22. Organic Chemistry
Homologous Series
Characteristics of a homologous series
Structural Formula
Isomers
Alkanes
Types of Addition Reactions
To Manufacture Alkenes
To Manufacture Margarine
To Form Alcohols
Oxidation of Alcohols
Carboxylic Acids
Carboxylic Acids Reactions
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.
- Each carbon atom can form four covalent bonds.
- Each hydrogen atom forms one covalent bond.
- 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 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 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
- Reaction with carbonates: Carboxylic acids react with carbonate to form salt, water and carbon dioxide.
- Reaction with bases: Carboxylic acids react with alkalis to form salt and water.
- Reaction with metals: Carboxylic acids react with reactive metals to produce hydrogen gas.
- Esterification: An alcohol and carboxylic acid react to form a sweet-smelling compound called an ester.
- Conditions: Heated with concentrated sulfuric acid as catalyst