- LPG’ and ‘CNG’ used as fuels.
- LPG - liquified petroleum gas
- CNG - compressed natural gas.
- ‘LNG’ -Liquified natural gas -a fuel and is obtained by liquifaction of natural gas.
- Petrol, diesel and kerosene oil are obtained by the fractional distillation of petroleum
- Coal gas - destructive distillation of coal.
- Petrol and CNG operated automobiles cause less pollution.
- All these fuels contain mixture of hydrocarbons.
- Hydrocarbons are also used for - Manufacture of polymers like polythene, polypropene, polystyrene etc.
- Higher hydrocarbons are used as
- solvents for paints.
- starting materials for manufacture of many dyes and drugs.
ClassiFication :
Three main categories – (i) saturated (ii) unsaturated and (iii) aromatic hydrocarbons.
Saturated hydrocarbons
- Contain carbon-carbon and carbon-hydrogen single bonds - alkanes
- if carbon atoms form a closed chain or a ring, they are termed as cycloalkanes.
Unsaturated hydrocarbons :
• Contain carbon-carbon multiple bonds – double bonds, triple bonds or both.
Aromatic hydrocarbons :
- are a special type of cyclic compounds.
- Carbon is tetravalent and hydrogen is monovalent.
- For making models of alkanes, you can use toothpicks for bonds and plasticine balls for atoms
- For alkenes, alkynes and aromatic hydrocarbons, spring models can be constructed.
ALKANES :
- Saturated open chain hydrocarbons containing carbon - carbon single bonds.
- Methane - first member of this family.
- Methane is a gas found in coal mines and marshy places.
- Hydrocarbon with molecular formula C2H6 is known as ethane.
- Hydrocarbons are inert under normal conditions
- Hence - paraffins (latin : parum, little; affinis, affinity).
- General formula for alkanes is CnH2n+2
According to VSEPR theory, Methane
- Tetrahedral structure
- Carbon atom lies at the centre.
- Four hydrogen atoms lie at the four corners of a regular tetrahedron.
- All H-C-H bond angles are of 109.5°
- In alkanes,C-C bond lengths is 154 pm
- C-H bond lengths is 112 pm
- C–C and C–H σ bonds are formed by head-on overlapping of sp3 hybrid orbitals of carbon and 1s orbitals of hydrogen atoms.
+ POint :
Isomers : 1>2>3
example :1. Isomers of C4H10 :
- Butane (n- butane), (b.p. 273 K) > 2-Methylpropane (isobutane) (b.p.261 K)
2. Isomers Of C5H12 :
- Pentane (n-pentane) (b.p. 309 K) > 2-Methylbutane isopentane) (b.p. 301 K) > 2,2-Dimethylpropane (neopentane) (b.p. 282.5 K)
Types Of Carbon Atom :
- Terminal carbon atoms are always primary.
- Carbon atom attached to two carbon atoms is known as secondary.
- Tertiary carbon is attached to three carbon atoms.
- Neo or quaternary carbon is attached to four carbon atoms.
Alkane : No of isomers
- C5H12 : 05
- C6H14 : 05
- C7H16 : 09
- C10H22 : 75
Preparation of Alkanes :
1. From unsaturated hydrocarbons :
- This process is called hydrogenation.
- These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen – hydrogen bond.
- Platinum and palladium catalyse the reaction at room temperature.
- Relatively higher temperature and pressure are required with nickel catalysts.
2. From alkyl halides :
- Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes
2. Wurtz reaction :
3. From carboxylic acids
- 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.
4. Kolbe’s electrolytic method:
- An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode.
Physical properties Of Alkane :
- Almost non-polar molecules [ covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms. ]
- Weak van der Waals forces.
- C1 to C4 - gases, C5 to C17 - liquids and those containing 18 carbon atoms or more are solids at 298 K.
- Colourless and odourless.
- Petrol is a mixture of hydrocarbons and is used as a fuel for automobiles.
- Petrol and lower fractions of petroleum are also used for dry cleaning of clothes to remove grease stains.
- Grease (mixture of higher alkanes) is non polar and, hence, hydrophobic in nature.
- Polar substances are soluble in polar solvents, whereas the non-polar ones in non-polar solvents i.e., like dissolves like.
- Steady increase in boiling point with increase in molecular mass. [ Reason : intermolecular van der Waals forces increase with increase of the molecular size ]
- Boiling points of three isomeric pentanes : pentane having a continuous chain of five carbon atoms has the highest boiling point (309.1K) whereas 2,2 – dimethylpropane boils at 282.5K.
- Pentane > 2-methylbutane > 2,2-dimethylpropane.
- With increase in number of branched chains, the molecule attains the shape of a sphere.- smaller area of contact - weak intermolecular forces
Chemical properties :
1. Substitution reactions :
- One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group.
- Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light.
- Lower alkanes do not undergo nitration and sulphonation reactions.
• Chlorination of methane
- Rate of reaction of alkanes with halogens - F2 > Cl2 > Br2 > I2.
- Rate of replacement of hydrogens of alkanes is : 3° > 2° > 1°.
- Fluorination is too violent to be controlled.
- Iodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like HIO3 or HNO3
Halogenation - involving three steps namely
- Initiation
- Propagation and
- Termination
1. Initiation :
- The Cl–Cl bond is weaker than the C–C and C–H bond and hence, is easiest to break.
2. Propagation :
- Chlorine free radical attacks the methane molecule
- The methyl radical thus obtained attacks the second molecule of chlorine to form CH3 – Cl with the liberation of another chlorine free radical by homolysis of chlorine molecule.
- Two such steps given below explain how more highly haloginated products are formed.
3. Termination:
- The possible chain terminating steps are:
- The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.
2. Combustion :
- Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
- During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed
3. Controlled oxidation :
- Ordinarily alkanes resist oxidation but alkanes having tertiary H atom can be oxidized to corresponding alcohols by potassium permanganate.
4.Isomerisation :
- n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.
- Major products are given below.
5. Aromatization :
- n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues.
- This reaction is known as aromatization or reforming.
- Toluene (C7H8) [C6H5 - CH3 ] is methyl derivative of benzene.
6. Reaction with steam :
- Methane reacts with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen.
- This method is used for industrial preparation of dihydrogen gas
7. pyrolysis or cracking :
- Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc.
- Pyrolysis of alkanes is believed to be a free radical reaction.
- Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis.
Conformations :
- Spatial arrangements of atoms which can be converted into one another by rotation around a C-C single bond are called conformations or conformers or rotamers.
- Torsional strain [type of repulsive interaction ] : small energy barrier of 1-20 kJ mol–1 due to weak repulsive interaction between the adjacent bonds.
Conformations in Ethane :
- There are infinite number of conformations of ethane.
- However, there are two extreme cases. [ BUT Three Types ]
- Eclipsed conformation
- Skew conformation
- Staggered conformation
1. Eclipsed conformation :
- Hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation
2. Skew conformation :
- Any other intermediate conformation between Eclipsed and Staggered is called a skew conformation.
3. Staggered conformation :
- Hydrogens are as far apart as possible is known as the staggered conformation.
- In all the conformations, the bond angles and the bond lengths remain the same.
• Eclipsed and the staggered conformations can be represented by
- Sawhorse projections
- Newman projections.
1. Sawhorse projections :
- Molecule is viewed along the molecular axis.
- Central C–C bond as a somewhat longer straight line.
- Upper end of the line is slightly tilted towards right or left hand side.
- The front carbon is shown at the lower end of the line, whereas the rear carbon is shown at the upper end.
- Each carbon has three lines attached to it corresponding to three hydrogen atoms.
- The lines are inclined at an angle of 120° to each other.
2. Newman projections :
- In this projection, the molecule is viewed at the C–C bond head on.
- The carbon atom nearer to the eye is represented by a point. Three hydrogen atoms attached to the front carbon atom are shown by three lines drawn at an angle of 120° to each other.
- The rear carbon atom (the carbon atom away from the eye) is represented by a circle.
- The three hydrogen atoms are shown attached to it by the shorter lines drawn at an angle of 120° to each other.
Relative stability of conformations:
- In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible.
- Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule.
- On the other hand, when the staggered form changes into the eclipsed form, the electron clouds of the carbon – hydrogen bonds come closer to each other resulting in increase in electron cloud repulsions.
- To check the increased repulsive forces, molecule will have to possess more energy and thus has lesser stability.
- As already mentioned, the repulsive interaction between the electron clouds, which affects stability of a conformation, is called torsional strain.
- Magnitude of torsional strain depends upon the angle of rotation about C–C bond. This angle is also called dihedral angle or torsional angle.
- Of all the conformations of ethane, the staggered form has the least torsional strain and the eclipsed form, the maximum torsional strain.
- Therefore, staggered conformation is more stable than the eclipsed conformation.
- Hence, molecule largely remains in staggered conformation.
- Thus it may be inferred that rotation around C–C bond in ethane is not completely free.
- The energy difference between the two extreme forms is of the order of 12.5 kJ mol–1, which is very small.
- Even at ordinary temperatures, the ethane molecule gains thermal or kinetic energy sufficient enough to overcome this energy barrier of 12.5 kJ mol–1 through intermolecular collisions.
- Thus, it can be said that rotation about carbon-carbon single bond in ethane is almost free for all practical purposes.
- It has not been possible to separate and isolate different conformational isomers of ethane