Classification and Preparation of Alkanes: A Comprehensive Guide - Prof. Anjum, Slides of Organic Chemistry

Download Classification and Preparation of Alkanes: A Comprehensive Guide - Prof. Anjum and more Slides Organic Chemistry in PDF only on Docsity! ALKANES AND THEIR PREPARATION Prepared By: Ramisa Anjum Department of Pharmacy University of Asia Pacific CLASSIFICATION OF ALKANES Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecule. They are also called Paraffins. At high temperatures and pressure do undergo some reaction. The alkanes may be divided as: L\ O 1) Open chain or Acyclic alkanes. ae eee 2) Cycloalkanes or cyclic alkanes. CO C) C) From Unsaturated Hydrocarbon i)Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes. Foreg: oy,-cl +H," cH, + HCI Chloromethane Methane C,H,-Cl + H,""> C,H, +HCl Chloroethane Ethane Wurtz reaction of preparation of alkanes Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture)solution give higher alkanes. This reaction is known as Wurtz reaction and is used for the preparation of higher alkanes . containing even number of carbon = atom. CH,Br+2Na+BrCH, aL ~CH,+2NaBr Bromomethane Ethane KOLBE’S ELECTROLYTIC METHOD * This METHOD IS USED FOR THE REDUCTION OF HERMANN KOLBE COs. An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode. AT THE CATHODE THERE IS REDUCTION OF WATER INTO HYDROGEN FREE RADICALS AND THAT IS CONVERTED INTO HYDROGEN AND GIVEN OFF. 2CH,COO-Na*~+ 2H,O Sodium acetate J Electrolysis CH,,-CH,,+2CO, +H, +2NaOH Reaction mechanism of Kolbe's electrolytic method * Inamechanism there are three steps :- Initiation , propagation and termination._ INITIATION Here sodium acetate is converted into acetate and sodium ions. Oo I| 2CH,COO-Na* — 2CH,—C-—O- +2Na* Substitution reactions ¢ The reaction in which an atom or a group of atoms in a molecule is replaced by some other atom or group of atom. Alkanes undergo substitution reaction in which one or more hydrogen atoms are replaced or substituted by different atoms or groups such as halogen atom (Cl, Br or I), nitro group(-NO,) or sulphonic acid (-SO.H) group. Types of substitution reactions Mechanism of halogenation reaction Initiation The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat, the Cl- Cl bond is weaker than the C-C and C-H bond and hence, is easiest to break c-ad-——"_, @q@ + @ homolysis Chlorine free radicals Propagation Chlorine free radicals attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl. CH, + Cl—™5CH, + H-Cl The methyl radical thus obtained attacks the second molecule of chlorine to form CH3-Cl with the liberation of another chlorine free radical by homolysis of chlorine molecule. CH,+Cl-Cl—™ >CH, -Cl +Cl Chlorine free radical Termination The reaction stops after some time due to consumption of reactants and/or due to following side reaction: The possible chain terminating steps are: ) Cl+ Cl > cl_cl b) H,C + CH, >H,C-CH, ?) H,C + Cl—> H,C-Cl Though in (c) CH;-Cl, the one of the product is formed bur free radicals are consumed and the chain is terminated. Reaction of steam On passing a mixture of steam and methane over heated nickle (supported over alumina, Al,O;) catalyst at 1273 K, methane is oxidised to carbon monoxide and hydrogen is evolved. CH, +H,0——=> CO + 3H, Pyrolysis When higher alkanes are heated to high temperature in the presence of alumina or silica catalysts, the alkanes break down to lower alkanes and alkenes. For eg: C3He C3H,,CH,or C3He+H, This reaction is called Fragmentation or Cracking or Pyrolysis. Pyrolysis of hexane gives following product : > C,H, + H, 773K CoH, > C,H, + C,H, > C,H, + C,H, + CH, Alkane isomerise to branched chain alkanes when heated with anhydrous aluminium chloride (AICl;) and hydrogen chloride at 573 K under a pressure of about 30-35 atmosphere. CH,CH,CH,CH, lll Galea oleae n-butane isopropane The alkanes containing six or more carbon atoms when heated at about 773K under high pressure of 10-20 atm in the presence of catalyst on alumina gel get converted to aromatic compounds. This process is called aromatization. CH,-(CH,),-CH,_ 773K: 10-20 atm C3 Hexane Benzene