Esters Notes

Esters

Definition

Esters are organic compound derivatives of carboxylic acid where the hydroxyl group (–OH) has been replaced with an “–OR” group. Their characteristic functional group is the "carboxy" group (–COO).

Where:

• R and R’ are Alkyl, Aryl, or Cyclic groups.

• R – C=O is the acid part.

• –O – R’ is the alcohol part.

Nomenclature of Esters

Ester names have two words: the first is the alcohol portion, and the second is the acid portion.

• The first word is the carbon group named as a substituent.

• The second word has the ester ending -oate.

Example:

• Alcohol portion: 3-methylbutyl.

• Acid portion: ethanoic acid, changed to ethanoate.

• IUPAC name: 3-methylbutyl ethanoate.

More Examples:

• Alcohol portion: methyl group.

• Acid portion: 2-hydroxybenzoic acid.

• Ester name: methyl-2-hydroxybenzoate.

Another Example:

• Ester name: 2-chloropropyl propanoate.

Synthesis of Esters

Acylation of Alcohols or Phenols

Esters are prepared by the acylation of alcohols or phenols; this formation is known as esterification.

• Esterification of carboxylic acids with alcohols requires a mineral acid catalyst, such as concentrated $H<em>2SO</em>4$ or HCl gas.

• The esterification of carboxylic acids with alcohols is a nucleophilic acyl substitution.

• Mechanism supported by using isotopically labeled methanol with acetic acid to give methyl acetate and water without any isotopic oxygen.

• Esters of phenols are prepared by reversible acylation of phenols with acyl chloride or anhydrides, rather than the reaction with carboxylic acid in which all steps are reversible.

Fischer’s Esterification

Commonly prepared from carboxylic acids and alcohols.

Chemical Properties of Esters

Transesterification

Esters react with alcohols in the presence of an acid catalyst to undergo exchange of alcohol (alkoxy parts).

The equilibrium mixture consists of the reactants and a new ester and a new alcohol.

The reaction involves acyl substitution of the alkoxy group of the ester with the alkoxy group of the alcohol and is known as transesterification.

Structure of Esters

Esters have an acid part and an alcohol part.

• Isoamyl Acetate is the major odor-producing chemical in bananas.

• Methyl Salicylate is the major odor-producing chemical in oil of wintergreen.

Hydrolysis of Esters

Esters are hydrolyzed by boiling water slowly to carboxylic acids and phenols.

• The hydrolysis is accelerated in the presence of mineral acid or alkali.

Saponification

The alkaline hydrolysis is known as saponification, because esters with high molecular mass ($C<em>{12} - C</em>{17}$) give soap on hydrolysis with a base.

• Soaps are sodium hydroxide (hard soap) or potassium hydroxide salts (soft soaps) of carboxylic acids with high molecular mass ($C<em>{12} - C</em>{17}$).

• The carboxylic acid is obtained by acidification of the salt with mineral acid ($H<em>2SO</em>4$ or HCl).

Reactions of Esters

1. Reaction with Ammonia and Amines: form Amides

2. Reduction of the Acyl Group of Ester with $LiAlH<em>4$ (but NOT with $NaBH</em>4$): to Primary Alcohol

3. Catalytic Hydrogenation of Esters: to Alcohols

   • The reaction requires high temperature and pressure.

   • The catalyst used is a mixture of oxides “copper chromite.”

   • The alkoxy part of the ester gives alcohol as a byproduct.

4. Claisen Condensation

   • This is one method of synthesizing beta-dicarbonyl compounds, specifically a beta-keto ester.

     • Step 1: A strong base, such as sodium ethoxide, removes a hydrogen ion from the carbon atom adjacent to the carbonyl group in the ester.

     • Step 2: The anion can then attack a second molecule of the ester, which begins a series of mechanistic steps until the anion of the beta-dicarbonyl compound forms—the second step gives the product.

   • The reaction is named after a German chemist, Rainer Ludwig Claisen, in 1881.

   • It is a nucleophilic acyl substitution on an ester, in which the attacking nucleophile is an enolate ion.

5. Bouveault Blanc Reduction

   • Reaction of esters with sodium in ethanol

   • It is an organic reaction in which the esters are reduced to primary alcohols in the presence of absolute alcohol and sodium metal.

   • Carboxylic esters can also be reduced with dissolving sodium.

   • Different products are obtained depending on whether the reduction is carried out in ethanol or xylene.

6. Blaise Reaction

   • An organic reaction in which α-Keto esters are prepared from nitriles and α-haloesters using zinc.

   • This reaction is a condensation reaction.

   • Condensation of cyanides with α-bromoester in the presence of Zn.

Properties of Esters

• Esters cannot form hydrogen bonds to each other.

• The boiling point of esters is lower than those of alcohols and acids of comparable mass.

• Esters are more like ethers in properties.

• Low molecular weight esters are soluble in water.

• Solubility rapidly decreases as the carbon chain length increases.

• Low and intermediate molecular mass esters are usually colorless liquids at room temperature.

• Most esters have pleasant odors or aromas.

Uses of Esters

Pheromones

• Isoamyl Acetate is the alarm pheromone for honey bees.

• Methyl – p – hydroxybenzoate is the sexual attractant for canine species.

Flavoring Agents

Esters are largely responsible for the flavor and fragrance of fruits and flowers.

Medications

• Benzocaine is a local anesthesia.

• Aspirin is produced from the reaction of acetic acid with the alcohol group of salicylic acid.

  • A drug that is able to decrease pain, lower body temperature, and reduce inflammation.

  • When ingested, it undergoes hydrolysis to produce salicylic acid and acetic acid.

• Oil of Wintergreen (Methyl Salicylate) is produced from the reaction of methanol with the acid group of salicylic acid.

  • It is used in skin rubs and liniments to help decrease the pain of sore muscles.

  • Absorbed through the skin when it is hydrolyzed to produce salicylic acid.

Polyesters

Polymers in which the monomers are joined through a series of ester linkages.

• Dicarboxylic acids and dialcohols are the monomers used to form polyesters.

  • Monomer is comprised of one (1) unit of a “molecule” or a “mer.”

  • Polymer is composed of many “mers.”

Polyester: Dacron

Dacron is a polyester produced from the monomers: terephthalic acid and ethylene glycol.

• Used to produce synthetic artery grafts that replace diseased arteries and manufacture heart valve replacements.

Polyhydroxyalkanoates

Poly(hydroxyalkanoates) are natural biopolymers that have been developed for use in the manufacturing of degradable plastic containers.

• Examples: Poly(3–hydroxybutyrate), PHB, and Poly(3–hydroxyvalerate), PHV are naturally occurring polyesters produced by certain bacteria.

• PHB has many physical properties in common with polypropylene.

• PHB-PHV copolymer (BIOPOL) has recently been used to manufacture plastic shampoo bottles.

  • It is of special interest because it is biodegradable.

  • It is a naturally occurring polymer and is easily degraded by enzymes produced by soil microorganisms and, therefore, does not persist in the environment after disposal.

• Polyesters of ε-Caprolactone and Lactic acid are also biodegradable polymers of importance.

Degradable Polymers

Polymers that are degraded in the environment once they are no longer useful.

I am sorry, I cannot provide images or visual content.

I am sorry, I cannot provide images or visual content.

I am sorry, I cannot provide images or visual content.

I am sorry, I cannot provide images or visual content.

I am sorry, I cannot provide images or visual content.