Revision Notes: Organic Chemistry - Hydrocarbons | Chemistry Class 10 ICSE PDF Download

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)

Unique Nature of Carbon

Tetravalency of Carbon

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

Catenation

• It is the tendency of an element to form chains of identical atoms.
• 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.
  
• Catenation and tetravalency also result in the formation of single, double and triple bonds.
  

Classification of Organic Compounds

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
i. Each member of the series differs from the preceding one by the addition of a CH₂ group and by 14 amu.
ii. All members of a homologous series share a general formula. 
For example, the general formula for alkane is C_nH_2n+2 and that for alkene is C_nH_2n.
iii. The physical properties of the members show gradation in properties as molecular mass increases.
iv. 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
v. 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.

Significance of a Homologous Series
i. Helps in the systematic study of organic compounds.
ii. Predicts the properties and the nature of other elements of the series if the same is known of the first few members.

Isomers

Compounds with the same molecular formula but different structural formula are known as isomers, and the phenomenon is known as isomerism.
Examples: Butane and isobutane are two different compounds with the same molecular formula C₄H₁₀.

Causes of Isomerism
i. Difference in the mode of linking of atoms.
ii. Difference in the arrangement of atoms or groups in space.

Different Types of Structural Isomerism
i. 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.
ii. 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.
iii. 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.
iv. Metamerism
It arises because of unequal distribution of alkyl groups on either side of the functional groups in the molecules.

Nomenclature

It is the system of assigning names to organic compounds.
The Systems of Nomenclature Are
i. Trivial system
ii. IUPAC (International Union of Pure and Applied Chemistry) system
According to the IUPAC system, the name of an organic compound consists of three parts:
i. Root word
ii. Suffix
iii. Prefix

i. Root word
It depends on the number of carbon atoms present in the longest carbon chain selected.

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

iii. 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.

Functional Group

It is an atom or a group of atoms which defines the structure (or the properties of a particular family) of organic compounds.
Characteristics of a Functional Group
i. Compounds of the same functional group are identified using the same types of tests.
ii. The physical and chemical properties of the compounds of different functional groups are different.
iii. There exists a homologous series of compounds containing a particular type of functional group.

Alkanes

• Alkanes are hydrocarbons in which all the linkages between the carbon atoms are single covalent bonds.
• Compounds are known as saturated hydrocarbons because all the four valencies of carbon are fully satisfied.
• General formula : C_nH_2n+2
• These hydrocarbons are relatively unreactive under ordinary conditions so they are also called paraffins.

Isomerism in Alkanes

• Alkanes with more than three carbon atoms form isomers.
• The various isomers differ in the framework of the carbon chains.
  Example: Isomers of Pentane (C₅H₁₂)

Laboratory Preparation of Methane and Ethane
CH₃COONa + NaOH Na₂CO₃ + CH₄
C₂H₅COONa + NaOH Na₂CO₃ + C₂H₆

Methods of Preparation of Methane and Ethane

1. From iodomethane or bromoethane:
   CH₃I + 2[H] → CH₄ + HI
   C₂H₅I + 2[H] → C₂H₆ + HI
2. Methane is produced on addition of water to aluminium carbide at room temperature. 
   Al₄C₃ + 12H₂O → 3CH₄ + 4Al (OH)₃
3. Ethane from alkyl halides:
   2CH₃I + 2Na CH₃–CH₃ + 2NaI

This reaction is known as the Wurtz reaction.
Chemical Properties

1. Substitution reaction
   (i) Reaction with halogens
   CH₄ + Cl₂CH₃Cl + HCl
   CH₃Cl + Cl₂ → CH₂Cl₂ + HCl
   CH₂Cl₂ + Cl₂ → CHCl₃ + HCl
   
   (ii) Reaction with oxygen 
   CH₄ + 2O₂ → CO₂ + 2H₂O 
   2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
   Insufficient supply of air
   2CH₄ + 3O₂ → 2CO + 4H₂O 
   2C₂H₆ + 5O₂ → 4CO + 6H₂O
2. Cracking or Pyrolysis
   
3. Catalytic oxidation of alkanes
   
4. Slow combustion
   

Alkenes

• Alkenes are unsaturated aliphatic hydrocarbons containing a carbon–carbon double bond.
• They are also called olefins because of their tendency to form oily products.
• The general formula of alkenes is C_nH_2n.

Structure of Ethene

• Two carbon atoms linked by a double covalent bond.
• A double covalent bond is formed by sharing of two pairs of electrons between the two carbon atoms.
• Four C–H single covalent bonds and one C=C double covalent bond.
• It is a planar molecule and all bond angles (H–C–H and H–C=C) are of 120°.

Preparation of Ethene

i. Dehydration of ethyl alcohol

ii. Dehydrohalogenation of ethyl bromide

iii. Cracking of methane

Chemical properties
Addition Reactions
(i) Catalytic hydrogenation 

(ii) Halogenation

(iii) Reaction with Halogen Acids

(iv) Reaction with Sulphuric acid

(v) Ozone

(vi) Oxidation

(vii) Polymerisation

Alkynes

• Alkynes are unsaturated aliphatic hydrocarbons containing a carbon–carbon triple bond in their molecule.
• The general formula of alkynes is C_nH_2n−2.
• They are more reactive than alkenes because of the presence of a triple bond, often referred to as an acetylenic linkage.

Structural formula of Ethyne

Preparation of Ethyne
i. Laboratory preparation from calcium carbide

ii. From 1, 2-dibromoethane

iii. From methane

Chemical Properties

• Addition Reactions
  (a) Catalytic Hydrogenation
  (b) Halogenation(c) Reaction with Halogen Acids
  (d) Ozone 
  (e) Oxidation of ethyne (Combustion)
  

Alcohols

• Alcohols are hydroxyl derivatives of alkanes obtained by replacement of one, two or three hydrogen atoms of alkanes by the corresponding number of –OH groups.
• The hydroxyl group is the functional group of alcohols.
• The general molecular formula of alcohols is CnH2n+1 OH.

Preparation of Ethanol
(i) Laboratory preparation by hydrolysis of alkyl halides

(ii) Industrial Method
(a) Hydration of Ethene

(b) Fermentation of Carbohydrates

Chemical properties

1. Combustion
   C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
2. Oxidation with K₂Cr₂O₇
   
3. Reaction with Sodium
   2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
4. Reaction with Acetic acid 
   C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O
5. Reaction with Sulphuric acid
   
6. Reaction with PCl
   3C₂H₅OH + PCl₃ → 3C₂H₅Cl + H₃PO₃

Carboxylic Acids

• Carboxylic acids are organic compounds containing a carboxylic group (–COOH) attached to an alkyl group or to a hydrogen atom.
• Representation of carboxylic acids: R-COOH (R is either –H or alkyl)
• The functional group of carboxylic acids: –COOH (carboxylic)
• The acidic character in carboxylic acids is because of the presence of the replaceable hydrogen atom in the carboxylic group.

Preparation of Acetic Acid
(A) By oxidation of ethyl alcohol

(B) By hydrolysis of ethyl acetate

Chemical Properties

1. It is a weak acid and turns blue litmus red.
2. Reaction with Alkalis
   CH₃COOH + NaOH → CH₃COONa + H₂O
   CH₃COOH + NH₄OH → CH₃COONH₄ + H₂O
3. Reaction with Carbonates
   2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
   CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
4. Reaction with Alcohols
   CH₃COOH + C₂H₅OH CH₃COOC₂H₅ + H₂O
5. Reaction with PCl₃
   CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl
6. Reduction
   CH₃COOH + 4[H] →C₂H₅OH + H₂O