Detailed class notes of HYDROCARBONS based on the NCERT Chemistry textbook for Class 11. These notes make an excellent reference for revision and is entrance oriented, making it perfect for NEET and JEE aspirants as well. *Disclaimer - This document is part of a series of notes for CBSE Class 11 ch...
Hydrocarbons
yarocarbons are compounds which contain only carbon and hydrogen
Example: Methane (CH,)
Ethane(CH,-CH,)
Benzene (CH)
supposed to be derived froom
nese are fundamental organic compounds. All other organic compounds
are
hydrocarbons.
Hydrocarbons
open chain compounds closed chain compounds
Aromatic
saturated unsaturated
Alicyclic (characteristc smell
(only single bond (double bond or majority have benzene
between carbon triple bond between ring)
atoms) carbon atoms) saturated unsaturated
O
Nomenclature of alkanes
Aliphatic compounds
Formulae Common name IUPAC name
Open chain compounds & alicyclic compounds are
collectively known as aliphatic compounds.
CH, Methane Methane
Openchain + Alicyclic > Aliphatic
CH-CH Ethane Ethane
(together)
ALKANES (Paraffins) CH,-CH-CH Propane Propane
with only single bond
Open chain hydrocarbon
between carbon atoms is called alkanes
CH-CH-CH,-CH, n-Butane Butane
CH-CHCH, (Propane) CH-CH - CH-CH3
lso-pentane 2-Methyl butane
These are also known as paraffins due to theirlow CH
reactivity (paraffins means low affinity)
In alkane, the carbon-carbon bond length is 154 CH3
pm and that of C- H bond is 112 pm CH-C-CH Neo-pentane 22-Diethyl propane
H 112pm
154pm CH
c-C
, Preparation of Alkanes b. By Kolbe's electrolysis
Alkanes can be produced by the electrolysis of
From unsaturated hydrocarbons.
sodium or potassium salts of carboxylic acids.
(From alkenes and alkynes)
Unsaturated hydrocarbons can be converted This is known as Kolle's electrolysis.
into alkanes by adding hydrogen in presence 2CH,COONa + 2H,O cirolyis
of catalyst like nickel, platinum, palladium Sodiuniethanoate
This process of addition of hydrogen is called
CH-CH, +2C0, +2NaOH+H2
hydrogenation. Ethane
CH =CH+H, t o PCH -CH MECHANISM
Ethene Ethane
CH,COONa->CH, -CO0 +Na
CH-C CH+2H Ni Ptor PdCH -CH2 -CH3
Propyne Propane Anode
. Fromalkyl halides
There are 3 methods for conversion of CH-CO0>CH,-COO+le"
alkylhalides to alkanes. CH-CO 0->CH3 +CO2
a. By reduction
CH +CH>CH3-CH
Alkyl halides are reduced to alkanes using Zn/ Cathode
HCI.
H,0+le >OH+H
CH-CH2 -Br+ Zn-HCCH,-CH, +HBr
Ethylbromide Nascent or
hydrogenZn-Cu/HC)
Ethane
H+HH2
Methane cannot be prepared by Kolle's electrolysis.
b. By Wurtz reaction
When alkyl halides are treated with metallic Iv. Using Grignard reagent
sodium in dry ether higher alkanes are formed Alkyl magnesium halides are known as
C. By Frankland reaction Eg: CH-CH2 - Br + Mg CH, -CH2 -
Mg Br
Ethyl magnesium bromide
Alkanes can be prepared by the reaction of alkyl Grignard reagent on hydrolysis form alkanes
hatide with metallic zinc.
CH-CH2 -MgBr+H,Oydrolysis
CH-CH, -Br+ Zn+Br-CH2-CH, Ethylmagnesiunbromide
Ethylbromide OH
CH3-CH2 -CH2 -CH, +ZnBr
Butane
CH,-CH++Mg, Br
III. From carboxylic acids Physical properties of Alkanes
In alkanes, the intermolecular forces are weak
Carboxylic acids can be converted to alkanes dispersion forces. Due to these weak forces alkanes
by 2 methods.
with C, to C, are gases,
By decarboxylation using sodalime
C to C,are liquids,
[NaOH+CaO]
When sodium or potassium salt of carboxylic C and above are solids
acid on heating with soda lime, alkanes are The boiling point of alkanes regularly increases
formed. with molecular mass. This can be explained on
the basis of vanderwaal forces. As carbon
CH-CH, -COONa + NaOHCa0
number increases, molecular size increases and
Sodium propanoate
surface area increases.
CH-CHy+NajCO3
Ethane Therefore the magnitude of vanderwaal forces
increases.
Here, carbon no decreases by one unit. This
process is called "STEPPING DOWN". Among isomeric alkanes, the branched chain
isomers have relatively low boiling point.
, Cl
CH-CH, -CH, - CH, -CH,>CH, -CH, CH CH,>CH, -C-CH,
CH
n Pentane
Iso-pentane Neo-pentane
BP 309 K BP 301K BP- 282.5k
s Is beause, as branc hing inereases, surtace area of molec ule decreases and hence magnitude of
vanderwaal force decreases.
However, melting point of neopentane is
Sreater than that of normal pentane CH,C1, +C1 CHCI +HCI
Trichloromethane
Reason: Melting point depends upon the
packing of molecules in the crystal lattice. The CHCI, +Cl, CCI, +HCI
carbontetachioride
neo-pentane with a spherical shape has more
compact packing & therefore has the highest Mechanism of Initiation
melting point.
Alkanes with odd number of carbon atoms
have lower melting point than those with even
Propagation
no. of carbon atoms.
H
Reason: In alkane, carbon atoms form zig-zag
chain. Alkanes with even no. of carbon atoms
are symmetrical and fit in the crystal structure,
H-C-H+CI>CH,+HCI
but alkanes having odd no. of carbon atoms
are unsymmetrical and do not fit in the crystal
structure. Molecules which can be closely CH+C1Ci>CH, -Cl+CI
packed in the crystal lattice have higher melting
Termination
point.
CH CH2 Cl+CI>Cl,
CH3
H, +H, ->CH, -CH,
CH CH2 CH2 CH+C1 CH,CI
b. Combustion
Hexane() Symmetrical In presence of oxygen or air, alkanes are completely
oxidised to carbon dioxide and water, liberating a
CH2 CH2 large amount of energy.
CH +202>CO2 +2H,0+Energy AH
=-890k]/mol
CH CH2 CH2 CH General expression for combustion
Heptane (G) Unsymmetrical
RH2n+2 + 2nCO) +(n+1)H,O
Alkanes are colourless & odourless substances.
Due to this reaction, alkanes are used as fuels.
They are insoluble in water.
c. Incomplete combustion
They are good solvent for non-polar molecules.
In insufficient amount of air/oxygen, alkanes
Chemical properties of alkanes
undergo incomplete combustion and carbon black
a. Halogenation is formed.
When alkanes are treated with halogen in presence CH+O2C+2H,O
of UV light, hydrogen atoms are replaced by halogen. dControlled oxidation
This reaction is known as substitution reaction.
Alkanes can undergo oxidation under special
conditions to yield a variety of products.
CH +Cl2 CH -
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