituent is generally retained, e.g. formate, acetate, propionate, butyrate.^[1] For esters from more complex carboxylic acids, the systematic name for the acid is used, followed by the suffix -oate. For example, methyl formate is the ester of methanol and methanoic acid (formic acid): the simplest ester. It could also be called methyl methanoate.^[2]

Esters of aromatic acids are also encountered, including 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 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 analyzed 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:

Ester Name	Molar Mass (g/mol)	◆	◆	◆	Structure	Odor or Occurrence
Allyl hexanoate	156.22					pineapple
Benzyl acetate	150.18	1	1	0		pear, strawberry, jasmine
Bornyl acetate	196.29					pine tree flavor
Butyl butyrate	144.21	2	2	0		pineapple
Ethyl acetate	88.12	1	3	0		nail polish remover, model paint, model airplane glue
Ethyl butyrate	116.16					banana, pineapple, strawberry
Ethyl hexanoate	144.21					strawberry
Ethyl cinnamate	176.21					cinnamon
Ethyl formate	74.08					lemon, rum, strawberry
Ethyl heptanoate	158.27					apricot, cherry, grape, raspberry
Ethyl isovalerate	130.18					apple
Ethyl lactate	118.13	1	1	0		butter, cream
Ethyl nonanoate	186.29					grape
Ethyl pentanoate	130.18	1	3	0		apple
Geranyl acetate	196.29	0	1	0		geranium
Geranyl butyrate	224.34					cherry
Geranyl pentanoate	238.37					apple
Isobutyl acetate	116.16	1	3	0		cherry, raspberry, strawberry
Isobutyl formate	102.13					raspberries
Isoamyl acetate	130.19					pear, banana (flavoring in Pear Drops)
Isopropyl acetate	102.1	1	3	0		fruity
Linalyl acetate	196.29					lavender, sage
Linalyl butyrate	224.34					peach
Linalyl formate	182.26					apple, peach
Methyl acetate	74.08	1	3	0		peppermint
Methyl anthranilate	151.165					grape, jasmine
Methyl benzoate	136.15					fruity, ylang ylang, feijoa
Methyl benzyl acetate	164.20					cherry
Methyl butyrate	102.13					pineapple, apple
Methyl cinnamate	162.185					strawberry
Methyl pentanoate	116.16					flowery
Methyl phenyl acetate	150.17					honey
Methyl salicylate (oil of wintergreen)	152.1494					root beer, wintergreen, Germolene™ and Ralgex™ ointments (UK)
Nonyl caprylate						orange
Octyl acetate	172.27					fruity-orange
Octyl butyrate	200.32					parsnip
Amyl acetate (pentyl acetate)	130.19					apple, banana
Pentyl butyrate (amyl butyrate)	158.24					apricot, pear, pineapple
Pentyl hexanoate (amyl caproate)	186.29					apple, pineapple
Pentyl pentanoate (amyl valerate)	172.15					apple
Propyl ethanoate	102.13					pear
Propyl isobutyrate	130.18					rum
Terpenyl butyrate						cherry

Ester synthesis

"Esterification" (condensation of an alcohol and an acid) is not the only way to synthesize an ester. Esters can be prepared in the laboratory in a number of other ways:

• by transesterifications between other esters
• by Dieckmann condensation or Claisen condensation of esters carrying acidic α-protons
• by Favorskii rearrangement of α-haloketones in presence of base
• by nucleophilic displacement of alkyl halides with carboxylic acid salts
• by Baeyer-Villiger oxidation of ketones with peroxides
• by Pinner reaction of nitriles with an alcohol

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 amides.
• Phenyl esters react to hydroxyarylketones in the Fries rearrangement.
• Di-esters such as diethyl malonate react as nucleophile with alkyl halides in the malonic ester synthesis after deprotonation.
• Specific esters are functionalized with an α-hydroxyl group in the Chan rearrangement
• Esters are converted to isocyanates through intermediate hydroxamic acids in the Lossen rearrangement.
• Esters with β-hydrogen atoms can be converted to alkenes in ester pyrolysis

External links

• An introduction to esters
• Molecule of the month: Ethyl acetate and other esters
• Making an Ester A simple guide to naming and making esters, as well as the chemistry behind it.

References

1. ^ IUPAC parent groups using traditional names
2. ^ IUPAC naming of esters

Functional groups
Chemical class: Alcohol • Aldehyde • Alkane • Alkene • Alkyne • Amide • Amine • Azo compound • Benzene derivative • Carboxylic acid • Cyanate • Disulfide • Ester • Ether • Haloalkane • Imine • Isocyanide • Isocyanate • Ketone • Nitrile • Nitro compound • Nitroso compound • Peroxide • Phosphoric acid • Pyridine derivative • Sulfone • Sulfonic acid • Sulfoxide • Thioester • Thioether • Thiol

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