Hydrocarbons are of different types depending upon the types of carbon-carbon bonds present, they can be classified into three main categories:
Alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds.
) is the first member of this family. Methane is a gas found in coal mines and marshy places.
Chapter 13 : Hydrocarbons
First three alkanes– methane, ethane and propane have only one structure but higher alkanes can have more than one structure.
Structures for C4
. Four carbon atoms of C4
can be joined either in a continuous chain or with a branched chain.
13.2.1 Nomenclature and Isomerism
From Alkyl Halides: Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.
From carboxylic acid: Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as decarboxylation.
Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms.
Some chemical properties are:
Controlled Oxidation: Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.
Isomerisation: n-alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.
Alkanes contain carbon-carbon sigma bonds.
Spatial arrangements of atoms which can be converted into one another by rotation around a C-C single bond are called conformations.
Ethane molecule (C2
) contains a carbon – carbon single bond with each carbon atom attached to three hydrogen atoms.
Conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed. And it’s represented by:
Alkenes are unsaturated hydrocarbons containing at least one double bond.
Carbon-carbon double bond in alkenes consists of one strong sigma (ó) bond (bond enthalpy about 397 kJ mol–1) due to head-on overlapping of sp2
hybridized orbitals and one weak pi (ð) bond (bond enthalpy about 284 kJ mol–1) obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms.
Orbital diagrams of ethene molecule.
13.3.1 Structure of Double bond
The suffix ‘ene’ replaces ‘ane of alkanes. It may be remembered that first member of alkene series is: CH2
(replacing n by 1 in CnH2
n) known as methene but has a very short life.
Alkenes show both structural isomerism and geometrical isomerism.
From Alkyl Halides: Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol say, ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation.
Ethene is a colourless gas with a faint sweet smell.
Addition Reaction of HBr to Unsymmetrical Alkenes:
Hydrogen bromide provides an electrophile. H+
. which attacks the double bond to form carbocation.
The secondary carbocation (b) is more stable than the primary carbocation (a); therefore, the former predominates because it is formed at a faster rate.
The carbocation (b) is attacked by Br– ion to form the product as
Alkynes are also unsaturated hydrocarbons. They contain at least one triple bond between two carbon atoms.
Alkanes replacing ‘ane’ by the suffix ‘yne’.
There are two possible structures for butyne – (i) but-1-yne and (ii) but-2-yne. Since these two compounds differ in their structures due to the position of the triple bond, they are known as position isomers.
13.4.1 Nomenclature and Isomerism
13.4.2 Structure of Triple Bond
From Calcium Carbide: Ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with Coke.
Some properties of alkynes are:
All alkynes are colourless. Ethyene has characteristic odour. Other members are odourless.
Addition of Dihydrogen
Addition of Halogens
Cyclic Polymerization: Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization.
These hydrocarbons are also known as ‘arenes’. Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids.
Three different position isomers are possible. The 1, 2 or 1, 6 is known as the ortho (o–), the 1, 3 or 1, 5 as meta (m–) and the 1, 4 as para (p–) disubstituted compounds.
13.5.1 Nomenclature and Isomerism
The molecular formula of benzene is C6
The Kekulé structure indicates the possibility of two isomeric 1, 2-dibromobenzenes.
13.5.2 Structure of Benzene
Benzene was considered as parent ‘aromatic’ compound. And possessing following characteristics:
- Complete delocalisation of the π electron
- Presence of(4n+2) π electron in the ring.
Reduction of phenol: Phenol is reduced to benzene by passing its vapours over heated zinc dust.
13.5.4 Preparation of Benzene
Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma.
13.5.5 Physical Properties
Nitration: Benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid.
13.5.6 Chemical Properties
Friedel-Crafts Alkylation Reaction: benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenzene is formed.
Ortho and para products or meta product is predominantly formed. It has also been observed that this behaviour depends on the nature of the substituent already present in the benzene ring and not on the nature of the entering group.
Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property.
Some of the carcinogenic hydrocarbons are given:
13.5.6 Directive influence of a functional group in monosubstituted benzene