Reactions of Alkenes: Addition Reactions

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Reactions of Alkenes:Addition Reactions

Addition is the opposite of elimination.

Here are some of the many addition reactions that will be considered.

Addition Reactions

Addition of Halogensto Alkenes

+ X2 X X

electrophilic addition to double bondforms a vicinal dihalide

C CC C

General features

CH3CHCHCH(CH3)2

(100%)

CHCH(CH3)2CH3CHBr2

Example

F2 addition proceeds with explosive violence

I2 addition is endothermic: vicinal diiodides

dissociate to an alkene and I2

limited to Cl2 and Br2

Halogenation

anti addition

Stereochemistry of Halogen Addition

trans-1,2-Dibromocyclopentane80% yield; only product

Example

trans-1,2-Dichlorocyclooctane73% yield; only product

Example

anti addition

Mechanism of Halogen Addition to Alkenes: Halonium Ions

Br2 is not polar, but it is polarizable

two steps(1) formation of bromonium ion(2) nucleophilic attack on

bromonium ion by bromide

Mechanism is electrophilic addition

Addition of Halogens to Alkenes

ethylene H2C=CH2 1

propene CH3CH=CH2 61

2-methylpropene (CH3)2C=CH2 5400

2,3-dimethyl-2-butene (CH3)2C=C(CH3)2

920,000

More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore “electron rich.”

Relative Rates of Bromination

Question

Arrange the alkenes in order of decreasing rate of reaction toward bromine addition:2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-buteneA) 2-methyl-1-butene > 3-methyl-1-butene >

2-methyl-2-buteneB) 3-methyl-1-butene > 2-methyl-1-butene >

2-methyl-2-buteneC) 2-methyl-2-butene > 2-methyl-1-butene >

3-methyl-1-buteneD) 2-methyl-2-butene > 3-methyl-1-butene >

2-methyl-1-butene

trans-1,2-dibromocyclopentane80% yield; 2 asymmetric carbon atoms,

Chiral product,Optically inactive,Racemic mixture, only products formed

Example

Diastereomers

In general, diastereomers fall into two categories:geometric isomers

cis-trans

stereoisomers containing two or more asymmetric atoms; (that are not enantiomers)

diastereomers

Diastereomers

Diastereomers have different physical properties:BP, MP, density, refractive index, solubilityCan be separated through conventional

means (distillation, recrystallization, chromatography)

MP = 158oC MP = 256oC

Diastereomers

A compound with “n” asymmetric carbon atoms can have a maximum of 2n stereoisomers.

2,3-dibromobutane

Enantiomers Same compound (meso)diastereomers

(S) (R)

Only 3 stereoisomers for 2,3-dibromobutane.

Three stereoisomers of 2,3-dibromobutane

2R,3R 2S,3S 2R,3S

chiral chiral achiral

HBrH Br

The addition of bromine to cis-2-butene produces:

A) A single enantiomerB) A product with one asymmetric carbon atomC) An optically inactive meso productD) A racemic mixtureE) 4 different stereoisomers each with 2 chiral

carbon atoms

Question

The addition of bromine to trans-2-butene produces:

A) A single enantiomerB) A product with one asymmetric carbon atomC) An optically inactive meso productD) A racemic mixtureE) 4 different stereoisomers each with 2 chiral

carbon atoms

Question

Diastereomers

When naming compounds containing multiple chiral atoms, you must give the configuration around each chiral atom:position number and configuration of each

chiral atom, separated by commas, all in ( ) at the start of the compound name

(S)(2S, 3S)-2-bromo-3-chlorobutane

Diastereomers

Which ones are chiral? Name each one. Give the stereochemical relationship between them.

All of them are chiral.

Diastereomers

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

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

Diastereomers

A B C DA and B: enantiomersC and D: enantiomers

A and C: diastereomersA and D: diastereomersB and C: diastereomersB and D: diastereomers

Hydrogenation of AlkenesHeterogeneous Catalysis

The characteristic reaction of alkenes is addition to the double bond.

+ A—BC C A C C B

Reactions of Alkenes

+ H—Hs

exergonic H° = –136 kJ/mol

catalyzed by finely divided (heterogeneous) metals

(Insoluble) Pt, Pd, Rh, Ni

C C H C C

Hydrogenation of Ethylene

H2, Pt

Example

CH2H3C

What three alkenes yield 2-methylbutane on

catalytic hydrogenation?

H2, Pt

Problem

Question

Which one of the following terms best applies to the hydrogenation of an alkene in thepresence of finely divided platinum? (in ethanol as solvent)A) anti additionB) concerted reactionC) heterogeneous catalysisD) endothermic reaction

If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is HETEROGENEOUS.

HOMOGENEOUS catalysts also exist.

What advantage might a homogeneous catalyst have?

Catalytic Hydrogenation

Hydrogenation of AlkenesHomogeneous Catalysis

In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand.

A chiral catalyst can produce one desired enantiomer over another.

Asymmetric Hydrogenation

A chiral catalyst allows one enantiomer to be formed.

Some chiral catalysts give better enantioselectivity than others.

BINAP is a chiral ligand that gives very pronounced enantioselectivity.

For any reaction, stereoselectivity can only be occur if at least one reagent (reactant or catalyst) is chiral.

Question

Which of the alkenes below will produce 2-methylbutane on catalytic hydrogenation?

A) 1 and 3B) 1, 2 and 3C) 2 and 4D) 2, 3 and 4

can be used to measure relative stability of isomeric alkenescorrelation with structure is same as when heats of combustion are measured

Heats of Hydrogenation

CH3CH2CH2CH3

119115

Heats of Hydrogenation of Isomers

Ethylene 136

Monosubstituted 125-126

cis-Disubstituted 117-119

trans-Disubstituted 114-115

Terminally disubstituted 116-117

Trisubstituted 112

Tetrasubstituted 110

Heats of Hydrogenation (kJ/mol)

Question

Rank the following alkenes in order of decreasing heat of hydrogenation.

A) 1 > 3 > 2B) 3 > 2 > 1C) 2 > 3 > 1D) 1 > 2 > 3

Match each alkene with its correctheat of hydrogenation.

126 kJ/mol

118 kJ/mol

112 kJ/mol

highest heat ofhydrogenation;least stable isomer

lowest heat ofhydrogenation;most stable isomer

Problem

Question

Which alkene has the lowest heat of hydrogenation?

Stereochemistry of Alkene Hydrogenation

Catalytic Hydrogenation of an Alkene

Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation

(1) syn addition of both H atoms to double bond (adds from the same side)(2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

(1) syn addition of both H atoms to double bond

syn addition anti addition

syn Addition versus anti Addition

CO2CH3

(100%)

H2, Pt

Example of syn-Addition

CO2CH3

H2, Pt

Question

CO2CH3

Select one: True (A) or False (B) for the following statement.

“The hydrogenation product above is chiral.”

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

A reaction in which a single starting materialcan give two or more stereoisomeric productsbut yields one of them in greater amounts thanthe other (or even to the exclusion of the other)is said to be stereoselective.

H3C CH3

H2, cat

Both productscorrespond tosyn additionof H2.

Example of Stereoselective

Reaction

CH3H3C

H2, cat

But only thisone is formed.

Reaction

H3C CH3

CH3H3C

H3C CH3

H2, catTop face of doublebond blocked bythis methyl group

Reaction

H2, cat

H2 adds to bottom face of double bond.

Reaction

CH3H3C

H3C CH3

Electrophilic Addition of Hydrogen Halides to

Alkenes

Electrophilic Additions: Alkenes

+ H—X –

C C H XC C

When the Electrophile is a Hydrogen Halide

CH3CH2 CH2CH3

CH3CH2CH2CHCH2CH3

CHCl3, -30°CC C

Example

Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate.

Mechanism

Electrons flow fromthe system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.

Mechanism

HC C....X:

..:X: –

Regioselectivity of Hydrogen Halide Addition(Markovnikov's Rule)

When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

Markovnikov's Rule

acetic acidBr

CH3CH2CHCH3

Example 1

CH2CH3CH2CHHBr

Markovnikov's Rule

Br(90%)

Example 2

acetic acid

HBrCH3

Markovnikov's Rule

(100%)

Example 3

Markovnikov's Rule

Cl0°C

Question

What is the product of the reaction of 1-methylcyclohexene with HCl?

Protonation of double bond occurs in direction that gives more stable of two

possible carbocations.

Mechanistic Basis for Markovnikov's Rule

In many addition reactions, chirality centers are formed.

There are two possible Markovnikov products:

Stereochemical Aspects

The geometry of the carbocation:

CONCEPTUAL CHECKPOINT 9.6.

Stereochemical Aspects

CH3CH2CHCH3CH2CH3CH2CHacetic acid

Mechanistic Basis for Markovnikov's Rule:Example 1

CH3CH2CHCH3CH2CH3CH2CH

CH3CH2CH—CH3 + Br – +

CH3CH2CH2—CH2

primary carbocation is less stable: not formed

Question

Which carbocation forms when 3-methyl-2-pentene is protonated?

Question

The reaction of 3-methyl-1-butene with HBr produces 2-bromo-3-methylbutane and which other alkyl halide?

secondary carbocation is less stable: not formed

Question

Which alkene reacts with HCl (electrophilic addition) at the fastest rate?

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

HCl, 0°C

CH3CHCH(CH3)2 +

H2C CHCH(CH3)2

+CH3CHC(CH3)2

CH3CHCH(CH3)2

Cl (40%)

CH3CH2C(CH3)2

Cl(60%)

Rearrangements Can Occur

Carbocation Rearrangements

1,2-hydride shift a more stable carbocation

a more stable carbocation1,2-methyl shift

Carbocation Rearrangement Mechanism

a more stablecarbocation

Ring Expansion

NOTE: Carbocation does not always rearrange …

The "peroxide effect"

Free-radical Addition of HBr to Alkenes

acetic acidBr

CH3CH2CHCH3CH2CH3CH2CHHBr

Markovnikov's Rule

CH3CH2CH2CH2Br

Carbocation product :only one in absence of peroxides

Free Radical product only when peroxides added to reaction mixture

CH3CH2CHCH3

CH2CH3CH2CH

Addition of HBr to 1-Butene

CH3CH2CH2CH2Br

only product when peroxides added to reaction mixture

CH2CH3CH2CH

Addition opposite to Markovnikov's rule

Addition of HBr to 1-Butene

Addition of HBr with a regiochemistry oppositeto Markovnikov's rule also occur wheninitiated with light with or without added peroxides.

+ HBrh

Photochemical Addition of HBr

Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism.

Initiation steps:

Mechanism

..O RR O....

O .R ....

..O R.+

+ BrO .R ....

H ....

: +OR ....

H . Br....

Propagation steps: Mechanism

+CH3CH2CH CH2. Br..

.CH3CH2CH CH2 Br:

+.CH3CH2CH CH2 Br:

H Br....

CH3CH2CH2CH2 Br:..

. Br....

(More stable free radical)

Examples of Termination Steps(Initiation Propagation Termination)

Question

What is the correct IUPAC name of the compound isolated from the reaction of2-methyl-2-pentene with HBr in the presence of peroxides?A) 3-bromo-4-methylpentaneB) 3-bromo-2-methylpentaneC) 2-bromo-2-methylpentaneD) 2,3-dibromo-2-methylpentane

Reactions of Alkenes: Addition Reactions

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