22. Organic Chemistry

Homologous Series
Characteristics of a homologous series
  • Each member of the series is represented by a general formula.
  • The compounds in each family have the same reactive group of atoms called the functional group.
  • Each member differs from the next consecutive member by a -CH2- group
  • All members show a gradual change in physical properties as a result of increase in size and mass of the molecules.
  • All members have similar chemical properties.

Structural Formula
  • The structural formula shows how the various atoms in each molecule are bonded.
  1. Each carbon atom can form four covalent bonds.
  2. Each hydrogen atom forms one covalent bond.
  3. Each oxygen forms two covalent bonds.

Isomers
  • Organic compounds with the same molecular formula but different structural formula.

Alkanes
  • Alkanes have the general molecular formula CnH2n+2, where n = 1,2,3 ……
  • Alkanes are called saturated hydrocarbons, with single covalent bonds between carbon atoms; hence they are generally unreactive.
  • Substitution reactions is a reaction in which an atom or group of atoms from one molecule is replaced by another atom or group of atoms from another molecule.
  • Alkanes react with chlorine gas in the presence of ultraviolet light to form halogenoalkanes.

Alkenes
  • Alkenes have the general molecular formula CnH2n, where n = 2,3,4… . 
  • Alkenes are called unsaturated hydrocarbons, with double covalent bonds C = C bond between carbon atoms; hence they are generally reactive.
  • Unsaturated : contains carbon to carbon double covalent bonds and  can undergo further chemical reactions.

Types of Addition Reactions
  • Bromination:  Forms halogenoalkanes - Aqueous bromine is used as a test to distinguish between a saturated and unsaturated compound.
  • Hydrogenation: Forms alkanes (Condition: Nickel catalyst at 200°C)
  • Hydration: Forms alcohol (Condition: Steam, 300°C, 60 atm, phosphoric(V) acid as catalyst)

To Manufacture Alkenes
  • By catalytic cracking of large alkane molecules
  • Conditions: Heated catalyst (aluminium oxide or silicon dioxide) and High temperature
  • Cracking produces alkanes and alkenes OR alkenes and hydrogen.

To Manufacture Margarine
  • Margarine is made by reacting vegetable oil with hydrogen in the presence of a nickel catalyst.
  • The hydrogen molecules are added to the C=C bonds using hydrogenation.

Alcohols
  • Alcohols have the general molecular formula CnH2n+1OH, where n = 1,2,3…..
  • The functional group in alcohols is the -OH group.
  • Alcohols have low boiling points
  • The first four members are liquid at r.t.p.
  • They are generally soluble in water.

To Form Alcohols
  • Hydration of alkenes
  • Fermentation of sugar (glucose): Mixture of sugar and yeast undergo fermentation
  • Conditions: yeast, temperature of about 37°C in the absence of oxygen

Oxidation of Alcohols
  • Alcohols are oxidized to carboxylic acids
  • Conditions: Presence of an oxidizing agent (acidified potassium manganate(VII) or acidified potassium dichromate(VI)) OR oxygen in the air in the presence of bacteria

Carboxylic Acids
  • Carboxylic acids have the general molecular formula CnH2n+1COOH where n = 0,1,2,3……..
  • The functional group in carboxylic acids is the -COOH group.

Carboxylic Acids Reactions
  1. Reaction with carbonates: Carboxylic acids react with carbonate to form salt, water and carbon dioxide.
  2. Reaction with bases: Carboxylic acids react with alkalis to form salt and water.
  3. Reaction with metals: Carboxylic acids react with reactive metals to produce hydrogen gas.
  4. Esterification: An alcohol and carboxylic acid react to form a sweet-smelling compound called an ester.
  • Conditions: Heated with concentrated sulfuric acid as catalyst