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Alkyl Halides

General Introduction, Chemical Properties, Physical Properties

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Alkyl Halides

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  1. Alkyl Halides By Priyanka

  2. General Introduction • Alkyl halides are the compounds derived from the saturated hydrocarbons by replacement of one of the hydrogen atom by halogen atom. • Thus they can be represented by general formula R-X where R= alkyl group; X= Cl, Br, I or F.Halogen atom may be attached to 1o, 2o or 3o Carbon atom of alkyl group. Thus alkyl halide can be classified as primary (1o), secondary(2o) or tertiary (3o) depending upon whether the halogen is attached to primary, secondary of tertiary carbon atom.

  3. Methods of preparation of Alkyl Halides • By direct Halogenation of Alkanes • Alkanes when treated with chlorine or bromine in the presence of U.V light or at high temperature (400oC) gives alkyl chloride or alkyl bromide along with poly halogen derivatives. For example, • But iodoalkanes can be prepared only in the presence of oxidizing agents like HIO3, HIO4. Or Conc. HNO3 etc. because HI formed during the reaction reduces back alkyl iodide to alkane. But the products are difficult to separate.

  4. Methods of preparation of Alkyl Halides by treating Alkenes with hydrogen halides • When alkenes are treated with hydrogen halides, addition of hydrogen halide takes place in accordance with Markonikov’s rule except the addition of HBr in presence of organic peroxide [R-O-O-R].

  5. Methods of preparation of Alkyl Halides by By treatment of Alcohol with Halogen Acid • Alcohols react with HBr or HI to give alkyl bromide or alkyl iodide. They react with HCl in presence of zinc chloride catalyst.

  6. Methods of preparation of Alkyl Halides by treatment of Alcohol with Phosphorous Halides • Alcohols react with phosphorous halides (PX5 or PX3) to give alkyl halides.

  7. Methods of preparation of Alkyl Halides by treatment of Alcohol with Thionyl Chloride • Alcohols when treated with thionyl chloride in the presence of pyridine give alkyl chloride. Pyridine acts as scavenger for HCl formed.

  8. Methods of preparation of Alkyl Halides by By Halogen Exchange Reaction [Finkelstein reaction] • This method is suitable for the preparation of alkyl iodides and fluorides. This involves the treatment of alkyl bromide or chloride with concentrated solution of sodium iodide in acetone.For the preparation of alkyl fluoride alkyl bromides or chlorides are heated with inorganic fluorides.

  9. Physical Properties of Alkyl Halides • Methyl chloride, Methyl bromide, Methyl fluoride and Ethyl chloride are gases at room temperature. Other alkyl halides up to C18 are colourless liquids and beyond C18 are colorless gases. • Although alkyl halides are polar in nature but they are insoluble in water but soluble in organic solvents. Insolubility in water is due to the inability to form hydrogen bonding with water and to break already existing hydrogen bonding in water molecule. • Their densities decrease in the following order; Iodide> bromide>Chloride > fluoride. Alkyl bromides and iodides are denser than water while alkyl chlorides and fluorides are lighter than water. • For a given alkyl group boiling points of alkyl halides follow the following order. Alkyl iodide> Alkyl bromide> Alkyl Chloride> Alkyl fluoride. • For a given halogen atom, the boiling points of alkyl halides increase with the increase in the size of the alkyl group.

  10. Chemical Properties of Alkyl Halides In alkyl halides (C-X) bond is highly polar because of the high electronegativity of the halogen atom. Thus alkyl halides are very reactive compounds alkyl halides can be nearly all types of organic compounds and thus synthetically they are very important. They undergo substitution, elimination, and reduction reactions.

  11. Substitution Chemical Reactionof Alkyl Halides

  12. Saytzeff’s Rule • Hydrogen is eliminated from the Carbon atom of alkyl halide which bears the least number of hydrogen atoms. • For example, when 2-bromobutane undergo elimination with alc. KOH, two alkenes are possible and major one is more substituted alkene. (In accordance with Saytzeff’s rule)

  13. Reduction reaction of Alkyl Halides • Reduction: Alkyl halides are reduced to alkanes by different reducing agents like LiAlH4, Zn/HCl and H2/Ni or Pd etc.

  14. Wurtz Reaction • When alkyl halides react with metallic sodium in dry ether two molecules join together to give alkane with double number of carbon atoms.

  15. Raction with Mg and Li • Alkyl halides react with Magnesium in dry ether to give alkyl magnesium halide, the Grignard reagent.Grignard reagent is an important group of organometallic complex which has the great synthetic utility in organic chemistry. • With Lithium in dry ether alkyl halides give alkyl Lithium.Alkyl Lithium behaves in the same way as Grignard reagent but with increase reactivity.

  16. Friedel Crafts Reaction • Alkyl halides react with Arenes in the presence of anhydrous AlCl3 to form alkyl benzene.

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