Carboxylic Acid

General FormulaCnH2n+1COOH, n = 0, 1, 2, …..

Functional Group:

Carboxyl

First 3 Members:

  1. Carboxylic Acids are weak acid.
  2. The functional group of carboxylic acids is carboxyl group.
  3. When dissolve in water, the carboxyl group will ionise to produce a hydrogen ion.

Naming Carboxylic Acids

  1. All carboxylic acid names end with the suffix ‘oic’. For example, methanoic acid, ethanoic acid, propanoic acid and butanoic acid.
  2. The carboxylic acid is named by replace the ending “ane” from the name of alkane with “anoic acid”.
  3. Unlike alcohol, the position of the functional group (the carboxyl group) need not be stated in the name of carboxylic acid.
Example
Number of Carbon Name of Alkane Name of Carboxylic Acid
1 methane methanoic acid
2 ethane ethanoic acid
3 propane propanoic acid
4 butane butanoic acid
5 pentane pentanoic acid
6 hexane hexanoic acid

Physical Properties of Carboxylic Acids

Boiling points

  1. The greater the number of carbons in the molecule, the higher the boiling point of a carboxylic acid.
  2. The boiling points of carboxylic acids are higher than the alcohols of same number of carbons.

Solubility in water

  1. The small molecule carboxylic acid such as methanoic acid and ethanoic acid are soluble in water.
  2. The solubility of the bigger acids decreases as the size of the molecule increases. This is because the longer hydrocarbon “tails” of the molecules get between water molecules and break hydrogen bonds. 

Preparing Carboxylic Acids

Making Carboxylic Acids by Oxidising Primary Alcohols 

  1. Alcohols can be oxidised to carboxylic acids using potassium dichromate(VI) solution in the presence of dilute sulphuric acid.
  2. Chemical Equation below represents the reaction.
     C2H5OH+2[O] → CH3COOH + H2O
  3. The figure below shows the apparatus set up to prepare ethanoic acid.
  4. The mixture is heated under reflux to prevent the ethanol to evaporate and escape to the surrounding before it has time to be oxidised to the carboxylic acid.
  5. Ethanol vapour will condense into liquid in the Liebig condenser and flow back into the flask to complete the reaction.
  6. The mixture is heated by using a water bath:
    1. to keep the temperature constant
    2. to ensure a uniform heating (heat is evenly supplied)

Chemical Properties of Carboxylic Acids

  1. Ethanoic acid is a weak monoprotic acid. As such, its chemical properties are similar to acids such as hydrochloric acid, nitric acid and sulphuric acid.
  2. Ethanoic acid can react with:
    1. base
    2. metal carbonate
    3. metal
    4. alcohol

Reactions of Carboxylic Acids with Reactive Metals

Carboxylic acids react with the more reactive metals to produce a salt and hydrogen.

Carboxylic Acid + Reactive Metal → Salt + H2

Example:

CH3COOH + Zn → Zn(CH3COO)2 + H2

CH3COOH + Ca → Ca(CH3COO)2 + H2

HCOOH + Mg → Mg(HCOO)2 + H2

Reactions of Carboxylic Acids with Carbonates

As other acids, carboxylic acids react with carbonate metal to produce a salt, water and carbon dioxide.

Carboxylic Acid + Metal Carbonate → Salt + Carbon Dioxide + Water

Example:

HCOOH + Na2CO3 → HCOONa + CO2 + H2O

CH3COOH + CaCO3 → Ca(CH3COO)2 + CO2 + H2O

Neutralisation

As other acids, carboxylic acids react with base oxide or alkali to produce salt and water.

Carboxylic Acid + Base Oxide/Alkali → Salt + Water

Example:

HCOOH + CaO → Ca(HCOO)2 + H2O

CH3COOH + NaOH → CH3COONa + H2O

CH3COOH + Al2O3 → Al(CH3COO)3 + H2O

Esterification

Carboxylic acid reacts with alcohol with the presence of .concentrated sulphuric acid as catalyst to produce ester and water.

Carboxylic Acids + Alcohol → Ester + Water

Example:
CH3COOH + C2H5OH → CH3COOC2H+ H2O

CH3COOH + CH3OH → CH3COOCH3 + H2O

  1. The concentrated sulphuric acid acts as
    1. a drying agent to absorb the water that is produced
    2. a catalyst
  2. The reactant mixture needs to be heated under reflux.

Preparing of Propyl Ethanoate (ester) in Laboratory

  1. Propyl ethanoate, CH3COOC2H5 can be prepared in the laboratory by heating a mixture of propanoic acid and ethanol, with concentrated sulphuric acid under reflux.
  2. The mixture is heated under reflux to prevent the alcohol and propyl ethanoate to evaporate and escape to the surrounding.
  3. The mixture is heated in a water bath:
    1. to maintain temperature so that the temperature is not too high
    2. to ensure uniform heating

Ester

  1. Esters are non-hydrocarbon organic compounds containing carbon, hydrogen and oxygen.
  2. It is formed when a carboxylic acid reacts with an alcohol.

General Formula:

CmH2m + 1COOCnH2n + 1

where
m = 0, 1, 2, 3
n = 1, 2, 3

Example

Ester Formula 
 Methyl methanoate
(m=0, n=1)
 HCOOCH3
 Ethyl methanoate
(m=1, n=1)
 CH3COOCH3
 Ethyl ethanoate
(m=1, n=2)
 CH3COOC2H5
 Propylethanoate
(m=2, n=2)
 C2H5COO C2H5

Functional group:

Formation:

  1. Esters are produced when carboxylic acid reacts with alcohol in the presence of concentrated sulphuric acid as a catalyst.
  2. This reaction is called the esterification reaction.
Physical Properties of Ester
  1. Ester has the following properties:
    1. having fragrant similar to fruits
    2. insoluble in water but soluble in organic solvent
    3. low boiling point
    4. less dense than water
    5. colourless liquid
    6. volatile
  2. Ester can be found naturally in plants.

Uses of Ester in Daily Life

  1.  As food flavourings
  2. as a fragrant in soap and detergent
  3. To make cosmetics (perfumes)
  4. To make medicine