Ester - SpeedyLook encyclopedia

Nomenclature

The name of an ester comprises two terms:

the first, which finishes in - oate , indicates the principal chain which comes from the carboxylic acid. It is related to carbon and is numbered when it is necessary from this one.
the second, who finishes in - yle , is the name of the alkyl group coming from alcohol. This chain is numbered starting from the carbon atom related to the oxygen atom.

Example: the ethyl pentanoate

Synthesis of esters: esterification

Synthesis starting from carboxylic acids

Reaction

The chemical reaction which produces a Ester and H₂O, starting from a alcohol (R'-OH) and of a Carboxylic acid (R-COOH) is called esterification of Fisher . The general equation (simplified) of this reaction is:

The opposite reaction is a Hydrolyze. In both cases, it requires a Catalyseur: a free proton (hydrogen ionized) coming is of a strong acid (rich in free protons) in aqueous solution (to accelerate esterification), or water in which the carboxylic acid is in solution (esterification often slower if the carboxylic solution has a raised pH, low in free protons).

Properties

This reaction is capsizable (Saponification) and reversible (the opposite reaction is a " retro-estérification"), slow and limited (because precisely of its opposite reaction, the hydrolysis). It is also quasi athermic (it does not release nor absorbs heat); the constant of balance of this reaction is thus independent of the temperature.

Output

The output depends very little on the nature of the carboxylic acid used. It depends especially on the class of alcohol used: for reagents introduced in équimolaires quantities, it is of 67% with a primary education alcohol (methanol for example), of 60% with a secondary alcohol (ex: isopropanol or propan-2-ol) and from only 5% if alcohol is tertiary (ex: tertbutanol or 2-methylpropan-2-ol).

historical Note: these results, experimental, (athermiticity, output depend on the class of alcohol and little the carboxylic acid, etc) must mainly with work of Marcellin Berthelot and Armand Péan of Saint-Gilles (Memory of Berthelot and Péan of Saint-Gilles, 1861).

As the reaction is athermic, a variation in the temperature does not have any influence on the output (experimental Loi of Van' T Hoff). In the same way, a variation of the pressure does not involve any displacement of balance (considering almost all the reagents and products are liquids, experimental Loi of Châtelier). At best, an increase in the temperature accelerates the reaction and makes it possible more quickly to reach the limit of the balance of esterification. Balance is not moved, but it more quickly is reached (see kinetic).

To increase the output, there exist various methods:

To increase the quantity of the reagent in excess (in general the least expensive), which will modify the rate of final advance, therefore the output.
To increase the quantity of catalyst (free protons), by making the reaction with a strong acid in aqueous solution (possibly only heated to increase its ionization rate, or electric field which produces the same effect on the catalytic solution) does not modify the output of the reaction only its kinetics (see below).
All methods allowing to prevent the hydrolysis from occurring, and thus allowing to move balance in the direct direction (esterification). One can with the choice:

Distiller ester progressively its formation, if it is most volatile (what is often the case).
To eliminate water (to avoid its ionization source of protons catalysts opposite). For that two methods are possible:

To carry out a drive with the vapor, by adding to the reactional system a relatively volatile solvent and forming with water a hétéroazéotrope. One in general chooses Cyclohexane or Toluène, and one uses for this method the apparatus of Dean and Stark.
To incorporate in the reactional mixture a dessicant substance. That poses more problems, because on the one hand, even if it is put in excess, all water can not be consumed. In addition, it is necessary thereafter to separate ester from this product, which can involve complications (and moreover cause a drop in the output).

Kinetics

As one saw, the reaction is rather slow (to reach the maximum output, it takes several months). Speed also evolves/moves according to the class of alcohols: it decrease when one passes from a primary education alcohol to a secondary alcohol, then with a tertiary alcohol. At all events, one thus seeks means of accelerating the reaction.

Increase in the temperature: if it does not have any influence on the output, it improves the kinetics largely.
Use of a catalyst. One uses for that an acid, which makes it possible to increase the character électrophile group Carboxyle.

The majority of the reactions make it possible to use sulphuric acid, but that is not the case of all (some compounds " supportent" not the " treatment of choc" with the sulphuric acid hot, powerful oxidizing, which is thus likely to oxidize alcohol, or to even dehydrate it). One can also use anhydrous HCl, or a less strong acid, like the phosphoric acid, H₃PO₄, or the paratoluenesulfonic Acide (APTS), even, if it is rather strong (example: the Methanoic acid , pK_A=3.77), reactive carboxylic acid (=> Autocatalyse), put in excess. That has a double advantage: it catalyzes the reaction thus, and moreover, as one saw previously, that allows to improve the output.

Mechanism

According to the class of alcohol, there exist various mechanisms.

The first mechanism presented is valid for primary education and secondary alcohols, the second for tertiary alcohols.

The general case here is taken and one chooses for H⁺ catalyst.

Primary education and secondary alcohols

This mechanism is described in five stages (including two fast balances of protonation-deprotonation).

First stage: protonation of the carboxylic acid. Two possibilities arise:

the protonation of the group carbonyl

Here, the formed ion is stabilized by mesomery:

the protonation of the group hydroxyl

Here not only the formed ion (ion acyloxonium) does not have formulas mésomères which stabilizes it, but in more this state does not allow to continue the reaction. As in more this reaction is a balance, the possible protonic forms on the level of the grouping hydroxyl are consumed to form the other protonic form which will be it consumed by the following stages (displacement of balance, Principe of Châtelier).

the first stage is thus the protonation of the group carbonyl (balance quickly reached):

the second phase is the attacks nucleophilic alcohol on the site électrophile of the protonic carboxylic acid:

note: for this stage, one directly left the 2nd formula mésomère of the protonic acid, in order to simplify the mechanism.

the third stage is the transfer of the proton (H⁺) of the group resulting from alcohol on one of the hydroxyls groups ( reaction acid-bases intern )

the fourth stage, kinetically limiting, and the departure of a water molecule (H₂O).

the last stage is simple a deprotonation (restitution of catalyst)

Note: the mechanism was checked by using water with an atom ¹⁸O, while following the reaction by Spectrometry mass.

Tertiary alcohols

Here also the mechanism takes place in 4 stages

First stage: protonation of alcohol

Second phase: departure of H₂O , formation of the tertiary carbocation.

These two stages (especially 2nd) being impossible with an alcohol promaire or secondary, the carbocation formed not being stable enough.

Third stage: addition of the carbocation on the function carbonyl of the carboxylic acid.

The intermédaire thus substituted is relatively stable, because it has several forms mésomères:

(one leaves besides the last form mésomère for the last stage)

Last stage: it is just about the deprotonation of the preceding intermediary (restitution of catalyst).

Synthesis from derived from carboxylic acids

As one has just seen it, the synthesis of esters starting from carboxylic acids present of many disadvantages: a maximum output of about 2/3 in the most favorable cases (primary education alcohols) and more than poor in the most unfavourable cases (5% for tertiary alcohols), slow kinetics, even catalyzed (if the reaction is stopped too quickly, the output drops even more).

A solution thus consists in using derivatives of acids rather, like the chlorides acyles or the anhydrides of acid S.

Reactions

From acyles chlorides:

From anhydrides of acid:

Properties

Advantages:

These reactions are total .
They are in general relatively fast, but require a catalysis rather regularly. The catalysts are not useful which has to activate and make even more reactive the derivative of acid. Typically, the catalysts are, the pyridine or the triéthylamine (which then play the part of trap with HCl).

Disadvantages:

the acyle chlorides and the anhydrides of acids react easily with water (they are hydrolisent very easily). That obliges to take precautions of handling (to avoid any contact with the mucous membranes).
Although at the base, the reactions are total, the output is not 100%. Indeed, once the made reaction, it is necessary to extract ester from the medium, and the stages to arrive there are generally physicochemical balances.

the reactions starting from acyles chlorides are sharp: one needs to cool the mixture.

One has moreover need to trap formed HCl:

by using for example a base such pyridine or the triéthylamine.
a trap made up of a bottle containing of soda.

the reactions starting from anhydrides are less sharp and slower (the anhydrides are less reactive).

In industry, one thus uses rather the way of the carboxylic acids, easier in the implementation. However, in the case of the drug company or cosmetic, the chlorides or the anhydrides can be used because the products are with high added value.

Mechanisms

Starting from acyle chlorides

1st stage: nucleophilic addition of alcohol on acyle chloride.

2nd stage: departure of HCl

Starting from anhydrides of acid

1st stage: nucleophilic addition of alcohol on anhydride.

2nd stage: departure of RCOOH

Synthesis starting from other esters (Cross esterification)

Cross esterification transforms an ester and an alcohol into another ester and another alcohol. An acid or a base is often used as catalyst.

Reaction

Utility

Cross esterification is used in the manufacture of polyester and the biodiesel. It is also the mechanism which allows the épissage will introns during the maturation of ARNm.

Utility of esters

In addition to the fact of obtaining an ester, useful in agribusiness industry, in perfumery or other industrial sectors, esterification is useful, from its reversible character (for the carboxylic acids and alcohols, all at least), within the framework of the protection of functions.

Since the transformation is reversible, it makes it possible to protect either the function alcohol, or the carboxylic acid function, or both. Indeed, if one imagines for example that one wants to protect an alcohol, one makes it react with a carboxylic acid to form an ester; one makes the reaction which one wanted to carry out; once this one finished, one reverses the reaction of esterification to find alcohol. There exist two methods to reverse esterification:

One uses the same reaction (retro-estérifcation) while exploiting the quantities of matter so that balance is favorable to the carboxylic acid + alcohol.
One uses the reaction of Saponification or hydrolysis in basic medium of esters.

The esters are also constituent basic in the industry of the plastic . They are at the base of one of the most used plastics, the Polyester.

It acts of a means of forming the Lactone S: intramolecular esterification starting from a carboxylic hydroxyacide.

the esters can be reduced:

out of primary education alcohols by the action of the Tétrahydruroaluminate of lithium (LiAlH₄), in the diethylic ether (ether) or THF (tétrahydrofurane)
out of aldehydes by the action of DIBAL, in a nonpolar solvent (Toluene for example), at low temperature (- 60°C).

the definition of a ester on the wiktionnaire -->

Some esters :

Céride
Glycéride
Acetate of isoamyl

Table of certain esters and their odors

(This table only requires to be supplemented)

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