Structural Biochemistry/Organic Chemistry/Organic Functional Group/Carbonyl/Ester

Esters are another carboxylic derivative. Out of all the carboxylic acid derivatives, esters are considered to be the most important. They are especially present in nature, as they are responsible for the smell of flowers and fruit. The general structure of an ester is shown in the picture to the right.

IUPAC Nomenclature

Esters are named as alkyl alkanoates. The part of the ester, known as the ester grouping: the carbon doubly bonded to the oxygen and single bonded to the OR group, is called the alkoxy carbonyl.

Generally speaking, a cyclic ester is called a lactone. The IUPAC naming is detailed below as follows:

IUPAC Nomeclature:

1. The carbonyl carbon is designated as carbon number 1. This will be true unless there are more important functional groups present on the molecule but for these purposes, assume the ester is the predominant functional group. Other rules and modifications become necessary as the molecule becomes more and more complex and as more functional groups exist.
2. It is easiest to name the compound by treating it as two parts:
(a) The first part is the alkyl group attached the terminal
(b) Second portion is the alkyl group bonded to carbon #1.
3. Once the two groups have been identified, the alkyl groups should be named according to the usual IUPAC nomenclature for alkyl groups. Remember to include the carbonyl carbon when this chain is being named.
4. Once the two chains have been named, they need to be modified to represent the fact that an ester is present in the compound. The chain branching off the terminal oxygen is not modified. For example if a methyl group were attached, it would remain methyl. On the other hand, drop the “-ane”, “-ene”, or “yne,” suffix and add the suffic “oate” to signify that an ester is present.

Physical and Structural Properties

The carboxylate group is a polar group in it of itself but nonpolar carbon and hydrogen chains surround it. Because of their presence, ester molecules are only slightly polar because of the dipole-dipole bonding between them. Because of this, the esters with few carbons are liquid at room temperature while the esters with more carbon chains are solids.

Resonance is prominent in esters. Recall that resonance is the movement of electrons within a molecule. Though it has many meaning, resonance implies the flow of electron density from one atom to another. It means that the electron density does not belong to just one of the atoms, but is shared within numerous atoms in the molecule. This property is the reason why the carbonyl carbon is not as electrophilic. One of the lone pairs of electrons from the terminal oxygen can form a double bond with the carbonyl carbon that breaks its double bond with the other oxygen thereby giving it a negative charge. Because electron density is being essentially given to the carbon, it becomes less electrophilic meaning that it in essence is less susceptible to nucleophilic attack.

Reactions

Esters undergo numerous reactions though the majority of them follow the same mechanism. If one reaction is understood and the mechanism clear, the rest of the reactions and their mechanism inevitably become much easier to follow and understand.

1. Esters hydrolyze to carboxylic acids: Like the other carboxylic derivatives, esters also are susceptible to addition-elimination pathways. However, because of the resonance present in esters, they are less susceptible, though they still do occur. Because they are not as electrophilic, they need to be acid or base catalyzed in order for the reaction to take place before water creates the carboxylic acid.

2. Transesterification with alcohols: Transesterification means that the alkyl group attached to the terminal oxygen is switched with the alkyl group attached to the alcohol. To do this, it is necessary for the reaction to be acid or base catalyzed like it was in the previous reaction. Like the esterification process, this reaction is reversible.

The mechanism of this reaction is identical to the hydrolysis to the carboxylic acids. The carbonyl oxygen is protonated with the acid, after which the carbonyl carbon is attacked via nucleophilic attack. On the other hand, under base catalyzed reaction, the alcohol is deprotonated, and the alkoxide group that results from the deprotonation adds to the ester carbonyl group.

3. Amines and esters form amides: Amines are much more nucleophilic than the alcohols, thus they form amides. Because it is more nucleophilic than alcohols, acid and base are not needed to activate this reaction, though heat is sometimes needed. In this mechanism, the lone pair on the ammonia attacks the carbonyl carbon of the ester; the double bond breaks, and the lone pair moves to the top of the oxygen, creating a negative charge on the oxygen. When the lone pair comes back down to reform the carbon-oxygen double bond, the OR’ group leaves to form the amide.

4. Grignard reagents transform esters into alcohols: Esters converted into alcohols by using two equivalent ratios of Grignard reagents. All esters save that for formate esters -- which form secondary alcohols – form tertiary alcohols. The mechanism to form an alcohol is in two steps:

(1) The first step is for the first Grignard to attack the carbonyl carbon. The double bond between the carbon and the oxygen break, giving the oxygen a negative charge. When the lone pair comes down to reform the double bond, the OR’ group leaves to form the alkoxide ion. (2) The second step is virtually identical to the first step. The difference is that the lone pair on the oxygen does not come back down to form the double bond because there is no good leaving group. In this case, in order to furnish the alcohol, which is now a tertiary alcohol, is made by an acid workup. This gives the tertiary or in that one specific case, the secondary alcohol.

5. Esters can be reduced to give alcohols or aldehydes: Esters can be reduced to give alcohols and aldehydes. The reducing agent that is used is often LiAlH4. However, in this case, only 0.5 equivalent is required instead of 1 equivalent. If an aldehyde is desired, a milder reducing agent is required such as bis(2-methylpropyl)aluminum hydride.

Relation to Biochemistry

Esters bonds are found in naturally occurring fats and lipids as the fatty acid ester bonds to glycerol. As mentioned earlier esters are found in naturally occurring aromas and flavors. Phosphodiester bonds form the backbone of DNA.

References

1. Berg, Jeremy M. (2007). Biochemistry, 6th Ed., Sara Tenney. ISBN0-7167-8724-5.

2. Vollhardt and Schore (2007). Organic Chemistry: Structure and Function, 5th Ed.