DAY SIXTEEN
Hydrocarbons
Learning & Revision for the Day
u Alkanes u Alkenes u Aromatic Hydrocarbons
u Conformations u Alkynes u Benzene
Hydrocarbons means compounds containing carbon and hydrogen only. Single bond containing
hydrocarbons is saturated hydrocarbon and multiple bond hydrocarbons are called
unsaturated hydrocarbon.
Hydrocarbons are of different types and classified into three main categories :
(i) Saturated hydrocarbons (alkanes)
(ii) Unsaturated hydrocarbons (alkenes and alkynes)
(iii) Aromatic hydrocarbons.
Alkanes
l These are the simplest organic compounds formed by carbon and hydrogen only.
Chemically, these are almost inert that’s why also called Paraffins (Latin, Parum affinis =
Little affinity). The general formula for this series is C nH2n + 2. Hence, the first member is
CH4 (methane).
l Nomenclature Alkanes are named on the basis of their chain structure and adding suffix
‘ane’ in the end. IUPAC names of some simple alkanes are given below :
Molecular Formula IUPAC Name
Methane CH 4
Ethane C2 H 6
Propane C3 H 8
Decane C10H22
l Isomerism Alkanes exhibit only chain isomerism. e.g. possible chain isomers of C 5H12 ar
as follows :
CH3
CH3 CH2 CH2 CH2 CH3 , CH3 CH2 CH2 CH3 , H3C C CH3
Pentane
CH3 CH3
2 − methylbutane 2,2-dimethyl propane
l Structure All the carbon-carbon bonds in alkanes are sp3 -sp3 hybridised where σ-bond
is quite strong. They have three dimensional structures where all the four bonds are
tetrahedrally arranged making a normal angle of 109 °28′, e.g. in CH4 , the carbon at
, the centre is bonded by four hydrogen atoms, where all l Action of water on aluminium carbide or beryllium
the bonds lie in different planes giving it a three carbide gives methane.
dimensional structure. Al4C3 + 12H2O → 4Al(OH)3 + 3CH4
H Be2C + 4H2O → CH4 + 2Be(OH)2
H These reactions produce methane, hence called
sp3-hybridised methanoids.
109°28′
C
H
C
H H
H Physical Properties
Physical properties depend upon the intermolecular forces of
attraction.
H H (i) Boiling point of alkanes increases steadily with increase
Structures of methane (CH4) molecule in molecular mass. If molecular mass is same, as the
contact surface area increases, boiling point increases.
Methods of Preparation More branched chain has less contact surface area so, less
These can be prepared by the following ways : has boiling point.
l Unsaturated hydrocarbons (alkenes or alkynes), (ii) Melting point increases with increase in molecular mass.
reduction in the presence of finely divided catalyst But symmetric molecules have higher melting point than
(e.g. Pt/Pd/Ni) give alkanes. This reaction is known as asymmetric ones. Therefore, alkanes with even number of
Sabatier-Sanderens reduction. carbon atoms have higher melting point than the
successive alkanes with odd number of carbon atoms.
Pt /Pd /Ni
CH2 ==CH2 + H2 → CH3 CH3 (iii) Solubility of a compound is governed by the principle
Ethene that like dissolves like. As alkanes are non-polar,
Pt /Pd /Ni therefore soluble in non-polar solvents such as
CH3 C ≡≡C H+ 2H2 → CH3 CH2 CH3
petroleum, benzene etc.
Propyne
(iv) The densities of alkanes increase with increase in the
l Alkyl halides (except fluorides) on reduction give alkanes molecular masses till the limiting value of about 0.8 g/cm3
in the presence of Zn and HCl. is reached.
Zn, H+
CH3Cl + H2 → CH4 + HCl
Methane Chemical Properties
l Alkyl halides when treated with sodium metal in the Alkanes being saturated compounds undergo only
presence of dry ether gives higher alkanes. This reaction substitution reactions at C H bond. Some of these are
is known as Wurtz reaction. discussed below :
Dry ether
2C2 H5Br + 2Na → C2 H5 C2 H5 + 2NaBr Halogenation
Ethyl bromide l Halogenation of alkanes is a free radical substitution
l Sodium salt of carboxylic acid (RCOONa) is heated with reaction which occurs in the presence of UV- light.
hot sodalime (CaO + NaOH) to give alkane and this Reactivity towards halogenation is
reaction is called as decarboxylation. 3°H >2°H>1°H
CH3COONa + NaOH →
CaO
∆
CH4 + Na2CO3 CH4 + Cl2 → CH3Cl →
hν 2 Cl / hν
CH2Cl2
Chloromethane Dichloromethane
l Sodium or potassium salt of carboxylic acid on
Cl 2 / hν Cl / hν
electrolysis gives alkane (R R). This reaction is known → CHCl3 →
2
CCl4
as Kolbe’s electrolysis method. Trichloro Tetrachloro
methane methane
Electrolysis Bromination and chlorination proceed approx by same
2CH3COONa + 2H2O → l
way. Iodination is reversible but fluorination of alkanes is
C2 H6 + 2CO2 ↑ + H2 + 2NaOH
1442443 1442443 too vigorous.
Anode Cathode
l Grignard reagent (RMgX ) when reacts with H2O or ROH, Mechanism
gives alkane. The mechanism of halogenation can be discussed in the
− + following three steps:
CH3 C H2 MgBr + HO H →
(i) In initiation step, the homolysis of chlorine molecule
OH takes place in the presence of light or heat.
CH3 CH3 + Mg • •
Cl Cl hν→ Cl + Cl
Br Homolysis Chlorine free
radicals
, (ii) In propagation step, chlorine free radical reacts with Aromatisation
methane molecule and generates methyl free radical.
• •
Aromatisation take place when alkanes get converted into
CH4 + Cl hν
→ C H3 + H Cl benzene in the presence of Cr2O3 or V2O 5.
• • It is applicable to alkanes from hexane to decane.
C H3 + Cl Cl hν
→ CH3 Cl + Cl e.g.
• • Cr2O3 or V2O5
CH3Cl + Cl → C H2Cl + HCl n-hexane
773K, 10 – 20 atm
• •
Benzene
CH2 Cl+Cl Cl → CH2Cl2 + Cl
Reaction with Steam
(iii) Termination step occurs as follows :
• • Alkanes reacts with steam as follows :
Cl + Cl → Cl Cl Ni
• • CH4 + H2O(g) → CO + 3H2
1273 K
H3 C + CH3 → CH3 CH3
• • This method is used for industrial preparation of H2 gas.
H3 C + Cl → CH3 Cl
Pyrolysis or Cracking
Due to consumption of free radicals, chain is terminated.
Pyrolysis or cracking is a process in which higher alkanes on
Combustion heating to higher temperature decompose into lower alkanes.
l Complete combustion of alkanes give CO2 and H2O. C6H12 + H2
773K
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O(l ) C6H14 C4H8 + C2H6
l During incomplete combustion, alkanes give carbon C3H6 + C2H4 + CH4
black which is used in the manufacture of ink, printer
ink, black pigments and as fillers in tyre industry.
CH4 (g) + O2 (g) →
Incomplete
C(s) + 2H2O(l )
Octane Number
combustion
The quality of petrol or gasoline is measured in terms of octane
l On controlled oxidation, alkanes give different number while that of diesel is measured in terms of cetane
products as : number. Octane number gives the percentage of iso-octane (by
volume) in a mixture of n-heptane and iso-octane while cetane
Cu /523K /100 atm
2CH4 + O2 → 2CH3OH number gives the percentage of octane (by volume) in a mixture
Methanol of octane ( 16 34 ) and α-methyl naphthalene.
Mo O, ∆
CH4 + O2 → HCHO+ H2O
Methanal Conformations (or Rotamers or Conformers)
(CH3COO)2 Mn Spatial arrangements of atoms which can be converted into one
2CH3 CH3 + 3O2 → 2CH3COOH+2H2O
∆
Ethanoic acid
another by rotation around a C C single bond.
l Ordinarily, alkanes resist oxidation but iso-alkanes (3°H) Ethane has three main conformations:
on oxidation with KMnO 4 gives tertiary alcohol. (i) Eclipsed conformations are those in which the hydrogen
atoms attached to two carbons are as closed together as
KMnO
(CH3 )3 CH →
4
(CH3 )3 C OH possible. This conformation is highly unstable and has
Oxidation 3 °alcohol maximum energy.
(ii) Staggered conformations are those in which hydrogens
Isomerisation are as far apart as possible. It is highly stable and has
n-alkane on heating in the presence of AlCl3 (anhydrous) and minimum energy.
HCl (g) isomerises to give branched chain alkanes. This (iii) Skew conformation is an intermediate conformation of
reaction is called isomerisation. the above two conformations.
AlCl 3 (anhy) + HCl H
CH3 (CH2 )4 CH3 →
n-hexane H H
CH3 CH CH2 CH2 CH3 C C
H H
H
CH3 H
2-methyl pentane C H
C
+ CH3 CH 2 CH CH2 CH3 H H H H
Eclipsed Staggered
CH3
3-methyl pentane Sawhorse projection of ethane
Hydrocarbons
Learning & Revision for the Day
u Alkanes u Alkenes u Aromatic Hydrocarbons
u Conformations u Alkynes u Benzene
Hydrocarbons means compounds containing carbon and hydrogen only. Single bond containing
hydrocarbons is saturated hydrocarbon and multiple bond hydrocarbons are called
unsaturated hydrocarbon.
Hydrocarbons are of different types and classified into three main categories :
(i) Saturated hydrocarbons (alkanes)
(ii) Unsaturated hydrocarbons (alkenes and alkynes)
(iii) Aromatic hydrocarbons.
Alkanes
l These are the simplest organic compounds formed by carbon and hydrogen only.
Chemically, these are almost inert that’s why also called Paraffins (Latin, Parum affinis =
Little affinity). The general formula for this series is C nH2n + 2. Hence, the first member is
CH4 (methane).
l Nomenclature Alkanes are named on the basis of their chain structure and adding suffix
‘ane’ in the end. IUPAC names of some simple alkanes are given below :
Molecular Formula IUPAC Name
Methane CH 4
Ethane C2 H 6
Propane C3 H 8
Decane C10H22
l Isomerism Alkanes exhibit only chain isomerism. e.g. possible chain isomers of C 5H12 ar
as follows :
CH3
CH3 CH2 CH2 CH2 CH3 , CH3 CH2 CH2 CH3 , H3C C CH3
Pentane
CH3 CH3
2 − methylbutane 2,2-dimethyl propane
l Structure All the carbon-carbon bonds in alkanes are sp3 -sp3 hybridised where σ-bond
is quite strong. They have three dimensional structures where all the four bonds are
tetrahedrally arranged making a normal angle of 109 °28′, e.g. in CH4 , the carbon at
, the centre is bonded by four hydrogen atoms, where all l Action of water on aluminium carbide or beryllium
the bonds lie in different planes giving it a three carbide gives methane.
dimensional structure. Al4C3 + 12H2O → 4Al(OH)3 + 3CH4
H Be2C + 4H2O → CH4 + 2Be(OH)2
H These reactions produce methane, hence called
sp3-hybridised methanoids.
109°28′
C
H
C
H H
H Physical Properties
Physical properties depend upon the intermolecular forces of
attraction.
H H (i) Boiling point of alkanes increases steadily with increase
Structures of methane (CH4) molecule in molecular mass. If molecular mass is same, as the
contact surface area increases, boiling point increases.
Methods of Preparation More branched chain has less contact surface area so, less
These can be prepared by the following ways : has boiling point.
l Unsaturated hydrocarbons (alkenes or alkynes), (ii) Melting point increases with increase in molecular mass.
reduction in the presence of finely divided catalyst But symmetric molecules have higher melting point than
(e.g. Pt/Pd/Ni) give alkanes. This reaction is known as asymmetric ones. Therefore, alkanes with even number of
Sabatier-Sanderens reduction. carbon atoms have higher melting point than the
successive alkanes with odd number of carbon atoms.
Pt /Pd /Ni
CH2 ==CH2 + H2 → CH3 CH3 (iii) Solubility of a compound is governed by the principle
Ethene that like dissolves like. As alkanes are non-polar,
Pt /Pd /Ni therefore soluble in non-polar solvents such as
CH3 C ≡≡C H+ 2H2 → CH3 CH2 CH3
petroleum, benzene etc.
Propyne
(iv) The densities of alkanes increase with increase in the
l Alkyl halides (except fluorides) on reduction give alkanes molecular masses till the limiting value of about 0.8 g/cm3
in the presence of Zn and HCl. is reached.
Zn, H+
CH3Cl + H2 → CH4 + HCl
Methane Chemical Properties
l Alkyl halides when treated with sodium metal in the Alkanes being saturated compounds undergo only
presence of dry ether gives higher alkanes. This reaction substitution reactions at C H bond. Some of these are
is known as Wurtz reaction. discussed below :
Dry ether
2C2 H5Br + 2Na → C2 H5 C2 H5 + 2NaBr Halogenation
Ethyl bromide l Halogenation of alkanes is a free radical substitution
l Sodium salt of carboxylic acid (RCOONa) is heated with reaction which occurs in the presence of UV- light.
hot sodalime (CaO + NaOH) to give alkane and this Reactivity towards halogenation is
reaction is called as decarboxylation. 3°H >2°H>1°H
CH3COONa + NaOH →
CaO
∆
CH4 + Na2CO3 CH4 + Cl2 → CH3Cl →
hν 2 Cl / hν
CH2Cl2
Chloromethane Dichloromethane
l Sodium or potassium salt of carboxylic acid on
Cl 2 / hν Cl / hν
electrolysis gives alkane (R R). This reaction is known → CHCl3 →
2
CCl4
as Kolbe’s electrolysis method. Trichloro Tetrachloro
methane methane
Electrolysis Bromination and chlorination proceed approx by same
2CH3COONa + 2H2O → l
way. Iodination is reversible but fluorination of alkanes is
C2 H6 + 2CO2 ↑ + H2 + 2NaOH
1442443 1442443 too vigorous.
Anode Cathode
l Grignard reagent (RMgX ) when reacts with H2O or ROH, Mechanism
gives alkane. The mechanism of halogenation can be discussed in the
− + following three steps:
CH3 C H2 MgBr + HO H →
(i) In initiation step, the homolysis of chlorine molecule
OH takes place in the presence of light or heat.
CH3 CH3 + Mg • •
Cl Cl hν→ Cl + Cl
Br Homolysis Chlorine free
radicals
, (ii) In propagation step, chlorine free radical reacts with Aromatisation
methane molecule and generates methyl free radical.
• •
Aromatisation take place when alkanes get converted into
CH4 + Cl hν
→ C H3 + H Cl benzene in the presence of Cr2O3 or V2O 5.
• • It is applicable to alkanes from hexane to decane.
C H3 + Cl Cl hν
→ CH3 Cl + Cl e.g.
• • Cr2O3 or V2O5
CH3Cl + Cl → C H2Cl + HCl n-hexane
773K, 10 – 20 atm
• •
Benzene
CH2 Cl+Cl Cl → CH2Cl2 + Cl
Reaction with Steam
(iii) Termination step occurs as follows :
• • Alkanes reacts with steam as follows :
Cl + Cl → Cl Cl Ni
• • CH4 + H2O(g) → CO + 3H2
1273 K
H3 C + CH3 → CH3 CH3
• • This method is used for industrial preparation of H2 gas.
H3 C + Cl → CH3 Cl
Pyrolysis or Cracking
Due to consumption of free radicals, chain is terminated.
Pyrolysis or cracking is a process in which higher alkanes on
Combustion heating to higher temperature decompose into lower alkanes.
l Complete combustion of alkanes give CO2 and H2O. C6H12 + H2
773K
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O(l ) C6H14 C4H8 + C2H6
l During incomplete combustion, alkanes give carbon C3H6 + C2H4 + CH4
black which is used in the manufacture of ink, printer
ink, black pigments and as fillers in tyre industry.
CH4 (g) + O2 (g) →
Incomplete
C(s) + 2H2O(l )
Octane Number
combustion
The quality of petrol or gasoline is measured in terms of octane
l On controlled oxidation, alkanes give different number while that of diesel is measured in terms of cetane
products as : number. Octane number gives the percentage of iso-octane (by
volume) in a mixture of n-heptane and iso-octane while cetane
Cu /523K /100 atm
2CH4 + O2 → 2CH3OH number gives the percentage of octane (by volume) in a mixture
Methanol of octane ( 16 34 ) and α-methyl naphthalene.
Mo O, ∆
CH4 + O2 → HCHO+ H2O
Methanal Conformations (or Rotamers or Conformers)
(CH3COO)2 Mn Spatial arrangements of atoms which can be converted into one
2CH3 CH3 + 3O2 → 2CH3COOH+2H2O
∆
Ethanoic acid
another by rotation around a C C single bond.
l Ordinarily, alkanes resist oxidation but iso-alkanes (3°H) Ethane has three main conformations:
on oxidation with KMnO 4 gives tertiary alcohol. (i) Eclipsed conformations are those in which the hydrogen
atoms attached to two carbons are as closed together as
KMnO
(CH3 )3 CH →
4
(CH3 )3 C OH possible. This conformation is highly unstable and has
Oxidation 3 °alcohol maximum energy.
(ii) Staggered conformations are those in which hydrogens
Isomerisation are as far apart as possible. It is highly stable and has
n-alkane on heating in the presence of AlCl3 (anhydrous) and minimum energy.
HCl (g) isomerises to give branched chain alkanes. This (iii) Skew conformation is an intermediate conformation of
reaction is called isomerisation. the above two conformations.
AlCl 3 (anhy) + HCl H
CH3 (CH2 )4 CH3 →
n-hexane H H
CH3 CH CH2 CH2 CH3 C C
H H
H
CH3 H
2-methyl pentane C H
C
+ CH3 CH 2 CH CH2 CH3 H H H H
Eclipsed Staggered
CH3
3-methyl pentane Sawhorse projection of ethane
