Planar Carbon: Esters

What are Esters?

Esters are organic compounds formed by the reaction of a carboxylic acid with an alcohol, a process known as esterification.

The general formula for an ester is $RCOOR'$, where:

• $R$ represents a hydrocarbon chain from the carboxylic acid (usually an alkyl group), and
• $R'$ represents a hydrocarbon chain from the alcohol.

Nomenclature of Esters

To name an ester:

1. Identify the alkyl group ($R'$) from the alcohol component.
2. Identify the carboxylate group ($RCOO$) from the acid component.
3. Name the ester by writing the alkyl group first followed by the name of the acid, but replace the "-ic" ending of the acid with "-ate."

For Esters up to C4:

• Methyl methanoate ($HCOOCH₃$)
• Ethyl methanoate ($HCOOCH₂CH₃$)
• Methyl ethanoate ($CH₃COOCH₃$)
• Propyl ethanoate ($CH₃COOCH₂CH₂CH₃$)

Physical Properties of Esters

• Physical state: Most simple esters are liquids at room temperature.
• Solubility: Esters have moderate polarity, making them more soluble in non-polar solvents like cyclohexane than in polar solvents like water.
  • Ethyl ethanoate: It is soluble in cyclohexane (non-polar solvent), but has limited solubility in water (polar solvent) due to its inability to form strong hydrogen bonds with water molecules.

Fats as Natural Esters

Fats are naturally occurring esters formed from the reaction of long-chain carboxylic acids (fatty acids) with glycerol (a triol). These esters are called triglycerides, which are a major component of biological lipids and are essential for energy storage in living organisms.

Ethyl Ethanoate as a Solvent

Ethyl ethanoate is widely used as an industrial solvent due to its moderate polarity and ability to dissolve a wide range of substances, including:

• Paints and coatings
• Adhesives
• Nail polish removers

Aromas of Esters

Esters are responsible for many pleasant fragrances found in nature. They are commonly used in perfumes and flavourings. For example:

• Ethyl butanoate: Has a pineapple-like aroma.
• Isoamyl acetate: Smells like bananas.

Benzene and Bonding in Aromatic Compounds

Benzene ($C₆H₆$) is an important aromatic compound in organic chemistry. Its structure consists of six carbon atoms arranged in a ring, with alternating single and double bonds. However, these bonds are actually intermediate between single and double bonds.

• Representation of benzene: The circle inside the hexagon represents delocalized $π$-electrons, which are spread evenly across all six carbon atoms.
• Sigma ($σ$) and Pi ($π$) bonds:
  • Sigma bonds are the single covalent bonds between atoms, formed by the overlap of orbitals.
  • Pi bonds are formed from the sideways overlap of p-orbitals. In benzene, the $π$-electrons are delocalized, contributing to its stability.