ORGANIC CHEMISTRY. PART 2

 Isomers 

Isomerism is the phenomena whereby certain compounds, with the same molecular formula, exist in different forms owing to their different organisation of atoms. Isomers are atoms with the same molecular formula but different structural formulas.

The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry. Structural Isomers Structural isomers have different structural formulas because their atoms are linked together in different ways. 

This arises owing to: 

1. Arrangement of Carbon skeleton e.g. The formula C4H10 represents two possible structural formulae, butane and mythylpropane 

2. Position of Functional group e.g. propan-1-ol and propan-2-ol 

3. Different Functional groups e.g. the molecular formula C2H60 represents both ethanol and methoxymethane Cyclic alkanes are isomeric with alkenes, e.g. cyclopropane and propene 

Alkanoic acids 

These are organic compound with the functional group -COOH and the general formula CnH2n+1COOH, where n is the number of carbon atoms. 

 Naming of alkanoic acids 

Alkanoic acids are named by replacing the last letter '-e' of the parent alkane with the ending '-oic'. If the number of carbon atoms is 3 the formula is C3H7COOH. Since this compound contains four carbon atoms, its name is butanoic acid. Remember that the general formula already contains a carbon (C) atom. 

 Alkanoic acids are commonly known as fatty acids. This is because many of them were obtained from fat sources. 

**********The table below gives some alkanoic acids and their natural sources. 

Properties of alkanoic acids

 

1. Alkanoic acids undergo neutralization reaction with alkalis to produce salt and water. CH3COOH + NaOH → CH3COON + H2O 

2. They react with reactive metals to give a salt and a hydrogen gas. 2CH3COOH + Zn → (CH3COO)2Zn + H2 

3. They react with trioxocarbonates (IV) to give carbon dioxide and water. Na2CO3+2CH3COOH → 2CH3COON + CO2 + H2O 

Uses of methanoic acid 

1.It is used to remove calcium trioxocarbonate (IV) (chalk) deposits from containers. 

2. It is used as drying agent in making textiles. 

3. It is used as starting chemical in the production of other chemicals. 

Uses of ethanoic acid 

1. It is used as solvent to dissolve other carbon compounds. 

2. It is used as vinegar in flavouring food. 

3. It is used for cleaning dirty metal surfaces at home. 

4. It is used in laundries for brightening colours. 

5. It is used in drug production. 

6. It is used to make cellulose ethanoate for use in vanishes, lacquers, cinema films and some synthetic fibres such as rayon. 

Alkyl alkanoates (esters) 

Esters are produced when an alkanol reacts with an alkanoic acid. alkanol + alkanoic acid ⇌ ester + water .

This process is known as esterification. 

 Esters have a characteristic pleasant smell. Esters occur naturally in fruits (such as banana, pineapple etc.) and flowers (such as queen of the night, frangipani, thumbergia etc), giving them their pleasant smell. 

They have the general formula RCOOR’, where R and R’ belong to the same or different alkyl groups.

 Structure and naming of esters 

 To write the correct structure of the ester, the alkanoic acid loses the –OH group and the alkanol loses the –H atom. 

 The naming is done by combining the alkanol and the alkanoic acid. 

 The alkyl of the alkanol is stated first, followed by the acid name with the – oic replaced by –oate. 

For example if methanol, CH3OH, reacts with ethanoinc acid, CH3COOH, the ester, methyl ethanoate CH3COOCH3, is formed. CH3OH + CH3COOH → CH3COOCH3 + H2O 

 The alkyl group of methanol is methyl and the acid part of ethanoic acid becomes ethanoate, hence the name, methyl ethanoate to the resulting ester. 

 The reaction takes place in the presence of heat, with concentrated H2SO4, acting as a catalyst to speed up the reaction.

Properties of esters 

Physical properties 

1. Esters have pleasant smell. 

2. They are usually colourless. 

3. Esters with short chains are soluble in water. 

Chemical properties 

 Since the esterification reaction is reversible, an ester can be hydrolysed to form an alkanol and alkanoic acid. CH3COOH2CH3 ⇌ CH3COOH + CH3CH2OH 

 The addition of lithium aluminium hydride can reduce an ester to ethanol. CH3COOCH3 LiAIH CH3CH2OH + CH3OH 

Uses of esters 

1. Esters are used for making artificial flavours and essences.  

2. These are used in cold drinks, icecreams, sweets and perfumes. 

3. They are used as solvents for oils, fats, gums, resins, cellulose, paints, varnishes, etc. 

4. Esters are used as plasticizers (added to plastics or other materials to make or keep them soft or pliable). 

Fats and oils 

Fats and oils are formed when one or more of the hydrogen atoms of the glycerol group are replaced by a long chain ester (acid alkanoate). 

This is also known as glyceride. Esters with all the three hydrogen atoms replaced are called triglycerides or triester of glycerol. Both fats and oils contain acid and alkanol parts, which make them esters of the glycerol, long-chain fatty acids and propanetriol. 

 CH2 – OH 

  | 

 CH - OH

  | 

 CH2 - OH   (Structure of glycerol )

Butyric acid (butanoic acid) is one of the saturated short-chain fatty acids responsible for the characteristic flavour of butter CH3CH2CH2COOH or CH3(CH2)2COOH 

(Butanoic acid )

Differences between fats and oils 

Fats                               Oils 

1. Usually have Saturated hydrocarbon chain                        Have unsaturated hydrocarbon chain 

2.  Solid at normal temperature                                   Liquid at normal temperature 

3. Have high melting point                                        Low melting point

Uses of fats and oils 

1. Dietary fats supply energy, carry fat-soluble vitamins (A, D, E, K), and are a source of antioxidants and bioactive compounds. 

2. Fats are incorporated as structural components of the brain and cell membranes. 

3. Fats and oils are used in the manufacture of soap, drugs, margarine etc. 

4. They are used to make candles, paints, glycerols etc. 

Esterification and neutralization 

Esterification is the reaction between alkanols and alkanoic acids to produce esters and water. 

This reaction is reversible, and the catalyst, concentrated H2SO4 is used to dehydrate the ester. 

The following example is an esterification reaction. CH3COOH + C2H5OH ⇌ CH3COOH5C2 + H2O NB: The acid is named by counting up the total number of carbon atoms in the chain - including the one in the -COOH group. 

 For example, CH3CH2COOH is propanoic acid, and CH3CH2COO is the propanoate group. Since esterification reaction is reversible, the ester produced can be broken up into the constituent alkanol and alkanoic acid. 

 This process is known as saponification. 

 Saponification is the reaction between an ester and an alkali to produce soap and glycerol. Water may be produced but in small amount. fat or oil + sodium hydroxide→soap + glycerol 

Esterification and neutralization (reaction of acid and base) have some differences as well as a similarity. 

Differences between esterification and neutralization 

Esterification                                                  Neutralization 

 Reaction is reversible                               Reaction is irreversible 

Reaction is slow                             Reaction is fast 

Heat involved                                    No heat involved 

Catalyst needed                                      No catalyst needed 

Products are ester and water                               Products are salt and water 

Reactants are alkanol and alkanoic acid                            Reactants are acid and base (alkali).

Similarity between esterification and neatralization 

One similarity between esterification and neutralization is water is produced as a by-product in both reactions.