- Properties
- Water solubility
- Hydrolysis reaction
- Reduction reaction
- Transesterification reaction
- Structure
- Hydrogen bond acceptor
- Nomenclature
- How are they formed?
- Esterification
- Esters from acyl chlorides
- Applications
- Examples
- References
The esters are organic compounds having a carboxylic acid component and another alcohol. Its general chemical formula is RCO 2 R ' or RCOOR '. The right side, RCOO, corresponds to the carboxyl group, while the right side, OR ' is the alcohol. The two share an oxygen atom and share some resemblance to ethers (ROR ').
For this reason ethyl acetate, CH 3 COOCH 2 CH 3, the simplest of the esters, was regarded as the ether of acetic acid or vinegar, hence the etymological origin of the name 'ester'. So an ester consists of the substitution of the acidic hydrogen of the COOH group by an alkyl group from an alcohol.
Source: Pixabay
Where are esters found? From soils to organic chemistry, its natural sources are many. The pleasant smell of fruits, such as those of bananas, pears and apples, is the product of the interaction of esters with many other components. They are also found in the form of triglycerides in oils or fats.
Our body manufactures triglycerides from fatty acids, which have long carbon chains, and glycerol alcohol. What differentiates some esters from others resides both in R, the chain of the acid component, and R ', that of the alcoholic component.
A low molecular weight ester must have few carbons in R and R ', while others, such as waxes, have many carbons especially in R', the alcoholic component, and therefore high molecular weights.
However, not all esters are strictly organic. If the carbon atom of the carbonyl group is replaced by one of phosphorus, then we will have RPOOR '. This is known as a phosphate ester, and they are of crucial importance in the structure of DNA.
Thus, as long as an atom can efficiently bond to carbon or oxygen, such as sulfur (RSOOR '), it can consequently form an inorganic ester.
Properties
Esters are neither acids nor alcohols, so they do not behave as such. Their melting and boiling points, for example, are lower than those with similar molecular weights, but closer in values to those of aldehydes and ketones.
Butanoic acid, CH 3 CH 2 CH 2 COOH, has a boiling point of 164 ° C, while ethyl acetate, CH 3 COOCH 2 CH 3, has a boiling point of 77.1 ° C.
Apart from the recent example, the boiling points of 2-methylbutane, CH 3 CH (CH 3) CH 2 CH 3, of methyl acetate, CH 3 COOCH 3, and of 2-butanol, CH 3, CH (OH) CH 2 CH 3 are as follows: 28, 57 and 99 ° C. All three compounds have molecular weights 72 and 74 g / mol.
Low molecular weight esters tend to be volatile and have pleasant odors, which is why their content in fruits gives them their familiar fragrances. On the other hand, when their molecular weights are high, they are colorless and odorless crystalline solids, or depending on their structure, exhibit greasy characteristics.
Water solubility
Carboxylic acids and alcohols are usually soluble in water, unless they have a high hydrophobic character in their molecular structures. The same is true for esters. When R or R 'are short chains, the ester can interact with water molecules through dipole-dipole forces and London forces.
This is because esters are hydrogen bond acceptors. How? By its two oxygen atoms RCOOR '. Water molecules form hydrogen bonds with any of these oxygens. But when the R or R 'chains are very long, they repel the water in their environment, making it impossible to dissolve them.
An obvious example of this occurs with triglyceride esters. Its side chains are long and make oils and fats insoluble in water, unless they are in contact with a less polar solvent, more related to these chains.
Hydrolysis reaction
Esters can also react with water molecules in what is known as a hydrolysis reaction. However, they require a sufficiently acidic or basic medium to promote the mechanism of said reaction:
RCOOR '+ H 2 O <=> RCO OH + R'O H
(Acid medium)
The water molecule adds to the carbonyl group, C = O. Acid hydrolysis is summarized in the substitution of each R 'of the alcoholic component by an OH from water. Note also how the ester "breaks" into its two components: the carboxylic acid, RCOOH, and the alcohol R'OH.
RCOOR '+ OH - => RCO O - + R'O H
(Basic medium)
When hydrolysis is carried out in a basic medium, an irreversible reaction known as saponification occurs. This is widely used and is the cornerstone in the production of artisanal or industrial soaps.
RCOO - is the stable carboxylate anion, which associates electrostatically with the predominant cation in the medium.
If the base used is NaOH, the RCOONa salt is formed. When the ester is a triglyceride, which by definition has three R side chains, three fatty acid salts are formed, RCOONa, and the alcohol glycerol.
Reduction reaction
Esters are highly oxidized compounds. What does it mean? It means that it has several covalent bonds with oxygen. By eliminating the CO bonds, a break occurs that ends up separating the acidic and alcoholic components; and furthermore, the acid is reduced to a less oxidized form, to an alcohol:
RCOOR '=> RCH 2 OH + R'OH
This is the reduction reaction. You need a strong reducing agent, such as lithium aluminum hydride, LiAlH 4, and an acidic medium that promotes electron migration. Alcohols are the smallest forms, that is, those with the fewest covalent bonds with oxygen (only one: C – OH).
The two alcohols, RCH 2 OH + R'OH, come from the two respective chains of the original RCOOR 'ester. This is a method of synthesis of value-added alcohols from their esters. For example, if you wanted to make an alcohol from an exotic source of esters, this would be a good route for that purpose.
Transesterification reaction
Esters can be transformed into others if they react in acidic or basic environments with alcohols:
RCOOR '+ R''OH <=> RCO OR' ' + R'O H
Structure
Source: Ben Mills via Wikipedia
The top image represents the general structure of all organic esters. Note that R, the carbonyl group C = O, and OR ', form a flat triangle, product of sp 2 hybridization of the central carbon atom. However, the other atoms can adopt other geometries, and their structures depend on the intrinsic nature of R or R '.
If R or R 'are simple alkyl chains, for example, of the type (CH 2) n CH 3, they will appear zigzagged in space. This is the case of Pentyl Butanoate, CH 3 CH 2 CH 2 COOCH 2 CH 2 CH 2 CH 2 CH 3.
But in any of the carbons of these chains a branching or unsaturation could be found (C = C, C≡C), which would modify the global structure of the ester. And for this reason its physical properties, such as solubility and its boiling and melting points, vary with each compound.
For example, unsaturated fats have double bonds in their R chains, which negatively influence intermolecular interactions. As a result, their melting points drop, until they are liquid, or oils, at room temperature.
Hydrogen bond acceptor
Although the triangle of the ester skeleton stands out more in the image, it is the R and R 'chains that are responsible for the diversity in their structures.
However, the triangle does deserve a structural characteristic of esters: they are hydrogen bond acceptors. How? Through the oxygen of the carbonyl and alkoxide groups (the –OR ').
These have pairs of free electrons, which can attract the partially positive charged hydrogen atoms from water molecules.
Therefore, it is a special type of dipole-dipole interactions. The water molecules approach the ester (if not prevented by the R or R 'chains) and the C = OH 2 O, or OH 2 -O-R' bridges are formed.
Nomenclature
How are esters named? To properly name an ester it is necessary to take into account the carbon numbers of the R and R 'chains. Likewise, any possible branching, substituent or unsaturations.
Once this is done, to the name of each R 'of the alkoxide group –OR' the suffix –yl is added, while to the chain R of the carboxyl group –COOR, the suffix –ate. Leg R is mentioned first, followed by the word 'of' and then the name of leg R '.
For example, CH 3 CH 2 CH 2 COOCH 2 CH 2 CH 2 CH 2 CH 3 has five carbons on the right side, that is, they correspond to R '. And on the left side there are four carbon atoms (including the carbonyl group C = O). Therefore, R 'is a pentyl group, and R a butane (to include the carbonyl and consider the main chain).
Then, to give the compound name, simply add the suffixes and name in the appropriate order: butane ato pent yl.
How to name the following compound: CH 3 CH 2 COOC (CH 3) 3 ? The chain -C (CH 3) 3 corresponds to the tert-butyl alkyl substituent. Since the left side has three carbons, it is a "propane." His name then it is: propane ato tert-but yl.
How are they formed?
Esterification
There are many routes to synthesize ester, some of which may even be novel. However, they all converge on the fact that the triangle of the structure image must be formed, that is, the CO-O bond. For that, you must start with a compound that previously has the carbonyl group: such as a carboxylic acid.
And what should carboxylic acid bind to? To an alcohol, otherwise it would not have the alcoholic component that characterizes esters. However, carboxylic acids require heat and acidity to allow the reaction mechanism to proceed. The following chemical equation represents the above said:
RCOOH + R'OH <=> RCOOR '+ H 2 O
(Acid medium)
This is known as the esterification reaction.
For example, fatty acids can be esterified with methanol, CH 3 OH, to replace their H acids with methyl groups, so this reaction can also be considered as methylation. This is an important step in determining the fatty acid profile of certain oils or fats.
Esters from acyl chlorides
Another way to synthesize esters is from acyl chlorides, RCOCl. In them, instead of substituting an OH hydroxyl group, the Cl atom is substituted:
RCOCl + R'OH => RCOOR '+ HCl
And unlike the esterification of a carboxylic acid, not water is released but hydrochloric acid.
Other methods are available within the world of organic chemistry, such as the Baeyer-Villiger oxidation, which uses peroxyacids (RCOOOH).
Applications
Source: Pixnio
Among the main uses of esters are:
-In making candles or tapers, like the one in the image above. Very long side chain esters are used for this purpose.
-As preservatives of medicines or foods. This is due to the action of parabens, which are nothing more than esters of para-hydroxybenzoic acid. Although they preserve the quality of the product, there are studies that question its positive effect on the body.
-Serve for the manufacture of artificial fragrances that imitate the smell and taste of many fruits or flowers. Thus, esters are present in candy, ice cream, perfumes, cosmetics, soaps, shampoos, among other commercial products that deserve attractive aromas or flavors.
-Esters can also have a positive pharmacological effect. For this reason, the pharmaceutical industry has dedicated itself to synthesizing esters derived from acids present in the body to evaluate any possible improvement in the treatment of diseases. Aspirin is one of the simplest examples of these esters.
-Liquid esters, such as ethyl acetate, are suitable solvents for certain types of polymers, such as nitrocellulose and a wide range of resins.
Examples
Some additional examples of esters are as follows:
-Pentyl butanoate, CH 3 CH 2 CH 2 COOCH 2 CH 2 CH 2 CH 2 CH 3, which smells of apricot and pears.
-Vinyl acetate, CH 3 COOCH 2 = CH 2, from which the polyvinyl acetate polymer is produced.
-Isopentyl pentanoate, CH 3 CH 2 CH 2 CH 2 COOCH 2 CH 2 CH (CH 3) 2, which mimics the taste of apples.
-Ethyl Propanoate, CH 3 CH 2 COOCH 2 CH 3.
-Propyl methanoate, HCOOCH 2 CH 2 CH 3.
References
- TW Graham Solomons, Craigh B. Fryhle. Organic Chemistry. (Tenth Edition, p 797-802, 820) Wiley Plus.
- Carey, FA Organic Chemistry (2006) Sixth Edition. Editorial Mc Graw Hill-
- Chemistry LibreTexts. Nomenclature of Esters. Recovered from: chem.libretexts.org
- Admin. (2015, September 19). Esters: Its Chemical Nature, Properties and Uses. Taken from: pure-chemical.com
- Organic Chemistry in our daily life. (March 9, 2014). What are the uses of esters? Recovered from: gen2chemistassignment.weebly.com
- Quimicas.net (2018). Examples of Esters. Recovered from: quimicas.net
- Paz María de Lourdes Cornejo Arteaga. Esters main applications. Taken from: uaeh.edu.mx
- Jim Clark. (January 2016). Introducing Esters. Taken from: chemguide.co.uk