Hydrocarbons can be classified into saturated, unsaturated, and aromatic categories based on carbon-carbon bonds. Alkanes, alkenes, and alkynes each have distinct structures and reactions, with specific nomenclature and properties. Aromatic hydrocarbons, characterized by unique stability due to resonance, undergo electrophilic substitution reactions and are fundamental in organic chemistry.

1. HYDROCARBONS

2. Classification
Depending upon the types of carbon-carbon bonds present,
hydrocarbons can be classified into three main categories
Saturated Hydrocarbons
Unsaturated Hydrocarbons
Aromatic Hydrocarbons
Saturated hydrocarbons contain carbon-carbon and carbon-
hydrogen single bonds.
If carbon atoms form a closed chain or a ring, they are termed
as cycloalkanes. Aromatic hydrocarbons are a special type of
cyclic compounds
Unsaturated hydrocarbons contain carbon-carbon multiple
bonds –double bonds, triple bonds or both.

3. ALKANES
Hydrocarbons have adjacent carbon atoms with single bond
General formula for Alkanes is CnH2n+2

4. Nomenclature and Isomerism
Methane, Ethane and Propane have only one structure but
higher alkanes have more than one structure
Consider structure of Butane C4H10
These two structures have same molecular formula which are
known as Structural Isomers
Butane (n- butane) 2-Methylpropane (isobutane)

5. Structural isomers which differ in chain of carbon atoms are
known as Chain Isomers
Isomers of pentane
2,2-Dimethylpropane (neopentane)

6. Preparation
From Unsaturated Hydrocarbons
Dihydrogen is added to alkenes and alkynes in the presence
of finely divided catalysts like platinum, palladium or nickel to
form alkanes. This process is called Hydrogenation.

7. From Alkyl Halides
Alkyl halides on reduction with zinc and dilute hydrochloric
acid give alkanes
Wurtz reaction: Alkyl halides on treatment with sodium
metal in dry ethereal solution give higher alkanes

8. From carboxylic acids
Decarboxylation: The process of elimination of carbon dioxide
from a carboxylic acid
Kolbe’s electrolytic method: An aqueous solution of sodium or
potassium salt of a carboxylic acid on electrolysis gives alkane
Electrolysis

9. Physical properties
Alkanes are non-polar molecules.
They are colourless and odourless.
They possess weak van der Waals forces.
Boiling point (b.p.) of alkanes increase with increase in
molecular mass.
Alkanes can be separated by distillation. They are
produced in industry by fractional distillation of
petroleum and natural gas

10. Chemical properties
Substitution reactions:
Halogenation: The replacement of one or more hydrogen atoms
in an organic compound by a halogens, nitro group and
sulphonic acid group.
In this a C-H bond is broken and a new C-X bond is formed
CH4 (g)+ Cl2 (g)  CH3Cl(g) + HCl (g)
CH3Cl (g) + Cl2 (g)  CH2Cl2 (g) + HCl (g)
CH2Cl2 (g) + Cl2 (g)  CHCl3 (g) + HCl (g)
CHCl3 (g) + Cl2 (g)  CCl4 (g)+ HCl (g)
Light
chloromethane
Light
dichloromethane
trichloromethane
Light
Tetrechloromethane
Carbon tetrachloride
Light

11. Halogenation is proceeded via free radical chain mechanism
involving three steps
Initiation: Formation of radicals
Cl2  Cl• + Cl•
Propagation: Radicals attack substrates making new molecules
and new radicals
CH4(g) + Cl•(g)  CH3• (g) + HCl (g)
CH3 •(g) + Cl•(g)  CH3Cl(g)
Termination: Radical recombines with another and the reaction
is terminated. Cl• (g) + Cl• (g)  Cl2 (g)
H3C• + CH3•  H3C - CH3
H3C• + Cl•  H3C - Cl

12. At high temperatures alkanes combust
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) ΔHo
= -890.4 kJ
C2H6(g) + 7O2(g)  4CO2(g) + 6 H2O(g) ΔHo
= -3119 kJ
The reactions are all highly exothermic.
The general combustion equation for any alkane is
Incomplete Combustion:
CH4 (g) + O2 (g)  C (s) + 2H2O(l)
Carbon black
Combustion:

13. Controlled oxidation
Alkanes on heating with air at high pressure and in presence
of catalyst gives oxidation products
2CH4 + O2  2CH3OH
CH4 + O2  HCHO + H2O
2CH3CH3 + 3O2  2CH3COOH + 2H2O
Cu/523K
100atm
Methanol
Mo2O3
(CH3COOH)2 Mn
Methanal
Ethanoic acid
Δ
Δ
Alkanes having tertiary H atom can be oxidized to
corresponding alcohols by potassium permanganate
(CH3)3CH  (CH3)3COH
KMnO4
2-Methylpropane 2-Methylpropan-2-ol

14. Isomerisation
Aromatization
n-Alkanes on heating in the presence of anhydrous aluminium
chloride and hydrogen chloride gas isomerizes to branched
chain alkanes
Anhy. AlCl3/HCl
Alkanes having six or more carbon atoms on heating in the
presence of oxides of vanadium molybdenum or chromium get
dehydrogenated and cyclised to benzene

15. Reaction with steam
Pyrolysis
CH4 + CO + 3H2
Ni
Δ
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
Higher alkanes on heating to higher temperature decompose
into lower alkanes, alkenes etc. Such a decomposition reaction
is called Pyrolysis.

16. Conformations
The 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
Eclipsed Conformation: In this conformation the hydrogen
atoms attached to carbons are as closed together as possible
Staggered Conformation: In this conformation the hydrogen
atoms attached to carbons are as far apart as possible

17. Sawhorse Projections
Newman Projections

18. Alkenes
Structure of Double Bond
Alkenes are unsaturated hydrocarbons containing at least one
double bond.
Alkenes have general molecular formula CnH2n
Carbon-carbon double bond in alkenes consists of one strong
sigma (σ) bond and one weak pi (π) bond.

19. Nomenclature
Identify the longest chain containing both C of the double
bond.
Replace –ane to –ene.
The longest chain is numbered from the end nearest to the
double bond.
The compound having two double bonds ends in –adiene.
A cyclic compound is named as cycloalkene.
Numbering in cycloalkene starts from the carbon containing
double bond.

21. Isomerism
Structural Isomerism
Position Isomers

22. Geometrical isomerism
Geometric isomers are possible only when each carbon of
the double bond is a stereocenters.
Cis Isomer: Two identical atoms or groups lie on the same
side of the double bond.
Trans Isomer: Identical atoms or groups lie on the opposite
sides of the double bond

23. Preparation
From alkynes
Alkynes on partial reduction with calculated amount of
dihydrogen in the presence of palladised charcoal partially
deactivated with poisons like sulphur compounds or
quinoline give alkenes.
Partially deactivated palletized charcoal is known as
Lindlar’s Catalyst

24. Alkynes on reduction with sodium in liquid ammonia form trans
alkenes.

25. From alkyl halides
Dehydrohalogenation: Alkyl halides (R-X) on heating with
alcoholic potash eliminate one molecule of halogen acid to
form alkenes. This reaction is known as
dehydrohalogenation
The removal of halogen acid.
Hydrogen atom is eliminated from the β carbon atom it is
β-elimination reaction.

26. From Vicinal Dihalides
Dihalides in which two halogen atoms are attached to two
adjacent carbon atoms are known as Vicinal Dihalides
From alcohols by acidic dehydration
A water molecule is eliminated from the alcohol molecule in
the presence of an acid, this reaction is known as acidic
dehydration of alcohols

27. Physical properties
Alkenes are non polar compounds.
They are insoluble in water and soluble in non polar organic
solvents.
They are less dense than water.
The alkenes has a boiling point which is a lower than the
corresponding alkanes.
Straight chain alkenes have higher boiling point than isomeric
branched chain compounds.

28. Chemical properties
Addition of dihydrogen:
Alkenes add up one molecule of dihydrogen gas to form
alkanes
Addition of halogens :
Halogens react with alkene to form vicinal dihalides.

29. Addition of hydrogen halides:
Hydrogen halides (HCl, HBr,HI) add up to alkenes to form alkyl
halides
Markovnikov Rule: (Addition reaction of HBr to unsymmetrical
alkenes) Markovnikov's rule states that the hydrogen atom
adds to the carbon atom that already has the larger number of
hydrogen atoms when HX adds to an alkene.

30. Anti-Markovnikff’s addition or Peroxide effect or
Kharash Effect
In the presence of peroxide, addition of HBr to
unsymmetrical alkenes like propene takes place contrary to
the Markovnikov rule.
This happens only with HBr but not with HCl and Hl.

31. Addition of sulphuric acid:
Cold concentrated sulphuric acid adds to alkenes to form
alkyl hydrogen sulphate by the electrophilic addition
reaction.

32. Addition of water :
In the presence of a few drops of concentrated sulphuric acid
alkenes react with water to form alcohols
Oxidation:
Alkenes on reaction with cold, dilute, aqueous solution of
potassium permanganate produce vicinal glycols

33. Acidic potassium permanganate or acidic potassium
dichromate oxidises alkenes to ketones and/or acids
depending upon the nature of the alkene

34. Ozonolysis:
Ozonolysis of alkenes involves the addition of ozone
molecule to alkene to form ozonide.
This reaction is highly useful in detecting the position of
the double bond in alkenes
Zn+H2O

35. Polymerisation:
Polythene is obtained by the combination of large number
of ethene molecules at high temperature, high pressurein
the presence of a catalyst. This reaction is known as
polymerisation.
The simple compounds from which polymers are made are
called monomers.

36. Alkynes
Hydrocarbons have adjacent carbon atoms with triple
bonds are alkynes.
Alkynes have general molecular formula CnH 2n – 2.
The first stable member of alkyne series is ethyne which is
popularly known as acetylene.

37. Nomenclature and Isomerism
In IUPAC alkynes are named as derivatives of the
corresponding alkanes replacing ‘ane’ by the suffix ‘yne’.

38. Structures I and II are position isomers and structures I
and III or II and III are chain isomers.

39. Structure of Triple Bond
The carbon atom of ethyne has two sp hybridised orbitals.
Ethyne molecule consists of one C–C σ bond, two C–H σ
bonds and two C–C π bonds

40. Preparation
From Calcium Carbide
Ethyne is prepared by treating calcium carbide with water

41. From Vicinal Dihalides
Vicinal dihalides on treatment with alcoholic potassium
hydroxide undergo Dehydrohalogenation

42. Physical properties
First three members are gases, the next eight are liquids and
the higher ones are solids.
All alkynes are colourless.
Ethyene has characteristic odour. Other members are
odourless.
Alkynes are weakly polar in nature.
They are lighter than water and immiscible with water but
soluble in organic solvents like ethers, carbon tetrachloride
and benzene

43. Chemical properties
Acidic character of alkyne:
Sodium metal react with ethyne to form sodium acetylide
with the liberation of dihydrogen gas
Ethyne is acidic in nature in comparison to ethene and
ethane since the hydrogen atoms of ethyne which are
attached to triply bonded carbon atom are acidic in
nature.

44. Addition reactions:
Addition of dihydrogen

45. Addition of halogens
Addition of hydrogen halides
Reddish orange colour of the solution of bromine in carbon
tetrachloride is decolourised.

46. Addition of water to alkynes on warming with mercuric
sulphate and dilute sulphuric acid at 333 K forms carbonyl
compounds
Addition of water

47. Polymerisation
Linear polymerisation:
Linear polymerisation of ethyne takes place to produce
polyacetylene or polyethyne.
—( CH = CH – CH = CH)n—
Thin film of polyacetylene can be used as electrodes in
batteries.
These films are good conductors, lighter and cheaper than
the metal conductors.

48. Ethyne on passing through red hot iron tube at 873K
undergoes cyclic polymerization.
Three molecules of ethyne polymerise to form benzene.
Cyclic polymerisation:
b

49. Aromatic Hydrocarbon
Benzenoids: Aromatic compounds which contains benzene
ring
Non-benzenoids: Aromatic compounds which does not
contains a benzene ring.
Aromatic hydrocarbons are also known as Arenes
Examples of Arenes

50. Nomenclature and Isomerism
Aromatic compounds are named with benzene as the
parent chain.
One side group is named in front of the name benzene.
No number is needed for mono-substituted benzene since
all the ring positions are identical
Methylbenzene Chlorobenzene
(toluene)

51. When two groups are attached to benzene, the ring is
numbered to give the lower numbers to the side groups
Some substituted benzene rings also use a common name.
Then naming with additional more side groups uses the ortho-,
meta-, para- system.

52. Structure of Benzene
Benzene is a flat, symmetrical molecule
It’s molecular formula C6H6
It has alternating three carbon-carbon double and three
single bonds
Benzene’s relatively lack of chemical reactivity is due to its
structure.
There are two possible structures with alternating double
and single bonds

53. Resonance and stability of benzene
The hybrid structure is represented by inserting a circle or
a dotted circle in the hexagon
All the six carbon atoms in benzene are hybridized.
Each carbon atom is now left with one unhybridisedp orbital
perpendicular to the plane of the ring.

54. The unhybridised p orbital of carbon atoms form a π bond
by lateral overlap.
Presence of delocalised π electrons in benzene makes it more
stable than the hypothetical cyclohexatriene.

55. Aromaticity
Hückel Rule: Characteristics
Planarity
Complete delocalisation of the π electrons in the ring
Presence of (4n+ 2) π electrons in the ring where nis an
integer (n= 0, 1, 2, . . .)

56. Preparation of Benzene
Cyclic polymerization of ethyne
Three molecules of ethyne polymerize to form benzene

57. Sodium salt of benzoic acid on heating with sodalime gives
benzene.
Decarboxylation of aromatic acids

58. Reduction of phenol
By passing Phenol vapours over heated zinc dust, benzene is
formed

59. Physical properties
Aromatic hydrocarbons are non- polar molecules and are
usually colourless liquids or solids with a characteristic
aroma.
Aromatic hydrocarbons are immiscible with water but are
readily miscible with organic solvents.

60. Chemical properties
Arenes are characterised by electrophilic substitution
reactions
Electrophilic substitution reactions
Nitration:
A nitro group is introduced into benzene ring when benzene is
heated with a mixture of concentrated nitric acid and concentrated
sulphuric acid
323 -333K

61. Halogenation:
Arenes react with halogens in the presence of a Lewis acid
to yield haloarenes.

62. Sulphonation:
The replacement of a hydrogen atom by a sulphonic acid
group in a ring is called sulphonation

63. Friedel-Crafts alkylation reaction:
When benzene is treated with an alkyl halide in the presence
of anhydrous aluminium chloride, alkylbenene is formed.

64. Friedel-Crafts acylation reaction:
The reaction of benzene with an acyl halide or acid anhydride
in the presence of Lewis acids yields acyl benzene.

65. If excess of electrophilic reagent is used, further substitution
reaction takes place.

66. Mechanism of Electrophilic Substitution Reactions
Three steps involved in reactions are:
• Generation of the eletrophile
• Formation of carbocation intermediate
• Removal of proton from the carbocation intermediate

67. Generation of Electrophile E⊕
During chlorination, alkylation and acylation of benzene,
anhydrous , being a Lewis acid helps in generation of the
electrophile , ,(acylium ion) respectively by combining with the
attacking reagent.

68. The electrophile, nitroniumion, is produced by transfer of a
proton (from sulphuric acid) to nitric acid.
In this process ,sulphuric acid serves as an acid and nitric
acid as a base.

69. Formation of Carbocation(arenium ion)
The arenium ion gets stabilised by resonance
Attack of electrophile results in the formation of σ-complex or
areniumion in which one of the carbon is hybridised.

70. Removal of proton:
Sigma complex or arenium ion loses its aromatic character
because delocalisation of electrons stops at hybridised
carbon.
To restore the aromatic character, σ-complex releases proton
from sp3
hybridised carbon on attack by [AlCl4]–
and [HSO4]–

71. Addition reactions
At high temperature and/ or pressure in the presence of
nickel catalyst, Hydrogenation of benzene gives
cyclohexane.
Under utra-violet light, three chlorine molecules add to
benzene to produce Benzene hexachloridecalled
gammaxane()

72. Combustion:
General combustion reaction for any hydrocarbon is:
When heated in air, benzene burns with sooty flame
producing and

73. Directive influence of a functional group in
Monosubstituted Benzene
When mono substituted benzene is subjected to further
substitution either ortho and para products or meta product
are predominantly formed .
This depends on the nature of the substituent already present
in the benzene ring and not on the nature of the entering
group. This is known as directive influence of substituents

74. The groups which direct the incoming group to ortho and para
positions are called ortho and para directing groups.
The electron density is more on o – and p – positions. Hence,
the substitution takes place mainly at these positions.
–OH group activates the benzene ring for the attack by an
Ortho and para directing groups:

75. Meta directing group:
The groups which direct the incoming group to meta
position are called meta directing groups.
The electron density on o– and p– position is comparatively
less than that at meta position. Hence the electrophile attacks
on meta position
The overall electron
density on benzene ring
decreases making further
substitution difficult, these
are also called
‘deactivating groups’.

76. Carcinogenicity And Toxicity
Benzene and polynuclear hydrocarbons containing more
than two benzene rings merged together are toxic and
possess carcinogenic property.
They enter into human body and undergo biochemical
reactions which damage DNA and cause cancer.
Eg: