Alkane

 Introduction:

Alkane are the simplest organic compounds. They are made up of carbon and hydrogens canes form a homologous series with the general formula CnH2n+2 where n=1, 2, 3, …….. etc. For example, the first two members of the series have molecular formulas CH4 and C2H6. They can be represented as:

Those compounds are called ‘saturated hydrocarbons’, this is due to the fact that the carbon skeleton of alkane is fully satisfied or saturated with hydrogens. Since they are relatively inert towards most of the chemical reagents, they are sometimes also called ‘paraffin (Latin; parum = little, affine = affinity ).

Structure:

The molecules of alkane are made up of carbon atoms joined to each other by single covalent bonds while the remaining valency of carbon is satisfied by hydrogen atoms. According to modern orbital theory, the excited carbon has the electronic configuration ;

 

 

                                 C-atom:   1S2, 2S2, 2px1, 2py1, 2pz0            (Ground State)

                            C-atom:  1S2, 2s1, 2px1, 2py1, 2pz1         (Excited state)

 

The 2s and 2p orbitals in the excited state hybridize to give four sp3 orbitals. These orbitals are directed toward the four corners of a regular tetrahedron. These sp3 hybrid orbitals are utilized for the formation of C-C or C-H single bonds in alkane. For example: In methane carbon forms single bonds with four hydrogen atoms. Each carbon-hydrogen bond is the result of the overlap of an sp3 orbital from carbon and a rom hydrogen. All carbon-hydrogen bonds are sp3 -s, o- bonds.

The Ethane molecule, sp3 hybrid orbital of one carbon overlaps with the same orbital of other carbon atom is form sp3 – sp3 ; carbon – carbon, o – bonds. The remaining three sp3 orbitals of each of the two carbons overlap with 1s – orbitals of hydrogens to form sp3 – s; carbon – hydrogen, o – bonds.

In all higher alkane, the carbon atoms are joined each other by sp3 – sp3 , o – bonds and to hydrogens by sp3 -s, o- bonds. Since these o-bonds are remarkably strong hence the alkane are chemically inert.

Nomenclature :

There are two systems of naming alkane:

The common system:

In trivial or common system they are named by putting the corresponding Greek numeral prefixes before the ending – ane. Thus, the names of an individual alkane is a one word name. The common names of first ten alkane are given below :

CH4 : Methane         CH4 : Hexane

C2H6 : Ethane      C7H15 : Heptane

C3H5 : Propane      C3H18 : Octane

C4H10: Butane       C9H20 : Nonane

C5H12: Pentane     C10H22: Decane

They are further names as :

Straight chain Hydrocarbons :

The alkanes having a straight or normal chain are called ‘normal’ (or n -) hydrocarbons. This is indicated by prefixing n- to the name of alkane. e.g.

CH3—CH2—CH2—CH3     (C—C—C-C )    CH3—CH2—CH2—CH2—CH3    (C—C—C—C )

Branched Chain Hydrocarbons : The alkanes in which the carbon chains has branches are called branched chain hydrocarbons. They are further named by following two ways:

Iso hydrocarbons (Greek, isos = equal) : such alkane possess a one carbon branch on the second carbon of the basic normal chain. e.g.

Neo hydrocarbons ( Greek; neos = new) : When the normal carbon chain has two one carbon branches on the second carbon from the end.

 

Types of Carbon atoms in Alkane :

The structural formulae of alkanes and their derivatives contain four pes of carbon atoms.

 

Primary (10) carbon atom which is attached to one or no other carbon atom.

Secondary (20) carbon atom which is attached to two other carbon atoms.

Tertiary (30) carbon atom which is attached to three other carbon atoms.

Quaternary(40) carbon atom which is attached to four other carbon atoms.

I.U.P.A.C. System:

1.The LII.P.A.C. names of alkane with a continuous carbon chain or straight carbon chain without a branch are similar to the common names except that the prefix ‘n’ is not used in the I.U.P.A.C. names. Procedure for ving I.U.P.A.C. names to branched alkanes involves the following steps:

2.Select the longest possible continuous chain as the parent chain, irrespective of its mode of writing.

3.Write the root word corresponding to the number of carbon atoms in the parent chain and add the suffix – ane.

4. Number the parent carbon chain from that end which gives the lowest number to the branching alkyl group( if only one branch is present) or the lowest sum of numbers for the branching alkyl groups.

5. Prefix the names of the branching alkyl groups along with their attachment number to the root number. If different alkyl groups constitute the branches, arrange them alphabetically.

General Methods of Preparation:

Alkane can be obtained by the fractional distillation of petroleum and natural gas. Synthetic methods are, however, more practical when a pure alkane is desired. Alkanes can be prepared by the following methods:

  1. Catalytic Reduction(Hydrogenation) of Unsaturated Hydrocarbons: Alkanes are formed by passing a mixture of an unsaturated hydrocarbon and hydrogen over finely divided nickel at 200 °C-300 °C. This reaction is also called as “Sabatier-Senderens Reaction”.For examples :
  2. Note :

    The number of carbon atoms present in alkanes are same as the parent alkenes or alkynes.

    Platinum or palladium can also be used as catalysts in place of Nickel.

    1. Direct Reduction of Alkyl halides :

    Chemical reducing agents like Zinc and hydrochloric acid or acetic acid; Zn-Cu couple and alcohol; magnesium amalgam and water etc. generally give good yields of alkane.

    This reaction proceeds through electron transfer as shown below :

Alkyl halides can be reduced catalytically by palletised charcoal and hydrogen also(catalytic hydrogenation).

  1. Indirect Reduction of Alkyl halides: Grignard reagents, prepared from alkyl brɔmides or iodides are decomposed by water to form alkanes. For example:

For example:

 

Mechanism :

The two possible mechanisms are given below :

 

(i) Free radical Mechanism : It involves the attacks of a sodium atom to form sodium halide and an alkyl free radical. Two alkyl free radicals then unite together to give alkane.

For Example

  1. ii) Through the Formation of an organosodium Intermediate(lonic Mechanism) :

This involves the formation of an ionic intermediate, the organosodium compound which provides the nucleophilic alkyl carbanion to displace the halide ion from other molecules of alkyl halide as shown below:

For Example

When a mixture of two different alkyl halides is used, a mixture of three alkanes is obtained as shown below:

Limitations :

(i)Two different alkyl halides in the Wurtz reaction always give a mixture of alkanes. The separation of these alkanes is not always easy due to little difference in their boiling points. Thus, this method is useful only for the preparation of symmetrical alkanes.

(ii)Methane can not be prepared by this method.

(iii)It fails with tert. alkyl halides.

  1. Decarboxylation of Monocarboxylic acids :

    When sodium salt of a suitable aliphatic saturated monocarboxylic acid (alkanoic acid) reacts with soda lime(NaOH + Cao), the carboxylic acid decarboxylates and the corresponding alkane is formed. For example :

Sodium hydroxide can replace sodalime, but sodalime is preferred because it permits the reaction to be carried out at a relatively higher temperature. This method is used for the preparation of alkane, one carbon less from the parent carboxylic acid.

 

  1. Corey-House Synthesis :

In this synthesis an alkyl halide is first converted to lithium dialkyl cooperate; LiR, Cu., this is then treated with a second molecule of the alkyl halide to give an alkane.

This method is suitable for the preparation of unsymmetrical alkanes. e.g.

Propane Mechanism :

It is a typical nucleophilic substitution reaction. The organolithium compound furnishes the nucleophilic alkyl carbanion which attacks the alkyl halide to displace the halide ion as shown below :

  1. Kolbe’s Electrolytic Method: When a concentrated solution of sodium or potassium salt of a carboxylic acid or a mixture of carboxylic acids is electrolyzed, an alkane is formed. For example :

    Mechanism :

    Kolbe’s reaction occurs through the free radical mechanism. For e.g. when sodium or potassium propanoate has been electrolyzed a mixture of butane, ethane, ethene, and ethyl propanoate is obtained. The propanoate ion discharges at the anode to form a free radical.

Physical Properties Alkane:

  • The first four alkane(C, to C4) i.e. from methane to butane are gases, next thirteen members (Cato C17) i.e. from pentane to heptadecane are liquids and the remaining higher alkanes (C18 onwards) are waxy solids.
  • Alkanes are non-polar molecules. They are soluble in non- polar solvents like benzene and carbon tetrachloride but insoluble in polar solvents like water, liq. ammonia, alcohols etc.
  • They are always lighter than water.
  • In general, the boiling points and specific gravities of alkane increase with increase in molecular weight. Branched chain isomer boils at lower temperature than corresponding straight chain isomer.

Chemical Properties Alkane:

Under ordinary conditions, the alkanes are inert towards reagents such as acids, alkalies, oxidizing reagents, reducing reagents etc, but reactive if the ‘right’ conditions are used. One possible reason for this lack of reactivity is that the C-H bond is very strong due to nearly same electronegativities.

Alkanes undergo most of their reactions through the formation of the highly reactive “free radical as a result of energetic collisions’ between their molecules at high temperature. Alkanes give two main types of reactions :

[A] Substitution Reactions and

[B] Thermal or Catalytic Reactions

[A] Substitution Reactions :

In these reactions, one or more hydrogen atoms of alkanes are substituted by either halogen atoms or by certain groups like nitro (-NO2), sulphonic (-SO3H) groups etc. Some of the common reactions are given below :

  • Halogenation :

This involves the substitution of H – atoms of alkanes with halogen atoms. The order of reactivity of halogen molecules is :

F2   >  Cl2   >  Br2   >  I2

  • Chlorination :

Chlorination can be brought about by ultraviolet light or at high temperature(300-400 °C). The extent of chlorination depends largely upon the amount of chlorine used. Generally a mixture of different chloroalkanes is obtained.e.g.

Thus, in practice all the four substituted products are obtained. Emane and other higher alkanes react with chlorine in a similar way and all the possible substituted products are obtained.e.g.

Thus, in practice all the four substituted products are obtained. Emane and other higher alkanes react with chlorine in a similar way and all the possible substituted products are obtained.e.g. ethane reacts with chlorine to give chloroethane as the monosubstituted product in step first.

When propane is chlorinated, gives 1 – chloro and 2 – chloropropanes.e.g.

Wechanism:

The chlorination of alkanes takes place through the formation of free radicals as intermediates. e.g. the chlorination of methane involves the following steps:

Chain initiation Step : In this step chlorine molecule is cleaved to form highly reactive chlorine atoms or chlorine free radicals (Cl-).

Chain Propagation Step : A chlorine free radical attacks on a methane molecule to produce methyl free radical and HCI

(b) Methyl free radical attacks chlorine molecule to give methyl chloride and chlorine free radical.

 

Step (a) and (b) are repeated over and over again.

  1. Chain Termination Step : The chain reactions stop when two free radicals combine. e.g. C. + Ci. → CI-CI Chlorine free radicals.

A chlorine free radical can further attacks on methyl chloride to form chloro methyl free radical. This free radical participates again in the chain reaction to give dichloromethane.

Similarly, chloroform and carbon tetrachloride are obtained by further chain reactions.

  • Bromination : Bromination is similar to chlorination but not so vigorous because followed selectivity order.
  • lodination : lodination is reversible. Due to reducing property of HI, it reduces iodoalkanes to alkanes.

 

However, iodination can be carried out in the presence of an oxidizing agent like HIO3, HNO3, HgO etc, which destroys the hydrogen iodide as it is formed and so drives the reaction to right. e.g.

5HI + HIO3 – > 3I2 + 3H2O

  • Fluorination :

    Fluorine is the most reactive of the halogens towards alkanes. Pure fluorine reacts with alkanes explosively under most conditions. Fluoroalkanes can, however, be obtained from alkanes by the action of fluorine diluted with nitrogen. In the halogenation of higher alkanes, all the possible halogenated products are formed. The order of reactivity of different hydrogens in alkanes is as follows:

Tertiary > Secondary > Primary

Thus, the chlorination of n – butane gives the isomers of 1 – chlorobutane and 2 – chlorobutane but at different extent.

  1. Sulphonation alkane :

It involves the substitution of a hydrogen atom of alkane with sulphonic( -SOZH) group. When alkane are treated with fuming sulphuric acid at a high temperature, they undergo sulphonation.

Mechanism :

It is also a free radical substitution reaction. It proceeds through the following steps.

  1. Nitration Alkane:

    It involves the substitution of a hydrogen atom of alkane by nitro (-NO,) group. When a mixture of an alkane and nitric acid vapours is treated at 400°C to 500°C, one hydrogen atom on the alkane is substituted by a nitro group.

 

In this reaction, the product is usually a mixture of nitoalkanes including those with smaller carbon chains than that of parent alkane. e.g.

Mechanism :

Like halogenation, it also proceeds through a free radical mechanism. e.g. the nitration of ethane can be given by following steps:

Thus, a mixture of nitroethane and nitromethane is obtained.

[B] Thermal (Combustion)and Catalytic Reactions :

(a) Oxidation : When alkanes are burnt in air or oxygen, they are completely oxidized to carbon dioxide, water and evolve large quantities of heat. For example :

Generally this reaction can be represented as :

Despite being highly exothermic, these reactions require too high temperature of a flame to start due to high activation energy. When burnt in insufficient supply of oxygen, they give carbon monoxide and carbon black. e.g.

(b) Pyrolysis (Cracking):

When an alkane is heated to a high temperature, it decomposes into smaller units including lower alkanes, alkenes and hydrogen. The decomposition of any compound into smaller units by application of heat is called ‘pyrolysis’. e.g. Ethane when heated to 500°C in the absence of air, gives a mixture of methane, ethylene and hydrogen.

Mechanism :

It consists of following steps :

Chain Propagation Step :

Step (b) and (c) are repeated several times.

Chain Termination Step :

The pyrolysis (or cracking) is of great importance to the petroleum industry because it provides means of making smaller, more volatile molecules from large, less volatile molecules of hydrocarbons.

  1. c) Aromatisation :

When alkanes containing six or seven carbon atoms are heated to high Temperatures(about 520°C) under high pressure(10-20 atm.) and in the presence of special catalysts (such as oxides of chromium, vanadium and molybdenum supported on alumina), they are converted into aromatic hydrocarbons. This process is called ‘aromatisation. This reaction involves cyclisation, isomerisation and dehydrogenation as shown below.

Thus, it provides an excellent method for synthesis of aromatic hydrocarbons from aliphatic hydrocarbons.

(d) Isomerisation :

When n-alkanes are heated in presence of anhydrous aluminium chloride and HCI, they isomerises to branched chain alkane. e.g.

Cycloalkanes or Alicyclic Compounds

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