Alkanes

Hydrocarbons:

  • Compounds containing carbon and hydrogen only are called hydrocarbons.
  • Examples: Methane (CH4), Ethane (C2H6), Benzene (C6H6), etc.

Aliphatic Hydrocarbons:

  • The hydrocarbons in which carbon atoms are joined form an open chain are called aliphatic hydrocarbons.
  • Examples: Methane (CH4), Ethane (C2H6), etc.
  • Aliphatic hydrocarbons are further classified into two types a) Saturated hydrocarbons and b) Unsaturated hydrocarbons


Saturated Hydrocarbons:

  • The hydrocarbons in which all valencies of each carbon atom are fully satisfied by single covalent bonds only are called saturated hydrocarbons.
  • Examples: Methane (CH4), Ethane (C2Hi.e. CH3–CH3), etc.

Unsaturated Hydrocarbons:

  • The hydrocarbons in which valencies of at least two carbon atoms are not fully satisfied by single covalent bonds are called unsaturated hydrocarbons.  They are further classified as alkenes (contain at least one carbon-carbon double bond) and alkynes (contain at least one carbon-carbon triple bond)

Alkanes:

  • Open chain (aliphatic) saturated hydrocarbons are called alkanes.
  • Due to their little tendency to undergo chemical changes, they are also known as paraffin.
  • Their general formula is CnH2n + 2. Where n is the number of carbon atoms.
  • In them, carbons are linked to each other by single covalent bonds.
  • All the four valencies of carbon are satisfied by four atoms or group of atoms.

First Ten Basic Alkane:

Sr. No. Molecular Formula Name of Compound Sr. No. Molecular Formula Name of Compound
1 CH4 Methane 6 C6H14 Hexane
2 C2H6 Ethane 7 C7H16 Heptane
3 C3H8 Propane 8 C8H18 Octane
4 C4H10 Butane 9 C9H20 Nonane
5 C5H12 pentane 10 C10H22 Decane

Example – 1:

  • Write molecular formula for alkane containing 21 carbons

Number of carbons = n = 21

Their general formula is CnH2n + 2.

Hence molecular formula of the alkane is C21H2×21 + 2.  i.e.  C21H44.



Example – 2:

  • The molecular mass of alkane was found to be 142. Find its molecular formula.

Let there are n carbon atoms in alkane, its formula is CnH2n + 2.

Molecular mass of alkane = n × 12 + (2n + 2) × 1 = 142

∴   12n + 2n + 2 = 142

∴   14n  = 140

∴   n  = 10

Hence molecular formula of the alkane is C10H2×10 + 2.  i.e.  C10H22.

Structural Formula:

  • The molecular formula of a compound indicates, the number of atoms of various elements present in the compound. But it does not give the idea about the arrangement of these atoms in space to form the molecule.
  • The formula which gives exact arrangement of atoms of different elements present in the molecule of the compound is called structural formula.

Structural Formula for Methane:

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Structural Formula for Ethane:

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Structure of Methane Molecule:

  • The molecular formula for methane is CH4. Thus one molecule of methane contains one atom of carbon and 4 atoms of hydrogen.
  • Carbon atom contains four electrons in its valence shell, while hydrogen contains one electron its valence shell. During the formation of the methane molecule, carbon atom shares its four valence electrons with four hydrogen atoms. Thus the octet of carbon and duplets of hydrogen are completed. The Structural formula for methane is as follows.
  • But due to SP³ hybridisation in carbon, the four valencies of carbon do not lie in the same plane. Actually, they are directed towards the four corners of a regular tetrahedron. Thus in methane, the carbon atom is at the centre of a regular tetrahedron and four hydrogens are at the four corners of this tetrahedron.
  • the C-C bond length is 154 pm and C-H bond length is 112 pm. the H-C-H bond is 109°28′.


Structure of Ethane:

  • The molecular formula for methane is C2H6. Thus one molecule of ethane contains two atom of carbon and 6 atoms of hydrogen. Carbon atom contains four electrons in its valence shell, while hydrogen contains one electron its valence shell. During the formation of ethane molecule, two carbon atoms share one electron each among themselves forming carbon-carbon single bond. Rest six valence electrons ( three per carbon) are shared with six hydrogen atoms. Thus the octet of carbons and duplets of hydrogen are completed.

Classification of Alkanes:

  • Alkanes are classified into two types viz. Straight chain alkanes and branched chain alkanes

Straight-Chain Alkanes:

  • The alkanes in which all the carbon atoms are in continuous (same) chain are called straight chain alkanes or normal alkanes and are represented as n – alkanes.

CH3–CH2–CH2–CH3                CH3–CH2–CH2–CH2–CH3

n-Butane                                            n-Pentane

Branched-Chain Alkanes:

  • The alkanes in which all the carbon atoms are not in continuous (same) chain are called branched-chain alkanes.

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Isoalkanes:

  • The alkanes in which one methyl group is attached to the second carbon atom of the normal chain of carbon atoms are called isoalkanes.
  • In isoalkanes, one carbon branch is attached to carbon atom next to end carbon atom of the continuous chain.
  • In these alkanes CH(CH3)2– (isopropyl) group is attached at one end of the normal chain of carbon atoms.

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Neoalkanes:

  • The alkanes in which two methyl groups are attached to the second carbon atom of normal chain of carbon atoms are called neoalaknes.
  • In neoalkanes to 1 carbon branches are attached to carbon atom next to end carbon atom of continuous carbon chain.
  • In these alkanes H(CH3)3– (tertiary butyl group) is attached at one end of the normal chain of carbon atoms.

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Type of Carbon Atoms:

Primary Carbon Atom:

  • Carbon atom which is attached to only one other or no carbon atom is called primary carbon. It is denoted by 1°.

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Secondary Carbon Atom:

  • Carbon atom which is attached to two other carbon atoms is called secondary carbon. It is denoted by 2°.

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Tertiary Carbon Atom:

  • Carbon atom which is attached to three other carbon atoms is called tertiary carbon. It is denoted by 3°.

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Quaternary Carbon Atom:

  • Carbon atom which is attached to four other carbon atoms is called quarternary carbon. It is denoted by 4°.

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Isomerism in Alkanes:

  • Two or more compounds having same molecular formula but different structural formulae are called isomers and the phenomenon is known as isomerism. Isomerism is a unique characteristic of organic compounds.
  • Alkanes show chain isomerism only. The isomerism exhibited by organic compounds due to different arrangements of carbon atoms or the nature of carbon chain in them is called chain isomerism.
  • Butane C4H10 has two isomers.

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  • Pentane has  three isomers

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Conformations of Ethane:

  • The different spatial arrangements of atoms in a molecule that can be converted into one another by free rotations about a single bonds are called conformations or conformers or rotamers.
  • These isomers cannot be usually isolated because they interconvert rapidly.
  • By Newmann projection the carbon-carbon bond directly end on and represents two carbon atoms by circle. Bonds attached to the front carbon atom are represented by lines drawn from centre of the circle. The bonds attached to the rear carbon atom are represented by lines drawn from the edge of the circle.

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  • These conformations are possible due to presence of carbon carbon single bond which is cylindrically symmetrical about a line joining the nuclei of the two carbons. The energy difference between these two structures is very small hence they can interconvert amomg themselves very easily.
  • The infinite number of intermediate conformations are called skew conformations. The energy required to rotate the molecule about carbon-carbon bond is called torsional energy. The potential energy is minimum in staggered conformation (more stable) while it is maximum at eclipse conformation (less stable). 99% ethane molecules have staggered conformation.
  • Each H-C-C-H unit in ethane  is characterized by a torsion angle. Eclipsed bonds are characterized by a torsion angle of 0° (syn). Gauche conformation is characterized by torsion angle 60°. Anti conformation is characterized by torsion angle 180°.

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Alkyl Groups:

  • An alkyl group is saturated hydrocarbon radical obtained by removing one hydrogen from an alkane. Alkyl group possesses one free valency which can be attached to any suitable radical. Alkyl group has a general formula is  CnH2n + 1  –.
  • To name the alkyl group the ending characteristic –ane is replaced by –yl.. Thus from methane, we get methyl group.
Sr. No. Alkane Formula Alkyl Group Formula
1 Methane CH4 Methyl CH3
2 Ethane C2H6 Ethyl C2H5
3 Propane C3H8 Propyl C3H7
4 Butane C4H10 Butyl C4H9
5 Pentane C5H12 Pentyl C5H11
6 Hexane C6H14 Hexyl C6H13

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IUPAC Nomenclature of Alkyl Groups:

IUPAC Nomenclature of Alkanes:

  • The longest continuous chain of carbon atoms in the compound is selected and treated as a parent alkane and the compound is supposed to be derived from that alkane.
  • If there is more than one longest chain containing the same number of carbon atoms, then the chain with maximum branching should be selected as the basic chain. Branched groups are called substituents.
  • The carbons in the basic chain are then numbered such that the first substituent encountered gets the least possible number.
  • The side chains (substituents) are indicated by proper prefixes to the root name.
  • The location of the substituent is indicated by the number of the carbon atom to which it is attached. This number is called the locant. The locant is placed before the name of the substituent.
  • The number of same substituents is denoted by proper prefix as per the table.
  • The substituents are then listed alphabetically without considering the prefixes like di, tri, tetra etc.
  • The comma is put to separate numbers and hyphens are put to separate a number from the letter.
  • Name of last alkyl group and alkane is written as one word.

Preparation of Alkanes:

From Unsaturated Hydrocarbons

  • From Alkenes and Alkynes:
  • Alkanes are obtained by passing the mixture of alkenes and hydrogen over Raney nickel (Ni), or platinum(Pt) or palladium (Pd) at about 473 K to 573 K, corresponding alkanes are obtained. This is hydrogenation process and is also known as Sabatier and Sanderson reaction.
  • General Reaction and Examples:
  • From alkynes:
  • Alkanes are obtained by passing the mixture of alkynes and hydrogen over Raney nickel (Ni), or platinum(Pt) or palladium (Pd) at about 473 K to 573 K, corresponding alkanes are obtained. This is hydrogenation process and is also known as Sabatier and Sanderson reaction.
  • General Reaction and Examples:

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From Alkyl Halides:

  • Method – I:
  • When alkyl halides are treated with a suitable reducing agent like a zinc-copper couple with alcohol, or aluminium – amalgam in water-alcohol or zinc in acetic acid corresponding alkanes are obtained.
  • General Reaction and Examples:

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  • Method – II (By Wurtz Reaction):
  • When alkyl halides are treated with active metal like sodium or zinc in presence of dry ether.  corresponding alkanes are obtained. In this method, a higher alkane containing a double number of carbon atoms as compared to that in alkyl halide are obtained.
  • General Reaction and Examples:

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From Sodium Salt of Carboxylic Acids (Fatty Acids):

  • When anhydrous sodium salt of fatty acid is fused with soda lime it gives alkanes containing one carbon less than the carboxylic acid. The process is called decarboxylation.
  • General Reaction and Examples:

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Physical Properties of Alkanes:

  • They are colourless, odourless, tasteless compounds.
  • The first four alkanes namely, methane, ethane, propane and butane are gaseous at room temperature. Those containing 5 carbons to 17 carbons are liquids with increasing boiling points. Those containing more than 17 carbons are waxy solids.
  • They are insoluble in water but soluble in organic solvents like benzene, acetone, ether etc.
  • They are lighter than in air and hence float on water.
  • They are bad conductors of heat and electricity.
  • Boiling points of branched alkanes are lower than that of straight chain alkanes.

Reactivity of Alkanes (Less Reactivity):

  • Alkanes are saturated hydrocarbons. They do not contain any functional group. Hence they are less reactive.
  • Under normal conditions, they are not acted upon by concentrated acids, alkalies, oxidising agents etc.
  • Branched-chain alkanes are more reactive then straight-chain alkanes.
  • Alkanes shows substitution reactions

Reactions of Alkanes:

Halogenation of Alkanes:

  • Reactivity: F2 > Cl2 > Br2 > I2.
  • Fluorination of alkanes: Reaction of fluorine with alkanes highly explosive and highly exothermic. The reaction takes place even in the dark. The reaction is so spontaneous that even the C-C bond is also broken. The reaction of fluorine with alkanes is carried out by diluting fluorine with nitrogen.
  • Chlorination of alkanes: Chlorine reacts with alkanes in presence of diffused sunlight in steps by successive replacement of hydrogen by chlorine. A mixture of mono, di, tri and tetrachlorides is obtained.

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  • Bromination of alkanes: A reaction of alkanes with bromine is similar to that of chlorine but the reaction is very slow. Hence it is carried out in presence of AlBr3 as a catalyst.

CH4 +  Br2    →       CH3Br     +          HBr

Methane       Methyl bromide

  • Iodination of alkanes: The reaction of alkanes with iodine is reversible hence direct iodination of alkanes is not possible.

CH4 +  I2  ⇌      CH3I     +     HI

Methane             Methyl iodide

If the reaction is carried out in the presence of HIO3, HNO or HgO, alkyl halides are obtained.

5CH4 +  2 I2  + HIO3 →   5CH3I  +  3H2O

Methane                          Methyl Iodide



Mechanism of Chlorination of Alkanes:

  • Chlorination of alkanes is a free radical reaction. The reaction takes in the following steps.
  • Chain initiation: In presence of sunlight the chlorine molecule is decomposed into chlorine atoms. These atoms are highly reactive as they possess unpaired electron.

In presence of sunlight

Cl–Cl    → Cl*   +   Cl*

  • Chain propagation: The active chlorine atoms attack methane molecule and remove one hydrogen atom. During this process hydrogen chloride and a free methyl radical is formed.

CH4 +  Cl *    →   CH3*  +  HCl

  • Methyl radical formed reacts with another chlorine atom to form methyl chloride and an active chlorine atom. This active chlorine atom again brings about the whole sequence of similar reactions.

CH3*   +  Cl2   →  CH3Cl +  Cl *

  • In this reaction, we can assume one active chlorine atom can bring about an unlimited number of conversions. Hence the reaction is called chain reaction or chain propagation.
  • Chain termination: Reaction of two species possessing unpaired electrons form neutral molecules. Thus free radicals are consumed and hence these reactions are called chain termination reactions.

Cl *   +   Cl *  → Cl2

CH3*   + CH3*  → CH3–CH3

CH3*   + Cl*  → CH3Cl

Nitration of Alkanes:

  • There is no effect of nitric acid at normal temperature on alkanes. When nitric acid is heated with alkanes to about 423 K to 698 K nitroalkanes are obtained.
  • General Reaction and Examples:

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Pyrolysis of Alkanes:

  • The decomposition of the compound using heat alone (in absence of air) is called pyrolysis In alkanes two types of pyrolysis reaction can take place. Dehydrogenation and cracking.

Cracking of Alkanes:

  • The decomposition of alkanes by heating to a very high temperature in the absence of air is called the cracking of alkanes. In this process, the carbon-carbon bonds are broken and a mixture of alkane and alkene is obtained.
  • When propane is heated to about 873 K in the absence of air, ethene and methane is obtained.

CH3–CH2–CH3   →   CH2=CH2   +   CH4

Propane                        ethene       methane

Dehydrogenation of Alkanes:

  • The decomposition of alkanes by heating to a very high temperature in the absence of air is called the dehydrogenation of alkanes. In this process, the carbon-hydrogen bonds are broken and a mixture of alkanes and alkenes is obtained.

CH3– CH3      →    CH2=CH2 +  H2

Ethane                    Ethene

CH3–CH2–CH3  →  CH3 –CH=CH2  + CH2=CH2 + CH4 +   H2

Propane     Propene         Ethene       Methane

Aromatization or Reforming Reaction of Alkanes:

  • Alkanes having more than five carbon atoms get cyclized to benzene and its homologues, on heating under pressure of 10 to 20 atm at about 773 K in presence of oxides of chromium, vanadium or molybdenum supported on alumina.

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Combustion of Alkanes:

  • On burning in air alkanes combine with oxygen to form carbon dioxide and water. This reaction is highly exothermic and large amount of heat is liberated during combustion.

CH4 +    2O2       →   CO2 +  2 H2O   + Heat

C2H6 +    7O2       →    4CO2 +  6 H2O   + Heat

Uses of Alkanes:

  • Methane is the main constituent of natural gas. Natural gas is used as industrial fuel and for illumination.
  • Iso-butane is liquefied petroleum gas (LPG) and is filled in steel cylinders and is used as fuel for cooking and industrial heating.
  • Petrol contains a mixture of alkanes containing 6 to 12 carbons, which is used as motor fuel.
  • Higher liquid alkanes like kerosene and furnace oil are used as industrial and domestic fuels.
  • Diesel is used in a diesel engine.
  • Propane is used as refrigerant in petroleum industry.
  • A mixture of alkanes containing 20 to 30 carbons is called petroleum jelly or Vaseline and are used in ointments and cosmetics.
  • Alkanes containing 17 to 20 carbons are thick liquids which are used as lubricating oils for machines.
  • Alkanes are used to manufacture carbon black which is used in paints, inks, boot polish and rubber.
  • Methane is used for the synthesis of ammonia
  • Methane is used as starting reactant for the manufacture of methanol, methylene chloride, chloroform, carbon tetrachloride, formaldehyde etc.

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