mechanism oxidation alkene

Sec. 10 - alkenes/alkynes 1

Addition Reactions to AlkenesAddition of Hydrogen Halides to Alkenes.

C CH

H

H

H

R2C=CR2 + HX R2HC-CR2X

Nucleophile(weak base)

+ H +

Electrophile(acid)

C CH

H

H

H

H+

General Reaction

X

Nucleophile(base)

Electrophile(acid)

C C

H

H

H

H

HX

Order of reactivity = HI > HBr > HCl > HF Why?

Slow

Fast

Reactivity parallels acid strength. The alkene is a very weak base so you need

a very strong acid to get a reaction.

Why?

Unsymmetrical alkene


Markovnikov’s Rule

In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom to the double bond as to yield the more stable carbocation as an intermediate (You already knew this from SN1 reactions. The most stable carbocation intermediate forms)

orHydrogen becomes

attached to the carbon atom of the double

bond with the greater number of hydrogens


Examples

C C

H

CH3H3C

H3CH Br+

nucleophile(weak base)

electrophile(acid)

C CH

CH3H3C

H3C H+

C CH2CH3

H3C

H3C

Br

sp2

sp2

sp3

sp3

major product

CH3

CH3

H Br+ CH3

CH3

H H

+

CH3

CH3

Br

Br

CH3

CH3

(R)

(R)

(R)

(S)

diastereomers

bottom

top

top

bottom

+


C

H3C

H3C

CH2

Examples

I – Cl

C

H3C

H3C

CH2

H2SO4

cold concentrated

S

O

O

O

H

OH

nucleophile(weak base)

electrophile(acid)

very weak basepoor nucleophile

no water

weak bondHSO4¯ great leaving group


C

H3C

H3C

CH2

dilute H2SO4

hydrationwater present

S

O

O

O

H

OH

O

H

H

H

or+

S

O

O

O OH

H2O - weak basepoor nucleophile but better

than HSO4¯ also in greater concentration

Examples

H2O


C CH2

H3C

H3C

C CH3

H3C

H3C+

coldconcentrated

H2SO4HSO4¯ C CH3

CH3

H3C

OSO3H

C CH3

CH3

H3C

OH concentratedH2SO4

C CH3

CH3

H3C

OH2

C CH3

H3C

H3C+ E1

diluteH2SO4

heat

NaBr

C CH3

CH3

H3C

Br

addH2Oheat

C CH3

H3C

H3C+

C CH3

CH3

H3C

OH+ HSO4¯

C CH3

H3C

H3C+

H2O-H+

SN1

SN1

+

-H2O

H2O-H+

dehydration

C CH2

H3C

H3C

hydration

hydration

What is the difference between hydration, elimination of alcohols and substitution reactions with alcohols, they all seem the same?

C CH3

H3C

H3C+

C CH2

H3C

H3C

C CH2

H3C

H3Cconcentrated

H2SO4

Don’t forget anytime a carbocation is formedrearrangement of the carbonskeleton can occur

cold

-H+

NaBr


Anti-Markovnikov addition

The hydrogen atom becomes attached to the carbon atom with the fewer hydrogen atomsThis is a free radical reaction not an ionic reaction. You do not have to memorize the

statement above if you understand the mechanism.

CH3CH=CH2

HBrROOR

a peroxide

CH3CH2CH2BrMechanism

RO OR 2 RO RO H Br BrROH +

C CH

H

CH3

H

CHH3C CH2

Br

CHH3C CH2 Br2° most stable than 1°same as carbocations

H Br CH2H3C CH2 Br Br+propagation

Br

C

H

C

HH C

H

HH

2°


Addition of Bromine and Chlorine to Alkenes

Addition produces vicinal dihalides


Mechanism

C CR

R

R

R

C CR

R

R

R

nucleophile(base)

Br

Br

δ+

δ−

C C

R

R

R

R

Br

C CR

R

R

R

Br +

electrophile(acid)

δ+

δ−Br

Brpolarized

not true diagrams of orbitalsbut okay for our needs

+ Br

Br

C C

R

R

RRBr

Br

electrophile(acid)

nucleophile(base)

+

bromonium ion intermediate anti addition

forms trans products


Example

Br2

CCl4

(solvent)

trans-1,2-dibromocyclohexane

+ enantiomer

Unsymmetrical Alkene

C CH2

H3C

H3C

+ Br – Br C CH3C

H3CBr

H

H

δ+

δ+

stronger bond

weaker bond

3° carbon can bear more of the positive charge.

3°

1°+ Br C C

H3CH3C

H

HBr

Br


Bromonium Ion


Halohydrin Formation (halogen + alcohol)

H2O(solvent)

Br – Br

Major product

Br2

H2O

OH

Br

Na OH

H2O

+


Oxidation States of Carbon

A reaction of an organic molecule usually corresponds to increasing its hydrogen content or decreasing its oxygen content.

RCH3

[O]

[H]RCH2OH

[O]

[H]R C

O

H[O]

[H]R C

O

OH

LowestOxidation State

HighestOxidationState

R CH ROH

R C R

O[O]

[H]

Oxidation → broad definition → a reaction that increases its content of any element more electronegative than carbon

CH3[O]

[H]CH2Cl

How to figure out the oxidation state of a carbon atom

a bond to hydrogen or anything less electronegative than carbon is electron donating → -1a bond to nitrogen, oxygen or anything more electronegative than carbon is electron withdrawing → +1a bond to a carbon → 0

C

H

H

H

H

C OOC

H

H

H

C

H

OH

H

OS = oxidation state


Oxidation States of Carbon

C C

H

HH

HH2/Pd

C

H

H

H

C

H

H

H

LEO goes GER – Loss of Electrons Oxidation, Gain of Electrons Reduction

C

H

H

H

C

H

OH

H

KMnO4C

H

H

H

C

O

OH

Reduction Oxidation

Oxidation Alkenes: Syn 1,2-dihydroxylation


Mechanism

one oxygenacts like anelectrophile

(acid) the other oxygenacts like annucleophile

(base)

Syn addition(same side)

causes cleavage without altering stereochemistry

COLD


Example

• OsO4, ether

2) Na2SO3

OOs

O

O

O

H

H

intermediate(if you remember this you know the answer)

Oxidative Cleavage of Alkenes (KMnO4 or K2Cr2O7)

BondCleavage


Mn

O O

O O

C(CH3)2H3CH2C

Mn

O O

O O

C C

CH3

CH3H3C

H

OS =-1 OS = 0

OS = +7 OS = +5

OS = 0 OS = +1

O

C

CH3H

O

CH3C CH3

cleavage

OS = +1 OS = +2

MnO O

OS = +3

Mechanism

further oxidation of carbon while manganese is

reduced

+

can’t break C-C bondsno further oxidation

O

C

CH3HO

OS = +3

+

In an acidic solutionMn2+ will be the finalmanganese product

In a basic solution Mn(IV)O2

is formed not MnO3¯ and is the final manganese

product

Do not need to know just for interest

Ox

carbonic acid

Ox Ox

decomposition

formaldehydeformic acidno further

oxidation

4 electron transfer


Example

1) KMnO4/NaOH heat

2) H3O+

Ox

intermediate

Ozonolysis of AlkenesOzone is in a class of compounds called 1,3-dipole reagents. They are so called

because good neutral resonance forms cannot be written for them.

Diazo Nitrile oxide Azides Nitrones

R2C N N+ +

RC N O+

R N N N+

R2C NO

R

+


Ozonolysis of Alkenes

O O

O +

O O

O

+

O O

O

+ 1,3-dipolar addition reaction

O

O O

+

( C H 3 ) C C C

C H 3

H C ( C H 3 ) 3

( C H 3 ) C C C

C H 3

H C ( C H 3 ) 3

O O

O

ozonide not stable

C ( C H 3 ) 3 C

H O

O O C

C H 3 C ( C H 3 ) 3 +

reverse 1,3-dipolar

addition

1,3-dipolar addition C

C H 3

C ( C H 3 ) 3

O

H

C ( C H 3 ) C

O O

ozinide more stable

This form is favoured because of the formation of stronger C-O bonds as opposed to weak O-O bonds

oxidative work up

H 2 O 2

( C H 3 ) 3 C

O

O H C H 3 C

O

C ( C H 3 ) +

a carboxylic acid a ketone

reductive work up

Z n , H 2 O o r Z n , C H 3 C O O H

( C H 3 ) 3 C

O

H

a ketone

C H 3 C

O

C ( C H 3 ) +

an aldehyde

1,3-dipolar addition

same products as KMnO4 or K2Cr2O7


Examples

H

H

O OO O

H+

O OO O

OH+

1) O3, CH2Cl2, -78°C

2) Zn/HOAc

1) O3, CH2Cl2, -78°C

2) H2O2

Could also use1) KMnO4/NaOH heat2) H3O+

What is A?

1) O3, CH2Cl2, -78°C

2) Zn/HOAc

1) O3, CH2Cl2, -78°C

2) H2O2

A

A

O

O O

O

H

O

OO

O

OH

H

OH


Addition of Bromine and Chlorine to Alkynes

Addition of halogen to alkynes can occur once or twice depending on how many equivalents of the halogen are added

Addition of one equivalent usually proceeds to give the trans dihalide


Addition of Hydrogen Halides to Alkynes

Addition of hydrogen halides occurs once or twice depending on how many molar equivalent of hydrogen halide are addedBoth additions are Markovnikov and give gem-halides

HBr can be generated by reaction of acetyl bromide and alumina

Anti-Markovnikov addition of HBr occurs in the presence of peroxide


Oxidative Cleavage of Alkynes

Reaction of alkynes with ozone or basic potassium permanganate both lead to formation of carboxylic acids


Not covered

Not covered


Not covered Not covered

H2O2

mechanism oxidation alkene

Documents

final manganese

br

oxidative

dipolar addition

hydrogen halides

kmno4naoh

oxidation

double bond