Esters: properties and nomenclature
Esters are volatile compounds, many of which have a pleasant odor. Due to this property, they are actively used in the food industry as flavoring essences. A classic example is the ethyl ester of butyric acid, also known as ethyl butyrate. This compound is formed as a result of the reaction between butyric acid and ethanol, giving products a characteristic pineapple scent.
Esters are named in two main ways:
- Systematic nomenclature. Names are constructed on the principle of naming salts. First, the hydrocarbon radical of the alcohol is indicated as a prefix (e.g., propyl-), followed by the name of the acid anion (e.g., formate). Result: propyl formate for $HCOOC_3H_7$.
- Traditional system. The following structure is used: name of the acid + name of the alcohol + the word "ester". The same $HCOOC_3H_7$ in this system is called propyl formate.
An important aspect is interclass isomerism. Esters are isomeric with carboxylic acids having the same number of carbon atoms (the general formulas of the homologous series coincide). However, their physical properties differ drastically. Esters are significantly more volatile and have lower boiling points because their structure lacks the conditions for forming strong hydrogen bonds that are characteristic of carboxylic acids.
Comparison using $C_4H_8O_2$ isomers as an example:
- Ethyl acetate (ester): boiling point $77.1 ^\circ C$.
- Butyric acid: boiling point $163.5 ^\circ C$.
Chemical reactions of esters
Chemically, esters are completely different from ethers, despite the similarity in name and volatility. The main chemical transformations are associated with bond cleavage processes.
Ester hydrolysis The ester formation reaction (esterification) is reversible, therefore esters are capable of undergoing hydrolysis. This process is significantly accelerated in the presence of alkali.
Saponification The reaction of an ester with an alkali is called saponification. Unlike regular aqueous hydrolysis, this process is irreversible. The products are a carboxylic acid salt and an alcohol. The equation for the saponification of ethyl acetate with sodium hydroxide: $CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH$ (Ethyl acetate reacts with sodium hydroxide to form sodium acetate and ethanol).
In addition, esters can form polymers — polyesters. The condition for obtaining polyesters via esterification is the use of functional monomers: the reaction must proceed between a dibasic carboxylic acid and a diol.
Preparation of functional derivatives: anhydrides and acyl chlorides
Specific reagents that replace the hydroxyl group are used to obtain more reactive carboxylic acid derivatives.
- Formation of acyl chlorides
When carboxylic acids interact with phosphorus pentachloride ($PCl_5$), the hydroxyl group is replaced by a chlorine atom. Reaction: $R-COOH + PCl_5 \rightarrow R-COCl + POCl_3 + HCl$
- Formation of anhydrides
When acids interact with strong dehydrating agents, such as phosphorus(V) oxide ($P_2O_5$), intermolecular dehydration occurs. Two acid molecules lose a water molecule, joining via an oxygen atom. Reaction: $2R-COOH + P_2O_5 \rightarrow (R-CO)_2O + 2HPO_3$
Acyl groups and acylation reactions
Acyl is the general name for the organic acid residue formed after the removal of a hydroxo group ($-OH$). It is important to know the names of the main acyl residues:
- The formic acid residue ($H-CO-$) is called formyl.
- The acetic acid residue ($CH_3-CO-$) is called acetyl.
Anhydrides and acyl halides exhibit extremely high reactivity. Due to this, they are widely used for acylation — the process of introducing an acyl residue into the structure of various molecules.
Examples of acylation reactions:
- Preparation of esters. When an acyl chloride interacts with alcoholates (e.g., sodium ethoxide), substitution occurs to form an ester.
$C_2H_5ONa + CH_3CO-Cl \rightarrow CH_3CO-OC_2H_5 \text{ (ethyl acetate)} + NaCl$
- Preparation of amides. The interaction of acyl chlorides with ammonia ($NH_3$) leads to the formation of amides — another important type of functional acid derivatives.
$CH_3CO-Cl + NH_3 \rightarrow CH_3CONH_2 \text{ (amide)} + HCl$
To easily formulate the systematic name of an ester, imagine it is structured like an inorganic salt: first name the alcohol radical (as the "cation" — e.g., ethyl), then the acid anion (e.g., acetate). Result: ethyl acetate.
Class and exam questions
What are the similarities between ethers and esters?
The similarity lies only in physical properties (volatility) and name resemblance. From a chemical standpoint, these are completely different classes of compounds that differ drastically in their properties.
Why do esters boil at a lower temperature than their isomeric acids?
Ester molecules lack hydroxyl groups. Because of this, they cannot form strong intermolecular hydrogen bonds, which are characteristic of carboxylic acids and elevate their boiling points.
What is saponification?
This is the reaction of irreversible ester hydrolysis under the action of an alkali. The resulting products are a carboxylic acid salt and the corresponding alcohol.
What does the term "acylation" mean?
Acylation is the introduction of an acyl residue (an acid moiety without the OH group) into a molecule. Acid anhydrides or acyl halides are usually used for this reaction due to their high activity.
What else to study on this topic
- Interclass isomerism of carboxylic acids and esters
- Properties and applications of acid amides
- Mechanisms of esterification and hydrolysis reactions
- Specifics of polycondensation in obtaining polyesters
- Structure and chemical activity of acyl chlorides
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