Classification
* Hydrocarbons are organic compounds containing only hydrogen and carbon.
* They play important role in our life as they meet more than 90 percent of the energy requirements of the world. Also they are the major sources of various other organic compounds which find use in different walks of life.
* On the basis of carbon skeleton, that is, type of carbon-carbon bonds, hydrocarbons can be classified into three main categories-saturated, unsaturated and aromatic hydrocarbons.
* Saturated hydrocarbons are the hydrocarbons consisting of carbon-to-carbon single bonds.
* Unsaturated hydrocarbons contain multiple carbon-to-carbon bonds.
* Aromatic hydrocarbons are cyclic hydrocarbons with alternative double bonds in conjugation.
Alkanes: Nomenclature and Isomerism
* Alkanes are the simplest form of hydrocarbons containing only carbon and hydrogen.
* Its molecules consist of carbon-carbon single bonds and carbon-hydrogen bonds.
* Their general formula is CnH2n+2.
* Alkane series can be obtained by replacing one of the hydrogen atoms of methane by the – CH3 group.
* Depending on the number of carbon atoms that a single carbon atom is attached to, the carbon atom can be called as either primary, secondary, tertiary or quarternary.
* Higher alkanes with more than one structure are called isomers.
* The properties of alkanes depend on their structure, and hence, it is important to be able to name them and to be able to draw their structures from a given name.
Alkanes: Preparation
* Unsaturated hydrocarbons like alkenes react with hydrogen in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes.
* Alkyl halides on reduction with reducing agents such as zinc copper couple in ethanol or zinc and dilute HCl gives alkanes.
* Alkyl halides on treatment with metallic sodium in presence of dry ether give alkanes with even number of carbon atoms. This reaction is known as wurtz reaction.
* Sodium salts of carboxylic acids on heating with soda lime give alkanes with one carbon atom less than the parent carboxylic acid.
* Kolbe’s electrolytic method is another method to prepare alkanes from carboxylic acids.
Alkanes: Properties
* Physical properties of alkanes depend on the Van der waals forces between molecules.
* Linear or straight chain alkanes have a higher boiling point than its branched isomers.
* Alkanes are mostly inert, but do undergo substitution reactions under certain conditions.
Alkanes: Conformation
* The spatial inter-convertible arrangements of atoms by rotation around a carbon – to – carbon single bond are called conformations or conformers or rotamers.
* The spatial arrangements of the hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom are called conformational isomers.
* The spatial arrangement in a molecule wherein the hydrogen atoms are closest to each other is known as eclipsed conformation.
* The spatial arrangement in a molecule wherein the hydrogen atoms are farthest from each other is known as staggered conformation.
* All the arrangements between the eclipsed and staggered positions are known as skew conformations.
* The repulsive interaction between the electron clouds, which affects the stability of a conformation, is called torsional strain.
* The magnitude of torsional strain depends upon the torsional angle or dihedral angle, which is the angle of rotation about the carbon – to – carbon bond.
Alkenes: Nomenclature and Isomerism
* The unsaturated hydrocarbons with a double bond in between carbon atoms are known as alkenes.
* The general molecular formula of Alkenes is CnH2n
* In the IUPAC system, alkenes are named by replacing the ‘ane’ of the corresponding alkanes by the suffix ‘ene’.
* Alkenes show structural as well as stereo isomerism.
* In structural isomers, alkenes show chain and position isomerism.
* In stereo isomerism, they show cis and trans isomerism.
* Trans isomers are more stable than cis isomers.
Alkenes: Preparation and Physical Properties
* Alkenes can be prepared in four ways:
• From alkynes: by reduction
• From alkyl halides: by dehydrohalogenation
• From vicinal dihalides: by dehalogenation
• From alcohols: by acidic dehydration
* The first three members of the alkene homologous series are gases, the fourth to fourteenth are liquids, and all members above fifteenth are solids.
Alkenes: Chemical Properties
* The typical reactions of alkenes involve the breaking of the weak pi bond to form two sigma bonds. Such reactions are called addition reactions. These addition reactions are usually electrophilic in nature as the pi electrons of carbon-carbon double bond are available to any species seeking electrons.
* Alkenes undergo the following reactions:
• Addition of dihydrogen: In this reaction, alkenes react with dihydrogens to form alkanes. It can take place only in the presence of catalysts such as finely divided nickel, palladium or platinum.
• Addition of halogens: In this reaction, alkenes form dihaloalkanes.
• Addition of halogen acid: In this reaction, alkenes from haloalkanes.
• Addition of sulphuric acid: In this reaction alkenes form alkyl hydrogen sulphates.
• Addition of dihydrogen: In this reaction, alkenes react with dihydrogens to form alkanes. It can take place only in the presence of catalysts such as finely divided nickel, palladium or platinum.
• Oxidation: Alkenes on reacting with cold, dilute, aqueous solution of KMnO4 called Baeyer’s reagent, produce vicinal glycols, that is 1, 2-diols.
• Ozonolysis: Alkenes react with ozone to form ozonides. The ozonides on hydrolysis with water in presence of zinc form either aldehydes or ketones.
• Polymerisation: In this, a large number of molecules of the same species join together to form a giant molecule, called a polymer. Alkenes undergo addition polymerization reactions when heated under pressure, in the presence of suitable catalysts.
Aromatic Hydrocarbons: Carcinogenicity and Toxicity
* Aromatic hydrocarbons containing two or more fused rings are called polycyclic aromatic compounds or polynuclear aromatic hydrocarbons.
* Coal tar is the main source of polynuclear aromatic hydrocarbons.
* Polynuclear aromatic hydrocarbons possess either a linear or an angular structure.
* Polynuclear aromatic hydrocarbons are carcinogenic.
* Polynuclear aromatic hydrocarbons enter the environment due to the incomplete combustion of coal, petroleum, tobacco, etc. These compounds damage DNA, thereby leading to cancer.
Alkynes: Nomenclature and Isomerism
* The group of unsaturated hydrocarbons with the general formula, CnH2n-2, is known as alkynes.
* Alkynes contain at least one triple bond between two carbon atoms.
* In the IUPAC system, alkynes are named by replacing the ‘ane’ of the corresponding alkanes by the suffix ‘yne’.
* When two compounds differ in their structure due to the position of the triple bond, they are known as position isomers.
* The chain isomers differ in their chain structure.
* As a triple bond is shorter, it is stronger compared to single and double bonds.
Alkynes: Preparation
* Alkynes can be prepared from Calcium Carbide or from Vicinal Dihalides.
* When Calcium Carbide is treated with Water, Ethyne or Acetylene is formed.
* When Vicinal Dihalide is treated with Alcoholic Potash, Alkenyl Halide is formed.
* Alkenyl Halide, when treated with Sodamide, gives an Alkyne.
Alkynes: Acidic Character
* The first three of the alkyne series are gases, from the fourth to the eleventh in the series are liquids, while the higher ones in the series are solids.
* All alkynes are colourless and odourless, except ethyne, which has a characteristic smell.
* The C-H bond attached to carbon atom with triple bond in alkynes exhibit some polarity, unlike alkanes and alkenes, which are non-polar.
* The melting point, boiling point and density of the alkynes are directly proportional to their molecular masses.
* Alkynes exhibit more acidic behaviour than alkenes and alkanes.
Alkynes: Reactions
* Alkynes undergo addition reactions with dihydrogen, halogens, halogen halide.
* Addition in unsymmetrical alkyne takes places according to Markovnikov’s rule.
* When heated in the presence of a catalyst, alkynes undergo polymerisation.
Aromatic Hydrocarbons
* Hydrocarbons containing conjugated cyclic rings are called aromatic hydrocarbons.
* The aromatic hydrocarbons are also called arenes.
* Aromatic hydrocarbons that contain benzene ring are known as benzenoids.
* Aromatic hydrocarbons that do not contain benzene ring are known as non-benzenoids.
Benzene: Structure
* Benzene, with the molecular formula C6H6, was first isolated by Michael Faraday in 1825.
* In 1865, Friedrich August Kekule gave the first insight into the structure of benzene. He proposed a structure, in which the six carbon atoms were arranged to form a hexagonal ring with each carbon atom carrying one hydrogen atom.
* The proposed structure of Kekule has alternate single and double bonds. The structure of benzene suggested by Kekule is known as the Kekule structure.
* The resonance structures or canonical structures of benzene are obtained by interchanging the positions of the double bonds, which takes place due to the delocalisation of the π bonds between the six carbon atoms in the benzene ring.
* Benzene undergoes substitution in preference to addition, due to resonance.
* In 1931, Eric Huckel proposed the modern theory of the aromaticity or aromatic character of compounds.
* A cyclic compound must contain a total of (4n+2) π electrons, where n is an integer, which is equal to 0, 1, 2, 3.
Benzene: Preparation
* Benzene was first isolated and identified by Michael Faraday. In 1845, Hofmann and his team isolated benzene from coal tar.
Benzene can be prepared in the laboratory by:
• Cyclic Polymerisation of Ethyne
• Decarboxylation of Aromatic Acids
• Reduction of Phenol
* Benzene is a colourless liquid with a characteristic smell, insoluble in water, but soluble in organic solvents such as alcohol and ether. It is inflammable and burns with a sooty flame.
Benzene: Electrophilic Substitution Reactions
* If a substitution reaction is initiated by an electrophile, it is called an electrophilic substitution reaction. Common electrophilic substitutions are:
• Halogenation
• Sulphonation
• Nitration
• Friedel – Crafts Alkylation
• Friedel – Crafts Acylation Reaction
* All types of electrophilic substitution reactions proceed via the following three steps:
• Generation of the electrophile
• Formation of the carbocation intermediate
• Removal of the proton from the carbocation intermediate
Benzene: Directive Influence of Substituents
* The nature of the group in monosubstituted benzene decides the position of the new incoming electrophile on the benzene ring; this is known as the directive influence of a group in monosubstituted benzene.
* This influence could be either ring activating or deactivating.
* Ring activating groups are ortho and para directing groups.
* Examples of ortho and para directing groups are – NH2, -NHR, -NHCOCH3,-OCH3 amongst others.
* Ring deactivating groups are meta directing groups.
* Examples of meta directing groups are – NO2, -CN, -CHO, -COR, -COOH,-COOR, -SO3H amongst others.