Reactions and Reaction Mechanisms

A reaction mechanism shows the actualflow of electrons and movement of the atoms during the reaction.

If one can understand a reaction mechanismone can predict the course of other reactions.

B

F

FFH N

H

H

Attack of a Lewis Base on a Lewis Acid

N B

H

FHH

FF

It is also useful to correlate changesin energy with the movement of atoms.

To do this one creates an energy profileof the reaction showing how the energychanges as the atoms move.

+

H3O+ Cl-+

H2O HClH

Energy Profile of a Exothermic Reaction

Potential Energy

H2O + HCl H3O+ + Cl-

+

H3O+ Cl-+

H2O HClH

Ea

-+H

O

H

H ClTransitionState

H = D (bonds broken) - D (bonds formed)

H = DH-H + DC=C - 2 DC-H - DC-C

C CH

CH3

H

H

H

HH2+

HC

CH3

CH

H

H = 432 + 614 - 2 x 413 - 347 = -127 kJ / mol

C CH

CH3

H

H

H

HH2+

HC

CH3

CH

H

But if you mix propene and H2 nothing happens!

C CH H

HCH3

H HMolecules repeleach other.

No good mechanismfor the reaction.

H = -127kJ/mol

CH3CH=CH2 H2+

CH3CH2CH3

? Ea too high thusno reaction

Try to find a new mechanismwith a lower Ea

How do you dothat? Use acatalyst.

Palladium metal can be used as a catalystfor the hydrogenation of alkenes.

H2 will actually dissolvein Pd metal. The H atomsdissociate and go intothe octahedral holes.

The bond of an alkene will bond to thesurface atom of the Pdmetal.

This is all we need!

C CH H

HCH3

Pd Pd PdPd Pd

PdPd

H H

C C

PdH

CH3

H HH

H

C

Pd

HH

H

H

CH3

H

C HH

H

H

H3C

H

Pd

C C

PdH

CH3

H HH

H

C

Pd

HH

H

H

CH3

H

C CCH3

H HH

+

H2Pd

C HH

H

H

H3C

HPd

C CCH3

H HH

+

H2

Pd

C HH

H

H

H3C

H

Palladium metalis a catalyst forthe hydrogenationof alkenes.

H2

Pd

H2

Pd

H2

Pd

BrBr

or

C C

H

H H

HC C

Br

H

HH

HH

Addition of HBr to ethylene gives bromoethane

But what does HBr give when it reacts with2-methylpropene?

BrBr

or

But what does HBr give when it reacts with2-methylpropene?

For this we need to know the mechansism.

The reaction involves a carbocationintermediate.

CH3

CCH3C

H

H

CH3

CCH3C

H

HH

CH3

CCH3C

H

HH

H Bror

Br

Br

Which carbocationis formed?

C

C

HH

H

HH

C

C

HH

HH

H

Alkyl groups stabilizecarbocations viaHyperconjugation.

It involves “no bond”resonance structures.

C

C

HH

H

H

H

C

C

HH

H

HH

C

HHC

H

HH

Empty p orbitalFilled sp3

hybrid orbital

Electrons from the C-H bond helpstabilize the empty p orbital

Order of Carbocation Stability

C

H

HHC

CH3

HH

C

CH3

CH3HC

CH3

CH3CH3

LeastStable

MostStable

<<<

primary

secondarytertiary

CH3

CCH3C

H

H

CH3

CCH3C

H

HH

CH3

CCH3C

H

HH

H Bror

Br

Br

Which carbocationis formed?

Tertiary most stable

C

CH3

CH3CH3

How does acarbocation react?

Br

C

CH3

BrCH3

CH3

Since the carbocationis a Lewis acid It can react with aLewis base.

The electron pair onthe base attacks theelectron deficient center

General Reaction:

The addition of a hydrogen halide, HCl, HBr orHI to an alkene gives an alkyl halide. Theregiochemistry is determined by which carbocation is the most stable.

HBr

Br

H

HCH3

H

more stable

H I

I CH3

H HH

H HH

more stable

HCl

Cl

Cl

and

Both carboncation intermediateswould be secondary, little selectivity

C C

H

H H

H H2SO4

H2OC C

H

HO

H

HH

H

What happens if there is no halide?

Hydration of alkenes to give alcohols.

H2SO4 is a catalyst

C C

H

H CH3

H H2SO4

H2OC C

H

HO

H

HH

CH3

or C C

H

H

H

HOH

CH3

What is the regiochemistry of the reaction?

What is the mechanism of the reaction?

CH3

CCH3C

H

H

CH3

CCH3C

H

HH

CH3

CCH3C

H

HH

or

The H2SO4 dissociatesto give hydronium ion.

H O

H

H

OH

H

OH

H

CH3

CCH3C

H

HH

O

H

H

O

CC

H

HHH3C

H3C

HH

O

CC

H

HHH3C

H3C

H

OH

H

H3O+

+

C C

H

H CH3

H H2SO4

H2OC C

H

HO

H

HH

CH3

or C C

H

H

H

HOH

CH3

What is the regiochemistry of the reaction?

The regiochemistry is determined by therelative stability of the intermediatecarbocation.

Reactions of alkenes so far:

H2

Pd

XHX

OHH2O

H2SO4

One more reaction for the sake of completeness

Br2 Br

Br

Bromine or chlorine easily add to a doublebond to give dibromo or dichloro compounds.

Br2

Br

Br

C

CH H

C

Cl C

Cl

H H

Cl2

Reactions of alkenes so far:

H2

PdX

HX

OHH2O

H2SO4

Br

Br

Br2

OHH2O

H2SO4

Enough of this simple stuff.

The acid catalyzed addition of water toan alkene gives an alcohol.

The regiochemistry is determined by the relative stability of the intermediate carbocation.

But what if you want 1-propyl alcohol?

What could you do to “trick” the regiochemistry?

The Trick

CH3C

C

H

H

H

CH3C

H

H

HH

H+

The observed regiochemistry is determinedby the first step of the reaction.

But we want an OH- group to add to theprimary carbon. How do we do that?

Add some other Lewis Acid instead of H+

then convert it to an OH group.

The Trick

CH3C

C

H

H

H

BH3

CH3C

H

B

HH

HH

H

CH3C B

HH

H

HH H2

B

tripropylborane

B

But what good is tripropylborane? We are tryingto make 1-propanol?

It turns out that oxidation of alkyl boraneswill give boric acid plus alcohols.

B

OH

OO

H

H

OH3

+

H2O2

-OH

Net Reaction

1. BH32. H2O2, -OH

OH

This is called hydroboration

1. BH32. H2O2, -OH

OH

Thus we have a choice in regiochemistry.

OH

H2O

H2SO4

HO2. H2O2, -OH 1. BH3

OHH2O

H2SO4

H2O

H2SO4

Oxidation states in carbon chemistry.

Oxidation and reduction are important chemicalprocesses. In organic chemistry it is not always obvious that you are carrying out an oxidation or a reduction.

Consider average oxidation states of the C atoms in each of the following molecules.Remember that H is counted as +1 and O as –2.

CH3 CH3

-3 C C

H

H H

H-2

C CH H

-1

CH3CH2

OH

-2

CH3C

H

O

-1

CH3C

O

O

H

0

C C

O

O O

O

H

H +3

Adding H2 to a double bond is a reduction.The reverse reaction ( removing H2) would be an oxidation.

-3-2

H2CH3 CH3C C

H

H H

H

-2-1CH3

CH2O

HH2

CH3C

H

O

Adding H2O is not an oxidation. Removing H2O is not a reduction.

-2CH3

CH2O

H

-2H2SO4

H2OC C

H

H H

H

Adding H2O is not an oxidation. Removing H2O is not a reduction.

-1-1

C C

H

H O

H

HH2O

H2SO4

C CH H

Unstable rearranges

-1CH3

CH

O

To go from an alcohol to an aldehyde (or a ketone )is an oxidation. Going from an aldehyde on to a carboxylic acid is a further oxidation.

SomeOxidant

CH3C

H

O

-1

CH3CH2

OH

-2

CH3C

O

O

H

0

SomeOxidant

Primary alcohols can be oxidized to carboxylic acids using CrO3. Chromium (VI) is a strong oxidizing agent. It is used with an acid catalyst.

OH

OH

O

CrO3

H2SO4

OH

O

OH H2SO4

CrO3

Alcohols can be oxidized to aldehydes or ketonesusing a modified form of CrO3 called PCC for pyridinium chlorochromate, (C5H6NCrO3Cl).

It is a milder reagent and if you use it carefully you can stop the reaction at the intermediate aldehyde step.

PCCO

H

OH

PCCO

HO

PCC

OH

O

Going Backwards: Reduction of aldehydes or ketones to an alcohol. To reduce a carbonyl group, C=O, to an alcohol one could use H2 gas, but a simpler way is to use a hydride as a source of H- and a acid as a source of H+.

The reagent of choice is NaBH4, followed by some acid.

1. NaBH4

2. H3O+O

HO

H

O

Na

B

H

H HH

B

H

HH

Na

H

O

H

H

O

H

Na

O

H

HH

H

O

H

H

O1. NaBH4

2. H3O+

OH

OH

1. NaBH4

2. H3O+O

H

Starting with an alkene how would you make?

Br

1. BH32. H2O2, -OH O

H

HBr

OOH

PCCH3O+

H2SO4

HBr

Br

Can you explain an unexpected result?

82%

+

? 18%

HBrBr

Br

and

82%

? 18%

CH3

H

H

H

H

CH3H

H

H

H