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: SN1 and SN2 reactions

Classification

Alkyl and aryl halides

MonohalogenDihalogen

10 20 30 vicinal Geminal

H

H

X

C C C H

X

H X HR

R

R XC

H

H

R XC

R

H

R XC

H

H

X

C C C H

H

H X H

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Classification

Alkyl and aryl halides

Trihalogen PolyhalogenExamples1. CHCl32. CHI3

Examples1. CCl42. PFC3. p–dichlorobenzene4. DDT5. BHC6. Westron7. Westrosol

Unsaturated

Examples1. Vinyl halide

H2C=CHX2. Allyl halide

CH2x–CH=CH2

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Nature of C–X Bond

C : X C — X+ –

or+ –

- -Nu +R - X R - Nu+ X :

Preparation of Alkyl Halides

By direct halogenation

4 2 3 2 2 3 4Mixture of products

CH +Cl CH Cl+CH Cl +CHCl + CCl U V

light

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

CH3 — CH CH2 + HBr CH3 — CH — CH3

Br

CH3 — CH CH2 ROORCH3 — CH — CH2Br CH3 — CH2 — CH2Br

H

Propene

HBr

sec-free radicalsstable

Preparation of Alkyl Halides

By alcohol

(a) Reaction with halogen acid

2ROH HX RX H O

(b) Reaction with phosphorous halides

3 3 3

5 3

3ROH PX 3RX H PO

ROH PX RX PX HX

(c) Reaction with thioxyl chloride

2 2ROH SOCl RCl SO HCl

Ask yourself

Which is the best method for preparation of alkyl chlorides?

Thioxyl chloride method because product obtained aregaseous in nature

Preparation of Alkyl Halides

By Finkelstein reaction(Halide exchange reaction)

3 2 3 2CH CH Cl NaI CH CH I NaCl

3 2 3 22CH Cl HgF 2CH F HgCl

From ethers

heat

2Excess

R O R 2HX 2RX H O

Preparation of Alkyl Halides

Borodine-Hunsdiecker reaction

4CCl2 2 2RCO Ag Br RBr CO AgBr

4CCl2 2RCOOAg Cl RCl CO AgCl

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)

2 2 2 22RCO Ag I RCO R CO 2AgI

Preparation of Alkyl Halides

Preparation of allylic and benzylic halides

CH3

X2

CH2X

HXh

CH3 — CH CH2CH2 — CH

Cl

CH2

Cl2 / h or

At 400° Cat low conc.

H2C CH — CH2 — CH3 H2C

Br

CH — CH — CH3NBS

Preparation of Alkyl Halides

Preparation of vinyl halides

C H 2C H 2 H 2C C H — C l

C H 3C O C l / a lu m in a

H C l

V in y l ch lo rid e

Physical and Chemical Properties of Alkyl HalidesPhysical 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 > RISince 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

2 5Na C H OH3 2 2 6CH CH Br 2[H] C H HBr

Red P

3 7 3 8 2150 C

C H I HI C H I

(ii) Reaction with metals

dry

ether2R — X 2Na R — R NaI

22R — X 2Na R — R ZnX

(iii) Friedel-Crafts’ reaction

3anhyd. AlCl3 6 6 6 5 3CH Br C H C H CH HBr

Chemical properties

(iv) Reaction with alc. KOH

alc KOH3 2 2 3 2CH CH CH Br CH CH CH HBr

Br

alc KOHCH3 — CH — CH2 — CH3

2-Bromobutane CH — CH2 — CH3CH3 – CH CH – CH3 + CH2

2-Butene (Major)

1-Butene (Minor)

Chemical properties

Reaction with aq. KOH

RX KOH(aq.) KOH KX

Reaction with moist Ag2O

2 22RX Ag O H O 2R – OH 2AgX

Reaction with dry Ag2O

22RX Ag O R – O – R 2AgX

Williamson synthesis

Rx RONa R – O – R NaX

Rx R`ONa R – O – R` NaX

Chemical properties

RX KCN RCN KX

Alkyl cyanide

H / OH2 3

(a) complete hydrolysis

RCN 2H O RCOOH NH

2 2H O2 2

(b) Partial hydrolysis

RCN H O RCONH

Na–alcohol2 2

(c) Reduction

RCN RCH NH

Reaction with alkyl cyanide

Chemical properties

Reaction with amines

3 2 3 2CH Br KNH CH NH KBr

Reaction with silver nitrite:

2 5 2 2 52C H Br 2AgNO C H ONO 2AgBr

Chemical properties

Alkyl halides react with sodium-lead alloy to form tetra-ethyl lead

2 5 2 5 4Sodium lead Tetra ethyl

alloy lead TEL

4C H Cl 4Pb Na C H Pb 4NaCl 3Pb

Reaction with magnesium:

dry

ether Alkylmagnesiumhalide

Alkyl lithium

RMgX

alkylhalide

Li

ether

RX Mg dry

R — X R — Li LiX

Nucleophilic aliphatic substitution

The SN1 reaction mechanism:

Ist step

C

CH3CH3

CH3

IInd step

(Racemic mixture)

C Cl

CH3

CH3

CH3 slow

C

CH3CH3

CH3

fast

OH

C OH

CH3

CH3

CH3

CH3

HO C CH3

CH3

+

Factors effecting substitution

1. Structure of RX

• Reactivity for SN1 3° > 2° > 1°.

• Reactivity for SN2 1° > 2° > 3°.

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

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

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°

Thank you