Alkane

Introduction –

  • Hydrocarbon is composed of hydrogen and carbon.
  • The important fuels like petrol, kerosene, coal gas etc all contain hydrocarbon and their mixture.
  • Petroleum and natural gas both are the major source of aliphatic hydrocarbon. The word petroleum (Petra – rock or oleum – oil) means rock oil. The oil which is found inside the rocks is called rock oil.
  • The main constituents of the natural gas are methane, ethane, propane and butane.
  • CH4 is used in CNG (compressed natural gas) as a fuel (in which 85% CH4 is used).
  • Butane and Iso – butane is present in LPG (Liquid petroleum gas).
Classification of Hydrocarbon –

Alkanes –

General Points of Alkanes –

  1. Alkanes are saturated, open chain hydrocarbons which containing carbon – carbon single bonds.
  2. The first alkane is methane (CH4), which is also known as Marsh gas.
  3. General formula of alkane – CnH2n+2.
  4. These are also called paraffins which means little affinity or reactivity.This name is justified because they are inert at normal conditions (at normal temperature and pressure, alkanes do not react with reagents like acids, bases, oxidizing agents and reducing agents).
  5. Each carbon atom of the alkane is sp3 hybridized and its shape is tetrahedral.
  6. The bond length between C — H and C — C bonds are 1.12A° and 1.54 A°.

Methods of preparation of alkanes –

  1. Decarboxylation –
  • Decarboxylation means the removal of CO2 from the molecules which having –COOH group.
  • (NaOH + CaO) – Soda lime is used for the decarboxylation.

Reaction –

  • The purpose of CaO in lime is to keep NaOH dry because it is quite hygroscopic (absorbs moisture from air).
  • It is the best method to produce methane.
  • In this process, we obtain one carbon less than the original acid. The yield is good in case of lower member but poor for the higher members.

2. Wurtz Reaction – Dehalogenation using metallic sodium

In wurtz reaction, alkyl halide reacts with sodium in presence of dry ether gives alkane.

Reaction –

  • The method gives symmetrical alkane with even number of carbon atoms. If two different alkyl halides are used in the wurtz reaction, then it will give all possible alkanes.
  • This is the best method to prepare ethane.
  • In case of unsymmetrical alkenes –

The separation of mixture into individual members is not easy because their boiling points are near to each other. So, the wurtz reaction is not suitable for the synthesis of alkanes containing odd umber of carbon atoms.

Mechanism of wurtz reaction –

In case of wurtz reaction, mechanism is not clear however two mechanisms are proposed for this reaction –ionic as well as free radical mechanism. But dry ether is used in the free radical mechanism.

Step – I –

Limitation of wurtz Reaction –

  1. Methane cannot be prepared by this method.
  2. This reaction fails in case of tertiary halides.

Note –

When Zn is used in the wurtz reaction in place of Na, then method is known as Frankland method.

3. For Reduction of alkenes /alkynes  – From unsaturated hydrocarbon

Ease of Hydrogenation depends on the stearic hinderence. More will be the stearic hinderence, less will be the reactivity towards hydrogenation.

Note –

  • By H2/Ni, H2/Pd cyclopropane and cyclobutane also shows breaking.

This is due to Baeyer strain or Ring strain. Ring strain is a type of instability that exists when bonds in the molecules from angles are substantially smaller than idealized values of approximately 109°.

  • When nickel is used as a catalyst, then reaction occurs at 525 – 533K. But if Pt or Pd is used as catalyst, then reaction takes place even at room temperature.

4. Reduction of alkyl halide –

  • Alkyl halide on reduction with nascent hydrogen forms alkanes.
  • Alkyl halides can also be reduced to alkane by LiAlH4, NaBH4 or Zn/H+.

NaBH4  

  • Milder reducing agent
  • It reduces only aldehyde, ketones, acid chlorides or imines.
  • This reagent has no effect on — C ═ C —, C ≡ C, N ═ N, esters, nitriles and nitro groups.
  • In case of NaBH4, 2° and 3° alkyl halides gives alkane while 1° has no effect.

LiAlH4

  • LiAlH4 is most powerful reducing agent than NaBH4 and reduces most of the organic functional group.
  • The anion of this reagent (AlH4) is strong nucleophile and normally attacks on polarized multiple bonds such as C ═ O, C ≡ N by the transfer of hydride ion to the most positive ion.
  • It does not usually reduce carbon – carbon double or triple bond.
  • 1° and 2° alkyl halide gives alkane with LiAlH4 but 3° gives alkene.

5. From Grignard Reagent (RMgX) –

Halogen can be removed via Grignard reagent.

Alkyl or aryl magnesium halide (R — MgX) are also known as Grignard reagents. Grignard reagents on double decomposition with water or other compounds having active hydrogen (the hydrogen attached with O, N, F or triple bonded carbon atom) gives alkane.

6. Frankland Reaction –

7. By the reduction of carbonyl compounds –

Carbonyl compounds can be reduced by following reagents –

  • Clemmensen’s reagent –  Zn(Hg)/HCl (Zinc Amalgam)

It is used for the reduction of ketones or aldehydes to alkanes.

  • Wolff – Kishner Reduction – NH2 — NH2/ KOH or NH2 — NH2/ NaOH. This reagent can not reduce acid or acid derivative.
  • Red P/HI – Red phosphorus is used to remove iodine. This is the best reagent to reduce carbonyl compound.

8. Kolbe’s Electrolysis Method –

In Kolbe’s electrolysis method, alkanes are formed by the electrolysis of concentrated aqueous solution of sodium or potassium salt of carboxylic acid.

Mechanism –

At anode – Oxidation

The reduction potential of CH3COO is more than OH. So, CH3COO is easily oxidized. Therefore, the preferred reaction is   –

At cathode – reduction

The reduction potential of H+ (0.00) is more than Na+ (-2.71). Therefore, the preferred reaction is –

  • If the number of carbon atom in sodium carboxylate is n. Then, the number of carbon atoms in alkane is 2(n – 1).

Limitation –

  • Methane can’t be prepared by this method.
  • It only produces alkanes that have an even number of carbons. If alkanes with an odd number of carbon atoms are used, then mixtures of products are formed.

Note – Fumaric acid (HOOC) CH ═ CH (COOH) and malic acid H2C ═ C (COOH)2 , they are same but they are geometrical isomers.

9. Hydrolysis of Aluminium and Beryllium carbide –

10. Corey House – Synthesis Reaction –

  • Corey – house synthesis reaction is the best method to increase carbon chain. In this, alkyl halide reacts with lithium in presence of ether.
  • R Li reacts with CuI, gives dialkyl cuprate.
  • Lithium dialkyl cuprate react with alkyl chloride gives alkane.

It forms a single product.

  • RCl should not be T – alkyl halide.
  • No side products are formed in Corey synthesis reaction and only a single product formation is there.

11. By Hydroboration of alkenes –

  • Trialkyl borane on treatment with acetic acid yields alkane.

Physical properties of alkanes –

  1. Nature –

Alkanes are non – polar molecules and their molecules are held by weak intermolecular forces depend on the size or surface area.

Due to weak forces –

C1 to C4 – Alkanes are gases

C5 to C7 –Liquids

C18 or more – Solids

2. Boiling Points –

  • Alkanes are generally low boiling point due to non- polar nature and weak forces are between them. As the number of carbon atom increases, molecular size increases, magnitude of vanderwaal forces decreases. So, boiling point increases.
  • The branched chain alkanes have more boiling point than straight chain alkanes due to the large surface area of straight chain alkanes. The branching makes the molecules more compact and decreases the surface area. The inter molecular forces which depend on the surface area become smaller in magnitude.

3. Melting Point –

  • Even number of carbon atoms have more melting point than odd number of carbon atoms because of more symmetrical structure and close packing in the crystal structure of even  number of carbon atoms. So, more will be the attractive force, more will be the melting point.

4. Solubility –

  • Alkanes are non – polar. So, they dissolves in non – polar solvent e.g. – ether, benzene, CCl4 etc. Generally, the molecular mass increases, solubility also increases.

5. Density –

  • Alkanes are lighter than water.
  • The number of carbon atoms increases, density increases.

Chemical properties –

  1. Halogenation

The reactivity of hydrogen towards free radical substitution is 3° > 2° > 1°. The rate of reaction of alkane with halogen is F2 > Cl2 > Br2 > I2.

In this reaction, if bromination is there then major product is tertiary and in case of chlorination, reactivity is depending on the type of carbon. Fluorination of alkanes is too vigorous to be controlled under normal conditions while iodination is very slow and a reversible reaction. This reaction is undergoes the free radical mechanism.

Mechanism –

a. Chain initiation step –

b. Chain propagation step –

It consists two steps –

i. The free radical of chlorine which formed in initiation step attacks the CH4 molecule, removes a H – atom and forms methyl free radical.

ii. Then after, CH3° reacts with Cl2 and form methyl chloride.

c. Chain Termination step –

In this step, same or different radicals terminate and produce new radicals.

2. Oxidaion –

Carbon black that is used in the manufacture of ink paints polishes etc which is prepared by incomplete combustion.

3. Controlled oxidation of alkanes –

4. Aromatization of alkanes –

5. Pyrolysis or thermal Cracking –

Higher alkane on heating gives smaller alkane through free radical formation. This is known as thermal cracking or Pyrolysis.

6. Nitration –

7. Sulphonation –

8. Isomerization

They show chain isomers.

Download as PDF

Leave a Reply

Your email address will not be published. Required fields are marked *