slide1
Download
Skip this Video
Download Presentation
Reactions of Alkenes

Loading in 2 Seconds...

play fullscreen
1 / 81

6.1 characteristic reactions, table 6-1 - PowerPoint PPT Presentation


  • 384 Views
  • Uploaded on

Reactions of Alkenes. Chapter 6. 6.1 Characteristic Reactions, Table 6-1. The reaction typical of alkenes is addition. Characteristic Reactions, Table 6-1. 6.2 Reaction Mechanisms. A reaction mechanism describes how a reaction occurs:

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about '6.1 characteristic reactions, table 6-1' - arleen


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
6 1 characteristic reactions table 6 1
6.1 Characteristic Reactions, Table 6-1
  • The reaction typical of alkenes is addition
6 2 reaction mechanisms
6.2 Reaction Mechanisms
  • A reaction mechanism describes how a reaction occurs:
    • which bonds are broken and which new ones are formed
    • the order and relative rates of the various bond-breaking and bond-forming steps
    • if in solution, the role of the solvent
    • if there is a catalyst, the role of a catalyst
    • the position of all atoms and energy of the entire system during the reaction
gibbs free energy
Gibbs Free Energy
  • Gibbs free energy change, DG0 (standard state): a thermodynamic function relating enthalpy, entropy, and temperature
    • exergonic reaction: a reaction in which the Gibbs free energy of the products is lower than that of the reactants; the position of equilibrium for an exergonic reaction favors products
    • endergonic reaction: a reaction in which the Gibbs free energy of the products is higher than that of the reactants; the position of equilibrium for an endergonic reaction favors starting materials
gibbs free energy6
Gibbs Free Energy
  • a change in Gibbs free energy is directly related to chemical equilibrium:
  • summary of the relationships between DG0, DH0, DS0, and the position of chemical equilibrium
energy diagrams
Energy Diagrams
  • Enthalpy change, DH0: the difference in total bond energy between reactants and products
    • a measure of bond making (exothermic) and bond breaking (endothermic)
  • Heat of reaction, DH0:the difference in enthalpy between reactants and products
    • exothermic reaction:a reaction in which the enthalpy of the products is lower than that of the reactants; a reaction in which heat is released
    • endothermic reaction:a reaction in which the enthalpy of the products is higher than that of the reactants; a reaction in which heat is absorbed
a energy diagrams
A. Energy Diagrams
  • Energy diagram: a graph showing the changes in energy that occur during a chemical reaction
  • Reaction coordinate:a measure in the change in positions of atoms during a reaction
activation energy
Activation Energy
  • Transition state:
    • an unstable species of maximum energy formed during the course of a reaction
    • a maximum on an energy diagram
  • Activation Energy, G‡:the difference in Gibbs free energy between reactants and a transition state
    • if G‡ islarge, few collisions occur with sufficient energy to reach the transition state; reaction is slow
    • if G‡ is small, many collisions occur with sufficient energy to reach the transition state; reaction is fast
energy diagram fig 6 1
Energy Diagram, Fig. 6-1
  • A one-step reaction with no intermediate
energy diagram fig 6 2
Energy Diagram, Fig. 6-2
  • A two-step reaction with one intermediate
b developing a reaction mechanism
B. Developing a Reaction Mechanism
  • How it is done
    • 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
why mechanisms
Why Mechanisms?
  • they are the framework within which to organize descriptive chemistry
  • they provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems
  • they are tools with which to search for new information and new understanding
  • they help us understand what events occur in the pathway from reactants to products
6 3 electrophilic additions
6.3 Electrophilic Additions
  • An alkene reacts using electrons from the pi bond as a nucleophile, the other reactant is an electrophile.
    • hydrohalogenation using HCl, HBr, HI
    • hydration using H2O in the presence of H2SO4
    • halogenation using Cl2, Br2
    • halohydrination using HOCl, HOBr
    • oxymercuration using Hg(OAc)2, H2O followed by reduction
a addition of hx
A. Addition of HX
  • Carried out with pure reagents or in a polar solvent such as acetic acid
  • Addition is regioselective
    • regioselective reaction:an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed
    • Markovnikov’s rule:in the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of Hs.
hbr 2 butene
HBr + 2-Butene
  • A two-step mechanism

Step 1: proton transfer from HBr to the alkene gives a carbocation intermediate

Step 2: reaction of the sec-butyl cation (an electrophile) with bromide ion (a nucleophile) completes the reaction

hbr 2 butene fig 6 4
HBr + 2-Butene, Fig. 6-4
  • An energy diagram for the two-step addition of HBr to 2-butene
    • the reaction is exergonic
carbocations
Carbocations
  • Carbocation:a species in which a carbon atom has only six electrons in its valence shell and bears positive charge
  • 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-loving
    • Lewis acids
carbocations fig 6 3
Carbocations, Fig. 6-3
  • bond angles about a positively charged carbon are approximately 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
carbocation stability
Carbocation Stability
  • a 3° carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation
  • a 2° carbocation is, in turn, more stable than a 1° carbocation,
  • methyl and 1° carbocations are so unstable that they are never observed in solution
carbocation stability21
Carbocation Stability
  • relative stability
  • methyl and primary carbocations are so unstable that they are never observed in solution
carbocation stability22
Carbocation Stability
  • we can account for the relative stability of carbocations if we assume that alkyl groups bonded to the positively charged carbon are electron releasing and thereby delocalize the positive charge of the cation
  • we account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation
the inductive effect fig 6 5
The Inductive Effect, Fig. 6-5
  • the positively charged carbon polarizes electrons of adjacent sigma bonds toward it
  • the positive charge on the cation is thus localized over nearby atoms
  • the larger the volume over which the positive charge is delocalized, the greater the stability of the cation
hyperconjugation fig 6 6
Hyperconjugation, Fig. 6-6
  • involves partial overlap of the -bonding orbital of an adjacent C-H or C-C bond with the vacant 2p orbital of the cationic carbon
  • the result is delocalization of the positive charge
b addition of h 2 o
B. Addition of H2O
  • addition of water is called hydration
  • acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond
  • HOH adds in accordance with Markovnikov’s rule
addition of h 2 o

slow, rate

determining

C

H

C

H

=

C

H

C

H

C

H

C

H

3

2

3

3

o

A 2

carbocation

C

H

C

H

C

H

C

H

C

H

C

H

O

-

H

3

3

3

3

Addition of H2O
  • Step 1: proton transfer from H3O+ to the alkene
  • Step 2: reaction of the carbocation (an electrophile) with water (a nucleophile) gives an oxonium ion
  • Step 3: proton transfer to water gives the alcohol

+

+

:

:

:

O

H

+

H

O

H

+

H

H

intermediate

+

:

fast

:

+

+

O

H

:

H

H

An oxonium ion

c carbocation rearrangements
C. Carbocation Rearrangements
  • In electrophilic addition to alkenes, there is the possibility for rearrangement
  • Rearrangement:a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material
carbocation rearrangements
Carbocation Rearrangements
  • in addition of HCl to an alkene
  • in acid-catalyzed hydration of an alkene
carbocation rearrangements29

C

H

C

H

3

3

C

H

C

-

C

H

C

H

C

H

C

-

C

H

C

H

3

3

3

3

Carbocation Rearrangements
  • the driving force is rearrangement of a less stable carbocation to a more stable one
  • the less stable 2° carbocation rearranges to a more stable 3° one by 1,2-shift of a hydride ion

fast

+

+

H

H

A 3° carbocation

carbocation rearrangements30

C

H

C

H

3

3

C

H

C

-

C

H

C

H

C

l

C

H

C

-

C

H

C

H

3

2

3

3

2

3

C

l

Carbocation Rearrangements
  • reaction of the more stable carbocation (an electrophile) with chloride ion (a nucleophile) completes the reaction

-

:

fast

:

:

+

:

+

:

:

:

2-Chloro-2-methylbutane

d addition of cl 2 and br 2
D. Addition of Cl2 and Br2
  • carried out with either the pure reagents or in an inert solvent such as CH2Cl2
  • addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane
  • addition occurs with anti stereoselectivity; halogen atoms add to opposite faces of the double bond
  • this selectivity discussed in detail in Section 6.7
addition of cl 2 and br 2
Addition of Cl2 and Br2
  • Step 1: formation of a bridged bromonium ion intermediate
addition of cl 2 and br 233
Addition of Cl2 and Br2
  • Step 2: attack of halide ion (a nucleophile) from the opposite side of the bromonium ion (an electrophile) opens the three-membered ring to give the product
addition of cl 2 and br 234
Addition of Cl2 and Br2
  • for a cyclohexene, anti coplanar addition corresponds to trans diaxial addition
  • the initial trans diaxial conformation is in equilibrium with the more stable trans diequatorial conformation
  • because the bromonium ion can form on either face of the alkene with equal probability, both trans enantiomers are formed as a racemic mixture
e addition of hocl and hobr
E. Addition of HOCl and HOBr
  • Treatment of an alkene with Br2 or Cl2 in water forms a halohydrin
  • Halohydrin:a compound containing -OH and -X on adjacent carbons
addition of hocl and hobr
Addition of HOCl and HOBr
  • reaction is both regiospecific (OH adds to the more substituted carbon) and anti stereoselective
  • both selectivities are illustrated by the addition of HOBr to 1-methylcyclopentene
  • to account for the regioselectivity and the anti stereoselectivity, chemists propose the three-step mechanism in the next screen
addition of hocl and hobr37
Addition of HOCl and HOBr

Step 1: forms a bridged halonium ion intermediate

Step 2: attack of H2O on the more substituted carbon opens the three-membered ring

addition of hocl and hobr38
Addition of HOCl and HOBr
    • Step 3: proton transfer to H2O completes the reaction
  • As the e-plot map on the next screen shows
    • the C-X bond to the more substituted carbon is longer than the one to the less substituted carbon
    • because of this difference in bond lengths, the transition state for ring opening can be reached more easily by attack of the nucleophile at the more substituted carbon
addition of hocl and hobr39
Addition of HOCl and HOBr
  • bridged bromonium ion from propene
f oxymercuration reduction
F. Oxymercuration/Reduction
  • Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond
    • oxymercuration
    • reduction
oxymercuration reduction
Oxymercuration/Reduction
  • an important feature of oxymercuration/reduction is that it occurs without rearrangement
  • oxymercuration occurs with anti stereoselectivity
oxymercuration reduction42
Oxymercuration/Reduction
  • Step 1: dissociation of mercury(II) acetate
  • Step 2: formation of a bridged mercurinium ion intermediate; a two-atom three-center bond
oxymercuration reduction43
Oxymercuration/Reduction
  • Step 3: regioselective attack of H2O (a nucleophile) on the bridged intermediate opens the three-membered ring
  • Step 4: reduction of the C-HgOAc bond
oxymercuration reduction44
Oxymercuration/Reduction
  • Anti stereoselective
    • we account for the stereoselectivity by formation of the bridged bromonium ion and anti attack of the nucleophile which opens the 3-membered ring
  • Regioselective
    • of the two carbons of the mercurinium ion intermediate, the more substituted carbon has the greater degree of partial positive character
    • alternatively, computer modeling indicates that the C-Hg bond to the more substituted carbon of the bridged intermediate is longer than the one to the less substituted carbon
    • therefore, the ring-opening transition state is reached more easily by attack at the more substituted carbon
6 4 hydroboration oxidation

C

H

C

H

2

3

3

C

H

=

C

H

C

H

C

H

2

2

3

2

C

H

C

H

2

3

Triethylborane

(a trialkylborane)

2

B

H

B

H

3

2

6

D

iborane

6.4 Hydroboration/Oxidation
  • Hydroboration:the addition of borane, BH3, to an alkene to form a trialkylborane
  • Borane dimerizes to diborane, B2H6

H

H

B

+

B

H

Borane

Borane

hydroboration oxidation
Hydroboration/Oxidation
  • borane forms a stable complex with ethers such as THF
  • the reagent is used most often as a commercially available solution of BH3 in THF
hydroboration oxidation47

H

C

B

H

3

3

C

H

3

B

R

2

(Syn addition of BH

)

3

(R = 2-methylcyclopentyl)

Hydroboration/Oxidation
  • Hydroboration is both
    • regioselective (boron to the less hindered carbon)
    • and syn stereoselective

H

+

H

H

1-Methylcyclopentene

hydroboration oxidation48
Hydroboration/Oxidation
  • concerted regioselective and syn stereoselective addition of B and H to the carbon-carbon double bond
  • trialkylboranes are rarely isolated
  • oxidation with alkaline hydrogen peroxide gives an alcohol and sodium borate
hydroboration oxidation49
Hydroboration/Oxidation
  • Hydrogen peroxide oxidation of a trialkylborane
    • step 1: hydroperoxide ion (a nucleophile) donates a pair of electrons to boron (an electrophile)
    • step 2: rearrangement of an R group with its pair of bonding electrons to an adjacent oxygen atom
hydroboration oxidation50
Hydroboration/Oxidation
  • step 3: reaction of the trialkylborane with aqueous NaOH gives the alcohol and sodium borate
6 5 oxidation reduction
6.5 Oxidation/Reduction
  • Oxidation:the loss of electrons
    • alternatively, the loss of H, the gain of O, or both
  • Reduction:the gain of electrons
    • alternatively, the gain of H, the loss of O, or both
  • Recognize using a balanced half-reaction

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

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

3. complete a charge balance using electrons, e-

oxidation reduction
Oxidation/Reduction
  • three balanced half-reactions
oxidation to glycols
Oxidation to glycols
  • Alkene is converted to a cis-1,2-diol
  • Two reagents:
    • Osmium tetroxide (expensive!), followed by hydrogen peroxide or
    • Cold (25o), dilute aqueous potassium permanganate, followed by hydrolysis with base
a oxidation with oso 4

O

H

O

s

O

HOOH

4

O

s

H

O

2

O

H

A

c

y

c

l

i

c

o

s

m

a

t

e

c

i

s

-

1

,

2

-

C

y

c

l

o

p

e

n

t

a

n

e

d

i

o

l

(

a

c

i

s

g

l

y

c

o

l

)

A. Oxidation with OsO4
  • OsO4 oxidizes an alkene to a glycol, a compound with OH groups on adjacent carbons.
    • oxidation is syn stereoselective

O

O

O

O

oxidation with oso 4
Oxidation with OsO4
  • OsO4 is both expensive and highly toxic
  • it is used in catalytic amounts with another oxidizing agent to reoxidize its reduced forms and, thus, recycle OsO4
b oxidation with dilute kmno 4

O

H

KMnO4

-OH

Mn

H

O

2

O

H

A

c

y

c

l

i

c

o

s

m

a

t

e

c

i

s

-

1

,

2

-

C

y

c

l

o

p

e

n

t

a

n

e

d

i

o

l

(

a

c

i

s

g

l

y

c

o

l

)

B. Oxidation with dilute KMnO4
  • KMnO4 (dilute) oxidizes an alkene to a glycol, as did OsO4, here the cyclic ester is hydrolyzed with base.
    • oxidation is syn stereoselective

O

O

O

O

cleavage of both bonds of an alkene
Cleavage of both bonds of an alkene
  • Both the pi and sigma bonds of the alkene are broken (cleavage of the double bond)
  • Two reagent systems:
    • Ozone followed by reduction or
    • Hot, concentrated potassium permanganate in base, followed by acidification
c oxidation with o 3

C

H

3

1

.

O

O

O

3

C

H

C

=

C

H

C

H

C

H

C

H

C

C

H

+

H

C

C

H

C

H

3

2

3

3

3

2

3

2

.

(

C

H

)

S

3

2

Propanone

Propanal

2-Methyl-2-pentene

(a ketone)

(an aldehyde)

C. Oxidation with O3
  • Treatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl groups in its place
oxidation with o 3

O

3

C

H

C

H

=

C

H

C

H

C

H

C

H

-

C

H

C

H

3

3

3

3

(

C

H

)

S

3

2

H

C

C

H

C

H

C

H

3

3

3

Oxidation with O3
  • the initial product is a molozonide which rearranges to an isomeric ozonide

O

O

O

2-Butene

A molozonide

H

H

O

O

C

C

O

O

Acetaldehyde

An ozonide

d oxidation with conc kmno 4

C

H

3

C

H

C

=

C

H

C

H

C

H

C

H

C

C

H

+

HOCCH2CH3

3

2

3

3

3

2. H+

Propanone

Propanoic acid

(a ketone)

(a carboxyic acid)

1. Conc. KMnO4,

OH-

D. Oxidation with conc. KMnO4
  • Treatment of an alkene with permanganate cleaves the C=C and and continues to oxidize the two alkene carbon atoms if they contain hydrogen atoms

O

O

2-Methyl-2-pentene

determining cleavage products
Determining cleavage products
  • Predicting products from cleavage of an alkene, C=C, depends upon the number of H’s on each C of the double bond
  • # hydrogens Products

on carbon: From O3 From conc KMnO4

None ketone ketone

One aldehyde carboxylic acid

Two formaldehyde CO2 + HOH

6 6 reduction of alkenes

+ H

2

6.6 Reduction 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
    • addition occurs with syn stereoselectivity

Pd

25°C, 3 atm

Cyclohexene

Cyclohexane

a reduction of alkenes
A. Reduction of Alkenes
  • Mechanism of catalytic hydrogenation
reduction of alkenes
Reduction of Alkenes
  • even though addition syn stereoselectivity, some product may appear to result from trans addition
  • reversal of the reaction after the addition of the first hydrogen gives an isomeric alkene, etc.
b h 0 of hydrogenation
B. H0 of Hydrogenation
  • Reduction of an alkene to an alkane is exothermic
    • there is net conversion of one pi bond to one sigma bond
  • H0 depends on the degree of substitution
    • the greater the substitution, the lower the value of H°
  • H0 for a trans alkene is lower than that of an isomeric cis alkene
    • a trans alkene is more stable than a cis alkene
6 7 reaction stereochemistry
6.7 Reaction Stereochemistry
  • In several of the reactions presented in this chapter, chiral centers are created
  • Where one or more chiral centers are created, is the product
    • one enantiomer and, if so, which one?
    • a pair of enantiomers as a racemic mixture?
    • a meso compound?
    • a mixture of stereoisomers?
  • As we will see, the stereochemistry of the product for some reactions depends on the stereochemistry of the starting material; that is, some reactions are stereospecific
a reaction stereochemistry
A. Reaction Stereochemistry
  • We saw in Section 6.3D that bromine adds to 2-butene to give 2,3-dibromobutane
    • two stereoisomers are possible for 2-butene; a pair of cis,trans isomers
    • three stereoisomers are possible for the product; a pair of enantiomers and a meso compound
    • if we start with the cis isomer, what is the stereochemistry of the product?
    • if we start with the trans isomer, what is the stereochemistry of the product?
bromination of cis 2 butene
Bromination of cis-2-Butene
  • reaction of cis-2-butene with bromine forms bridged bromonium ions which are meso and identical
bromination of cis 2 butene70
Bromination of cis-2-Butene
  • attack of bromide ion at carbons 2 and 3 occurs with equal probability to give enantiomeric products as a racemic mixture
bromination of trans 2 butene
Bromination of trans-2-Butene
  • reaction with bromine forms bridged bromonium ion intermediates which are enantiomers
bromination of trans 2 butene72
Bromination of trans-2-Butene
  • attack of bromide ion in either carbon of either enantiomer gives meso-2,3-dibromobutane
bromination of 2 butene
Bromination of 2-Butene
  • Given these results, we say that addition of Br2 or Cl2 to an alkene is stereospecific
    • bromination of cis-2-butene gives the enantiomers of 2,3-dibromobutane as a racemic mixture
    • bromination of trans-2-butene gives meso-2,3-dibromobutane
  • Stereospecific reaction:a reaction in which the stereochemistry of the product depends on the stereochemistry of the starting material
oxidation of 2 butene
Oxidation of 2-Butene
  • OsO4 oxidation of cis-2-butene gives meso-2,3-butanediol
oxidation of 2 butene75
Oxidation of 2-Butene
  • OsO4 oxidation of an alkene is stereospecific
    • oxidation of trans-2-butene gives the enantiomers of 2,3-butanediol as a racemic mixture (optically inactive)
    • and oxidation of cis-2-butene gives meso 2,3-butanediol (also optically inactive)
reaction stereochemistry
Reaction Stereochemistry
  • We have seen two examples in which reaction of achiral starting materials gives chiral products
    • in each case, the product is formed as a racemic mixture (which is optically inactive) or as a meso compound (which is also optically inactive)
  • These examples illustrate a very important point about the creation of chiral molecules
    • optically active (enantiomerically pure) products can never be produced from achiral starting materials and achiral reagents under achiral conditions
    • although the molecules of product may be chiral, the product is always optically inactive (either meso or a pair of enantiomers)
b reaction stereochemistry
B. Reaction Stereochemistry
  • Next let us consider the reaction of a chiral starting material in an achiral environment
    • the bromination of (R)-4-tert-butylcyclohexene
    • only a single diastereomer is formed
    • the presence of the bulky tert-butyl group controls the orientation of the two bromine atoms added to the ring
c reaction stereochemistry
C. Reaction Stereochemistry
  • Finally, consider the reaction of an achiral starting material in an chiral environment
    • BINAP can be resolved into its R and S enantiomers
reaction stereochemistry79
Reaction Stereochemistry
  • treating (R)-BINAP with ruthenium(III) chloride forms a complex in which ruthenium is bound in the chiral environment of the larger BINAP molecule
  • this complex is soluble in CH2Cl2 and can be used as a homogeneous hydrogenation catalyst
  • using (R)-BINAP-Ru as a hydrogenation catalyst, (S)-naproxen is formed in greater than 98% ee
reaction stereochemistry80
Reaction Stereochemistry
  • BINAP-Ru complexes are somewhat specific for the types of C=C they reduce
  • to be reduced, the double bond must have some kind of a neighboring group that serves a directing group
ad