CHEM 261 November 16, 2021 RECALL (REVIEW) Addition Reactions: General Mechanism

Water or Alcohol Addition

C C X2 C CX

X

C C X2R OH

R = H or AlkylC C

X

OR

X = F, Cl, Br, (I)

trans addition

trans addition

Like H2/ Cat. (Pd, Pt, Ni, Rh)

Hydroboration - Addition of Boron with Hydride (H: minus)

Structure of borane Exists as Diborane (B2H6), but behaves like BH3

Cis addition for both reactions

Stereospecific syn/cis addn.

concerted (bonds break & form at same time)

organoborane

H BH

H

B2H6

B

H2OHCl

H2O2NaOH OH

B

H O OHNaOH

O OH Na+ + H2O

BOOH O

B

R

R

OH

O

H OH

OH

HO BR

R

Example

Mechanism of addition of OH

Example

Syn addition, but anti-Markovnikov addn of HOH overall

*

*

Oxidation and reduction reactions Reduction adds electrons Oxidation removes electrons Reduction Reaction

12 e- 14 e- outer shell outer shell As there is an increase in the electron count in the outer shell, this is a reduction of ethylene. Ozonolysis - an Oxidation of Alkenes

Ozonolysis (lysis = cleavage)

Examples of carbonyl groups (part of many functional groups)

Reaction scheme of ozone

Example

Ozonolysis Mechanism (lysis = cleavage)

Example: Myrcene – C10H16

1) O3

2) Zn

O

H H

O

O

H

OO

H HO

Formaldehyde

Acetone

H2 Pd

Structure of Myrcene?

Therefore, structure of Myrcene is:

Oxidation Reactions of Alkenes (Addition Reactions) Epoxidation:

Mechanism:

Concerted process = all bond breaking and bond making occurs at the same time

C C C CO

Epoxide/OxiraneCO

O O HR

A Peracid

Syn/Cis AdditionStereospecific

CO

O O HH3CExample of a peracid

C CHH H

H

O O HC OH3C

a

bc

d C CO

HH

HH

+ H3C CO

OH

acetic acid

Concerted process Stereospecific

Example 2: 2-Methyl-7-octadecene

biologically, only one enantiomer is active (one shown) – racemate produced by peracetic acid

CO

O O HH3C

OSex pheromone for Gypsy Moth

Example: trans- vs cis-Butene

The possibility of epoxidation from the top is 50% and from the bottom is 50% so a 1:1 mixture of enantiomers is form (racemic mixture).

CO

OO

HH3C

OHH

Meso Compound

S RO

(R)

(S)

CO

OO

HH3C

(S) (S)

O

O

(R)(R)

O

O

+

Enantiomers1:1 Mixture

trans-2-Butene

cis-2-Butene

(R) (S)

OO

(R)

(S)

same

achiral

achiral

achiral

achiral

chiral chiral

Oxidation Reactions of Alkenes (Addition Reactions) Potassium Permanganate: KMnO4

General Scheme:

MnK O

O

O

O

C CH2O, H2SO4“Cold” (20 ºC)

KMnO4C COH OH

A diol

Cis Addition

H2O, H2SO4“Hot” (100 ºC)

KMnO4C COH OH

C O CO

Purple crystals in H2O and H2SO4

Reaction with aldehydes:

Example 2: Formaldehyde in box is unstable if formed under these conditions (hot KMnO4). The other product is acetone.

CO

R H

KMnO4

CO

R OH

CO

H H

KMnO4 CO2 + H2O

H2O, H2SO4Cold

KMnO4

OHH2O, H2SO4

Hot

KMnO4

C O COH

H+

Unstable inthese condition)

KMnO4H2O + CO2

OH

Mechanism:

Example 1:

Example 2:

MnO

O

O

O

O OMn

OO

Cold

H2OOH OH

cis-Diol

Hot

Cis Addition H2OC O CO

+ MnO2

+ MnO2

H2O, H2SO4Cold

KMnO4

OH OHH2O, H2SO4

Hot

KMnO4

C O COH

+

Acetaldehyde(Unstable inthese conditions)

COOH

KMnO4

trans-2-Butene

cis-2-ButeneCold

KMnO4 HO OH

OH

HOCold

KMnO4

HO OHH HR S

Achiral (Meso)

OHHOH

HS S

OHHOH

HR R

Top Add’n

Bottom Add’n

Enantiomers

Example 3:

Example 4:

OHO

OOH

Acetic acid

KMnO4

Hot

KMnO4

HotSame as above

O OH

OH

O

OH

OHKMnO4

KMnO4

Cold

Hot

Adipic acid

Meso Compound

O

O