Alkanoic Acids/Carboxylic Acid SS3 Chemistry Lesson Note

CARBOXYLIC  ACIDS (ALKANOIC ACIDS)

Occurrence and General Molecular Formula of Alkanoic Acids

1. The general molecular formula for monocarboxylic acid is CnH2n +1COOH where n starts from 0. The functional group is –COOH.
2. General properties of monocarboxylic acid

0-3 carbon atom monocarboxylic acids are colourless liquids with a sharp odour of vinegar. 

4-8 carbon atoms have an unpleasant odour while others are colourless and odourless.

1. There is a regular increase in b.p and viscosity as the relative molecular mass increases.
2. Among the isomeric alkanoic acids, b.p decreases as the branching increases. This is due to a decrease in the surface area and intermolecular attraction as the branching increases.
3. The b.p of an alkanoic acid is much higher than the corresponding alkanol.
4. Natural sources of alkanoic acids

Aliphatic carboxylic acids occur in nature as free acids or in the form of fats and oils. The pain and irritation experienced from the stings of ants, bees or wasps are due to the injection of methanoic acid into the bloodstream.

Other occurrences are ethanoic acid from vinegar, butanoic acid from human sweat.,citric acid from lemon, palmitic acid from palm fruits, lactic acid from sour milk e.t.c

Lab methods of preparing alkanoic acid

• The oxidation of primary alkanoic
• The hydrolysis of alkene in acid media
• The distillation of a hydrous sodium ethanoate with conc H2 SO4.

Lab. Preparation of Ethanoic acid

The oxidation of ethanoic ethanol 

The suitable oxidising agents are KMnO4 and KzCr2O7.

 Procedure

Add 30cm3 of H2SO4 to 20cm3 of water. Stir continually and allow to cool. Dissolve 70g of K2Cr2O7 in it and place it in a round-bottomed flask.

Mix 40cm3 of ethanol with 20cm3 of water, place in a dropping funnel and set the apparatus as shown. The teacher unfolds the chant illustrating the preparation of ethanol and continues:

 The mixture (acidified K2CrO7 and ethanol) is refluxed as the ethanol is being added from the dropping

funnel. When all the ethanol has been added (100cm3), the mixture is refluxed. 

CH3 CH2OH + [0]               CH3-CHO + H20

CH3-CHO + 2OJ                        CH3COOH                                                

NB

The apparatus must be all glass because hot CH3COOH vapours attack wooden / rubber cork.

HCL cannot be used because it is too volatile.

The oxidising agent is required more than the ethanol for complete oxidation.

 Industrially, ethanoic acid is prepared from ethyne using mercury II tetraoxosulphate vi as a catalyst. The chemistry of Alkanoic Acids

The functional group (–COOH), The carboxyl group is responsible for the high reactivity of alkanoic acids. Using ethanoic acid as a typical carboxylic acid, the following are some of the reactions.

As an acid: 

1. It slightly ionises in water to give hydronium

 CH3COOH + H2O   ———   CH3COO–     +    H3O+  

1. It liberates hydrogen from strongly electropositive metals.
2. it neutralises a base 

CH3COOH + 2Na        2CH3COONa + H

CH3COOH + NaOH    CH3COONa + H2O

• Esterification Reaction: This is a reaction of carboxy acid with an alkanol in the presence of a cone. H2SO4 as a catalyst to produce ester and water only. The reaction is reversible and accompanied by a fruity or fragment small of the ester formal.

CH3COOH –CH3CH2OH  ========== CH3COOCH2CH3 + H2O

                                                      Ethyl ethanoate

      (i)   Esterification   (ii)   Alkaline hydrolysis

• Reduction of Alkanoic acid: Alkanoic acids can be reduced to primary alcohols using lithium tetrahydridoaluminate 

LiAlH4 is a reducing agent to supply hydrogen.

CH3COOH    + 4 [H]          CH3CH2OH + H2O

Uses Ethanoic Acids

Ethanoic acid is an important organic solvent

It is used in the food industry as vinegar for preserving and flavouring food. It is also used in coagulating rubber latex.

Ethanoic acid is used in making compounds like ethyl ethanoate (vinyl acetate ) which is used in vinyl resins, ethanoic anhydride which is used in aspirin, cellulose acetate which is used in packing, in vanishes in filter tips of cigarettes as well as in manufacture of synthetic fibres like rayon and artificial silk.

ALKANOATES

Sources of Alkanoates:

Animal and vegetable fats and oils are esters with high relative molecular masses, simple esters are present in fruits of plants and are responsible for the natural sweet smell of flowers and flavours of fruits.

Ethyl ethanoate has the flavour of pineapple

Penthylethanoate has the flavour of a banana

Octyl Ethanoate has that of the orange

 General Molecular formula

 The general Molecular formula for alkanoates (esters) is Cn H2nO2  where n = 2, 3, 4 etc. The formula is represented as R COOR1. R1 replaces the tonisable H atoms of the Carboxylic and R is the allyl group of the alkanol. The functional group is =COOR1 when the two alkyl groups R and R1 are Saturated the esters are simply alkanoates. 

NAMING OF ALKANOATES:

In the systematic name, the alkanol portion R1 is first named as alkyl followed by the acid portion RCOO- named alkanoates.

C2H5OH + CH3COOH    CH3COOC2H5 + H2O

Ethanol ethanoic acid              Ethyl ethanoate

PREPARATION OF ETHYL ETHANOATE IN THE LAB.

Ethyl ethanoate is prepared by the esterification between glacial acid and ethanoate is prepared by the esterification between glacial ethanoic acid and ethanol in the presence of Conc. H2SO4.

Procedure: Add about 10 drops of conc. H2SO4 to about 2. Cm3 of glacial ethanoic acid in a dry test tube. Mix well, add 4 cm3 of ethanol and mix again. Warm the mixture in water for about 25 minutes. Pour into a beaker partially filled with sodium or calcium chloride solution, the ethyl ethanoate floats on the mixture as oil globules.

C2H5OH + CH3COOH    CH3COOC2H5 + H2O

REACTIONS OF ALKANOIC ACIDS

Alkanoates are generally unreactive and undergo few reactions.

1. Hydrolysis; This is a reverse of esterification producing its component acid and ethanol. Ethyl ethanoate can be hydrolysed by water into its component acid and alkanol again. The reaction is extremely slow but is catalysed by H+ or OH– i.e. a dil. Acid or alkali. 

CH3COOC2H5   +   H2O ======   CH3COOH   +   C2H5OH

If an alkali is used as a catalyst, the carboxylic acid reacts immediately to produce the sodium salt of the acid.

CH3COOC2H5   +   NaOH ======   CH3COONa  +   C2H5OH

1. Alkaline hydrolysis is not reversible and leads to the formation of soap.
2. Ammonolysis; Ethyl ethanoate reacts with aqueous ammonia to produce a mixture of ethanamide and ethanol.

Uses of Esters.

Esters are used mainly as solvents for cellulose nitrate and quick-drying substances.

They are also used in perfumes and cosmetics and as artificial flavouring for food.

ASSIGNMENT

1. Ethanoic acid reacts with both sodium hydroxide and ethanol. Using equations, compare the two reactions and classify their products. 
2. Write the structural formula of the following esters

   (i). ethyl ethanoate  (ii) propyl ethanoate

3. Briefly describe how you can prepare ethyl ethanoate with the appropriate equation.