Hydrocarbon Class 11th Chemistry Notes

 Hydrocarbon Class 11th Chemistry Notes

Hydrocarbons are the compounds which contains carbon and hydrogen. These hydrocarbon chemistry notes are written in simple and easy to understand language. So, start learning with these hydrocarbons class 11th chemistry notes.  


The compounds which contain only carbon and hydrogen are called hydrocarbons. Hydrocarbons are classified as aliphatic, alicyclic and aromatic hydrocarbons.

Aliphatic hydrocarbons contain open chains of carbon atoms, which are further classified as saturated and unsaturated hydrocarbons. 

Saturated hydrocarbon contains all C – C single bonds (alkanes) while unsaturated hydrocarbons contain C = C double bonds (alkenes) or C ≡ C  triple bonds (alkynes).

Alicyclic hydrocarbons contain rings of carbon atoms and have properties similar to that of aliphatic hydrocarbons.

Aromatic hydrocarbons contain one or more benzene rings in their structure.


Alkanes are aliphatic saturated hydrocarbons containing carbon – carbon and carbon – hydrogen single covalent bonds. Alkanes are also called paraffins because alkanes do not undergo chemical reactions easily. 

The general chemical formula of alkanes is CnH2n+2where n is an integer and indicates the number of carbon atoms in the molecule. Some common alkanes are methane (CH4), ethane (C2H6), propane (C3H8), Butane (C4H10), Pentane (C5H12) etc. Every carbon atom in alkanes is sp3 hybridized and has tetrahedral geometry. 

The tetrahedral are joined together and C – C and C – H bond lengths are 154pm and 112 pm respectively. The H – C – H, C – C – H and C – C – C bond angles are 109ᵒ28’.


The different spatial arrangements of atoms in a molecule that can be converted into one another by free rotation about single bonds are called conformations or conformers or rotamers. These isomers cannot be usually isolated because they interconvert rapidly. Conformations can be represented by different formula, such as sawhorse and Newmann projection formula.


From unsaturated hydrocarbons : By hydrogenation

Alkanes are obtained by passing a mixture of alkenes or alkynes and hydrogen over Raney nickel and catalyst at about 473K to 573K. This reaction is called Sabatier and Sanderson reaction. Platinum or Palladium also catalyses the reaction at room temperature.

e.g. CH2 = CH2  +   H2     Raney Ni  &  Δ   →    CH3 – CH3

          Ethene                                                        Ethane

       CH ≡ CH  +   2H2     Raney Ni  &  Δ   →    CH3 – CH3

        Ethene                                                        Ethane

From sodium of fatty acids : By decarboxylation

The reaction in which carbon dioxide is removed from carboxylic acid is called decarboxylation. When anhydrous sodium salt of fatty acid is fused with soda lime it gives alkane containing one carbon atom less than carboxylic acid.

Hydrocarbon Class 11th Chemistry Notes

From alkyl halides

By reduction of alkyl halides

Alkyl halides on reduction with zinc and dilute hydrochloric acid from alkane. The reduction of alkyl halides is due to nascent hydrogen obtained from reducing agent. Zn – Cu couple and alcohol can also be used as the reducing agents.

     R – X    +    2[H]      zn, HCl  →     R – H   +    HX

  Alkyl halide                                    Alkane

e.g.      CH3 – I    +    2[H]       Zn, HCl  →    CH4     +     HI

      Methyl iodide                                      Methane

By using active metal (Wurtz synthesis reaction)

Alkyl halides on heating with sodium metal in presence of dry ether as solvent give higher alkanes containing even number of carbon atoms.

R – X  +  2Na + X – R   dry ether  →  R – R  +  2NaX

e.g.      CH3 – Cl    +    2Na    +    Cl – CH3      dry ether  →    CH3 – CH3  +  2NaCl

       Methyl Chloride                                                            Ethane


  • Alkanes are colourless and odourless they are insoluble in water, but readily dissolve in non – polar solvents like benzene, ether, chloroform etc.
  • At room temperature, the first four alkanes are gases, next thirteen (C5 to C17) are liquids where as higher alkanes are waxy solids.
  • Alkanes are non polar as C – H and C-C bonds in alkanes are nonpolar covalent bonds. All alkanes are less dense than water and their relative density increases with increase in molecular mass.
  • Melting and boiling points increases with increase in molecular mass and decreases as branching increases.


Halogenations of alkanes

Alkanes reacts with halogens in presence of ultraviolet light as diffused sunlight or at high temperature (573K to 773K) to give mixture of alkyl halides.

Chlorination of Methane :

CH4  Cl2 – HCl →  CH3 – Cl   Cl2 – HCl →  CH2Cl2   Cl2 – HCl →  CHCl3   Cl2 – HCl →  CCl4

Nitration of alkanes

The reaction in which one hydrogen atom of alkanes in replaced by nitro (- NO2) group is called nitration of alkanes. It is carried out in vapour phase, by heating a mixture of an alkane and conc. HNO3 at about 423K to 698K.

e.g.  CH3 – CH3   +   HO – NO2      423 to 698K  →    CH3 – CH2 – NO2   +   H2O

        Ethane                                                              Nitroethane

Combustion of alkanes

Alkanes on heating in the presence of air or dioxygen (O2) are completely oxidized to CO2 and water with the evolution of large amount of heat.

e.g. CH4 + 2O2  →  CO2  +  2H2O


ΔcHᵒ = -890KJmol-1

Pyrolysis of alkanes

Alkanes on heating at higher temperature in absence of air decompose to give lower alkanes, alkenes and hydrogen etc. this is known as pyrolysis or cracking.

Pyrolysis of alkanes

Reforming of alkanes

Alkanes having more than five carbon atoms get cyclized to benzene and its homologues, on heating under 10 to 20 atm. pressure at about 773K in presence of oxides of Cr, V, Mo supported on alumina. This reaction is called aromatization or reforming.

aromatization or reforming reaction of hydrocarbons in our hydrocarbon chemistry notes website "Chemistry Notes Info" ChemistryNotesInfo.com


Alkenes are aliphatic unsaturated hydrocarbons containing one or more double bonds. The aliphatic unsaturated hydrocarbons are called alkadienes and alkatrienes respectively. Alkenes are also called Olefines (oil forming). The general formula of alkenes is CnH2n

Every doubly bonded carbon atom in alkene is sp2 hybridized and has trigonal planar geometry with bond angles 120. In alkenes the doubly bonded carbon atoms have one C – C sigma (σ) and one C – C pi (∏) bond. The ∏ bond enthalpy is 284 KJmol-1 and that of (σ) bond is 397KJmol. The C = C bond length in ethane is 133 pm.


From alcohols (by dehydration)

Dehydration by concentrated sulphuric acid

When an alcohol is heated with concentrated sulphuric acid (H2SO4) at about 443K, it loses a water molecule to give corresponding alkene. It is acidic dehydration.


Dehydration by alumina

When vapours of an alcohol are passed over heated alumina at 623K, it gives corresponding alkenes. It is known as vapour phase dehydration of alcohol.

METHOD OF PREPARATION OF ALKENES from dehydration by alumina

From alkyl halides (By dehydrohalogenation)

When an alkyl halide is boiled with alcoholic alkali it gives corresponding alkene. The order of reactivity of alkyl halides towards dehydrohalogenation is 3ᵒ > 2ᵒ > 1ᵒ and that of halogens is I > Br > Cl.

 e.g. CH3-CH2-Br   +   KOH      boil   ---> CH2=CH2  + KBr  +  H2O   

       Ethyle bromide Ethyne

From vicinal dihalides (By dehalogenation)

The dihalides in which two halogen atoms are attached to the adjacent carbon atoms are called vicinal dihalides when vicinal dihalide is heated with zinc metal in presence of alcohol, it gives alkene.

 e.g.      Br-CH2-CH2-Br   +   Zn       Alcohol   --->   CH2=CH2  +   ZnBr2

           Ethylene dibromide                                         Ethene

From alkynes

Alkynes on partial reduction by Lindlar’s catalyst gives alkenes. Lindlar’s catalyst is metallic palladium deposited on calcium carbonate, partially poisoned with lead acetate.

  e.g. CH=CH  +  H2       Pd, Lead acetate --->    CH2=CH2  

       Ethene                                                      Ethene


  • Alkenes resemble alkanes in most of their physical properties. Ethyne is colourless gas with sickly sweet odour while all other alkenes colourless and odourless.
  • First three alkenes are gases, next 14 are liquids and rest higher ones are solids. Alkenes are lighter than water and the density gradually increases with rise in molecules mass.
  • Alkenes are non – polar or weakly polar compounds and are insoluble in water but soluble in non – polar solvents like benzene, ether, chloroform, etc.
  • As in alkanes, boiling point of alkenes increases with increase in molecular mass and decreases with increase in branching.


Formation of alkanes (Hydrogenation) 

Alkenes on catalytic hydrogenation at about 443K to 453K, under pressure give corresponding alkanes.

 e.g. CH2=CH2  +  H2  Raney Ni, Δ  ---->  CH3-CH3

         Ethene                                             Ethane

Formation of vicinal dihalides (Halogenation)

Alkenes react with halogens to give vicinal dihalide. The order of reactivity of halogens is Cl2 >  Br2 > I2.

 e.g. . CH2=CH2   +  Cl2  CCl4   --->  Cl- CH2-CH2-Cl

          Ethene                                   Ethylene dichloride

Formation of alkyl halides (Hydrohalogenation)

Alkenes react with hydrogen halides to give corresponding alkyl halides. In case of symmetrical alkenes, addition of hydrogen halides leads to only one product. However unsymmetrical alkenes give two products. The major product is predicted by following rule.

Markownikoff’s rule:  

When an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part of reagents gets attached to that carbon atom which carries less number of hydrogen atoms.

Peroxide effect Or anti-Marjownijoff's rule: 

If addition takes place in presence of peroxide then negative part of reagent gets attached to that carbon atom which carries greater number of hydrogen atoms.

Peroxide effect Or anti-Marjownijoff's rule

Addition of sulphuric acid

Alkenes react with cold conc. Sulphuric acid to give alkyl hydrogen sulphate which on boiling with water give corresponding alcohol. Addition of alkyl hydrogen sulphate takes place according to morkownikoff’s rule.

CH2=CH2 (Ethene) +   H-OSO3H (Conc.) ----> CH3-CH2-OSO3H (Ethyl hydrogen sulphate) +  H-OH  boil--->  CH3-CH2-OH (Ethyl alcohol) +  H2SO4

Formation of aldehydes and ketones (Ozonolysis)

When a stream of ozonized oxygen is passed through a solution of alkenes in presence of organic solvent, it gives unstable ozonide which on reduction with zinc dust and water give either aldehyde or ketones or mixture of both.

CHEMICAL PROPERTIES OF ALKENES - Formation of aldehydes and ketones (ozonolysis)

Formation of glycols : Hydroxylation (Bayer’s test)

Alkenes react with cold and dilute alkaline potassium permanganate to give vicinal glycols (1, 2 - diols).

 e.g. CH2=CH2  +  H2O  +   (O)        Alkaline KMnO4--->       HO-CH2-CH2-OH

       ethane                                                                  Ethylene glycol


Ethene on polymerization at high temperature and pressure in presence of oxygen gives high molecular weight polymer called polythene.

 e.g. CH2=CH2  ---Polymerization---->    –[--CH2-CH2--]-   

              ethane Polythene


Alkynes are aliphatic unsaturated hydrocarbons containing at least one triple bond. The aliphatic unsaturated hydrocarbons containing two or three carbon – carbon triple bonds in their structure are called alkadiyenes and alkatriynes respectively. The general formula of alkynes is CnH2n-2 carbon atom in ethylene is sp hybridized and has linear geometry. 

Acetylene also called as ethyne is the chemical compound with the chemical formula C2H2 and have chemical structure H−C≡C−H. Ethyne is a hydrocarbon and it is the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block.

The H – C – C bond angle is 180ᵒ and  C≡C (823KJmol-1) bond length is 120pm. The triply bonded (C≡C) carbon atoms have one C – C sigma (σ) bond and 2 C – C pi (∏) bonds.


From calcium carbide (Industrial method)

Industrially the ethyne is prepared by the reaction of calcium carbide with water.

 CaC+  2H2O  ---->   C2H2   +   Ca(OH)2

By dehydrohalogenation

From vicinal dihalides

When vicinal dihalides react with alcoholic solution of potassium hydroxide to form vinyl halide which on further treating with sod amide forms alkyne. 


Br-CH2-CH2-Br (Ethylene dibromide) +  KOH ----> CH2=CH-Br  NaNH2 ---> CHCH (ethyne) +  NaBr  +NH3

From germinal dihalide

When germinal dihalide is boiled with alcoholic caustic potash it gives vinyl halide which on heating with sod amide gives acetylene.


CH-CH(Br)+ KOH  Δ  ---> CH2=CH-Br  NaNH2---> CH≡CH +  NaBr  +NH3

From tetra halide (by dehalogenation)

When acetylene tetra bromide is heated with zinc is presence of alcohol, acetylene is formed.

 (Br)2-CH-CH-(Br) +  2Zn  Alcohol--->  CH≡CH (Ethyne) +  ZnBr2



  • Alkynes resemble alkanes and alkenes in their physical properties. First three alkynes are gases, next eight are liquids and higher ones are solids.
  • All alkynes, except acetylene, are odourless. They are generally more polar and have slightly higher boiling points than corresponding alkanes and alkenes .
  • They are less dense than water and are insoluble in water but quite soluble in organic solvents like ether, benzene, etc.


Acidic character of alkynes

Alkynes react alkali metal sodium to give sodium acetylide with the liberation of hydrogen gas while alkanes as well as alkenes do not react with alkali metals. This reaction explains acidic character of alkynes.

  CH≡CH    +  2Na  ----> 2 CH≡C-Na+  + N2

Formation of alkanes (Hydrogenation)

Alkynes on hydrogenation in presence of catalyst like raney Ni to give first alkene which                     further reaction give alkanes.   

  CH≡CH (Ethyne) +  H2  Raney nickel Δ---> CH2=CH2  raney nickel H2--->  CH3-CH3(Ethane)

Formation of tetra halides (halogenations)

Ethyne on halogenations first gives ethyne dichloride which on further reaction gives ethyne tetrachloride.

 CH≡CH (Ethyne) +  Cl2 ----> (Cl)2-CH-CH-(Cl)2

Formation of aldehydes or ketones (hydration)

Alkynes react with water in presence of 40% sulphuric acid and 1% mercuric sulphate to form aldehyde or ketones i.e. carbonyl compounds.

 CH≡CH (Ethyne)   + H-OH  ---->  CH3-CHO (Acetaldehyde)                                

Formation of benzene (cyclization)

    When ethyne is passed through red hot iron quartz tube at873K, it trimerises to give benzene.

Formation of benzene (cyclization) from ethyne


Benzene and compounds that resembles benzene in their chemical properties are called aromatic compounds or arenes. Aromatic compounds posses pleasant fragrance (Aroma). Benzene is the parent compound in most of the aromatic compounds.  Benzene was originally called phene. It is colourless liquid having characteristic odour. 

Aromatic compounds contain high percentage of carbon and burn with sooty flame. They are cyclic compounds with alternate single and double bonds and are not attacked by normal oxidizing and reducing agents. Aromatic compounds do not undergo addition reactions easily and prefer substitution reaction.


The molecular formula of benzene is C6H6. All six carbon atoms in benzene are sp2 hybridized. Benzene is a planar molecule with bond angle 120° each and all C-C bond lengths in benzene are equal to 139pm. Benzene has boiling point 353K. It is insoluble in water and soluble in alcohol, ether and chloroform. The structure of benzene is as follows:




Huckel rule helps to predict whether a compound is aromatic or not. It states that any cyclic, planar and conjugated compound containing (4n+2)π electrons, where n=0,1,2,3,4… etc. is aromatic compound. Thus aromatic compounds have delocalized electron cloud of π electrons of 2,6,10,14 etc. e.g. Benzene (n=1) has 6π electrons. Naphthalene  (n=2) has 10π electrons. 


From acetylene (by cyclization)

    When ethyne is passed through red hot iron quartz tube at 873K, it trimerises to give benzene.


From sodium benzoate (by decarboxylation)

     When anhydrous sodium benzoate is heated with soda-lime it gives benzene.

METHODS OF PREPARATION OF BENZENE - Hydrocarbon chemistry notes

From phenol (By reduction)

     When vapour of phenol passed over heated zinc dust, it gives benzene. 

METHODS OF PREPARATION OF BENZENE from phenol - Hydrocarbon chemistry notes


Addition reactions

Addition of chlorine to Benzene

Benzene when treated with chlorine in presence of bright sunlight or UV light, add up three molecules of chlorine to give benzene hexachloride. 


Addition of hydrogen to Benzene

Benzene on heating with hydrogen gas in presence of catalyst like nickel at 453K, it adds up hydrogen molecules to give cyclohexane. 

CHEMICAL PROPERTIES OF BENZENE - Addition of hydrogen - Hydrocarbon

Substitution reaction 

Halogenation of Benzene

Substitution of hydrogen atom of benzene by a halogen atom is called halogenation of benzene. Chlorine Or bromine react with benzene in dark in presence of Fe, FeCl3, AlCl3 or red phosphorus to give chlorobenzene Or bromobenzene. 

Halogenation of Benzene - Hydrocarbon chemistry notes by ChemistryNotesInfo.com

Nitration of Benzene

Nitration is the substitution of - NO2 (nitro) group. When benzene is heated with a mixture of concentrated nitric acid and sulphuric acid in proportion 1:2 (nitrating mixture) at about 313K to 333K, it gives nitrobenzene.

Nitration of Benzene - Hydrocarbons Chemistry Notes by Chemistry Notes Info website

Sulphonation of Benzene

Sulphonation is the substitution of - SO3H  (sulphonic acid) group. When benzeblne is heated with fuming sulphuric acid (oleum) at 373K, it gives benzene sulphonic acid. 

Sulphonation of Benzene - Hydrocarbon Science Notes

Friedel – crafts alkylation reaction of Benzene

When benzene is treated with an alkyl halide in the presence of anhydrous AlCl3 it gives alkyl benzene. 

Friedel – crafts alkylation reaction of Benzene - Hydrocarbon notes

Friedel – crafts acylation reaction of Benzene

When benzene is heated with acyl halide or acid anhydride, in the presence of anhydrous AlCl3 , it gives corresponding acyl benzene. 

Friedel – crafts acylation reaction When benzene is heated with acyl halide or acid anhydride, in the presence of anhydrous AlCl3 , it gives corresponding acyl benzene.

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