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Organic Chemistry - I

Organic Chemistry -I Synopsis

Synopsis


Introduction
Organic Chemistry
It is the chemistry of specific carbon compounds except oxides, carbonates and carbides.

Hydrocarbons
Organic compounds composed of carbon and hydrogen only.
Examples: Methane [CH4], Ethane [C2H6]

Sources of Organic Compounds

  • Plants: Compounds like sugar, starch and cellulose.
  • Animals: Proteins, urea
  • Coal: Destructive distillation of coal produces dyes, drugs etc.
  • Petroleum: Organic compounds like gasoline, petrol and naptha are obtained from petroleum.
  • Fermentation: Compounds like ethyl alcohol and acetic acid are obtained by fermentation.
  • Wood: Methyl alcohol, acetone etc. are obtained by destructive distillation of wood.
  • Synthetic method: Most organic compounds are synthesised in the laboratory.

Unique Nature of Carbon Atoms

  • The number of carbon compounds already known at present is more than 5 million.
  • Every day more new compounds are isolated or prepared in the laboratories.
  • Two characteristic properties of the carbon element which lead to the formation of a very large number of organic compounds are

Tetravalency of Carbon

  • Carbon forms four covalent bonds by mutually sharing its four electrons with other atoms.
  • Carbon is hence tetravalent or exhibits tetravalency. 
     

Catenation

  • The property of carbon element due to which its atoms can join one another to form long carbon chains is called catenation.
  • Catenation is maximum in carbon because the value of the C–C bond energy is maximum.
  • Carbon undergoes self-linking forming straight, branched and closed chains.
  1. Straight chain of carbon atoms
  2. Branched chain of carbon atoms
  3. Closed chain or ring chain of carbon atoms

Catenation and tetravalency also result in the formation of single, double and triple bonds.

 

Types of Organic Compounds

Classification of Organic Compounds (Hydrocarbons)

Comparison of Saturated and Unsaturated Hydrocarbons

 

Isomers
Compounds with the same molecular formula but different structural formula are known as isomers, and the phenomenon is known as isomerism.
Isomers differ in physical properties or chemical properties or both.
Examples: Butane and isobutane are two different compounds with the same molecular formula C4H10.

              

Causes of Isomerism

  • Difference in the mode of linking of atoms.
    For example:C4H10O shows different types of linkages and this type of isomerism is called structural
    isomerism.



  • Difference in the arrangement of atoms or groups in space and this type of isomerism is called stereoisomerism.
    For example:
    1, 2 – dichloroethene

Different types of Structure Isomerism

  1. Chain isomerism
    Two or more compounds which have a similar molecular formula but different arrangement of carbon atoms in straight or branched chains are referred to as chain isomers, and the phenomenon is known as chain isomerism.
    For example: Butane (C4H10)
  2. Position isomerism
    When two or more compounds with the same molecular formula differ in the position of the substituent atom or functional group on the carbon atom, they are called position isomers, and the phenomenon is known as position isomerism.
    For example:
  3. Functional isomerism
    Two or more compounds with the same molecular formula but different functional groups are called functional isomers, and the phenomenon is known as functional isomerism.
    For example:
    CH3 CH2 OH  and  CH3 – O – CH3
    Ethanol                   Dimethyl ether
  4. Metamerism
    It arises because of unequal distribution of alkyl groups on either side of the functional groups in the molecules.
    For example:
    CH3 O C3H7        and       C2H5OC2H5
    Methoxy propane             Ethoxyethane

Homologous Series 

It is a group of organic compounds with a similar structure and similar chemical properties in which the successive compounds differ by a -CH2 group.

Characteristics of a Homologous Series

  1. Each member of the series differs from the preceding one by the addition of a CH2 group and by 14 amu.
  2. All members of a homologous series share a general formula.
    For example, the general formula for alkane is CnH2n+2 and that for alkene is CnH2n.
  3. Physical properties of the members show gradation in properties as the molecular mass increases.
  4. The chemical properties also show gradient similarity.
    For example, methane and ethane react with chlorine to form methyl chloride and ethyl chloride, respectively..
    CH4 + Cl2 → CH3Cl
    C2H6 + Cl2 → C2H5Cl
  5. All members of a homologous series can be prepared by the same general method of preparation.
    For example, alcohols can be prepared from alkyl halides.
    CH3Br + KOH  →  CH3OH + KBr
    C2H5Br + KOH →  C2H5OH + KBr

Examples of a Homologous Series

  1. Alkane



  2. Alkene

Significance of a Homologous Series

  1. It helps in the systematic study of organic compounds.
  2. iIt helps to predict the properties and the nature of other elements of the series if the same is known of the first few members.

Nomenclature
It is the system of assigning names to organic compounds.
Systems of Nomenclature

  1. Trivial system
  2. UPAC (International Union +of Pure and Applied Chemistry) system

According to the IUPAC system, the name of an organic compound consists of three parts:

  1. Root word
  2. Suffix
  3. Prefix
  • Root Word
    It depends on the number of carbon atoms present in the longest carbon chain selected.



  • Suffix
  • The root word is followed by an appropriate suffix which represents the nature of the bond in a carbon–carbon atom.

     
  • Prefix
    It denotes the substituent, alkyl or functional group and its position in the carbon chain.
    Di-, tri- and tetra- are used for two, three and four groups of the same type, respectively.
    In naming an organic compound, the following simple rules are followed:
  1. Selection of carbon chains: The longest continuous chain of ‘C’ atoms, known as parent chain, is selected. The longest chain need not be straight. For example:
    The longest chain is of 6 carbon atoms, so the root word is ‘hex’.
    (i)
     
    (ii)
           
  2. Here, the longest chain is of 7 carbon atoms, so the root word is ‘hept’, the remaining carbon atoms are substituents.
    The branch chains are considered substituents, and their positions are indicated by the number of carbon atoms to which they are attached.
    For example:
               
    2-methyl (methyl is attached to the 2nd carbon)

  3. The carbon atoms of the longest chain are numbered in such a way that the alkyl groups get the smallest possible number.
    For example:
                
  4. In case, any functional group is present in the chain, the carbon atoms are numbered in such a way that the functional group gets the smallest possible number.
    For example:

  5. In case, different types of substituents are attached to the chain, they are arranged and named alphabetically.
  6. The positions of alkyl groups are indicated by writing the position and name of the alkyl group just before the name of the parent hydrocarbon.

    3-ethylheptane

  7. Multiple alkyl groups are labelled with the Greek numerical prefixes such as ‘di’ for two, ‘tri’ for three, ‘tetra’ for four, ‘penta’ for five. If two alkyl groups are on the same carbon atom, then the numeral is repeated. 
                       
    Aliphatic Hydrocarbons Alkanes:
    Comparative study of Methane and Ethane:

    Structure of Methane and Ethane


  • Laboratory Preparation of Methane
    Reactants: Sodium ethanoate and soda lime
    Diagram: