Lecture 12 Organic Chemistry 1, Study notes of Organic Chemistry

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University of Illinois

at Chicago (^) UIC

CHEM 232

Organic Chemistry I

Lecture 12

Organic Chemistry 1

Professor Duncan Wardrop

February 18, 2010

UIC

University of

Illinois at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Which of the following reactions proceeds through a

carbocation intermediate?

Self Test Question

A

B

C

D

E

H Br H

Br

Br

Br^ Br

H Br

• Br Br

Br

Br

HBr

peroxides

HBr

CH 3

ONa

CH 3

OH

Br

HO

HCl

H 2

, Pd/C

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Addition of B-H Bonds to Alkenes

C C

• (^) X Y

addition

X C C Y

C C

H BH 2

hydroboration

H C C BH 2

alkene borane alkyl borane

alcohol

oxidation

H C C OH

oxidation is a separate step and requires

a separate set of conditions and reagents

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Boranes - Structure & Electronics

B H

H

H

boron is exception to octet rule

sp

2

-hybridized (only 3 valence e

s)

contains an empty p-orbital

strong Lewis acid (electron pair acceptor)

forms 3-bonds (neutral) & 4-bonds (-/ve)

“Free” borane (BH

3

) only exists in gas phase, otherwise

undergoes dimerization to diborane (B

2

H

6

borane

diborane

2 x BH

3

dimerization

B

2

H

6

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Conditions: Hydroboration

B H

H

H

boron is exception to octet rule

sp

2

-hybridized (only 3 valence e

s)

contains an empty p-orbital

strong Lewis acid (electron pair acceptor)

forms 3-bonds (neutral) & 4-bonds (-/ve)

“Free” borane (BH

3

) only exists in gas phase, otherwise

undergoes dimerization to diborane (B

2

H

6

borane diborane

B

H

H

H

B

H

H

H dimerization

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Stoichiometry of Hydroboration

O

O

O

CH

3

H

3

C

diglyme: di ethylene gly col di m ethyl e ther

diglyme is a common solvent

hydroborations typically at room temp.

generally very fast reactions

B

H

diglyme

6 x

2 x

B

1 - butene

tributylborane

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Oxidation Step

• boron is replaced by

more electronegative

atom (O); therefore:

oxidation

• oxidation using

peroxide (H 2 O 2 ) &

hydroxide (OH

• ); added

immediately after

hydroboration

• I will not ask you to

learn the mechanism

for this step; for the

curious, see textbook

page 250

H

BH 2

H 2

O 2

NaOH (aq)

oxidation

H

OH

B 2

H 6

diglyme

hydroboration

carried out in the same reaction vessel

(boranes are reactive)

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Hydroboration-Oxidation is Regioselective

usually written as a two-

step process

works with diborane (B 2 H 6 )

or a borane complex with

another Lewis base

diglyme is a common

solvent

affords alcohol products;

similar to hydration (H 2 O/

H 3 O

)

1. B

2

H

6

, diglyme

2. H

2

O

2

, NaOH

OH

CH

3

1. H

3

B•S(CH

3

2

2. H

2

O

2

, NaOH

CH

3

OH

1. H

3

B•THF

2. H

2

O

2

, NaOH

OH

11

Which carbon of the alkene becomes bonded to the -OH group? Do you see a pattern here?

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Mechanism of Hydroboration

Why this regioselectivity?

Syn Addition: B and H atoms add to same side

(face) of C-C double bond at same time

hydride (H:) adds to most substituted carbon

boron adds to least substituted carbon

this is NOT a PROTONATION

CH 3

H 2

B H

CH 3

H H 2

B

CH 3

H 2

B

CH 3

CH 3

H 2

B

not

13

Note that although the borane generated upon monohydroboration of methylcyclopentene is itself

trans-substituted, the hydroboration remains a syn-addition. In other words, it is the BH2 group

and beta-hydrogen on the carbon bearing the methyl group which are in a syn relationship.

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Regioselectivity - 1st Rationale

Steric Effects Control Regioselectivity...

boron is larger than hydrogen; it prefers to add to the less sterically

hindered side of the double bond, which is the least substituted side.

X

Steric strain in transition

state raises E

act

for this

pathway - slower reaction

C

B H

CH

3

H H

2

B

H

H

H

H

H

CH

3

H B

H

H

CH

3

BH

2

H

UIC

University of

Illinois at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

A.

B.

C.

D.

E.

What is the product of the following reaction sequence?

Self Test Question

HO

HO

HO

Br

HO

1. Br 2

, CH 2

Cl 2

, hv

2. NaOC(CH 3

) 2

, DMSO

3. B 2

H 6

, diglyme

4. H 2

O 2

, NaOH (aq)

University of Illinois

at Chicago (^) UIC

CHEM 232

Organic Chemistry I

Section 6.15-6.

Halide Addition to Alkenes

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Typical Reaction Conditions

for Halogen Addition

• only bromine (Br 2 ) and chlorine (Cl 2 ) undergo this reaction
• typically use a halogenated solvent like chloroform (CHCl 3 )
• low temperature (0 ºC)
• no light (avoid radical halogenation at saturated carbons)

UIC

University of Illinois

at Chicago

CHEM 232, Spring 2010

Slide

Lecture 12: February 18

Halogenation is Stereospecific

• anti^ addition of Br 2 or Cl 2 across double bonds
• two halogen atoms are trans to each other in rings