esters

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Physical propertiesSimple estersI am thinking here about things like ethyl ethanoate.Boiling pointsThe small esters have boiling points which are similar to those of aldehydes and ketones with the same number of carbon atoms.Like aldehydes and ketones, they are polar molecules and so have dipole-dipole interactions as well as vander Waals dispersion forces. However, they don't form hydrogen bonds, and so their boiling points aren't anything like as high as an acid with the same number of carbon atoms

For example:molecule type boiling point (°C)

CH3COOCH2CH3 ester 77.1

CH3CH2CH2COOH carboxylic acid 164

Solubility in waterThe small esters are fairly soluble in water but solubility falls with chain length.FOR-EXAMPLE:

ester formulasolubility (g per 100 g of

water)

ethyl methanoate HCOOCH2CH3 10.5

ethyl ethanoate CH3COOCH2CH3 8.7

ethyl propanoate CH3CH2COOCH2CH3 1.7

Some other Properties of Ester:Esters are neutral(do not change color of litmus, phenolphthalein, methyl orange or any other acid-base indicator) compoundsEsters are colorless and exist in liquid state at room temperatureEsters are polar due to the presence of carbonyl (=C=O)group and act as hydrogen bond acceptors not as donors. Esters are volatile in nature and have low boiling and melting points.

MECHANISM OF THE BASE HYDROLYSIS OF ESTERS

Step 1:The hydroxide Nucleophilic

attacks at the electrophilic C of the ester C=O, breaking the p

bond and creating the tetrahedral intermediate.

Step 2:The intermediate collapses,

reforming the C=Oresults in the loss of the leaving

group the alkoxide, leading to the carboxylic acid.

Step 3:An acid / base reaction. A very

rapid equilibrium where the alkoxide functions as a base deprotonating the carboxylic acid (an acidic work up would

allow the carboxylic acid to be obtained from the reaction).

ACID –HYDROLYSIS OF AN ESTER:

FIRST STEP:

The actual catalyst in this case is the hydroxonium ion, H3O+, present in all

solutions of acids in water. In the first step, the ester takes a proton (a hydrogen

ion) from the hydroxonium ion. The proton becomes attached to one of the lone

pairs on the oxygen which is double-bonded to the carbon.

SECOND STEP:

The positive charge on the carbon atom is attacked by one of the lone pairs on the oxygen of a water molecule.

THIRD STEP:

What happens next is that a proton (a hydrogen ion) gets transferred from the bottom oxygen atom to one of the others. It gets picked off by one of the other substances in the mixture (for example, by attaching to a lone pair on a water molecule), and then dumped back onto one of the oxygens more or less at random. Eventually, by chance, it will join to the oxygen with the ethyl group attached. When that happens, the net effect is:

The structure for the latest ion is just like the one we discussed at length back in step 1. The positive charge is actually delocalized all

over that end of the ion. The real structure will be a hybrid of these:

FIFTH STEPThe hydrogen is removed from the oxygen by reaction with a water

molecule.

EXAMPLE: Ethyl Ethanoate

Preparation Of Ethyl Acetate:

USES :In Glues

In Nail polish RemoversIn decaffeinating tea and coffee

In cigarettes.

As a solvents in industry, notably for lacquers and resins

Artificial fruit flavorIn organic synthesis.

In organic synthesis e.g., for making ethyl acetoacetate.