Substitution and Elimination

1

Substitution and EliminationReaction of Alkyl Halides

By: Ismiyarto, MSi

2

ALKIL HALIDA

1. Manfaat (Pestisida, Bahan Dasar Sintesis Alkohol, Alkena)

2. Struktur (Metil, Primer, Sekunder, Tersier, Benzil dan Vinil)

3. Reaksi (SN-2, SN-1, E-2 dan E-1)

3

PETA REAKSI ALKIL HALIDA

1. Metil Halida2. Alkil halida Primer3. Alkil Halida Sekunder4. Alkil Halida Tersier

5. Alil Halida6. Benzil Halida

SN-2

SN-2SN-2, SN-1 dan E-2

SN-2, SN-1 dan E-2

SN-2, SN-1

SN-2, SN-1

7. Vinil Halida8. Aril Halida

Dalam Pembahasan Tersendiri

4

-Organic compounds with an electronegative atom or an electron-withdrawing group bonded to a sp3 carbon undergo substitution or elimination reactions

Organic compounds with an electronegative atom or an electron-withdrawing group bonded to a sp3 carbon undergo substitution or elimination reactions

Substitution

Elimination

Halide ions are good leaving groups. Substitution reaction on these compounds are easy

and are used to get a wide variety of compounds

Halide ions are good leaving groups. Substitution reaction on these compounds are easy

and are used to get a wide variety of compounds

alkyl fluoride alkyl chloride alkyl bromide alkyl iodide

5

Alkyl Halides in Nature

Synthesized by red algae

Synthesized by sea harea sea hare

red algae

6

Substitution Reaction with Halides

If concentration of (1) is doubled, the rate of the

reaction is doubled.



bromomethane

(1) (2)





If concentration of (1) and (2) is doubled, the rate of the reaction quadruples.

If concentration of (1) and (2) is doubled, the rate of the reaction quadruples.

methanol

7

Substitution Reaction with Halides

bromomethane

(1) (2)

methanol

Rate law:

rate = k [bromoethane][OH-]

this reaction is an example of a SN2 reaction.S stands for substitutionN stands for nucleophilic 2 stands for bimolecular

Rate law:

rate = k [bromoethane][OH-]

this reaction is an example of a SN2 reaction.S stands for substitutionN stands for nucleophilic 2 stands for bimolecular

8

Mechanism of SN2 Reactions

The rate of reaction depends on the concentrations of both reactants.The rate of reaction depends on the concentrations of both reactants.

When the hydrogens of bromomethane are replaced with methyl groups the reaction rate slow down.

When the hydrogens of bromomethane are replaced with methyl groups the reaction rate slow down.

The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of only one stereoisomer

The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of only one stereoisomer

Alkyl halide Relative rate

1200

40

1

≈ 0

9


Hughes and Ingold proposed the following mechanism:Hughes and Ingold proposed the following mechanism:

Transition state

Increasing the concentration of either of the reactant makes their collision more probable.Increasing the concentration of either of the reactant makes their collision more probable.

10


activationenergy: G1

activationenergy: G2

Steric effectSteric effect

Inversion of configurationInversion of configuration

(S)-2-bromobutane (R)-2-butanol

Ener

gy

reaction coordinate reaction coordinate

11

Factor Affecting SN2 Reactions

relative rates of reaction pKa HX

HO- + RCH2I RCH2OH + I- 30 000 -10

HO- + RCH2Br RCH2OH + Br- 10 000 -9

HO- + RCH2Cl RCH2OH + Cl- 200 -7

HO- + RCH2F RCH2OH + F- 1 3.2

relative rates of reaction pKa HX

HO- + RCH2I RCH2OH + I- 30 000 -10

HO- + RCH2Br RCH2OH + Br- 10 000 -9

HO- + RCH2Cl RCH2OH + Cl- 200 -7

HO- + RCH2F RCH2OH + F- 1 3.2

The leaving group

The nucleophile

In general, for halogen substitution the strongest the base the better the

nucleophile.

In general, for halogen substitution the strongest the base the better the

nucleophile.

pKa Nuclephilicity

12

SN2 Reactions With Alkyl Halidesan alcohol

a thiol

an ether

a thioether

an amine

an alkyne

a nitrile

13

Substitution Reactions With Halides





If concentration of (2) is doubled, the rate of the reaction is not doubled.


Rate law:

rate = k [1-bromo-1,1-dimethylethane]

this reaction is an example of a SN1 reaction.

S stands for substitutionN stands for nucleophilic 1 stands for unimolecular

Rate law:


this reaction is an example of a SN1 reaction.

S stands for substitutionN stands for nucleophilic 1 stands for unimolecular

1-bromo-1,1-dimethylethane 1,1-dimethylethanol

14


The rate of reaction depends on the concentrations of the alkyl halide only.The rate of reaction depends on the concentrations of the alkyl halide only.

When the methyl groups of 1-bromo-1,1-dimethylethane are replaced with hydrogens the reaction rate slow down.

When the methyl groups of 1-bromo-1,1-dimethylethane are replaced with hydrogens the reaction rate slow down.

The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of two stereoisomers

The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of two stereoisomers

Alkyl halide Relative rate

≈ 0 *

≈ 0 *

12

1 200 000

* a small rate is actually observed as a result of a SN2

15


C-Br bond breaksC-Br bond breaks

nucleophile attacks the carbocation

nucleophile attacks the carbocation

Proton dissociationProton dissociation

slow

fast

16


G

Rate determining stepRate determining stepCarbocation intermediateCarbocation intermediate

R++ X-

R-OH2

+

R-OH

17


Same configuration as the alkyl halide

Same configuration as the alkyl halide

Inverted configuration

relative the alkyl halide

Inverted configuration

relative the alkyl halide

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Factor Affecting SN1 reaction

Two factors affect the rate of a SN1 reaction:• The ease with which the leaving group dissociate from the carbon• The stability of the carbocation

Two factors affect the rate of a SN1 reaction:• The ease with which the leaving group dissociate from the carbon• The stability of the carbocation

The more the substituted the carbocation is, the more

stable it is and therefore the easier it is to form.

The more the substituted the carbocation is, the more

stable it is and therefore the easier it is to form.

As in the case of SN2, the weaker base is the leaving group, the less tightly it is

bonded to the carbon and the easier it is to break the bond

As in the case of SN2, the weaker base is the leaving group, the less tightly it is

bonded to the carbon and the easier it is to break the bond

The reactivity of the nucleophile has no effect on the rate of a SN1 reaction

The reactivity of the nucleophile has no effect on the rate of a SN1 reaction

19

Comparison SN1 – SN2

SN1 SN2

A two-step mechanism A one-step mechanism

A unimolecular rate-determining step A bimolecular rate-determining step

Products have both retained and inverted configuration relative to the reactant

Product has inverted configuration relative to the reactant

Reactivity order:3o > 2o > 1o > methyl

Reactivity order:methyl > 1o > 2o > 3o

20

Kestabilan Karbokation

H

H

H

H

H

H

H

+

propan-2-ylium Ethanylium

+H2C

H

H

H

Methanylium

CH3+

21

22

Elimination Reactions

1-bromo-1,1-dimethylethane 2-methylpropene

Rate law:

rate = k [1-bromo-1,1-dimethylethane][OH-]

this reaction is an example of a E2 reaction.E stands for elimination2 stands for bimolecular

Rate law:

rate = k [1-bromo-1,1-dimethylethane][OH-]

this reaction is an example of a E2 reaction.E stands for elimination2 stands for bimolecular

23

The E2 Reaction

A proton is removed

A proton is removed

Br- is eliminatedBr- is eliminatedThe mechanism shows that an E2

reaction is a one-step reactionThe mechanism shows that an E2

reaction is a one-step reaction

24








Rate law:


this reaction is an example of a E1 reaction.

E stands for elimination1 stands for unimolecular

Rate law:


this reaction is an example of a E1 reaction.

E stands for elimination1 stands for unimolecular

1-bromo-1,1-dimethylethane 2-methylpropene

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The E1 Reaction

The alkyl halide dissociate, forming a

carbocation

The alkyl halide dissociate, forming a

carbocation

The base removes a

proton

The base removes a

proton

The mechanism shows that an E1 reaction is a two-step reaction

The mechanism shows that an E1 reaction is a two-step reaction

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Products of Elimination Reaction

2-bromobutane

2-butene

1-butene

80%

20%

The most stable alkene is the major product of the reaction

for both E1 and E2 reaction

The most stable alkene is the major product of the reaction

for both E1 and E2 reaction

The greater the number of alkyl substituent the more

stable is the alkene

The greater the number of alkyl substituent the more

stable is the alkeneFor both E1 and E2 reactions, tertiary alkyl halides

are the most reactive and primary alkyl halides are the least reactive

For both E1 and E2 reactions, tertiary alkyl halides are the most reactive and primary alkyl halides

are the least reactive

30% 50%

27

ELIMINATION REACTIONS:ALKENES, ALKYNES

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C C

X Y

C C + X Y

Dehydrohalogenation (-HX) and Dehydration (-H2O) are the main types of elimination reactions.

29

Dehydrohalogenation (-HX)

strong

base

X = Cl, Br, I

+ " "C C

X

H XC C

H

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The E2 mechanism

..:..

__

+

+ Br_

..:

concerted mechanism

H O

C C

Br

H

H O

H

C C

This reaction is done in strong base at high concentration, such as 1 M NaOH in water.

_

31

Kinetics

• The reaction in strong base at high concentration is second order (bimolecular):

Rate law: rate = k[OH-]1[R-Br]1

32

The E1 mechanism

1)

++ Br

_slow

+

2)..

:

+fast

O.. +O

C C

Br

C C

H

C C

HC C

H

H H

H

H

H

rate determining step

This reaction is done in strong base such as 0.01 M NaOH in water!! Actually, the base solution is weak!

33

Kinetics

• The reaction in weak base or under neutral conditions will be first order (unimolecular):

• Rate law: rate = k [R-Br]1

• The first step (slow step) is rate determining!

34

The E2 mechanism• Mechanism• Kinetics• Stereochemistry of reactants• Orientation of elimination (Zaitsev’s rule)• Stereochemistry of products• Competing reactions

35

E2 mechanism

..:..

__

+

+ Br_

..:

concerted mechanism

H O

C C

Br

H

H O

H

C C

This reaction is done in strong base at high concentration, such as 1 M NaOH in water.

36

Kinetics of an E2 reaction• The reactions are second order (bimolecular

reactions).

• Rate = k [R-Br]1[Base]1

second order reaction (1 + 1 = 2)High powered math!!

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energy

Reaction coordinate

C C

H OH

Br-

..:..

__H O

C C

Br

H

..:H O

C C

H

Br

Transition State

38

Stereochemistry of reactants

• E2 reactions must go by an anti elimination• This means that the hydrogen atom and

halogen atom must be 180o (coplanar) with respect to each other!!

• Draw a Newman projection formula and place the H and X on opposite sides.

39

Stereochemistry of E2 Reaction

KOH

AlcoholSolventH

Br

H

HH

CCH3

CH3

CH3

C

H

CH3

CH3

CH3H

H

H and Br are anti structure in conformation!!!!!!!!!

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(S,S)-diastereomer

KOHethanolheat

(E)-isomer (Z)-isomer

??? ???

C C

Br

HCH3

CH3

H

C C

CH3 CH3

H t-butyl

C C

H CH3

CH3 t-butyl

t-butyl

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(E)-isomer

C C

CH3 CH3

H T-butyl

This one is formed!

42

(R,S)-diastereomer

KOHethanolheat

(E)-isomer (Z)-isomer

??? ???

C C

Br

HH

CH3

CH3

t-butyl

C C

CH3 CH3

H T-butyl

C C

H CH3

CH3 t-butyl

43

(Z)-isomer

C C

H CH3

CH3 t-butyl

This one is formed!

44

Orientation of elimination: regiochemistry/ Zaitsev’s Rule• In reactions of removal of hydrogen halides from

alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched -carbon.

A. N. Zaitsev -- 1875 C C C C

H

H

H H

X

H

H

HH

CH3

Less branchedMore branched

45

Product formed from previous slide

C

C CC

H

HH

H

HCH3

HH

More substituted alkene is more stable!!!!!!!!

46

Typical bases used in E2 reactions

High concentration of the following >1MIf the concentration isn’t given, assumethat it is high concentration!• Na+ -OH• K+ -OH• Na+ -OR• Na+ -NH2

47

Orientation of elimination: regiochemistry/ Zaitsev’s Rule

Explaination of Zaitsev’s rule:When you remove a hydrogen atom from the more branched position, you are forming a more highly substituted alkene.

48

Stereochemistry of products• The H and X must be anti with respect to each

other in an E2 reaction!• You take what you get, especially with

diastereomers! See the previous slides of the reaction of diastereomers.

49

Competing reactions• The substitution reaction (SN2) competes with

the elimination reaction (E2).• Both reactions follow second order kinetics!

50

The E1 mechanism• Mechanism• Kinetics• Stereochemistry of reactants• Orientation of elimination (Zaitsev’s rule)

• Stereochemistry of products• Competing reactions

51

E1 mechanism

1)

++ Br

_slow

+

2)..

:

+fast

O..+O

C C

Br

C C

H

C C

HC C

H

H H

H

H

H

water helpsto stabilizecarbocation

This reaction is done in strong base at low concentration, such as 0.01 M NaOH in water)

52

E1 Reactions • These reactions proceed under neutral

conditions where a polar solvent helps to stabilize the carbocation intermediate.

• This solvent also acts as a weak base and removes a proton in the fast step.

• These types of reactions are referred to as solvolysis reactions.

53

• tertiary substrates go by E1 in polar solvents, with little or no base present!

• typical polar solvents are water, ethanol, methanol and acetic acid

• These polar solvents help stabilize carbocations

• E1 reactions also occur in a low concentration of base (i.e. 0.01M NaOH).

54

•With strong base (i.e. >1M), goes by E2

However!!!!

55

Structure of the Carbocation Intermediate

C CH3

CH3

CH3

56

Carbocation stability order

Tertiary (3o) > secondary (2o) > primary (1o)

It is hard (but not impossible) to get primary compounds to go by E1. The reason for this is that primary carbocations are not stable!

57

Kinetics of an E1 reaction• E1 reactions follow first order (unimolecular)

kinetics:Rate = k [R-X]1

• The solvent helps to stabilize the carbocation, but it doesn’t appear in the rate law!!

58

energy

Reaction coordinate

C

H

C

Br

C

H

C

Br-

C C

H

C C

H

C C + H+

intermediate

59

Stereochemistry of the reactants• E1 reactions do not require an anti coplanar

orientation of H and X. • Diastereomers give the same products with E1

reactions, including cis- and trans products.• Remember, E2 reactions usually give different

products with diastereomers.

60

Orientation of elimination• E1 reactions faithfully follow Zaitsev’s rule!• This means that the major product should be

the product that is the most highly substituted.

61

Stereochemistry of productsE1 reactions usually give the thermodynamically most stable product as the major product. This usually means that the largest groups should be on opposite sides of the double bond. Usually this means that the trans product is obtained.

62

Competing reactions

• The substitution reaction (SN1) competes with the elimination reaction (E1).

• Both reactions follow first order kinetics!

63

Whenever there are carbocations…• They can undergo elimination (E1)• They can undergo substitution (SN1)

• They can rearrange– and then undergo elimination– or substituion

64

Rearrangements• Alkyl groups and hydrogen can migrate in

rearrangement reactions to give more stable intermediate carbocations.

• You shouldn’t assume that rearrangements always occur in all E1 reactions, otherwise paranoia will set in!!

65

Comparison of E2 / E1• E1 reactions occur under essentially neutral

conditions with polar solvents, such as water, ethyl alcohol or acetic acid.

• E1 reactions can also occur with strong bases, but only at low concentration, about 0.01 to 0.1 M or below.

• E2 reactions require strong base in high concentration, about 1 M or above.

66

Comparison of E2 / E1• E1 is a stepwise mechanism (two or more);

Carbocation intermediate!• E2 is a concerted mechanism (one step)

No intermediate!• E1 reactions may give rearranged products• E2 reactions don’t give rearrangement• Alcohol dehydration reactions are E1

67

Bulky leaving groupsHofmann Elimination

+

OH_

heat

+

6%

94%

CH3 CH2 CH2 CH CH3

N

CH3

CH3CH3

CH3 CH2 CH CH CH3

CH3 CH2 CH2 CH CH2

This give the anti-Zaitsev product (least substituted product is formed)!

68

Orientation of elimination: regiochemistry/ Hofmann’s Rule • In bimolecular elimination reactions in the presence

of either a bulky leaving group or a bulky base, the hydrogen that is lost will come from the LEAST LEAST highly-branched -carbon.

C C C C

H

H

H H

X

H

H

HH

CH3

Less branchedMore branched

69

Product from previous slide

CC

C

H

H

H

HCH3

HH

C

H

70

Elimination with bulky bases

• Non-bulky bases, such as hydroxide and ethoxide, give Zaitsev products.

• Bulky bases, such as potassium tert-butoxide, give larger amounts of the least substituted alkene (Hoffmann) than with simple bases.

71

Comparing Ordinary and Bulky Bases

CH3 C CH CH3

Br

NaOC2H5

C2H5OHheat

C CHCH3 CH3

CH3 C CH CH3

Br

KOC(CH3)3

(CH3)3COHheat

C CHCH3 CH2

Major

H

CH3 CH3

CH3

H

CH3

Major

H

72

1-butene: watch out for competing reactions!

H3C CH2 CH2 CH2 Br

KOCH3

Non-bulky

SN2

H3C CH2 CH2 CH2 O-CH3

H3C CH2 CH CH2

bulky baseKO-t-butyl

E2

73

Highlights• Dehydrohalogenation -- E2 Mechanism• Zaitsev’s Rule• Dehydrohalogenation -- E1 Mechanism• Carbocation Rearrangements -- E1• Elimination with Bulky Leaving Groups and Bulky

Bases -- Hofmann Rule -- E2

74

Competition Between SN2/E2 and SN1/E1

rate = k1[alkyl halide] + k2[alkyl halide][nucleo.] + k3[alkyl halide] + k2[alkyl halide][base] rate = k1[alkyl halide] + k2[alkyl halide][nucleo.] + k3[alkyl halide] + k2[alkyl halide][base]

SN1SN1 SN2SN2 E1E1 E2E2

• SN2 and E2 are favoured by a high concentration of a good nucleophile/strong base• SN1 and E1 are favoured by a poor nucleophile/weak base, because a poor nucleophile/weak base disfavours SN2 and E2 reactions

• SN2 and E2 are favoured by a high concentration of a good nucleophile/strong base• SN1 and E1 are favoured by a poor nucleophile/weak base, because a poor nucleophile/weak base disfavours SN2 and E2 reactions

75

Competition Between Substitution and Elimination

• SN2/E2 conditions:In a SN2 reaction: 1o > 2o > 3o

In a E2 reaction: 3o > 2o > 1o In a SN2 reaction: 1o > 2o > 3o

In a E2 reaction: 3o > 2o > 1o

90% 10%

25% 75%

100%

76

Competition Between Substitution and Elimination

• SN1/E1 conditions:

All alkyl halides that react under SN1/E1 conditions will give both substitution and elimination products (≈50%/50%)

All alkyl halides that react under SN1/E1 conditions will give both substitution and elimination products (≈50%/50%)

77

Summary

• Alkyl halides undergo two kinds of nucleophilic subtitutions: SN1 and SN2, and two kinds of elimination: E1 and E2.

• SN2 and E2 are bimolecular one-step reactions• SN1 and E1 are unimolecular two step reactions• SN1 lead to a mixture of stereoisomers• SN2 inverts the configuration od an asymmetric carbon• The major product of a elimination is the most stable alkene• SN2 are E2 are favoured by strong nucleophile/strong base• SN2 reactions are favoured by primary alkyl halides• E2 reactions are favoured by tertiary alkyl halides

REAKSI ADISI ALKENA

78

Addition Reaction of Alkene

1. HX Addition• Electrophilic Addition (Markovnikov Product)• Free Radical Mechanism (Anti-Mark Product)

2. Hydration (+ H2O)

3. Halogenation/ Hydrohalogenation4. Reduction or Hydrogenation (+ H2 )

5. Oxidation6. Multi-step Synthesis

•electrophilic addition to double bond•forms a vicinal dihalide

++ XX22

XX XXCC CCCC CC

Addition of Halogens to Alkenes

XX22 = Cl = Cl22 or Br or Br22

FF22; explosive I; explosive I22 ; endothermic ; endothermic

CHCH33CHCHCHCHCH(CHCH(CH33))22

(100%)(100%)

CHCHCH(CHCH(CH33))22

CHCH33CHCH

BrBr22

Example

BrBr BrBr

BrBr22

transtrans-1,2-Dibromocyclopentane-1,2-Dibromocyclopentane80% yield; only product80% yield; only product

HH

HH

BrBr

BrBr

HH

HH

Anti Addition ; Two Bromines add to oppositeAnti Addition ; Two Bromines add to opposite sides of the ringsides of the ring

Stereochemistry of Halogen AdditionStereochemistry of Halogen Addition

•anti additionanti addition•anti additionanti addition

ClCl22

transtrans-1,2-Dichlorocyclooctane-1,2-Dichlorocyclooctane73% yield; only product73% yield; only product

Example HH

HH

HH

HH

ClCl

ClCl

•Br2 is not polar, but it is polarizable

•two steps(1) formation of bromonium ion &

• electrophilic attack

• (2) nucleophilic attack on bromonium ion by bromide

Mechanism is electrophilic addition

CHCH22=CH=CH22 + Br + Br22 -> Br-CH -> Br-CH22-CH-CH22-Br-Br

NET REACTIONNET REACTION

BrBr

BrBr

Mutual polarizationMutual polarizationof electron distributionsof electron distributionsof Brof Br22 and alkene and alkene

Step 1a: Formation of Bromonium Ion

BrBr

BrBr

––

++++

Electrons flow Electrons flow from alkenefrom alkenetoward Brtoward Br22

Step 1b; Electrophilic Addition to form Bromonium Ion

CC

H

H H

H

+ BrBr

CC

Br

H

H H

H+ + Br+ Br--

Part iPart i

Step 1b; Lone Pair on Bromine Stabalizes Carbocation and

Forms Cyclic Bromonium IonPart iiPart ii

CC

Br

H

H H

H+

CCH

H H

H

Br +

+ Br+ Br--

BrBr++

Step 2; Bromide Ion Must Attack from Oppositte Side of

Cyclic Bromonium Ion (anti addition)

CCH

H H

H

Br +

Br-

+

CC

Br

H

H

H

H

Br

BrBr++

BrBr

BrBr22

transtrans-1,2-Dibromocyclopentane-1,2-Dibromocyclopentane80% yield; only product80% yield; only product

Example HH

HH

BrBr

BrBr

HH

HH

++ X X22

XX XXCC CCCC CC

++ H H22OOOHOH

+ + HH—X—X

++ X X22

XXCC CCCC CC

alkenes react with Xalkenes react with X22 to form vicinal dihalides to form vicinal dihalides

alkenes react with Xalkenes react with X22 in water to give vicinal in water to give vicinal

halohydrinshalohydrins

ClCl22

anti anti addition: only productaddition: only product

HH22OO

HH22CC CHCH22

BrBrCHCH22CHCH22OOHH++ BrBr22

HH22OO

(70%)(70%)

Examples HH

HH

OOHH

ClCl

HH

HH

Mechanism; 1) Cl2 is polarized and adds across double bond. 2) Ion formed is stabalized by lone pair of Cl.

3) Water attacks chloronium ion from side opposite (anti addition) carbon-chlorine bond. This gives trans isomer

(77%)(77%)

HH33CC

CC CHCH22

HH33CC

CHCH33

OOHH

CC CHCH22BrBrCHCH33

•Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens.

BrBr22

HH22OO

Regioselectivity

+ H—H+ H—H

•exothermic H° = –136 kJ/mol

•catalyzed by finely divided Pt, Pd, Rh, Ni

CC CC HHCC CC

HH HH

HH HH

HH

HH

HH

HH

HH

Hydrogenation (Reduction, +H2) of Ethylene

MetalMetal

CatalystCatalyst

Two spatial (stereochemical) aspects ofalkene hydrogenation:

•(1) syn addition of both H atoms to double bond•(2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

COCO22CHCH33

COCO22CHCH33

HH22,, Pt PtCOCO22CHCH33

COCO22CHCH33

HH

HH

syn additionsyn addition anti additionanti addition

syn-Additon versus anti-Addition

H

CC CCAA

BB

XX

YY

H HH

syn-Addition; Metal catalyst breaks H-H bonds.

H H

H

CCCC

AA

BB

XXYY

H

syn-Addition; Addition of H2 across double bonds takes

place in two steps.

HH33CC CHCH33

HH33CC

HH

HH22, cat, cat

Both productsBoth productscorrespond tocorrespond tosyn additionsyn additionof Hof H22..

Example of Stereoselective Reaction

CHCH33HH33CC

HH33CCHH

HH

HH

CHCH33

HH33CC

HH33CC

HH

HH

HH

HH22, cat, cat

But only thisBut only thisone is formed.one is formed.

Example of Stereoselective Reaction

HH33CC CHCH33

HH33CC

HH

CHCH33HH33CC

HH33CCHH

HH

HH

Top face of doublebond blocked bythis methyl group

Top face of doublebond blocked bythis methyl group

H2 adds to bottom face of double bond.

H2 adds to bottom face of double bond.

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Substitution and Elimination

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