Lecture 16(ak 1)

Halogenation

σ* (p) π* (p) LUMO

σ (p) π (p) σ* (s) σ (s) Diatomic MOs for Br2

HOMO

A Br2

Side on Br2 addition is symmetry forbidden. End on addition is allowed.

LUMO

π* A

LUMO

S

C2H4 HOMO

π HOMO

S

A Br2

The addition of halogens (Br2, Cl2) to alkenes

Stereochemistry.

Anti-addition

Br Br

Br Br

+ Br

Br

The addition of halogens (Br2, Cl2) to alkenes.

Stereochemistry. Br Br

Br

Anti-addition

Br H

Br

Br

Br

H

+

Br

Br Br

H

Br H

Br

Br

Br

Br

Br




⊕

Br




⊕

Br H

Br H

H

H

Br H

Br H

The addition of halogens (Br2, Cl2) to alkenes.

Stereochemistry. Br Br

Br H

Br

S

Br H

Br

Br + R R Br

H

Br

Anti-addition

Br Br H

Br R R Br +

S

Br

Br S Br S

The addition of halogens (Br2, Cl2) to alkenes.

Stereochemistry.

Anti-addition H

Br

Br2

Br Br H

Br Br
H

H H

Br2

H

H

H

C

C H

Br H C

anti addition twist-boat

Br H H Br

H H

H

Br

anti addition chair H

lower energy TS

HH

Br Br

HH C H

Br

higher energy TS

H H

H

Br

The addition of Br2 in presence of H2O to alkenes. Regiochemistry and Stereochemistry. Anti-addition Br

Br2 H2O

OH

Br Br Br

H O H

Br

Br

Br

H

H H

O H H

weaker bond

O H

Br Br Br

H O H

Br

Br

Br

H

H H

O H H

H

Br

H H H

Br

H H

O H

Br-Br

Br

Br

H2O

H

O H

Br

Br

Br

equal

2.06Å

Bromonium ion is symmetrical

Br

Br-Br

H3C

H2O

H3C

H

H3C

H3C

H3C

O H

Br

Br

Br

Br

H3C

more important

2.06Å

2.17Å

2.03Å

Predict the product of the following reaction. CH3 H

Br2

CH3 OH

H 2O

H Br

+

eight isomers possible! only two are observed!!

Br CH3 OH H

CH3 H

CH3 OH

Br2

H Br

H 2O

CH3

+

OH CH3 Br H

CH3

Br H

Br H

Hydrogenation

Reaction Mechanisms for Hydrogenation How do the atoms of the reactant molecules rearrange to form the product molecules? H2 + CH2=CH2

?

CH3CH3

But first. Do we expect the reaction to occur? Look at the thermodynamics. ∆Gr°= Σproducts - Σreactants =

-100 kJ/mol

∆Gf°=0.0

? ∆Gf°=+68 kJ/mol

∆Gf°=-32 kJ/mol

Mechanism 1

H2 + CH2=CH2

CH3CH3

Mechanism 2

H2 + CH2=CH2

CH3CH3

Mechanism 3

H2 + CH2=CH2

CH3CH3

It looks good. What is wrong with this mechanism? H H H

C

H C

H

H H

H C

H

H

Mechanism 2

C H

H

H2 + CH2=CH2

Examine the M.O.s CH3CH3

LUMO

π* A

S

A

LUMO

H2

C2H4

π

σ*

HOMO HOMO

σ S

S (symmetric with respect to reflection)

π* A

LUMO

A (asymmetric with respect to reflection) It has a node.

A-S Wrong Symmetry No Interaction

σ HOMO

S

LUMO

π* A

π S

σ* A

LUMO

HOMO HOMO

σ S

A-S Wrong Symmetry No Interaction

σ* LUMO

A

This reaction path is forbidden by orbital symmetry

π HOMO

S

LUMO

π* A

σ* A

LUMO

The direct reaction of H2 and C2H4 is forbidden by orbital symmetry

π S

HOMO HOMO

σ S

Energy ∆Gr = -100 kJ/mol

∆G‡

reactants H2 + C2H4

Forbidden Reaction ∆G‡ is too high We need a catalyst!

-100 kJ

0 + 68 kJ

Reaction Coordinate

C2H6 -32 kJ products

What is a catalyst? An added component that changes the reaction mechanism to one with a lower energy pathway. A lower ∆G‡ The catalyst is neither produced or destroyed during the reaction. It does not change ∆G of the reaction. It does not change Keq of the reaction. A catalyst can be simple like H+ or a metal ion or it can be complex like an enzyme.

The platinum metals are often used in catalysis They have filled d orbitals and empty s or p orbitals. This means they can act as either an acid or a base.

Can H2 bind to a transition metal?

5p orbital S

LUMO

A

LUMO

A-A Allowed The reaction of H2 and a Ru ion is allowed by orbital symmetry

4d orbital A

S-S Allowed HOMO

Ruthenium ion

σ HOMO

H2

S

Can C2H4 bind to a transition metal?

5p orbital S

LUMO

A-A Allowed S-S Allowed Symmetry Allowed

4d orbital A

HOMO

Ruthenium atom

LUMO

A

HOMO

S

Catalyst Ru

H2 + CH2=CH2 H H

+

Ru

CH3CH3

H Ru H

H2C CH2

H Ru

Catalyst ready to be used again Ru + H3C CH3 product

H

H

H2 C Ru CH2 H

CH2 CH2

Catalyst H-H H2 + CH CH22=CH -CH22

Ru

CH3CH3

H

H

CH CH CH23CH 23

After the reaction the catalyst is unchanged and ready to go again.

Catalytic Hydrogenation: Heterogeneous

C

C

H

C H H

C

H

Pt or Pd surface

H

C

H2 / Pd

C

C H

C C

H

syn or cis addition

Both hydrogens add to the same face of the alkene

H

C

C H

C

H

Pt or Pd surface

Pd

H

H C

+ H

C

H

H2/Pd H

H

H C C

H

HH

HH both hydrogens add to the same face of the alkene

H

CH3

H CH 3 H

H

C

C

H

H

H CH 3 H

H2

and

Pd H

H

H

H H CH 3 H

Stereochemistry of hydrogenation is cis or syn.

not H

H

H H

Energy

C C

Symmetry forbidden High energy Pd

∆G‡ ∆G‡

H H

reactants ∆Gr

H

H

C

C

H

Catalyzed pathway Symmetry allowed Lower energy

products Reaction Coordinate

H

Simmons-Smith Reaction • The Simmons-Smith reaction is considered the best way to carry out a cyclopropanation reaction. • The reaction uses iodomethyl zinc iodide (ICH2ZnI) as the source of the third carbon. • This reagent, known as the Simmons-Smith reagent, is not a carbene but it reacts like it so it is called a carbenoid.