Question

Draw the structure for each of the following esters: (a) "propyl formate" (b) "ethyl acetate" (c) "phenyl propionate" (d) ethyl benzoate

Short answer

Propyl formate: HCOOCH2CH2CH3; Ethyl acetate: CH3COOCH2CH3; Phenyl propionate: CH3CH2COOC6H5; Ethyl benzoate: C6H5COOC2H5.

Step-by-step solution

Understanding Ester Structure

An ester consists of two parts: an acyl group (derived from a carboxylic acid) and an alkoxy group (derived from an alcohol). The general structure of an ester is written as \( RCOOR' \), where \( R \) is the acyl group and \( R' \) is the alkoxy group.

Drawing Propyl Formate

For propyl formate, the acyl group is 'formate', which is derived from formic acid (HCOOH). The alkoxy group is 'propyl', consisting of three carbon atoms (CH3-CH2-CH2). The structure is: HCOOCH2CH2CH3.

Drawing Ethyl Acetate

In ethyl acetate, the acyl component is 'acetate', derived from acetic acid, CH3COOH. The alkoxy part is 'ethyl', which has the formula CH3CH2. The resulting structure is CH3COOCH2CH3.

Drawing Phenyl Propionate

Phenyl propionate has the acyl group derived from propionic acid (CH3CH2COOH), and the alkoxy group is 'phenyl', which is a benzene ring. The overall structure is CH3CH2COOC6H5.

Drawing Ethyl Benzoate

For ethyl benzoate, the acyl group is 'benzoate', from benzoic acid (C6H5COOH), and the alkoxy group is 'ethyl', C2H5. The structure is C6H5COOC2H5.

Key concepts

Ester Structure

Understanding the structure of esters is essential in organic chemistry. An ester is a compound formed from the reaction between an alcohol and a carboxylic acid, where a hydrogen atom from the acid group is replaced with an alkoxy group from the alcohol. This reaction is known as esterification.
The general formula for an ester is represented as \( RCOOR' \). Here, \( R \) is the acyl group, which comes from the carboxylic acid, and \( R' \) is the alkoxy group originating from the alcohol. This formula suggests that an ester consists of two main parts: the carbonyl group and the ether-like oxygen bonded to another carbon chain.
Esters are often responsible for the fragrant aromas of many fruits and flowers. They are considered to be versatile intermediates in synthetic chemistry.

• *Example 1*: **Propyl Formate** - This ester has a "formate" acyl part (from formic acid) paired with a "propyl" alkoxy group (from propane).
• *Example 2*: **Ethyl Acetate** - Contains an "acetate" acyl group from acetic acid and an "ethyl" alkoxy group from ethanol.

Acyl Group

The acyl group is a crucial component of an ester’s structure. It is derived from a carboxylic acid and generally consists of a carbon double bonded to an oxygen (carbonyl group), along with another carbon chain or hydrogen attached to it. In the context of esters, the acyl group is what begins the ester molecule structure at the \( R \) position in \( RCOOR' \).
Acyl groups provide esters with particular properties due to their structural variations. The representation of acyl in the name of an ester often alludes to its corresponding acid.

• *Formate acyl group* - Derived from formic acid (HCOOH), characterized by a hydrogen atom bonded to the carbonyl carbon.
• *Acetate acyl group* - Originates from acetic acid (CH3COOH); features a methyl group attached to the carbonyl carbon.

The distinct structure of an acyl group influences reactivity and aromatic qualities, making acyl groups imperative in ester formation.

Alkoxy Group

The alkoxy group in an ester is derived from alcohol. It typically comprises an oxygen atom bonded to an alkyl group and is the part that occupies the \( R' \) position in the ester’s general formula \( RCOOR' \).
The alkoxy group plays a significant role in defining the physical properties and fragrance of an ester. Their variation makes them versatile in terms of applications in flavors and fragrances,
helping in the synthesis of diverse organic compounds.

• *Propyl alkoxy group* - Consists of a straight-chain of three carbon atoms, found in esters like propyl formate, which gives a fruity aroma.
• *Ethyl alkoxy group* - Contains two carbon atoms, typical in esters such as ethyl acetate and ethyl benzoate, contributing to their volatility and use as solvents.

Understanding how different alkoxy groups interact within esters allows chemists to manipulate and achieve desired ester characteristics in both industrial and natural settings.