Chemistry

Alkyl and Aryl halides–1

Session objectives 1.Classification 2.Nature of C — X bond 3.Preparation of alkyl halides 4.Physical and chemical properties of alkyl halides 5.Substitution reaction: S N 1 and S N 2 reactions

Classification Alkyl and aryl halides Monohalogen Dihalogen vicinal Geminal H H X CC C H X H X H R R RXC H H RXC R H RXC H H X CC C H H H X H Next slide

Classification Alkyl and aryl halides Trihalogen Polyhalogen Examples 1.CHCl 3 2.CHI 3 Examples 1.CCl 4 2.PFC 3.p–dichlorobenzene 4.DDT 5.BHC 6.Westron 7.Westrosol Unsaturated Examples 1.Vinyl halide H 2 C=CHX 2.Allyl halide CH 2 x–CH=CH 2 Previous slide

Nature of C–X Bond

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.

Preparation of Alkyl Halides By addition of hydrogen halides on alkenes

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

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

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

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)

Preparation of Alkyl Halides Preparation of allylic and benzylic halides

Preparation of Alkyl Halides Preparation of vinyl halides

Physical and Chemical Properties of Alkyl Halides Physical properties 1.Alkyl halides are insoluble in water but soluble in organic solvents. 2.Decreasing order of their densities is: Iodide > Bromide > Chloride > Fluoride 3.Decreasing order of their boiling points for the same alkyl or aryl group is: Iodide > Bromide > Chloride > Fluoride 4.Their stability order is: RF > RCl > RBr > RI Since C — X bond strength decreases in the same order. 5.Alkyl halides are insoluble in water although it has polar nature.

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

Chemical properties (iv) Reaction with alc. KOH

Chemical properties Reaction with aq. KOH Reaction with moist Ag 2 O Reaction with dry Ag 2 O

Williamson synthesis

Chemical properties Reaction with alkyl cyanide

Chemical properties Reaction with amines Reaction with silver nitrite:

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

Nucleophilic aliphatic substitution The S N 1 reaction mechanism: Ist step IInd step

Factors effecting substitution 1.Structure of RX Reactivity for S N 1 3° > 2° > 1°. Reactivity for S N 2 1° > 2° > 3°. 2. Solvent Polar protic solvents (e.g., H 2 O,Alcohol etc) S N 1 non-polar (CCl 4, CHCl 3 ) or polar aprotic (DMSO,DMF) solvents S N 2 reactions.

Factors effecting substitution 3. Nature of Nucleophile Low concentration favour S N 1 reactions while high conc. Favour S N 2 reactions. Basicity is the tendency to abstract H + and nucleophilicity is the tendency to attack on carbon. 4. Effect of leaving group same for S N 1 and S N 2 reaction. R – I > R – Br > R – Cl > R – F

Substitution versus Elimination S N 2 vs. E 2 Effect of substrate For S N 2, the reactivity order is 1° > 2° > 3° For E 2, the reactivity order is 3° > 2° > 1°

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