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Chapter 2 ALKANES Dr. Yasser Mostafa

Chapter 2 ALKANES Dr. Yasser Mostafa. For Dent a l S t ud e nts. 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 C n H 2n+2 ;

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Chapter 2 ALKANES Dr. Yasser Mostafa

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

  2. 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;

  3. 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)

  4. General methods for the Preparation of Alkenes 1-Byhydrogenation of alkenes and alkynes : Alkenesand alkyneson catalytic hydrogenationgivealkanes

  5. 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

  6. 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 RedP+2HI +H2O +I2 R H R OH ,200oC RedP+4HI + 2I2 R CH3 +H2O R R R CHO COR ,200oC RedP+4HI RCH R +HO +2I 2 2 2 ,200oC RedP+6HI RCH +2HO+ 3I 3 2 2 COOH

  7. 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 Kishnerreaction.

  8. 5- Wurtz reaction: (a)Asolutionofalkylhalideinether on heating with sodium gives alkane. (b)AnalkylhalideonWurtzreaction 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.

  9. The different steps are: CH3X + 2Na + C2H5X → CH3CH2CH3+ NaX + CH3X 2Na + C2H5X →CH3CH3+ 2NaX C2H5X + 2Na + C2H5X → C2H5C2H5 + 2 NaX Limitationsofwurtzreaction: (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

  10. 6- Kolbe's electrolysis method: Alkanesare formed,on electrolysisof concentrated aqueoussolutionof sodiumor potassiumsaltof 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.

  11. (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.

  12. Reactions of alkanes 1-Halogenation: Example: 14

  13. Example: 2-NitrationReaction: 16

  14. 3-Sulphonation: Insertion of methylene group- 4 Thisreactionisusedtoincreasethechainofalkaneby CH2 by usingdiazomethane(CH2N2) or ketene (CH2=C=O). RCH3 + N2 RH + CH2N2 diazomethane RCH3 + CO RH + CH2=C=O k etene

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