Hydrocarbons are composed of Carbon and hydrogen.
The important fuels like Petrol, kerosene, coal gas, CNG, LPG etc. are all hydrocarbons or their mixture.
Petroleum and natural gas are the major sources of aliphatic hydrocarbon while coal is an important source of aromatic hydrocarbons. The oil trapped inside the rocks is known as petroleum. PETRA – ROCK, OLEUM – OIL. The oil in the petroleum field is covered with a gaseous mixture known as natural gas. The main constituents of the natural gas are methane, ethane, propane and butane.
CLASSIFICATION OF HYDROCARBONS:
Acyclic or Aliphatic Carbocyclic or Cyclic
( Open chain)
Alkanes Alkenes Alkyne
Cycloalkanes Cycloalkenes Cycloalkynes
General formula CnH2n+2
C–C bond length 1.15 4 A0
Show chain, position and optical isomerism.
Heptane has 9 isomer, Octane 18 and Decane 75.
Follow mainly free radical mechanism
Useful in preparing an alkane containing
even number of carbon atoms
Stepping up reaction
From Grignard reagent (RMgX)
From unsaturated hydrocarbons:-
4. From carboxylic acids-
Kolbe’s electrolytic method-
(1) Nature:- Non-Polar due to covalent nature of C—C bond and C—H bond. C— C bond enrgy = 83 kj/mole and C—H bond energy = 99 kj/mole. C1—C4 = gases, C5—C17 = colourless odourless liquid and > C17 = Solid. (2) Solubility:- Like dissolve like
Viz, Polar compounds dissolve in polar solvent and Non-Polar compound dissolve in non polar solvent.
(3) Boiling point:- Low boiling point due to non polar in nature. Dry
3 2 3 2 2 3
2CH CH Br 2Na CH CH CH CH 2NaBr
R C CH H2 R CH2 CH3
R CH CH2 H2 R CH2 CH3
The molecules are held together only by weak Van der Waalls’ forces. Since we known that the magnitude of Van der Waalls’ forces is directly proportional to the molecular size. Therefore, the boiling point increases with increase the molecular size i.e. with increase in number of carbon atoms. Noted:- the boiling points of the branched chain Alkanes are less than the straight chain isomers.
This is due to the fact that branching of the chain makes the molecule more compact and thereby decreases the surface aria and consequently, the magnitudes of Van der Waalls’ forces also decrease.
(4) Melting point:- the melting point of Alkanes do not show regular variation with increase in molecular size. The Alkanes with even number of carbon atoms having higher melting point as compared to those Alkanes having immediately next lower and immediately next higher odd number of carbon atoms. Chemical properties
CH4 + Cl2 CH3Cl + HCl
CH3Cl CH2Cl2 CHCl3 CCl4
Noted:- Iodination is a reversible reaction. So it is carried out by heating alkane in the presence of some oxidizing agent like iodic acid (HIO3) or nitric acid (HNO3) or mercuric oxide (HgO) which oxidizes HI formed during the reaction. CH4 + I2 CH3I + HI
5HI + HIO3 3H2O + 3I2
2HI + 2HNO3 2H2O + I2 + 2NO2
Noted:- Fluorination of alkane takes place explosively resulting even in the rupture of C—C bond in higher alkanes.
CH4 2O2 CO2 2H2O
H 217.0 K cal/mole
4 2 3
CH O 2CH OH
4 2 2
CH O HCHO H O
boiling point = 309 K
H3C — CH — CH2CH3
boiling point = 301 K
H3C — C — CH3
boiling point = 282.5 K
Features of Halogenations:-
(i) The reactivity of Halogens:- F2 > Cl2 > Br2 > I2.
(ii) The rate of replacement of Hydrogens of alkanes is:
3° > 2° > 1°
Mechanism:- halogenations reaction...