Chapter 2 ALKANES Dr. Yasser Mostafa For Dental Students Chapter 2 ALKANES Dr. Yasser Mostafa

Learning outcomes: Students should be able to: Describe a homologous series and its general characteristics; Describe the alkanes as a homologous series of saturated hydrocarbons with the general formula CnH2n+2; Draw the structures of branched and unbranched alkanes, C1 to C6 and name the unbranched alkanes C1 to C10; Define different methods to prepare alkanes; Describe the properties of alkanes;

Some Abbreviations in Alkanes: Hydrogenation = addition of H2 Dehydrogenation = removal of H2 Reduction = addition of H2 Nitration = addition of HNO3 + H2SO4 Sulphonation = addition of H2SO4 Halogenation = addition of (I2, Br2, F2 and Cl2)

General methods for the Preparation of Alkenes 1- By hydrogenation of alkenes and alkynes :  Alkenes and alkynes on catalytic hydrogenation give alkanes

2-By the reduction of alkyl halides: (a) Alkyl halides on reduction with new hydrogen form alkanes. (b) The new hydrogen may be obtained by any one of the fallowing methods: (i) Zn + HCl (ii) Zn + CH3COOH

3 - By reduction of alcohols, aldehydes, ketones and their derivatives: The reduction of either of the above in presence of red P & HI gives corresponding alkane. , 200oC Red P + 2 HI R H + H2O + I2 R OH , 200oC Red P + 4 HI R CH3 + H2O + 2 I2 R R R CHO COR , 200oC Red P + 4 HI RCH R + H O + 2I 2 2 2 , 200oC Red P + 6 HI RCH + 2 H O + 3 I COOH 3 2 2

4- By Reduction of Carbonyl compounds: The reduction of carbonyl compounds by amalgamated zinc and conc. HCl also yields alkanes. This is clemmensen reduction. Carbonyl compounds may also be reduced to alkanes by Wolf Kishner reaction.

5- Wurtz reaction: (a) A solution of alkyl halide in ether on heating with sodium gives alkane. (b) An alkyl halide on Wurtz reaction leads to the formation of symmetrical alkane having an even number of carbon atoms. (c) Two different alkyl halides, on Wurtz reaction give all possible alkanes.

Limitations of wurtz reaction : The different steps are: CH3X + 2Na + C2H5X → CH3CH2CH3+ NaX + CH3X 2Na + C2H5X →CH3CH3 + 2NaX C2H5X + 2Na + C2H5X → C2H5C2H5 + 2 NaX Limitations of wurtz reaction : (a) Methane can not be obtained by this method (b) The reaction fails in case of tertiary halides Mechanism : The mechanism of Wurtz reaction is although not clear however two mechanisms are proposed for this reaction. Involving intermediate formation of an organometallic compound: RX + 2 Na → [RNa] + NaX Intermediate RX + [RNa] → R-R + NaX

6- Kolbe's electrolysis method: Alkanes are formed, on electrolysis of concentrated aqueous solution of sodium or potassium salt of saturated mono carboxylic acid. 7- By Grignard reagents: (i) Organic compounds in which a metal atom is directly linked to carbon atom are known as organometallic compound. Alkyl or aryl magnesium halide (R-MgX) are also called Grignard reagents or organometallic compounds.

(iii) Grignard reagent on double decomposition with water or with other compounds having active H (the hydrogen attached to O, N, F or triple bonded carbon atom are known as active hydrogen) give alkane. e.g.

Reactions of alkanes 1-Halogenation: Example: 14

 Example: 2-Nitration Reaction: 16

Insertion of methylene group- 4  3-Sulphonation: Insertion of methylene group- 4 This reaction is used to increase the chain of alkane by CH2 by using diazomethane (CH2N2) or ketene (CH2=C=O). RH + CH2N2 RCH3 + N2 d iazomethane RH + CH2=C=O k etene RCH3 + CO