Hydrocarbons Class 11 Notes Chemistry Chapter 9 - CBSE

Chapter : 9

What Are Hydrocarbons ?

Hydrocarbon

A compound of carbon and hydrogen is known as hydrocarbon.

Saturated Hydrocarbon

A hydrocarbon is said to be saturated if it contains only C—C single bonds. For example: Ethane

CH3 —CH3

Unsaturated Hydrocarbon

A hydrocarbon is said to be unsaturated if it contains only C = C or C = C multiple bonds. For example: ethene CH2 =CH2 , ethyene HC≡ HC.

Aromatic Hydrocarbon

Benzene and its derivatives are called aromatic compounds. For example:

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Alicyclic Compounds

Cyclic compounds which consist only of carbon atoms are called alicyclic or carbocyclic compounds.

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Heterocyclic Compounds

Cyclic compounds in which the ring atoms are of carbon and some other element (For example, N, S, or O) are called heterocyclic compounds.

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Alkanes

  • Alkanes are the simplest organic compounds made of carbon and hydrogen only.
  • They have the general formula CnHC2n+2 (where n = 1, 2, 3, etc.)
  • The carbon atoms in their molecules are bonded to each other by single covalent bonds. Since the carbon skeleton of alkanes is fully saturated’ with hydrogens, they are also called saturated hydrocarbons.

Alkanes contain strong C —C and C —H bonds.

Therefore, this class of hydrocarbons are relatively chemically inert. Hence, they are sometimes referred to as paraffins (Latin parum affinis = little affinity). First three members of this class can be represented as:

Nomenclature Of Alkanes

For nomenclature of alkanes in IUPAC system, the longest chain of carbon atoms containing single bond is selected. Numbering of the chain is done from the one end so that maximum carbon will included in the chain. IUPAC names of few members of alkanes are given below :

The Alkane family of Organic Compounds

IUPAC Name Formula
methane CH4(g)
ethane C2H6(g)
propane C3H8(g)
butane C4H10(g)
pentane C5H12(1)
hexane C6H14(1)
heptane C7H16(1)
octane C8H18(1)
nonane C9H20(1)
decane C10H22(1)
-ane C2H2n+2

Physical Properties Of Alkanes

  • Due to weak Van der Waal’s, the first four members C1 to C4 , i.e., methane, ethane, propane and butane are gases. From C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. They all are colourless and odourless.
  • Alkanes are generally insoluble in water or in polar solvents but they are soluble in non-polar solvents like, ether, benzene, carbon tetrachloride etc.
  • The boiling point of straight chain alkanes increase regularly with the increase of number of carbon atoms due to the intermolecular Van der Waal’s forces.

Chemical Properties Of Alkanes

  • Halogenation Reaction: It takes place either at high temperature (300–500°C) or in the presence of diffused sunlight or ultraviolet light.

$$\text{CH}_{4} +\text{CH}_{2}\xrightarrow{}\text{CH}_{3}\text{Cl} + \text{HCl} $$

  • Combustion Reaction: Alkanes on heating in presence of air gets completely oxidised to carbon dioxide and water. It is an exothermic process.

$$\text{CH}_{4} +2\text{O}_{2}\xrightarrow{}\text{CH}_{3}\text{Cl} + 2\text{H}_{2}\text{O} $$

  • Controlled Oxidation: When methane and dioxygen compressed at 100 atm are passed through heated copper tube at 523 K yield methanol.

$$2\text{CH}_{4} + \text{O}_{2}\xrightarrow{}\text{C2H}_{3}\text{OH}$$

  • Aromatisation: The conversion of aliphatic compounds into aromatic compounds is known as aromatisation.
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This reaction is also known as reforming.

  • Reaction with Steam: 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.

$$\text{CH}_{4} + \text{H}_{2}\text{O}\xrightarrow{}\text{CO + 3H}_{2}$$

Conformations

Sawhorse projections of eclipsed and staggered conformations of ethane are shown below:

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Newman’s projections of eclipsed and staggered conformations of ethane are shown below:

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Newman’s projections of ethane

Alkenes

Alkenes are hydrocarbons that contain atleast one carbon-carbon double bond (C=C) in their molecule. They have the general formula CnH2n. They are also known as olefins (oil forming) as the ethene (C2H4) was found to form an oily liquid on reaction with chlorine.

Nomenclature Of Alkenes

In IUPAC system

  • The name of the hydrocarbon is based on the parent alkene having the longest carbon chain of which double bond is apart.
  • This chain is numbered from the end near the double bond and its position is indicated by the number of the carbon atom not which the double bond originates.
  • The name of the parent alkene with the position number of the double bond is written first and then the names of other substituents prefixed to it.

$$\underset{\text{1-Butene}}{\text{CH}_{3}-\text{CH}_{2}-\text{CH}_{2}=\text{CH}_{2}}\\\underset{\text{2-Butene}}{\text{CH}_{3}-\text{CH}=\text{CH}-\text{CH}_{3}}$$

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  • When there are two or three double bonds in a molecule, the ending-one of the corresponding alkane is replaced by a diene to get the name.
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Structure Of Alkene

In ethylene, the carbon atoms are sp2 hybridised - They are attached to each other by a π bond and a σ bond.

The σ bond results from the head-on overlapping of sp2 hybridised orbitals of two sp2 hybrid orbitals. The π bond is formed by the lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms. Ethylene is a planar molecule.

Orbital picture of ethene showing formation of π bond

Physical Properties Of Alkenes

  • The first three members of alkenes are gases, the next fourteen are liquids and the higher ones are solids.
  • Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non-polar
    solvents like benzene, petroleum ether.
  • They show a regular increase in boiling point with increase in size i.e., every —CH2 group added increase the boiling point by 20-30K.

Chemical Properties Of Alkenes

  • Addition of Dihydrogen: Alkenes adds one mole of dihydrogen gas in presence of catalysts such as Ni at 200-250°C, or finely divided Pt or Pd at room temperature to give an alkane.

$$\text{CH}_{2}=\text{CH}_{2} + \text{H-H}\xrightarrow{}\\\text{CH}_{3}-\text{CH}_{3}$$

  • Addition of Halogens: Halogens like bromine or chlorine add up to alkene to form vicinal dihalides in presence of CCl4 as solvent. The order of reactivity of halogens is F > Cl > Br > l.

$$\text{CH}_{2}=\text{CH}_{2} + \text{Br–Br}\xrightarrow{}\\\text{Br}-\text{CH}_{2}-\text{CH}_{2}-\text{Br}$$

  • Addition of Hydrogen Halides : Hydrogen halide such as HCl, HBr, HI etc add upto alkenes to form alkyl halides. The order of reactivity of hydrogen halides is HI > HBr > HCl.

$$\text{CH}_{2}=\text{CH}_{2} +\text{H - Br}\xrightarrow{}\\\text{CH}_{3}-\text{CH}_{2}-\text{Br}$$

  • Addition of Hydrogen Halides : Hydrogen halide such as HCl, HBr, HI etc add upto alkenes to form alkyl halides. The order of reactivity of hydrogen halides is HI > HBr > HCl.

$$\text{CH}_{2}=\text{CH}_{2} + \text{H - Br}\xrightarrow{}\\\text{CH}_{3}-\text{CH}_{2}-\text{Br}$$

Markovnikov Rule

According to the rule, the negative part of the adding molecule adds to that carbon atom of the unsymmetrical alkene which is maximum substituted or which possesses lesser number of hydrogen atoms.

Anti Markovnikov Rule

Anti Markovnikov 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 reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule.

$$\text{CH}_{3}\text{CH}=\text{CH}_{2} +\text{HBr}\xrightarrow{}\\\text{CH}_{3}-\text{CH}_{2}-\text{CH}_{2}-\text{Br}$$

Polymerisation

Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst.The large molecules thus obtained are called polymers.

$$\text{n(CH}_{2} = \text{CH}_{2})\xrightarrow[\text{Catalyst}]{\text{High temperature/Pressure}}\\\underset{\text{Polythene}}{(- \text{CH} _{2} - \text{CH} _{2}-)_{n}}$$

Ozonolysis

It involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn-H2O to smaller molecules.

Oxidation

Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols.

$$\text{CH}_{2}=\text{CH}_{2} + \text{H}_{2}\text{O} +\text{O}\xrightarrow[\text{273K}]{\text{dil. KnO}_{4}}$$

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Addition Of Sulphuric Acid

Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.

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Alkynes

Alkynes are characterised by the presence of a triple bond in the molecule. Their general formula is CnH2n-2. The first and the most important member of this series of hydrocarbons is acetylene, HC=CH, and hence they are also called the Acetylenes.

Structure

Ethyne is a linear molecule. In ethyne, (HC=CH), the carbon atoms are sp hybridized. They are attached to each other by a σ-bond and two π-bonds.

The σ -bond results from the overlap of two sp hybrid orbitals. The π bonds are formed from the separate overlap of the two p-orbitals from the two adjacent carbon atoms. The other sp hybrid orbital of each carbon atom forms a σ bond with another carbon or hydrogen atom.

Nomenclature

In common system, alkynes are named as derivatives of acetylene. In IUPAC system, they are named as derivatives of the corresponding alkanes replacing ‘ane’ by the suffix ‘yne’. The position of the triple bond is indicated by the first triply bonded carbon.

Common and IUPAC names of a few members of alkyne series are given in the table below :

Structure IUPAC Name
C2H2 Ethyne
C3H4 Propyne
C4H6 Butyne
C5H8 Pentyne
C6H10 Hexyne

Physical Properties Of Alkynes

  • The first three members (acetylene, propyne and butynes) are gases, the next eight are liquids and higher ones are solids.
  • All alkynes are colourless and odourless, except ethyne which have an offensive characteristic odour.
  • Alkynes are weakly polar in nature and nearly insoluble in water. They are quite soluble in organic solvents like ethers, carbon tetrachloride and benzene.
  • Their melting point, boiling point and density increase with increase in molar mass.

Chemical Properties Of Alkynes

  • Addition of Dihydrogen: Alkynes contain a triple bond, so they add up, two molecules of dihydrogen.

$$\text{CH ≡ CH + H}_2 ——› \text{CH}_2 = \text{CH}_2\\ ——› \text{CH}_3 – \text{CH}_3$$

  • Addition of Halogens: Alkynes contain a triple bond, so they add up, two molecules of halogen.

$$\text{CH ≡ CH + Cl}_2 ——›\text{CH(Cl)=CH(Cl)}\\——›\text{CH(Cl)}_2 – \text{CH(Cl)}_2$$

  • Addition of Hydrogen Halides: Two molecules of hydrogen halides (HCl, HBr, Hl) add to alkynes to form gemdihalides (in which two halogens are attached to the same carbon atom).

$$\text{CH ≡ CH + HCl} ——›\\\text{CH}_2 – \text{CH(Cl)}$$

  • Addition of Water: Like alkanes and alkenes, alkynes are also immiscible and do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphric acid at 333 K to form carbonyl Compounds.

$$\text{CH ≡ CH} +\text{H}_{2}\text{O}\xrightarrow{}\\\text{CH}_{3}-\text{CHO}$$

  • Polymerisation: Ethyne on passing through red hot iron tube at 873 K undergoes cyclic polymerisation. Three molecules polymerise to form benzene, which is the starting molecule for the
    preparation of derivatives of benzene, dyes, drugs and large number of organic compounds.
  • Oxidation:

$$2\text{C}_{2}\text{H}_{2} + 5\text{O}_{2}\xrightarrow{}\\\text{4 CO}_{2} + 2\text{H}_{2}\text{O} $$

Aromatic Hydrocarbons

These hydrocarbons are also known as ‘arenes’. Most of such compounds were found to contain benzene ring. Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids. Some examples of arenes ar e given below.

Nomenclature And Isomerism

Benzene and its homologous are generally called by their common names which are accepted by the IUPAC system. The homologous of benzene having a single alkyl group are named as Alkyl benzenes.

Dimethylbenzenes have the common name Xylenes. The three isomeric xylenes are

Dimethylbenzenes have the common name Xylenes. The three isomeric xylenes are

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Structure Of Benzene

The molecular formula of benzene is C6H6 and it is a highly unsaturated compound. In 1865, Kekule gave the cyclic planar structure of benzene with six carbons with alternate double and single bonds.

The Kekule structure indicates the possibility of two isomeric 1,2-dibromobenzenes. In one of the isomers, the bromine atoms are attached to the doubly bonded carbon atoms whereas in the other they are attached to the singly bonded carbon.

However, benzene was found to form only one ortho disubstituted product. To overcome this problem Kekule suggested the concept of oscillating nature of double bonds in benzene as given below:

Failure of Kekule’s structure: Kekule structure of benzene failed to explain the unique stability and its preference to substitution reaction than addition reactions.

Resonance And Stability Of Benzene

The two structures, A and B given by Kekulé are the main contributing structures. The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon as shown in (C).

The circle represents the six electrons which are delocalised between the six carbon atoms of the benzene ring.

Aromaticity

It is a property of the sp2 hybridised planar rings in which the p orbitals allow cyclic delocalization of π electrons.

Conditions for Aromaticity

  • An aromatic compound is cyclic and planar.
  • Each atom in an aromatic ring has a p-orbital. These p-orbitals must be parallel so that a continuous overlap is possible around the ring.
  • The cyclic π molecular orbital (electron cloud) formed by overlap of p-orbitals must contain (4n + 2) π electrons. Where n = integer (0, 1, 2, 3, etc.). This is known as Huckel rule.

Some Examples of Atomic Compounds are given below:

Preparation Of Benzene

 Benzene is commercially isolated from coal tar. However, there are some synthetic methods which is applied in the laboratory for the preparation of benzene.

(i) By passing acetylene through red-hot tube at 500°C

(ii) Decarboxylation: By heating sodium salt of benzoic acid with sodalime (NaOH + CaO).

(iii) By heating phenol with Zinc dust.

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Physical Properties of Benzene

  • Benzene is a colourless liquid.
  • It is insoluble in water. It is soluble in alcohol, ether, chloroform etc.
  • Benzene itself is a good solvent for many organic and inorganic substances e.g., fat, resins, sulphur and iodine.
  • It burns with a luminous, sooty flame in contrast to alkanes and alkenes which usually burn with a bluish flame.

Addition Reactions

Under vigorous conditions, i.e., at high temperature and/ or pressure in the presence of nickel catalyst, hydrogenation of benzene gives cyclohexane.

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Benzene on treatment with excess of chlorine in the presence of anhydrous AlCl3 can be chlorinated to hexachlorobenzene (C6 Cl6), also called gammaxane.
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Combustion

When heated in air, benzene burns with sooty flame producing CO2 and H2O.

$$\text{C}_{6}\text{h}_{6} +\frac{15}{2}\text{O}_{2}\xrightarrow{} 6\text{CO}_{2} + 3\text{H}_{2}\text{O}$$

Directive Influence Of A Functional Group In Monosubstituted Benzene

  • Ortho and para directing groups: The groups which direct the incoming group to ortho and para positions are called ortho and para directing groups. Example, —OH; —NH2, —NHR, —NHCOCH3, —OCH3—CH3—C2H5etc.
  • Meta directing group: The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing
    groups are –NO2 , –CN, –CHO, –COR, –COOH, –COOR, –SO3H, etc.

Carcinogenicity And Toxicity

Some polynuclear hydrocarbons containing more than two benzene rings fused together become toxic and they are having cancer producing property. They are actually formed due to incomplete combustion of some organic materials like tobacco, coal and petroleum, etc.