Hydrocarbons and Methods of Preparation of Alkanes Class 11 Notes | EduRev

Chemistry Class 11

NEET : Hydrocarbons and Methods of Preparation of Alkanes Class 11 Notes | EduRev

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What are Hydrocarbons?

Hydrocarbons are organic compounds that are entirely made up of only two kinds of atoms – carbon and hydrogen. 

Hydrocarbons and Methods of Preparation of Alkanes Class 11 Notes | EduRev

Typically, hydrocarbons are colourless gases that have very weak odours. Hydrocarbons can feature simple or relatively complex structures and can be generally classified into four subcategories, namely alkanes, alkenes, alkynes, and aren’t. The study of hydrocarbons can provide insight into the chemical properties of other functional groups and their preparation.

Classification of Hydrocarbons

Hydrocarbons and Methods of Preparation of Alkanes Class 11 Notes | EduRev

  1. Saturated Hydrocarbons: 
    In these compounds, carbon-carbon atoms and carbon-hydrogen atoms are held together by single bonds. These single-bonded compounds are the simplest hydrocarbons. These types of hydrocarbons don’t have double or triple bonds. In terms of hybridization, they have sp3 hybridised carbon atom with no sp2 or sp hybridised carbon atoms. They are together called alkanes which have a general formula CnH2n+2. For example, CH4C3H6.
  2. Unsaturated Hydrocarbons: 
    These compounds consist of a single, double or triple bond between carbon-carbon atoms. The double-bonded compounds are called alkenes and the triple bonded compounds are called alkynes. The general formula for alkenes is CnH2n and for alkynes the general formula is CnH2n-2.
  3. Cycloalkanes:
    These hydrocarbons possess one or multiple carbon rings. The hydrogen atom is attached to the carbon ring.
  4. Aromatic Hydrocarbons: 
    These are also called arenes. Arenes are compounds that consist of at least one aromatic ring.
  5. Aliphatic Hydrocarbons:
    They are straight-chain structures having no rings in them.
  6. Alicyclic Hydrocarbons: 
    They are hydrocarbons having a ring structure in them. The carbons atoms can be sp, sp2 or sp3 hybridised.

Properties of Hydrocarbons

  • Due to their different molecular structures, the empirical formula of hydrocarbons is also different from each other. 
  • For instance, in alkanes, alkynes or alkenes, the amount of bonded hydrogen decreases in alkenes and alkynes. This is mainly due to the “self-bonding” or catenation of carbon that prevents the complete saturation of the hydrocarbon by the formation of double or triple bonds. 
  • The ability of hydrocarbons to bond to themselves is known as catenation. With such capabilities, they can form more complex molecules like cyclohexane and in rare instances aromatic hydrocarbons like benzene.
  • Meanwhile, the cracking of Hydrocarbons is a process in which heavy organic molecules are broken down into lighter molecules. This is accomplished by supplying an adequate amount of heat and pressure. 
  • Sometimes catalysts are used to speed up the reaction. This process plays a very important role in the commercial production of diesel fuel and gasoline.

Preparation of Hydrocarbons

Alkanes

➤ From alkenes and alkynes

The alkanes can be produced from alkenes or alkynes through hydrogenation. H2 gas is passed over a metal surface such as Ni, Pt along with the alkenes to produce alkane.

CH2=CH2 → CH3-CH3

The above reaction is called as “Sabatier-Sender son’s” reaction. Other catalysts which can be used are Pt, Pd-BaSo4, Adams catalyst (Pt2O) or Wilkinson catalyst (R3PRhCl), etc.

➤ From Alkyl Halides

Alkyl halides can be converted to alkanes through various methods. They are as follow is.

  • Using Zn/Protic solvents
  • Using courts reactions

Note: Alkanes with only even number of carbons atoms can be produced.

➤ Using Reducing Agents:

R-X → [H]R – H

The reducing agents which can be used are LiAlH4, NaBH4, NaNH2, etc.


Note:

LiAlH4  can’t reduce 3° halides.

NaBH4 can’t reduce 1° halide.

➤ From Aldehydes/Ketones:

  • Clemmensen’s Reduction
  • Wolf-Kishner reduction

➤ From Carboxylic acids through Decarboxylation:

  • Kolbe’s Electrolysis
  • Using soda-lime
Alkenes

General formula: CnH2n

Most of the reactions involving the preparation of alkenes involve the elimination process. There are 3 mechanisms suggested for the elimination reactions. All these eliminations are β- eliminations.

➤ E2 Mechanism

  • Second-order kinetics.
  • Single-step process.
  • Order & reactivity 1° > 2° > 3°

Because of steric hindrance

  • More favoured in non-polar, aprotic solvents.
  • Less substituted alkenes formed as a major product.

➤ E1 Mechanism

  • Two-step process.
  • 1st order kinetics
  • Order of reactivity: 3° > 2° > 1°

Because of the stability of carbonation.

  • More favoured by polar, protic solvents.
  • Rearrangement possible
  • Gives more substituted alkene as major products.

➤ Hydration

(i) Acid catalysed

  • Markovnikov product
  • Rearrangement is possible.

(ii) Hydroboration-oxidation:

  • Anti – Markonikov
  • No rearrangement

(iii) Oxymercuration-Demercuration:

  • Markonikov
  • No rearrangement
  • Oxidation Reactions
  • Using Baeyer’s reagent
  • Using hot KMnO4
  • Ozonolysis
  • Using O5O4
  • Addition of peroxy acid

Alkynes

Alkynes can be prepared from alkyl halides and alcohols.

➤ Addition reaction:

All addition reactions in alkenes are possible.

Benzene

  • From ethyne
  • From phenol
  • From aniline

Uses of Hydrocarbons

  • Hydrocarbons are widely used as fuels. For example LPG (liquefied petroleum gas), CNG (Liquefied natural gas).
  • They are used in the manufacturing of polymers such as polyethene, polystyrene etc.
  • These organic compounds find their application in the manufacturing of drugs and dyes as a starting material.
  • They serve as lubricating oil and grease.
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