6-1 6 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown

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Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Introduction to Introduction to Organic Organic

ChemistryChemistry2 ed2 ed

William H. BrownWilliam H. Brown

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Reactions of Reactions of AlkenesAlkenes

Chapter 6Chapter 6

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Characteristic ReactionsCharacteristic Reactions

+

Hydrochlorination (hydrohalogenation)

CC C C

H C l

H C l

+

Hydration

C C C C

H O H

H2

O

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+

Bromination (halogenation)

C C C C

B r B r

B r2

+

Hydroxylation (oxidation)

C C C C

H O O H

O s O4

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Polymerization

CC C C

i nitiator

n

n

+

Hydrogenation (reduction)

CC C C

HH

H2

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Reaction MechanismsReaction Mechanisms• A reaction mechanism describes how a reaction

occurs• which bonds are broken and which new ones are

formed• the order in which bond-breaking and bond-forming

steps take place• the role of the catalyst (if any is present)• the energy of the entire system during the reaction

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PE DiagramsPE Diagrams• Potential energy (PE)

diagram: a graph showing the changes in energy that occur during a chemical reaction

• Reaction coordinate: a measure of the change in position of atoms during a reaction

Reaction

coordinate

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PE DiagramsPE Diagrams

E a

Δ H

transition

state

+ A B

+ C D

Reaction Coordinate

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PE DiagramsPE Diagrams• Heat of reaction, ΔH: the difference in potential

energy between reactants and products• exothermic: products are lower in energy than

reactants; heat is released• endothermic: products are higher in energy than

reactants; heat is absorbed

• Transition state: an energy maximum on a PE diagram• represents an unstable species of maximum PE

formed during the course of a reaction

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Activation EnergyActivation Energy• Activation Energy, Ea: the difference in

potential energy between reactants and the transition state• determines the rate of reaction• if activation energy is large, only a few

molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow

• if activation energy is small, many collisions generate sufficient energy to reach the transition state, and the reaction is fast

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Developing a MechanismDeveloping a Mechanism• Design experiments to reveal details of a particular

chemical reaction• Propose a set or sets of steps that might account for the

overall transformation• A mechanism becomes established when it is shown to

be consistent with every test that can be devised• This does mean that the mechanism is correct, only that

it is the best explanation we are able to devise

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Why Mechanisms?Why Mechanisms?• A framework within which to organize descriptive

chemistry• The intellectual satisfaction derived from

constructing models that accurately reflect the behavior of chemical systems

• A tool with which to search for new information and new understanding

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Electrophilic AdditionsElectrophilic Additions• Hydrohalogenation using HCl, HBr, HI

• Hydration using H2O, H2SO4

• Halogenation using Cl2, Br2

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Addition of HXAddition of HX• Carried out with pure reagents or in a polar

solvent such as acetic acid• Addition is regioselective • Regioselective reaction: a reaction in which one

direction of bond-forming or bond-breaking occurs in preference to all other directions

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Addition of HXAddition of HX• Addition is regioselective

• Markovnikov’s ruleMarkovnikov’s rule: in the addition of HX or H2O to an alkene, H adds to the carbon of the double bond that has the greater number of hydrogens bonded to it

1-Chloropropane

(not observed)

2-Chloropropane

Propene

+

+

C lC l

C H 3 C H = C H 2H C l

C H3

C H - C H2

C H3

C H - C H2

H H

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HCl + 2-ButeneHCl + 2-Butene• A two-step mechanism

• Step 1: formation of sec-butyl cation, a 2° carbocation intermediate

-

+

+

δ -δ +

H - C lC H3

C H = C H C H3

C l

sec-Butyl cation

slow, rate-

limiting step

+

H

C H3

C H - C H C H3

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HCl + 2-ButeneHCl + 2-Butene• Step 2: reaction of the sec-butyl cation with chloride

ion

- +

sec-Butyl cation

(a Lewis acid)

+

C l

C l C H3

C H C H2

C H3

C H3

C H C H2

C H3

Chloride ion

(a Lewis base)

fast

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CarbocationsCarbocations• CarbocationCarbocation: a species containing a positively

charged carbon• Carbocations are

• classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge

• electrophiles; that is, they are “electron-lovers”

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Carbocation StructureCarbocation Structure• Bond angles about the

positively charged carbon are 120°

• Carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups

• The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons

R C

R

R+

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Carbocation StabilityCarbocation Stability• A 3° carbocation is more stable (requires a lower

activation energy for its formation) than a 2° carbocation

• A 2° carbocation is, in turn, more stable (requires a lower activation energy for its formation) than a 1° carbocation

• 1° and methyl cations is so unstable that they are rarely if ever observed in solution

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Carbocation StabilityCarbocation Stability• Relative stability

Methyl

cation

(methyl)

Ethyl

cation

(1°)

Isopropyl

cation

(2°)

tert -Butyl

cation

(3°)

Increasing carbocation stability

+ + + +C

H

H

CH 3CCH 3

CH 3

H

C

CH 3

CH 3

CH 3CH

H

H

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Carbocation StabilityCarbocation Stability• Carbocations are stabilized by the electron-

withdrawing inductive effect of the positively-charged carbon• according to molecular orbital calculations:

H

H

C

H

H3

C

C H3

C

C H3

+0.83

+0.06

+0.60

+0.13

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Addition of HAddition of H22OO• Addition of water is called hydration• Acid-catalyzed hydration of an alkene is

regioselective - hydrogen adds to the less substituted carbon of the double bond

Propene 2-Propanol

+

O H

C H3

C H = C H2 H

2O

H2

S O4

C H3

C H - C H2

H

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Addition of HAddition of H22OO• Step 1: proton transfer from solvent to the alkene

+

+

+

intermediate

A 2o

carbocation

+

HO

H

HOH

H

C H3

C H = C H2

C H3

C H C H3

slow, rate-

limiting step

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Addition of HAddition of H22OO• Step 2: reaction of the carbocation intermediate with

H2O to form an oxonium ion

• Step 3: proton transfer to solvent

+

+

+

An oxonium ion

H O

HH

C H3

C H C H3 O - H C H

3C H C H

3

fast

+

+

+O

H HO

H

H

H

H O HC H 3 C H C H 3

C H 3 C H C H 3

O H

fast

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Addition of ClAddition of Cl22 and Br and Br22• Carried out with either the pure reagents or in an

inert solvent such as CCl4 or CH2Cl2

2,3-Dibromobutane2-Butene

+

B r B r

B r2

C H3

C H = C H C H3 C H

3C H - C H C H

3C C l

4

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Addition of ClAddition of Cl22 and Br and Br22

• Addition is stereoselective

• Stereoselective reactionStereoselective reaction: a reaction in which one stereoisomer is formed or destroyed in preference to all others than might be formed or destroyed

trans -1,2-Dibromo-

cyclohexane

Cyclohexene

+ B r2

C C l4

B r

B r

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Addition of ClAddition of Cl22 and Br and Br22• Addition involves a two-step mechanism

• Step 1: formation of a bridged bromonium ion intermediate

C C

B r

C C

B r

B r

B r -

A bridged bromonium

ion intermediate

+

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Addition of ClAddition of Cl22 and Br and Br22• Step 2: Attack of halide ion from the opposite side of

the three-membered ring

Anti addition

-

C C

Br

C C

Br

Br

Br

+

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Addition of ClAddition of Cl22 and Br and Br22• For a cyclohexene, anti coplanar addition

corresponds to trans-diaxial addition

trans-Diaxial

(less stable)

trans-Diequatorial

(more stable)

Br

Br

Br

BrBr 2

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Oxidation/ReductionOxidation/Reduction• Oxidation: the loss of electrons• Reduction: the gain of electrons• Recognize using a balanced half-reaction

• write a half-reaction showing one reactant and its product(s)

• complete a material balance. Use H2O and H+ in acid solution; use H2O and OH- in basic solution

• complete a charge balance using electrons, e-

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Oxidation with OsOOxidation with OsO44• Oxidation by OsO4 converts an alkene to a glycol,

a compound with -OH groups on two adjacent carbons• oxidation is syn stereoselective

H

H O

H

O H

+R O O H

O s O4

cis- Cyclopentanediol

(a cis glycol)

Cyclopentene

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Oxidation with OsOOxidation with OsO44• Intermediate is a cyclic osmic ester containing a

five-membered ring

H

O

H

O

O s

O O

A cyclic osmic ester

O O

O s

O O

H

H O

H

O H

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Reduction of AlkenesReduction of Alkenes• Most alkenes react with H2 in the presence of a

transition metal catalyst to give alkanes• commonly used catalysts are Pt, Pd, Ru, and Ni

• The process is called catalytic reduction or, alternatively, catalytic hydrogenation

+ H2

Pt

Cyclohexene Cyclohexane

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Reduction of AlkenesReduction of Alkenes• the most common pattern is syn stereoselectivity

70% to 85%

cis -1,2-Dimethyl-

cyclohexane

1,2-Dimethyl-

cyclohexene

+

CH3

CH3

CH3

CH3

CH3

CH3

H2

/ Pt

30% to 15%

trans -1,2-Dimethyl-

cyclohexane

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Reduction of AlkenesReduction of Alkenes• Mechanism of catalytic hydrogenation

• H2 is absorbed on the metal surface with formation of metal-hydrogen bonds

• the alkene is also absorbed with formation of metal-carbon bonds

• a hydrogen atom is transferred to the alkene forming one new C-H bond

• a second hydrogen atom is transferred forming the second C-H bond

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Reaction StereochemistryReaction Stereochemistry• Consider the addition of bromine to 2-butene?

• three stereoisomers are possible for 2,3-dibromobutane; one pair of enantiomers

C H3

C H = C H2

C H3

C H3

C H - C H C H3

2-Butene 2,3-Dibromobutane

B r2

B r B r

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Reaction StereochemistryReaction Stereochemistry• how many stereoisomers are formed in the addition of

bromine to cis-2-butene?

C CHH

H3

C C H3

cis- - 2-Butene

C C

H

H

H 3 C

C H3

B r

B r

C C

H

H

H3

C

C H3

B r

B r

+

A pair of enantiomers

(a racemic mixture)

B r 2

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Reaction StereochemistryReaction Stereochemistry• Consider the oxidation of 2-butene by OsO4

• three stereoisomers are possible for 2,3-butanediol; one pair of enantiomers and one meso compound

• which are formed in this oxidation?

C H3

C H = C H2

C H3

C H3

C H - C H C H3

2-Butene 2,3-Butanediol

O s O4

O H O H

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Reaction StereochemistryReaction Stereochemistry• syn hydroxylation of cis-2-butene produces only the

meso compound

identical;

a meso

compound

(2S,3R)-2,3-Butanediol

(2R,3S)-2,3-Butanediol

2

2

3

3

C

HO

HO

C

H

C

OH

H

CH3

H3

C

C

OH

H3

C

H

CH3

H

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Reaction StereochemistryReaction Stereochemistry• Enantiomerically pure products can never be

formed from achiral starting materials and achiral reagents

• An enantiomerically pure product can be generated in a reaction if at least one of the reactants is enantiomerically pure, or if the reaction is carried out in an achiral environment

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Reactions of Reactions of AlkenesAlkenes

End Chapter 6End Chapter 6

Documents

6-1 6 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown