Addition of Br2 and Cl2

Stereochemistry

Anti Addition (halogens enter on opposite sides); Stereoselective

Syn addition (on same side) does not occur for this reaction.

Mechanism, Step 1Step 1, formation of cyclic bromonium ion.

Step 2

Detailed Stereochemistry, addition of Br2

H3C CH3

C3H7 C2H5

Br

Br

(S) (R)H3C CH3

C3H7 C2H5

Br

Br

Br

(S)

(S)

H3C

CH3C3H7

C2H5

Br

Br

(R)

(R)

H3C

CH3

C3H7

C2H5

Br

Br

(R) (S)

H3C CH3

C3H7 C2H5

Br

Br

Br

Br(R)

(R)

H3C

CH3

C3H7

C2H5

Br

Br

(S)

(S)

H3C

CH3

C3H7

C2H5

Br

Br

enantiomers

Alternatively, the bromine could have come in from the bottom!

enantiomers

S,S

S,S

R,R

R,R

Only two compounds (R,R and S,S) formed in equal amounts. Racemic mixture.

Bromide ion attacked the carbon on the right.

But can also attack the left-side carbon.

Number of products formed.

(S)

(S)

H3C

CH3

C3H7

C2H5

Br

Br

enantiomers

enantiomers

S,S

S,S

R,R

R,R

We have formed only two products even though there are two chiral carbons present. We know that there is a total of four stereoisomers. Half of them are eliminated because the addition is anti. Syn (both on same side) addition does not occur. (R)

(R)

H3C

CH3

C3H7

C2H5

Br

Br

(R)

(R)

H3C

CH3

C3H7

C2H5

Br

Br

(S)

(S)

H3C

CH3C3H7

C2H5

Br

Br

Attack of the Bromide Ion

(S) (R)H3C CH3

C3H7 C2H5

Br

Br

(S)

(S)

H3C

CH3C3H7

C2H5

Br

Br

Starts as R Becomes S

The carbon was originally R with the Br on the top-side. It became S when the Br was removed and a Br attached to the bottom.

In order to preserve a tetrahedral carbon these two substituents must move upwards. Inversion.

Progress of Attack

Things to watch for:

•Approach of the red Br anion from the bottom.

•Breaking of the C-Br bond.

•Inversion of the C on the left; Retention of the C on the right.

R1R2

R3R4

Br2

anti addition

R1R2

R3R4

Br

Br

+ enantiomer

Using Fischer Projections

Not a valid Fischer projection since top vertical bond is coming forward.

Convert to Fischer by doing 180 deg rotation of top carbon.

+ enantiomer

Br

R1 R2

Br

R4 R3

=

There are many variations on the addition of X2 to

an alkene. Each one involves anti addition.

Br -

+ enantiomer

Br

R1 R2

Br

R4 R3

R2 R4

R1 R3

Br

I -

+ enantiomer

I

R1 R2

Br

R4 R3

+ enantiomer

Br

R1 R2

I

R4 R3

+

The iodide can attach to either of the two carbons.

I -I -

Instead of iodide ion as nucleophile can use alcohols to yield ethers, water to yield alcohols, or amines.

R1R2

R3R4

Br2

Regioselectivity

If Br2 is added to propene there is no regioselectivity issue.

Br2

Br

Br

If Br2 is added in the presence of excess alternative nucleophile, such as CH3OH, regioselectivity may become important.

Br - BrOCH3

BrCH3O-H

Br

OCH3and/or

+ H + + Br - + H + + Br -

Regioselectivity - 2Consider, again, the cyclic bromonium ion and the resonance structures.

R

BrWeaker bond

More positive charge

Stronger bond

Expect the nucleophile to attack here. Remember inversion occurs.

Regioselectivity, Bromonium Ion

– Bridged bromonium ion from propene.

Et

H Me

Me

Me

Cl2/H2O

H

Example

Regioselectivity, addition of Cl and OH

Cl, from the electrophile Cl2, goes here

OH, the nucleophile, goes here

Stereochemistry: anti addition

Note: non-reacting fragment unchangedEt

H Me

Me

Me

Et

H Me

Me

Me

+

Cl

HH

OH

Cl

OH

Put in Fisher Projections. Be sure you can do this!!

Et

H Me

Me OH

Me

H Cl

Et

H Me

HO Me

Me

Cl H

+

Bromination of a substituted cyclohexene

Consider the following bromination.

C(CH3)3

Br2

Expect to form two bromonium ions, one on top and the other on bottom.

C(CH3)3

Br+

C(CH3)3

Br+

+

Expect the rings can be opened by attack on either carbon atom as before.

But NO, only one stereoisomer is formed. WHY?

C(CH3)3

Br-Br

Br

C(CH3)3

Br

Br

+

Addition to substituted cyclohexene

HH

Br2

The tert butyl group locks the conformation as shown.

Br

Br

H

H

H H

+

The cyclic bromonium ion can form on either the top or bottom of the ring.

How can the bromide ion come in? Review earlier slide showing that the bromide ion attacks directly on the side opposite to the ring.

Progress of Attack

Things to watch for:

•Approach of the red Br anion from the bottom.

•Breaking of the C-Br bond.

•Inversion of the C on the left; Retention of the C on the right.

Notice that the two bromines are maintained anti to each other!!!

Addition to substituted cyclohexene

Br2

Br

Br

+

ObserveRing is locked as shown. No ring flipping.

Attack as shown in red by incoming Br ion will put both Br into equatorial positions, not anti.

Br

Br

This stereoisomer is not observed. The bromines have not been kept anti to each other but have become gauche as displacement proceeds.

Br-

Br-

Be sure to allow for the inversion motion at the carbon attacked by the bromide ion.

Addition to substituted cyclohexene

Br2

Br

Br

+

Attack as shown in green by the incoming Br will result in both Br being axial and anti to each other

Br

Br

This is the observed diastereomer. We have kept the bromines anti to each other.

Br-

Br-

Oxymercuration-Reduction

Regioselective: Markovnikov Orientation

Occurs without 1,2 rearrangement, contrast the following

3,3-dimethylbut-1-ene

H2O

H2SO4

OH

formed viarearrangement

1 Hg(OAc)2

2. NaBH4OH

No rearrangement

Alkene Alcohol

Mechanism1

2

3

4

Hydroboration-Oxidation

Alkene Alcohol

Anti-Markovnikov orientation

Syn addition

1. BH3

2. H2O2

HHO

HHO

Borane, a digression

Isoelectronic with a carbocation

B B

H

HH

H

HH

MechanismSyn stereochemistry, anti-Markovnikov orientation now established.

Two reasons why anti-Markovnikov:

1. Less crowded transition state for B to approach the terminal carbon.

2. A small positive charge is placed on the more highly substituted carbon.

Just call the circled group R. Eventually have BR3.

Next…

Cont’d

Oxidation and Reduction Reactions

We think in terms of Half Reactions

• Write reactants and products of each half reaction.

Cr2O7 2- + CH3CH2OH Cr 3+ + CH3CO2H

Cr2O7 2- 2 Cr 3+

Balance oxygen by adding water

+ 7 H2O

In acid balance H by adding H +

14 H+ +

Balance charge by adding electrons

6 e - +

Inorganic half reaction…

If reaction is in base: first balance as above for acid and then add OH- to both sides to neutralize H +. Cancel extra H2O.

Will be oxidized.

Will be reduced.

Cont’d

Now the organic half reaction…

Balance oxygen by adding water

In acid balance H by adding H +

Balance charge by adding electrons

CH3CH2OH CH3CO2HH2O + + 4 H+ + 4 e-

Combine half reactions so as to cancel electrons…

CH3CH2OH CH3CO2HH2O + + 4 H+ + 4 e-

Cr2O7 2- 2 Cr 3+ + 7 H2O14 H+ +6 e - +

3 x ( )

16 H+ + 2 Cr2O7 2- + 3 CH3CH2OH 4 Cr 3+ + 3 CH3CO2H + 11 H2O

2 x ( )

Formation of glycols with Syn Addition

Osmium tetroxide

Syn addition

KMnO4cold, dilute, slightly alkaline

also KMnO4

Anti glycols

PhCO3H, a peracid

O

H+

O

H

H2O

HO

OH

Using a peracid, RCO3H, to form an epoxide which is opened by aq. acid.

Peracid: for example, perbenzoic acid

O O

OH

The protonated epoxide is analagous to the cyclic bromonium ion.

epoxide

An example

chiral, optically active

(S)-3-methylcyclohex-1-ene

PhCO3HO + O

aq. acid

OH

OH

OH

OHOH

OH

OH

OH

Are these unique?

Diastereomers, separable (in theory) by distillation, each optically active

OzonolysisR3

R4

R1

R2

1. O3

2. (CH3)2SR4

R3

O

R1

R2

O+

Reaction can be used to break larger molecule down into smaller parts for easy identification.

Ozonolysis Example

For example, suppose an unknown compound had the formula C8H12 and upon ozonolysis yielded only 3-oxobutanal. What is the structure of the unknown?

The hydrogen deficiency is 18-12 = 6. 6/2 = 3 pi bonds or rings.

The original compound has 8 carbons and the ozonolysis product has only 4

Conclude: Unknown two 3-oxobutanal.

Unknown

C8H12

ozonolysysO

O

O

O

Simply remove the new oxygens and join to make double bonds.But there is a second possibility.

O

O

Another Example

2. An unknown compound (derived from the gall bladder of the gila monster) has the formula C10H14 . When subjected to ozonolysis the following compound is isolated

O

O O

O

Suggest a reasonable structure for the unknown.

Hydrogen Deficiency = 8. Four pi bonds/rings.

Unknown has no oxygens. Ozonolysis product has four. Each double bond produces two carbonyl groups. Expect unknown to have 2 pi bonds and two rings.

To construct unknown cross out the oxygens and then connect. But there are many ways the connections can be made.

a

bc

d

a-b & c-d

a b

c

da-c & b-d

ac

d

b

a-d & b-c

ad

c

b

Look for a structure that obeys the isoprene rule.

Mechanism

OO

O OO

O OO

O OO

O

Consider the resonance structures of ozone.

These two, charged at each end, are the useful ones to think about.

Electrophile capability.

Nucleophile capability.

Mechanism - 2

OO

O OO

O OO

O OO

O

Mechanism - 3

Mechanism - 4

Hydrogenation

No regioselectivity

Syn addition

Heats of Hydrogenation

Consider the cis vs trans heats of hydrogenation in more detail…

Heats of Hydrogenation - 2The trans alkene has a lower heat of hydrogenation.

Conclusion:

Trans alkenes with lower heats of hydrogenation are more stable than cis.

We saw same kind of reasoning when we talked about heats of combustion of isomeric alkanes to give CO2 and H2O

Heats of Hydrogenation

Incr

easi

ng s

ubst

itutio

n

Red

uced

hea

t of

Hyd

roge

natio

n

By same reasoning higher degree of substitution provide lower heat of hydrogenation and are, therefore, more stable.

Acid Catalyzed Polymerization

Principle: Reactive pi electrons (Lewis base) can react with Lewis acid. Recall

Which now reacts with a Lewis base, such as halide ion to complete addition of HX yielding 2-halopropane

Variation: there are other Lewis bases available. THE ALKENE.

+ HH

The new carbocation now reacts with a Lewis base such as halide ion to yield halide ion to yield 2-halo-4-methyl pentane (dimerization) but could react with another propene to yield higher polymers.

the carbocation is an acid!

+

Examples of Synthetic Planning

Give a synthesis of 2-hexanol from any alkene.OH

Planning:

Alkene is a hydrocarbon, thus we have to introduce the OH group

How is OH group introduced (into an alkene): hydration

What are hydration reactions and what are their characteristics:

•Mercuration/Reduction: Markovnikov

•Hydroboration/Oxidation: Anti-Markovnikov and syn addition

What alkene to use? Must involve C2 in double bond.

Which reaction to use with which alkene?

Markovnikov rule can be applied here. CH vs CH2.

Want Markovnikov!

Use Mercuration/Reduction!!!

Markovnkov Rule cannot be used here. Both are CH.

Do not have control over regioselectivity.

Do not use this alkene.

For yourself : how would you make 1 hexanol, and 3-hexanol?

Another synthetic example…

How would you prepare meso 2,3 dibromobutane from an alkene?

Analysis:

Alkene must be 2-butene. But wait that could be either cis or trans!

We want meso. Have to worry about stereochemistry

Know bromine addition to an alkene is anti addition (cyclic bromonium ion)

trans

Br2

BrBr

H

Br

Br

rotate lower unit

Br H

Br H

meso

This worked! How about starting with the cis?

cis

Br2

H Br

Br H

racemic mixture

+ enantiomer

This did not work, gave us the wrong stereochemistry!

Addition Reaction General Rule…

Characterize Reactant as cis or trans, C or T

Characterize Reaction as syn or anti, S or A

Characterize Product as meso or racemic mixture, M or R

Relationship

C RA

cis

Br2H Br

Br H

racemic mixture

+ enantiomer

Characteristics can be changed in pairs and C A R will remain true.

Want meso instead?? Have to use trans. Two changed!!

AT M

trans

Br H

Br H

meso

Br2