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Chemistry

Chemistry. Alkyl and Aryl halides–1. Session objectives. Classification Nature of C — X bond Preparation of alkyl halides Physical and chemical properties of alkyl halides Substitution reaction: S N 1 and S N 2 reactions. X. X. H. H. H. H. H. C. C. C. C. H. C. C. H. H. R.

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Chemistry

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  1. Chemistry

  2. Alkyl and Aryl halides–1

  3. Session objectives • Classification • Nature of C — X bond • Preparation of alkyl halides • Physical and chemical properties of alkyl halides • Substitution reaction: SN1 and SN2 reactions

  4. X X H H H H H C C C C H C C H H R H R X X H H X H R R R C C C X X X R H H Classification Alkyl and aryl halides Dihalogen Next slide Monohalogen Geminal vicinal 10 30 20

  5. Classification Alkyl and aryl halides Unsaturated Polyhalogen Trihalogen Previousslide • Examples • Vinyl halide H2C=CHX • Allyl halideCH2x–CH=CH2 • Examples • CCl4 • PFC • p–dichlorobenzene • DDT • BHC • Westron • Westrosol • Examples • CHCl3 • CHI3

  6. Nature of C–X Bond

  7. Preparation of Alkyl Halides By direct halogenation The substitution of H-atom in halides follows the order: tert-hydrogen > sec-hydrogen > p-hydrogen To get maximum yield of mono halogenated compound we should take alkene in excess.

  8. Preparation of Alkyl Halides By addition of hydrogen halides on alkenes

  9. Preparation of Alkyl Halides By alcohol (a) Reaction with halogen acid (b) Reaction with phosphorous halides (c) Reaction with thioxyl chloride

  10. Ask yourself Which is the best method for preparation of alkyl chlorides? Thioxyl chloride method because product obtained aregaseous in nature

  11. Preparation of Alkyl Halides By Finkelstein reaction(Halide exchange reaction) From ethers

  12. Preparation of Alkyl Halides Borodine-Hunsdiecker reaction The yield of halide decreases in the order: 1° > 2° > 3° Yield of alkyl chloride is less than alkyl bromide. Mechanism is through free radical formation. Iodides cannot be prepared by this method since iodine forms esters instead of alkyl halide (Birnbaum-Simonini reaction)

  13. Preparation of Alkyl Halides Preparation of allylic and benzylic halides

  14. Preparation of Alkyl Halides Preparation of vinyl halides

  15. Physical and Chemical Properties of Alkyl Halides Physical properties • Alkyl halides are insoluble in water but soluble in organic solvents. • Decreasing order of their densities is:Iodide > Bromide > Chloride > Fluoride • Decreasing order of their boiling points for the same alkyl or aryl group is:Iodide > Bromide > Chloride > Fluoride • Their stability order is:RF > RCl > RBr > RISince C — X bond strength decreases in the same order. • Alkyl halides are insoluble in water although it has polar nature.

  16. Chemical properties (i) Reduction (ii) Reaction with metals (iii) Friedel-Crafts’ reaction

  17. Chemical properties (iv) Reaction with alc. KOH

  18. Chemical properties Reaction with aq. KOH Reaction with moist Ag2O Reaction with dry Ag2O

  19. Williamson synthesis

  20. Chemical properties Reaction with alkyl cyanide

  21. Chemical properties Reaction with amines Reaction with silver nitrite:

  22. Chemical properties Alkyl halides react with sodium-lead alloy to form tetra-ethyl lead Reaction with magnesium:

  23. Nucleophilic aliphatic substitution The SN1 reaction mechanism: Ist step IInd step

  24. 2. Solvent • Polar protic solvents (e.g., H2O,Alcohol etc) SN1 • non-polar (CCl4 , CHCl3) or polar aprotic (DMSO,DMF) solvents SN2 reactions. Factors effecting substitution • Structure of RX • Reactivity for SN1 3° > 2° > 1°. • Reactivity for SN2 1° > 2° > 3°.

  25. Factors effecting substitution 3. Nature of Nucleophile Low concentration favour SN1 reactions while high conc. FavourSN2 reactions. Basicity is the tendency toabstract H+ and nucleophilicity isthe tendency to attack on carbon. 4. Effect of leaving group same for SN1 and SN2 reaction.R – I > R – Br > R – Cl > R – F

  26. Substitution versus Elimination SN2 vs. E2 Effect of substrate For SN2, the reactivity order is 1° > 2° > 3° For E2, the reactivity order is 3° > 2° > 1°

  27. Thank you

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