Addition Reactions of Alkenes

The majority of the reactions of alkenes that will be described 246 fall intothree basic categories:

1. an electrophile interacts with the alkene π-cloud, activating the alkenecarbons to nucleophilic addition in a second step,

2. syn addition reactions which are additions occurring on one side of thealkene π-cloud, by concerted mechanisms and,

3. oxidative cleavage reactions in which the carbon-carbon double bond iscleaved to form di-carbonyl derivatives (next term).

• Many of these reactions have mechanisms in common, involvingLewis acid-Lewis-base reactions.

• If you can identify the Lewis-acid, Lewis-base and lowest energyintermediates and transition states, you can make good predictionsfor reaction mechanisms and products without memorizing eachreaction.

THIS IS OUR GOAL!!!!!!

2

Everything You Ever Wanted to Know about Additions to Alkenes andThen Some

CH2CH3CH3C

H3CH3C

CH2CH3CH3C

NuE

H

E

H

HHH3C

H3C

ENu

Nu

Nu

Nu

CH2CH3CH3C

E

Nuδ

δ

δ

E E

H

H3CH3C

HNu

E

H

H3CH3C

HNu

Nu attacks on opposite face of E giving anti product

Nu attacks on either face since carbocation is planar, giving both syn and anti products

anti

anti

syn

3

So, an alkene π-cloud, acts as a Lewis base (an electron donor), it donateselectron density to a Lewis acid or electrophile (E) (e.g., a proton, a halogencation (halonium ion), or mercuric ion).

When the nucleophile is of low reactivity or if a very stable carbocationintermediate can be formed, the initial complex may rearrange to form a σ-bond to the Lewis acid, leaving a full carbocation on the adjacent carbon.

• In some cases the symmetrically bound complex of the Lewis acid and theπ-system is a transition state along the path to a carbocation intermediate andin others it is an intermediate, depending upon the details of the Lewis acidand the nucleophile.

If the complex bearing a positive charge is an intermediate, it will highlyreactive towards nucleophiles in the system (anions or water) and willundergoes attack, to form the final addition products.

• In the case where the Lewis acid bridges between the carbons as anintermediate, attack is anti, i.e. from the opposite face of the molecule.

4

Detailed Look into Mechanism of HBr Addition to Alkenes

H3CH3C

CH2CH3CH3C

BrH

H

H

HH3CH3C

HH

Stan

dard

Fre

e En

ergy

Reaction Coordinate

δ

H

Br

δ

δ

Br

• The alkene π-cloud, functioning as a Lewis base (an electron donor),donates electron density to a Lewis acid (in this example, a proton).

The transition state involves bond formation between the carbon andhydrogen AND bond breaking between the hydrogen and Br.

• A full positive charge develops on the left-hand carbon forming a planarcarbocation intermediate and the bromine develops a full negative charge.

• The carbon that gets the hydrogen begins to rehybridize from sp2 to sp3.

• Transition states that are polar are stabilized by polar solvents that orienttheir dipoles in a manner so as to stabilize any charge that builds up. Thisshould be obvious by now.

5

Stability of Carbocations

Alkyl groups are inductively donating, and thus carbocations are stabilizedby substitution with such groups. Thus, in simple unstrained non-conjugatedsystems, without adjacent heteroatoms, the order of stability of carbocationswill be tertiary > secondary > primary.

This can be explained by the concepts of hyperconjugation wherein a C-Hσ-bond aligns with a dihedral angle of nearly 0º with the empty p-orbital andcan donate some electron density to the empty orbital thereby stabilizing it.

R'

R

HH

H

CH3+ CH3CH2

+ (CH3)2CH+ (CH3)3C+

Gas phase stability relative to(CH3)3C+ (kcal/mol), +82.7 +44.8 +17.3 0The difference of stability in solution will be less due to solventstabilization!

6

Resonance stabilization of Carbocations

Positive charge is stabilized by resonance. For example allyl cation (left) ismuch more stable than propyl cation

Gas phase stability relative to (CH3)3C+ (kcal/mol), +24.1 +38.1

Likewise benzylic cations are very stable relative to alkyl cations.

R'

R

R'

R

R'

R

...etc

Gas phase stability relative to (CH3)3C+ (kcal/mol), where R, R’ = H, +7.1

Since tertiary centers have no attached hydrogens, secondary centers haveone and primary centers have two, there is an apparent inverse relationshipbetween the "number of attached hydrogens" and the likelihood that thecarbocation will form at that center.

7

Markovnikov's Rule

This is the origin of Markovnikov's Rule, which states that... ...in the addition of HX to an alkene, the proton will attach to thecenter having the greatest number of hydrogens...often restated as "them that has, gets".• While the rule is a useful guide, you should remember that the selectivity is

actually to place the carbocation on the carbon that can best stabilize thecharge.

Once the carbocation is formed, the most favorable reaction will involve theaddition of a nucleophile to form an sp3 center.• In the reaction with HX (HCl and HBr), the most nucleophilic species in

the system will be the chloride or the bromide anions.

Attack of these on the planar (sp2) carbocation can occur from either above,or below the plane defined by the sp2 center, and the net addition of HX cantherefore occur either syn (cis; on the same side) or anti (trans; on theopposite side), relative to the hydrogen atom.

H3CH3C

CH2CH3CH3C

BrD

HH

D

HH

H3CH3C

DBr

Br

Br

CH2CH3CH3C

DBrδ

δδ

DH

H3CH3C

HBrantisyn

top face attack gives syn

bottom faceattack gives anti

8

Examples of Hammond PostulateThe acid-catalyzed addition of water to alkenes, as well as the addition of HXare examples of reactions involving carbocation intermediates

BrBr + HBr

T.S. 3º

T.S. 1º

ΔG‡

3º carbocation

ΔG‡

1º carbocation

Stan

dard

Fre

e En

ergy

Reaction Coordinate

• The addition of halogen acids to alkenes is a stepwise process thatgenerally involves a solvent-equilibrated carbocation intermediate.

• If the alkene is asymmetrical, two carbocation intermediates can be formed

that have different energies, as for 2-methylpropene shown above..

• The carbocation that is best able to stabilize the cationic center will, belower in energy. Here the 3o carbocation on the left in red.

• In general, the transition state leading to the more stable intermediate willbe of lower energy and will be the preferred pathway. (the red path)

9

Rearrangement Reactions of Alkenes

Once an alkene is protonated to form the initially more stable cation (throughthe lower energy transition state), it is possible that there is a kineticallyaccessible cation which is even lower in energy that can be created by arearrangement. Such rearrangements occur typically by a 1,2-alkyl or 1,2-hydride shift.

Reaction involving a 1,2-methyl shift:

H Cl

H3CH

CH3

ClCl+

secondary cation tertiary cationOR rearrangement

+

minor product major product

Clnucleophilic attack

nucleophilic attack

k

Clnucleophilic attack

1,2 alkyl shiftk

Notice that the reaction yields two products. The distribution of the productsis determined by the relative rates for trapping by the nucleophile (to give thenon-rearranged product) or by the intramolecular 1,2 alkyl shift, followed bytrapping by the nucleophile (to give the rearranged product).

10

Since the rearrangement simply involves a migration of the methyl group withits electrons this reaction can be surprisingly fast and can compete withtrapping by the nucleophile.

The actual distribution of products will be sensitive to the details of thereactants and the reaction conditions.• The nucleophiliticity of the nucleophile will be important in determining

the relative rates for rearrangement and trapping.

Rearrangement will occur when the mechanism involves the formation ofcarbocation intermediates and when the rearrangement yields a significantlymore stable carbocation. For example a secondary carbocation rearranges toa tertiary or resonance stabilized carbocation as shown for the alkyl shiftearlier and the hydride shift below.

Reactions involving a 1,2-hydride shift:

HH Cl

HH

H

Cl+

+ Clsecondary cation tertiary cation

1,2 hydride shift

Rearrangement

In general 1,2 hydride shifts can be even faster than 1,2 methyl shifts since thehydride is much smaller.

11

Hydration of Alkenes

If an alkene is treated with a trace of acid in water as a solvent, then ratherthan the conjugate base of the acid acting as a nucleophile, water will act asthe nucleophile because it is present in huge excess. Subsequent loss of aproton (to an additional molecules water in the system) yields an alcohol andregenerates a proton that can attack another molecule of an alkene.

In this manner, the proton is not consumed in the reaction but it greatlyenhances the rate of addition of water to the alkene by providing a lowerenergy reaction pathway. Thus the proton fulfills the requirements of being acatalyst for this reaction.

+ H2O

OH

CH3

H

CH3H

X

H

O

H

O

H

H

H+ H

OH

more stable intermediate

Note above the more stable tertiary carbocation is formed.

12

+ H2O

OH

H2C

HCH2

H

X

H+

Note above the more stable carbocation is formed.

+ H2O

OH

CH3

H

CH3

H

H

1,2 hydride shift

H+

Because of the possibility of rearrangement reaction, hydration of alkenes isnot a preferred procedure to make alcohols in the laboratory, except when itis clear that only the desired carbocation will be formed.

Later in the course we will explore alternative routes to synthesizing alcohols.

13

Addition of X2, Hg(CH3COO)2 (Hg(OAc)2) and HOX to Alkenes.

The addition of halogens, mercuric acetate and hypohalous acids to alkenesare all useful reactions.

CH2CH3CH3C

H

H3C

X

XH3C H

H

H3C

HO

XH3C H

H

H3C

HO

Hg(OAc)H3C H

where X = Cl , Br

X2

HOX, or X2 in H2O

1) Hg(OAc)2 in H2O

2) NaBH4

Addition of Halogen

Addition of Hypohalous Acid

Addition of Mercuric Acetate

H

H3C

HO

HH3C H

Overall Reaction: Oxymercuration-ReductionReduction

14

In each case the reactions proceed by a common initial mechanism:1 ) Coordination of the Lewis acid to the π-system to make a cyclic

intermediate, with concomitant expulsion of a leaving group from theinitial electrophile.

2) Attack of a nucleophile (either the leaving group or solvent) from theopposite face of the alkene at the site that will give the most stabilizedtransition state, hence the anti addition product is formed.

CH2CH3CH3C

H3CH3C

CH2CH3CH3C

NuE

HH H

H3C

Nu

Nu

E

E

H3CH3C

HH

Nu

E

δ

δ

Nu

EH3C H

H3C

H

Nu

EHH3C

X

Note that the δ+ by the carbons is to remind you of the build up of charge positve in the transition state.

The reaction will follow the path that gives the more stable transition state (i.e. more stable carbocation).

Hence the nucleophile will be bound to the more substituted carbon.

Anti- addition

15

Addition of Halogens to AlkeneAn electrophilic halogen is attacked by the alkene π-system, to form thecyclic "halonium" ion intermediate (i.e., bromonium or chloronium ion), withconcomitant loss of a halogen anion (i.e., bromide or chloride).

This cationic intermediate is highly electrophilic and reacts rapidly with thehalide anion that was formed in the previous step.

BrBr

H3C

Br

H3C

Br

Br

H3C

H H H

Br

Bromonium ion

Anti-addition

The halonium ion effectively blocks one face of the intermediate from attackby halide. Therefore the halide anion attacks from the opposite face of thehalonium ion to form the trans-1,2-dihalide. Thus in the above reactionplease note the stereochemistry, i.e. that the addition is anti across the doublebond.

This is an example of a nucleophilic substitution reaction in which anucleophile attacks at the carbon and displaces the leaving group in a single,smooth, concerted process without formation of a carbocation intermediate.Thus, rearrangement reactions are not observed.

16

Notes:Thus, the addition of Br2 to 1-methylcyclopentene has anti stereochemistry.

BrBr

H3C

Br

H3C

Br

Br

H3C

H H H

Br

Bromonium ion

Anti-addition

It is important to choose a solvent that is unreactive with Br2 or Cl2 whenperforming these reactions, typically dichloromethane (CH2Cl2 (A.K.A.methylene chloride) or carbontetrachloride (CCl4) are used. Sale of CCl4 hasbeen severely restricted due to environmental reasons, and thus its use willbecome less and less common.

Bromine reacts extremely quickly with alkenes, and, as a result, its red coloris completely discharged immediately upon contact with a solution of analkene. Discoloration of Br2 is used as a qualitative test for alkenes.

17

Formation of Halohydrins

CH2CH3CH3C

H

H3C

HO

XH3C H

HOX, or X2 in H2O

N

O

O

Br

N-bromosuccinimde, in H2O, DMSO

SH3C CH3

O

NBS

a Halohydrin

OR

As with the addition of halogens, the addition of hypohalous acid (HOX) toalkenes is a stepwise process that also involves the "halonium" ionintermediate.

The mechanism is completely analogous to the addition of Br2.

BrOH

H3C

Br

H3C

Br

HO

H3C

H H H

OH

18

Notes:

BrOH

H3C

Br

H3C

Br

HO

H3C

H H H

OH

• If the alkene is asymmetrical, hydroxide can potentially attack at eithercarbon.

• In general, hydroxide anion will attack the carbon which would form themost stable carbocation, (i.e. carbon most able to stabilize δ+ charge intransition state).

• This determines the regiochemical selectivity of the reaction.

Attack of hydroxide is by a nucleophilic substitution mechanism giving theanti addition product.

• Note the use of NBS. This is used because NBS is a solid that slowly,effectively liberates Br+ and is easier to use than bromine.

• DMSO is used as solvent because, in general, alkenes are relativelyinsoluble in water, yet DMSO is both a good solvent for alkenes andwater-miscible..

19

Oxymercuration-Reduction of Alkenes to Give Alcohols.The reaction of alkenes with mercuric acetate follows the general mechanismfor Lewis acid activation of alkene addition reactions.

HgOAcOAc

H3C

HgOAc

H3CH H

H2O

H2O, THF

HgOAc

O

H3C

HH H

OAc

HgOAc

HO

H3C

H

Note that the large excess of water leads to attack by water at the oppositeface and deprotonation gives an organomercurial that is stable (and toxic).

This species can be reduced with sodium borohydride, which replaces themercury atom with a hydrogen atom.

NaBH4

H

HO

H3C

H

+ Hg

HgOAc

HO

H3C

H

This reaction sequence occurs without formation of carbocation intermediateand therefore, without rearrangements to give Markovnikov addition ofwater.

Note the use of the isotopic label to illustrate something about the mechanismof the reaction (i.e. the addition of hydride (deuteride)). Deutrium isincorporated to both the same and opposite face of the alkene.

HgOAc

HO

H3C

H

NaBD4

D

HO

H3C

H

+ Hg

H

HO

H3C

D

+

20

Notes:Tetrahydrofuran, THF (shown below) is used to solubilize the alkene in thewater.

O

THF

If an alcohol is used as the solvent instead of water then an ether will beformed as the final product as shown below, because ROH will act as thenucleophile instead of water.

HgOAcOAc

H3C

HgOAc

H3C

HgOAcH3C

H HH

ROH, THF

R

OR H

-H+

O

HH3C

HRO

NaBH4

21

HydroborationImportant reaction! Discovered by H. C. Brown who won a Nobel Prize in1979 for this reaction.

H3C H3C H OH

32) H2O2, HO-

1) BH3

BH3 acts as a strong Lewis acid and adds a B-H bond to an alkene. All threeboron hydride bonds can add across three alkenes to give the trialkyl boraneshown below. (Note that the boron is attached to the less substituted carbon).

+ BH3

H3C

BH H

CH3

B

CH3

H3C

H3CB

CH3

H3C

H3CH

H3C

H

H

HHH

H

Notes:

In practice, typically a THF or diethyl ether complex of BH3 is used becauseBH3 is so reactive it does not exist as such (it dimerizes). The Lewis acid-Lewis base complex with THF or diethyl ether “tames” it a bit.

22

Mechanism of the Hydroboration Reaction

The initial reaction of BH3 with a substituted alkene can occur in two ways, leading to two different transition states wherein the boron atoms is nearer themore substituted carbon, or the less, substituted carbon.

The reaction occurs in a concerted manner without the appearance ofcarbocation intermediates. Thus, bonds are forming between the hydrogen inthe borane and one of the carbons of the alkene and between the boron atomand the other carbon of the alkene. At the same time, the B-H bond and theπ-bond are being broken.

Therefore the reaction occurs without rearrangements.

The overall reaction of hydroboration followed by reaction with hydrogenperoxide is complementary to the oxymercuration reaction in that it gives theopposite regiochemical alcohol, i.e. anti- Markovnikov addition of water.

23

Mechanism of Hydroboration

Because the boron is Lewis acidic, it develops a partial negative charge andthe carbon to which the hydrogen will be bound gains partial positive charge.Accordingly, it is electronically more stable to bond the boron to the lesssubstituted carbon. In addition, steric repulsion between the boron and themore substituted carbon also favors bonding the boron to the less substitutedcarbon atom of the alkene

.

H BH2

H3C H3C

BH2

H3C

H

H H H

H BH2

H3C H

H2B HHH2B

H3C H

H

H3C

H2B

H

more favored on both steric and electronic grounds

less favored on both steric and electronic grounds

δ

δ

δ

δ

Subsequent reaction with basic hydrogen peroxides cleaves the boron carbonbond to give a stereochemistry of the addition reaction that is syn (cis) and aregiochemistry of the product is generally anti-Markovnikov.

24

Oxidative Decomposition of an Organoborane to an AlcoholThe organoborane that is formed can be oxidized by alkaline peroxide toform the alcohol by the following mechanism:

O OH

HO O

H

+ OH + H2O

pKa = 14 pKa = 15.5First, hydroxide deprotonates the peroxide.

The deprotonated peroxide anion (a Lewis base) attacks the Lewis acidicorganoborane. It forms a tetrahedral intermediate that rearranges by alkylmigration to oxygen with concomitant loss of hydroxide. (Note that thisreaction is analogous to carbocation rearrangements that we have alreadyexamined.)

R BRRO O

HO

RBR

R

OH

R BROR + OH

This reaction can occur two more times to give B(OR)3 as follows:

R BRORO O

HR B

OROR

R BORORO O

HO

RBRO

RO

OH

RO BOROR

O

RBR

RO

OH

+ OH

+ OH

The resulting borate ester is rapidly hydrolyzed by the alkaline conditions:

RO BOROR

OH

ORBRO

RO

H OHO B

OHOH

H2O

+ 3 ROH

25

Comparison of Oxymercuration-reduction and Hydroboration-oxidation Reactions

Regiochemistry:anti-Markovnikov MarkovnikovStereochemistry: syn addition anti addition (first step)

In both reactions the electrophilic species is attacked by the π-cloud.

For hydroboration there is a concerted addition of the B-H bond across thedouble bond. In the transition state, positive charge builds up on the carbonwhich receives the hydrogen. Thus, the carbon atom that is better able tostabilize a δ+ charge will get the hydrogen.

This is typically the more sterically congested carbon. Since boron is largerthan hydrogen, this also favors boron bonding to the less substituted carbon.Thus both electronic and steric arguments favor the same regiochemistry ofaddition.For oxymercuration, the electrophile Hg atom is attacked by the π-systemforming a bridging mercurium ion with concomitant loss of acetate. In thenext step there is a nucleophilic attack of OH- from the opposite face of thealkene. In the transition state for the nucleophilic displacement reaction, thecarbon that will bond to the OH- develops a δ+ charge. If there are twopossible regiochemical addition products, the transition state that is better ableto stabilize a δ+ charge will lead to the major product.

H

OHH H

OH

H

1) BH3 THF

2) HOOH, OH

1) Hg(OAc)2 H2O

2) Na BH4

26

Radical Reactions

To date most of the compounds and intermediates that we have consideredhave an even number of electrons--eight if they have a complete octet, six ifthey are Lewis acids (except for H+ which has 0).

Radical species have an odd number of electrons. We briefly mentionedthem when considering a thermodynamic cycle to predict acidities.

H Br H + Br

H + Br

heterolytic cleavage

homolytic cleavage

note "fish hook" arrow formalism to indicate motion of one electon

H Br

Peroxides are known to cleave by homolytic cleavage.

OO

Ohomolytic cleavage

heat or light

2

The radical species thus created tend to be very reactive and can recombine toform a bond. The ΔH0 for recombination (i.e. two radicals forming a bond) isvery negative.

Br Br Br Br

27

A radical can abstract an atom generating another radical:

H BrO OH + Br

A radical can also add to a double bond to generate an alkyl radical:

H2C CH2O O CH2CH2

A radical can also cleave in an intramolecular (within one molecule) manneras shown below, by a β scission reaction.

R R

αβ

H2C CH2+

β refers to the position of the carbon relative to the reactive species (theradical carbon): the adjacent carbon is the α carbon, and as one moves awayfrom that site each carbon is assigned a higher letter in the Greek alphabet.

For a radical abstraction or addition reaction to effectively compete with arecombination reaction, the ΔH0 must not be significantly positive.

• If ΔH0 is positive then recombination (i.e. two radicals forming abond for which ΔH0 is very negative) will preferentially occur.

Other factors disfavoring radical recombination reactions are the conditions ofthe reaction.

• In general the reaction conditions are designed such that the radicalsare always present in very low concentration relative to other reactivespecies in the solution.

28

Radical Chain Reactions

Radical reactions typically have three steps, which taken together are calledradical chain reactions.

1. Initiation. In this step reactive radicals are created.• Homolysis, abstraction and addition reactions, can be the first steps

of a cycle in which radicals are generated (initiation). • Typically, bonds that are very weak are broken in the initial

homolysis reactions.2. Propagation. In these steps radicals react to create new chemically

equivalent radicals (or nearly equivalent) to the previous radical.• A radical generated in the initiation step reacts with a closed shell

species in solution by addition or abstraction to make and/or break abond, regenerating a reactive radical.

• The reaction can happen many times until the concentration of thereactant is very low.

3. Termination. In this step two radicals combine to make a covalent bonddestroying both radicals.

• The reaction is very exothermic relative to propagation steps.• Termination steps occur when the concentration of the radical

becomes significant relative to the concentration of the reactants.

29

Free radical Initiated Polymerization

Many polymerizations occur by radical chain reactions and are of industrialimportance. Consider the free radical polymerization steps for ethylene:

Initiation: Tert-butoxy radical is created by homolysis of di-tert-butylperoxide. This radical initiates a polymerization reaction by reactingwith a molecule of ethylene to create an alkyl radical.

H2C CH2O O CH2CH2

Propagation: This alkyl radical can react with a molecule of ethylenegenerating a new alkyl radical.

This reaction can happen many times in what are known as propagation stepsto yield a polymer with a terminal radical. Note that in the propagation stepthe radical always reacts with a closed shell species. This is called a radicalchain reaction.

O CH2CH2

O (CH2CH2)n H2C CH2 O (CH2CH2)n+1

H2C CH2 O CH2CH2

H2C CH2n-1

30

Termination: If the radical reacts with another radical, a bond will be formedannihilating both radicals and the reaction will stop. This is called atermination reaction.

O (CH2CH2)n+1 O O (CH2CH2)n+1 O

Summary: Radical chain reaction cycles thus involve:i) initiation steps that generate the active radicals;ii) propagation steps, (radical chain), to make new bonds and regenerate

active radicals, andiii) termination steps that end the cycle and destroy the radicals.

31

Radical Addition of HBr to Alkenes

If HBr is added to an alkene in the presence of traces of peroxides theregiochemistry of the product is generally anti-Markovnikov.

CH2CH3CH3C

H

H

H3CH3C

BrH

HBr, peroxides (trace)

The addition of HBr to an alkene in the presence of peroxides occurs via aradical chain reaction.

Initiation: Tert-butoxy radical is created by homolysis of di-tert-butylperoxide. This radical reacts with HBr: the most favorable initiationreaction is abstraction of a hydrogen atom from HBr to generate the bromineradical.

H BrO O H + Br

32

The bromine radical is electrophilic and can react with the alkene π-system,forming a σ-bond to the bromine and leaving an unpaired electron (a radical)and the remaining carbon from the alkene. • In the case of unsymmetrical alkenes, two radical intermediates can be

formed. • In general, the pathway leading to the more stable intermediate will have a

lower energy transition state (Hammond Postulate) and will be thepreferred pathway, which here, is the path to the right. (Patience--I’llexplain).

H3CH3C

CCH3CH3C

HBr

H

Br

HH

Br

top arrows

HHH3C

CH3

Br

bottom arrowsX

HHH3C

CH3

Br

H3CH3C

HBr

H

Like carbocation intermediates, carbon radicals are planar (sp2) and areelectrophilic (which should give you some idea about why the addition of Brradical takes the red pathway!).

33

Radical Addition of HBr to Alkenes (cont.)

Propagation: This radical can then abstract a hydrogen atom from anothermolecule of HBr to generate the HBr addition product (a closed shellspecies):

H3CH3C

HBr

H H

H

H3CH3C

BrH

H

Br

H

H

HH3C

BrH3C

+ + Br

and a new Br radical which can continue the chain reaction (n times) byreacting with another molecule of alkene:

Br

CCH3CH3C

HH

H3CH3C

HBr

H

H Br

H

H

H3CH3C

BrH

H

H

H

H3C Br

+

n times

H3C

34

Radical Addition of HBr to Alkenes (cont.)

Termination steps:

Br Br Br Br

CH3H3C

H

BrH

H3C CH3

H

BrH

BrBr

H3CH3C

HBr

H

Br

H

H

BrH3C

BrH3C

35

Stability of Radicals

The resulting radical is formed on the carbon of the alkene that is best able tostabilize the electrophilic site (the unpaired electron).• In simple unstrained non-conjugated systems, without adjacent

heteroatoms, the order of stability of carbon radicals parallels that ofcarbocations, with tertiary > secondary > primary > methyl.

CH3 CH3CH2

(CH3)2CH (CH3)3C

Stability relative to(CH3)3C (kcal/mol), 11.9 +5.0 +1.9 +0

This can also be explained hyperconjugation wherein aC-H σ-bond donates some electron density to thepartially filled orbital thereby stabilizing it.• Stabilization is not as sensitive to alkyl groups on

adjoining carbons, since there isn’t a positive charge

Radicals are stabilized by resonance. • For example allyl radical (left) is more stable than propyl radical.

Stability relative to (CH3)3C (kcal/mol), –6.9 +4.7

• Likewise benzyl radical is stable relative to alkyl radicals.

H

H

H

H

H

H

...etc

Stability relative to (CH3)3C (kcal/mol), –5.2

R'

R

HH

H

36

Catalytic Hydrogenation of Alkenes

Catalytic hydrogenation of alkenes produces the corresponding alkane, withsyn (cis) addition of hydrogen.• The reaction requires a metal catalyst, usually Pt, Pd or Ni, and the

mechanism involves adsorption of hydrogen to the metal surface, followedby adsorption of the alkene (probably through Lewis acid complex of theπ-system).

• Hydrogen transfer occurs and syn addition

+ H2

HH

syn addition

H H H HH H

Pd(catalyst)

Reactants

Products

uncatalyzed

catalyzed

A catalyst can provide an alternative mechanism for a reaction which lowers the transition state energy for the rate determining step of the reaction thereby enhancing the rate of the reaction

Catalysts only change the barrier of the reaction, not the energy of theproducts and reactants. Therefore a catalyst can accelerate the rate, but notchange the equilibrium constant for the reaction.

37

Notes:One criterion for a catalyst is that at the end of the reaction the catalyticspecies is regenerated in an unchanged state. Therefore the catalyst can be used again to catalyze a reaction for another setof reactants.

Each time a catalyst goes through a complete reaction path, generates aproduct, and regenerates itself is called a catalytic cycle.

The number of times that a catalyst proceeds through a complete cycle iscalled the number of turnovers.

Enzymes are naturally occurring catalysts that are typically made of proteins.RNA has been also shown in some cases to have catalytic activity. Thisactivity is thought to be of critical importance to the early development of life.

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StabilityHeats of Hydrogenation give an estimate of relative stabilities.

R2C=CR2 + H2 → HCR2-CR2H;• The more negative the ΔH0

hydrog the less stable the starting alkene.

H

HH3C

H CH3

HH3C

H CH3

HH3C

H3C CH3

CH3H3C

H3C

HH

H HΔH0

hydrog (Kcal/mol) –32.8 –30.1 –27.6 –26.9 –26.6relative stability un < mono < di < tri < tetra

• Greater substitution leads to morestability due to hyperconjugation:The stabilizing interaction between afilled σ-bond with and unfilled p-type orbital.

Butenes

CH3

HH3C

HH

CH3H3C

HH

HH3C

H3CH

HH3CH2C

H

ΔH0hydrog (Kcal/mol) –30.1 –28.4 –28.6 –27.6

relative stability mono < 1,1-di ≅ cis < trans

• And in general sp2-sp3 bonds are stronger than sp3-sp3 bonds, thus mono <than 1,1 di- for example.

There are exceptions, for example when dealing with very bulky substituents,but we will not worry about them here.

HAntibonding C-C π-orbital (unfilled)

Bonding C-Horbital (filled)