Addition Reaction1

8/13/2019 Addition Reaction1

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Lecture - 8

Organic Chemistry

Topic Addition Reactions

Electrophilic Addition Reactions

Nuceophilic Addition Reactions


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Chapter 8 2

Reactivity of C=C

Electrons in pi bond are loosely held.

Electrophiles are attracted to the pi electrons.

Reaction can be :

Ionic- Polar medium Free radical- non polar medium

Ionic:

Carbocation intermediate forms.s

Nucleophile adds to the carbocation. Net result is addition to the double bond.

=>


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Chapter 8 3

Electrophilic Addition

Step 1: Pi electrons attack the electrophile.

C C+

E+

C

E

C +

C

E

C+ + Nuc:

_

C

E

C

Nuc

=>

Step 2: Nucleophile attacks the carbocation.

Bond Broken -1 & ! Sigma

Formed 2 sigma


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Chapter 8 4

Types of Additions

=>


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Chapter 8 5

Hydrogenation

Alkene + H2 Alkane

Catalyst required, usually Pt, Pd, or Ni.

Finely divided metal, heterogeneous

Syn addition

=>


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Chapter 8 6

Addition of Halogens

Cl2, Br2, and sometimes I2add to a doublebond to form a vicinal dibromide.

Anti addition, so reaction is stereospecific.

CC + Br2 C C

Br

Br

=>


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Chapter 8 7

Mechanism for Halogenation

Pi electrons attack the bromine molecule.

A bromide ion splits off.

Intermediate is a cyclic bromonium ion.

CC + Br Br CC

Br

+ Br =>


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Chapter 8 8

Mechanism (2)

Halide ion approaches from side opposite the

three-membered ring.

CC

Br

Br

CC

Br

Br

=>
http://d/WADE/CHAP08/ANIMATE/AN08_013.MOV


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Chapter 8 9

Examples of Stereospecificity

=>


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Chapter 8 10

Test for Unsaturation

Add Br2in CCl4(dark, red-brown color) to analkene in the presence of light.

The color quickly disappears as the bromineadds to the double bond.

Decolorizing bromine is the chemical test for

the presence of a double bond.

=>


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Chapter 8 11

Formation of Halohydrin

If a halogen is added in the presence ofwater, a halohydrin is formed.

Water is the nucleophile, instead of halide. Product is Markovnikov and anti.

CC

Br

H2O

CC

Br

OH H

H2O

CC

Br

OH

+ H3O+

=>


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Chapter 8 12

Regiospecificity

The most highly substituted carbon has the

most positive charge, so nucleophile

attacks there.

=>


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Chapter 8 13

Addition of HX (1)

Protonation of double bond yields the most stable

carbocation. Positive charge goes to the carbon

that was not protonated.

X

=>

+ Br

_

+

+CH3 C

CH3

CH CH3

H

CH3 C

CH3

CH CH3

H

H Br

CH3 C

CH3

CH CH3


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Chapter 8 14

Addition of HX (2)

CH3 C

CH3

CH CH3

H Br

CH3 C

CH3

CH CH3

H+

+ Br_

CH3 C

CH3

CH CH3

H+

Br_

CH3 C

CH3

CH CH3

HBr

=>
http://d/WADE/CHAP08/ANIMATE/AN08_003.MOVhttp://d/WADE/CHAP08/ANIMATE/AN08_001.MOV


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Chapter 8 15

Regiospecificity

Markovnikovs Rule: The proton of an acidadds to the carbon in the double bond thatalready has the most Hs. Rich get richer.

More general Markovnikovs Rule: In anelectrophilic addition to an alkene, theelectrophile adds in such a way as to form

the most stable intermediate. HCl, HBr, and HI add to alkenes to form

Markovnikov products. =>


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Chapter 8 16

Free-Radical

Addition of HBr

In the presence of peroxides, HBr adds to

an alkene to form the anti-Markovnikov

product. Only HBr has the right bond energy.

HCl bond is too strong.

HI bond tends to break heterolytically toform ions.

=>


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Chapter 8 17

Free Radical Initiation

Peroxide O-O bond breaks easily to form

free radicals.

+R O H Br R O H + Br

O OR R +R O O R heat

Hydrogen is abstracted from HBr.

Electrophile

=>


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Chapter 8 18

Propagation Steps

Bromine adds to the double bond.

+C

Br

C H Br+ C

Br

C

H

Br

Electrophile =>

CBr

CC CBr +

Hydrogen is abstracted from HBr.


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Chapter 8 19

Anti-Markovnikov ??

Tertiary radical is more stable, so that

intermediate forms faster. =>

CH3 C

CH3

CH CH3 Br+

CH3 C

CH3

CH CH3

Br

CH3 C

CH3

CH CH3

Br

X


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Chapter 8 20

Hydration of Alkenes

Reverse of dehydration of alcohol

Use very dilute solutions of H2SO4or H3PO4to drive equilibrium toward hydration.

=>

C C + H2OH

+

C

H

C

OH

alkene alcohol


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Chapter 8 21

Mechanism for Hydration

+C

H

C+

H2O C

H

C

O H

H+

+ H2OCH

CO H

H+

C

H

C

O

H

H3O+=>

C C OH H

H

++ + H2O

C

H

C

+
http://d/WADE/CHAP08/ANIMATE/AN08_006.MOVhttp://d/WADE/CHAP08/ANIMATE/AN08_005.MOVhttp://d/WADE/CHAP08/ANIMATE/AN08_004.MOV


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Chapter 8 22

Orientation for Hydration

Markovnikov product is formed.

+

CH3 C

CH3

CH CH3 OH H

H

++ H2O+

H

CH3CH

CH3

CCH3

H2O

CH3 C

CH3

CH CH3

HOH H

+

H2OCH3 C

CH3

CH CH3

HOH

=>
http://d/WADE/CHAP08/ANIMATE/AN08_007.MOVhttp://d/WADE/CHAP08/ANIMATE/AN08_008.MOV


23/37

Chapter 8 23

Epoxidation

Alkene reacts with a peroxyacid to form an

epoxide (also called oxirane).

Usual reagent is peroxybenzoic acid.

CC + R C

O

O O H CC

O

R C

O

O H+=>


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Chapter 8 24

Mechanism

One-step concerted reaction. Several bonds

break and form simultaneously.

OC

O

R

H

C

C

OO

H

OC

O

RC

C

+

=>


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Chapter 8 25

Epoxide Stereochemistry

Since there is no opportunity for rotation

around the double-bonded carbons, cisor

trans stereochemistry is maintained.

CCCH3 CH3

H H Ph C

O

O O H

CCCH3 CH3

H H

O =>


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Chapter 8 26

Opening the

Epoxide Ring

Acid catalyzed.

Water attacks the protonated epoxide.

Trans diol is formed.

CC

O

H3O+

CC

O

H

H2O

CC

O

OH

H H H2O

CC

O

OH

H

=>


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Chapter 8 27

One-Step Reaction

To synthesize the glycol without isolating

the epoxide, use aqueous peroxyacetic acid

or peroxyformic acid. The reaction is stereospecific.

CH3COOH

O

OH

H

OH

H

=>


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Chapter 8 28

Syn Hydroxylation

of Alkenes

Alkene is converted to a cis-1,2-diol,

Two reagents:

Osmium tetroxide (expensive!), followed by

hydrogen peroxide or

Cold, dilute aqueous potassium permanganate,

followed by hydrolysis with base

=>


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Chapter 8 29

Mechanism with OsO4

Concerted synaddition of two oxygens toform a cyclic ester.

C

COs

O O

OO

C

C

O O

OO

Os

C

C

OH

OH + OsO4

H2O2=>


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Chapter 8 30

Oxidative Cleavage

Both the pi and sigma bonds break.

C=C becomes C=O.

Two methods:

Warm or concentrated or acidic KMnO4.

Ozonolysis

Used to determine the position of a double

bond in an unknown.=>


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Chapter 8 31

Cleavage with MnO4-

Permanganate is a strong oxidizing agent.

Glycol initially formed is further oxidized.

Disubstituted carbons become ketones.

Monosubstituted carbons become

carboxylic acids.

Terminal =CH2becomes CO2.

=>


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Chapter 8 32

Example

CCCH3 CH3

H CH3 KMnO4

(warm, conc.)C C

CH3

CH3

OHOH

H3C

H

C

O

H3C

H

C

CH3

CH3

O

C

O

H3COH

+

=>


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Chapter 8 33

Ozonolysis

Reaction with ozone forms an ozonide.

Ozonides are not isolated, but are treated

with a mild reducing agent like Zn ordimethyl sulfide.

Milder oxidation than permanganate.

Products formed are ketones or aldehydes.

=>


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Chapter 8 34

Ozonolysis Example

CCCH

3 CH

3

H CH3 O3C

H3C

H

O O

CCH3

CH3

O

Ozonide

+

(CH3)2S

C

H3C

H

O CCH3

CH3

O CH3 S

O

CH3

DMSO

=>


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Addition to C=O

Highly polar carbonyl bond>C+=O-

Attack on +ve charge carbon-=carbanion

Attack on -ve charge carbon-=carbanium ion

Structure & Reactivity of Carbonyl Compounds:

+I effect groups - decreases reactivity

- I effect groups -Increases reactivity


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addition of hydrogen cyanide to the carbonyl

group of an aldehyde or ketone

R d ti f ld h d d k t ith


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Reduction of aldehydes and ketones with

sodium tetrahydridoborate(III) or lithium

tetrahydridoaluminate(III)

Addition Reaction1

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