ALKANOIC ACID at a glance

 

ALKANOIC ACIDS

The alkanoic acids are a homologous series of organic compounds containing a carbonyl (-CO) group attached to the hydroxyl group (-OH).   They have the general molecular formula of C_nH_2n+1COOH and a functional group of R– COOH.

They are named by replacing the ending ‘e’ of the corresponding parent alkane with – oic acid. 

In the IUPAC method we take into consideration the functional group

    O
    ||
(–C–OH) and the positions of other substituents on the carbon chain.  The lowest number is given to the C- atom carrying the functional group.  

NAMING OF ALKANOIC ACIDS

The formular of the first 10 members of the series shown in the table below. Members. By applying the general molecular formular (C_nH_2n+1COOH) we have

When n=	General Molecular Formulae  CnH2n+1COOH	Name
0.	C0H2x0+1COOH =     HCOOH	Methanoic acid
1.	C1H2x1+1COOH   = CH3COOH	Ethanoic acid
2.	C2H2x2+1COOH    = C2H5COOH	Propanoic acid
3.	C3H2x3+1COOH    = C3H7COOH	Butanoic acid
4.	C4H2x4+1COOH     = C4H9COOH	Pentanoic acid
5.	C5H2x5+1COOH     = C5H11COOH	Hexanoic acid
6.	C6H2x6+1COOH     = C6H13COOH	Heptanoic acid
7.	C7H2x7+1COOH     = C7H15COOH	Octanoic acid
8.	C8H2x8+1COOH     =   C8H17COOH	Nonanoic acid
9.	C9H2x9+1COOH     =   C9H19COOH	Decanoic acid
10.	C10H2x10+1COOH  =  C10H21COOH	Undacanoic acid

 MOLECULAR STRUCTURES OF ALKANOIC ACIDS

N	ALKANOIC ACID	STRUCTURAL FORMULAR	MOLECULAR FORMULAR
1.	HCOOH Methanoic acid	O           ||          C -OH           |           H	HCOOH
2.	CH3COOH Ethanol	H   O         |   ||    H-C-C-OH         |           H	CH3COOH
3.	C2H5COOH Propanoic acid	H H  O       |   |   ||  H-C-C-C-OH       |   |          H H	CH3CH2COOH
4.	C3H7COOH Butanoic acid	H H H O         |   |   |   ||   H-C-C-C-C-OH         |   |   |           H H H	CH3(CH2)2COOH
5.	C5H11COOH Pentanoic acid	H H H H  O        |   |    |   |   ||   H-C-C-C-C-C-OH         |   |   |   |            H H H H	CH3(CH2)3COOH
6.	C6H13COOH Hexanoic acid	H H H H H  O        |   |   |    |   |   ||   H-C-C-C-C-C-C-OH        |   |   |    |   |            H H H H H	CH3(CH2)4COOH
7.	C7H15COOH Heptanoic acid	H H H H H H  O       |   |    |   |   |   |   ||  H-C-C-C-C-C-C-C-OH        |   |   |   |   |   |           H H H H H H	CH3(CH2)5COOH
8.	C8H17COOH Octanoic acid	H H H H H H H  O        |   |   |    |   |   |   |   ||   H-C-C-C-C-C-C-C-C-OH         |   |   |   |   |   |   |            H H H H H H H	CH3(CH2)6COOH
9.	C9H19COOH Nonanoic acid	H H H H H H H H  O       |    |   |   |   |   |    |   |   ||  H-C-C-C-C-C-C-C-C-C-OH       |   |   |    |    |   |   |   |          H H H H H H H H	CH3(CH2)7COOH
10.	C10H21COOH Decanoic acid	H H H H H H H H H  O      |   |    |  |    |   |   |   |   |   || H-C-C-C-C-C-C-C-C-C-C-OH       |   |   |   |   |   |   |   |    |   |       H H H H H H H H H H	CH3(CH2)8COOH

                                                                                                    

The alkanoic acids like the alkanols are classified into groups based on the number of caboxyl group present in the molecule. Thus we have  

1. Monocarboxylic acids: These are carboxylic acid which have only one -COOH per molecule. Examples include 

ii.   H   O

        |   ||
   H-C-C-OH
        |   
       H

2. Dicarboxylic acids: alkanoic acids with two -COOH groups. Examples include 

 (i)    C₁OOH   

         |              Ethanedioic acid              C₁OOH

    

(ii)  C₂OOH   

         |                              

        C₂H₂
                                   
                                                             C₃OOH
                                                            Propane-1,3-dioic acid 
3.  Tricarboxylic acids:-  These are carboxylic acids containig three carboxylic groups per molecule           

                  C₁OOH
|
                  C₂H – CH₃
|
          CH₃-C₃ – CH₃
|       
                  C₄OOH
            2,2,3- trimethyl butan-1,3- dioic acid

In this chapter we will be concentrating on monocarboxylic acids.

-They are colourless liquid at room temperatures

-lower members behave as typical acids, but as the number of carbon atom increases their solubility in water as well as their acidic nature decreases

-they have higher boiling points than normal because of the presence of hydrogen bonding

The first two members of the series are methanoic acid HCOOH and ethanoic acid with general formula CH₃COOH.

ETHANOIC ACID :- This is the second member of the series, it is a liquid at room temperature. it has a characteristic pungent smell.

LABORATORY PREPARATION OF ETHANOIC ACID

Ethanoic acid can be prepared in the laboratory in two ways or stages by oxidation of ethanol with potassium hexaoxodichromate (iv) (K₂Cr₂O₇) acidified with tetraoxosulphate (vi) (H₂SO₄)

STAGE 1:              K₂Cr₂O₇
               C₂H₅OH   →     CH₃CHO + H₂O
                Ethanol               Ethanal

                                     

STAGE 2:                      K₂Cr₂O₇ 
                       CH₃CHO   →     CH₃COOH
                        Ethanal                  ethanoic acid

PHYSICAL PROPERTIES OF ETHANOIC ACID

1. It is a colourless liquid

2. It has a pungent and characteristic  of vinegar odour

3. It has a boiling point of 118⁰C and freezes at temperature below 17⁰C (glass-like crystals known as glacier ethanoic acids

4. It is very soluble in water

5. It is weak electrolyte.

CHEMICAL PROPERTIES OF ETHANOIC ACID

1.      i).  It turns blue litmus paper red

2.      2) As an acid it reacts with alkalis and base to form salts called ethanoates (esters) and water    

 (i).  It reacts with sodium hydroxide (NaOH) to form sodium ethanoate (CH₃COONa) and water

          CH₃COOH + NaOH →CH₃COONa + H₂O

4.      ii) And with moderately reactive metals such as magnessium to liberate hydrogen gas.

→            2CH₃COOH + Mg   ^heat (CH₃COO)₂Mg+ H₂

5.      When heated with soda lime (NaOH) it forms methane gas (CH₄) and carbon (iv) oxide

          CH₃COOH + NaOH →   CH₄ + CO₂ 

3). ESTERIFICATION: -This is the process whereby alkanoic acids reacts with alkanol to form  alkanoate (ester) and water in the [presence of an acid as catalyst) 

RCOOH + ROH →   RCOOR + H2O        

 

 CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O
                                          Ethyl ethanoate

CH₃COOH + PCl5  → CH₃COCl(l) + HCl(l) +  PCl3(l)

USES OF ETHANOIC ACID

1. As vinegar for preserving food

2. For making cellulose ethanoate

3 . For making non-inflammable safety film

4. For making textile fibres such as rayon

5. For making vinylethanoate which is used in emulsion paints,

6. It is used in making adhesives for wood, glass and paper

7. It is used  to coagulate rubber latex.

TEST FOR ALKANOIC ACIDS

1. It has a characteristic Pungent and sharp smell and turns blue litmus paper red.

2. Put some of the unknown substance into a solution sodium hydrogentrioxocarbonate (IV) (NaHCO₃).  If there is effervescence and the of a gas is colourless, odourless and tasteless then the substance is ethanoic acid.