ester

Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. This could be an alkyl group like methyl or ethyl, or one containing a benzene ring like phenyl.

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. Polyesters are also a crucial part of the textile industry.
 * Esters** are [|chemical compounds] derived by reacting an [|oxoacid] with a [|hydroxyl] compound such as an [|alcohol] or [|phenol] . [|[1]] 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.



The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:
 * A common ester - ethyl ethanoate**

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. [|[4]]

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. [|[3]]

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.

Reactions Esters react with nucleophiles at the carbonyl carbonyl carbon. The carbonyl is weakly electrophilic but is attacked by strong nucleophilies (amines, alkoxides, hydride sources, organolithium compounds, etc.). The C-H bonds adjacent to the carbonyl are weakly acidic but undergo deprotonation with strong bases. This process is the one that usually initiates condensation reactions. The carbonyl oxygen is weakly basic (less so than in amides) but forms [|adducts].

Addition of nucleophiles at carbonyl
Esterification is a reversible reaction. Esters undergo [|hydrolysis] under acid and basic conditions. Under acidic conditions, the reaction is the reverse reaction of the [|Fischer esterification]. Under basic conditions, [|hydroxide] acts as a nucleophile, while an alkoxide is the leaving group. This reaction, [|saponification], is the basis of soap making. The alkoxide group may also be displaced by stronger nucleophiles such as [|ammonia] or primary or secondary [|amines] to give [|amides] : RCO2R' + NH2R" → RCONHR" + R'OH This reaction is not usually reversible. Hydrazines and hydroxylamine can be used in place of amines. Esters can be converted to [|isocyanates] through intermediate [|hydroxamic acids] in the [|Lossen rearrangement]. Sources of carbon nucleophiles, e.g., [|Grignard reagents] and organolithium compounds, add readily to the carbonyl.

Reduction
Compared to ketones and aldehydes, esters are relatively resistant to reduction. The introduction of catalytic hydrogenation in the early part of the 20th century was a breakthrough; esters of fatty acids are hydrogenated to [|fatty alcohols]. RCO2R' + 2 H2 → RCH2OH + R'OH A typical catalyst is [|copper chromite]. Prior to the development of [|catalytic hydrogenation], esters were reduced on a large scale using the [|Bouveault-Blanc reduction]. This method, which is largely obsolete, uses sodium in the presence of proton sources. Especially for fine chemical syntheses, [|lithium aluminium hydride] is used to reduce esters to two primary alcohols. The related reagent sodium borohydride is slow in this reaction. [|DIBAH] reduces esters to aldehydes. [|[10]]