acid: proton donor roh to rx ta k - webassignncsu/ch221... · alkyl halide alcohol section 4.0 r x...

1

Strong/weak acid

1

1. 2. 3. 4. 5. 6. 7. 8.

0%

13%

58%

3%1%0%

3%

22%

TA K1. Strong acid Strong base

2. Strong acid Weak base

3. Weak acid Weak base

4. Weak acid Strong base

5. Strong base Weak acid

6. Strong base Strong acid

7. Weak base Strong acid

8. Weak base Weak acid2

ROH to RXAcid: proton donor

© Dr. Kay Sandberg

Strong acids donate very easily

Weak acids do not donate easily

In other words, weak acids hold ontightly to their protons. (counterintuitive)

In other words, weak bases cannot“grab” protons away from acids. (intuitive)

3

ROH to RX

R OH

Transformation wheel

alkylhalide

alcohol

Section 4.0

R XHX

H2O+

© Dr. Kay Sandberg

Starting material

Product

Reagent

What

Why – energy profile

How – mechanismExamine simultaneously

4

Substitution rxn

Section 4.9

R—OH + H—X R—X + H —OH

Substitution rxn

Hydroxyl and halide switch partners

© Dr. Kay Sandberg

Hydroxyl = hydroxy

5

Step 1R O + H Cl

H

Section 4.9

R O + Cl

H

H

pKa ~ -8 pKa ~ -2

Is this a “downhill” reaction?

stronger acid

weaker base

There are only a few species that can make anelectronegative O stay positive

the REALLY good negative charge “handlers”Cl1-, Br1-, I1-

© Dr. Kay Sandberg

weaker acid

stronger base

6

Step 1 continuedO + H Cl

H

Section 4.9

O + Cl

H

H

© Kay SandbergReaction Coordinate

Go

Step 1

1

2

7

Step 1 continuedR O + H Cl

H

Section 4.9

R O + Cl

H

H

Stronger acid

weaker base

We do have an electronegative atomcarrying a +1 formal charge

Is there anything that can be doneto relieve this misery???????

© Dr. Kay Sandberg

8

Step 2O + H Cl

H

Section 4.9

O + Cl

H

H

© Kay Sandberg

OH

H

+ C

“sexteted”

carbocation

Reaction Coordinate

Go

Step 1

1

2

Step 2

Now look at the O

no longer +

9

Step 3O + H Cl

H

Section 4.9

O + Cl

H

H

© Kay Sandberg

O

H

H+

C

carbocation

Reaction Coordinate

Go

Step 1

1

2

Step 2

Step 3

3

Cl10

All 3 steps

alkyloxonium ion = protonated alcohol

R O + H Cl

H

Section 4.9

R O + Cl

H

H

RO

H

H+

R Cl

•Basic alcohol attacks HClcreating alkyloxonium ion & Cl-

•Alkyloxonium dissociates forming carbocation & water

•Cl- captures carbocationcreating alkyl halide

intermediates

© Dr. Kay Sandberg

11

Step 1 TSO + H Cl

H

Section 4.9

O + Cl

H

H


Go

Step 1

1

R O H Cl

H

d+ d-

•bimolecular Step 1

O

H Cl

H

O

Cl

H

H

d charges?

pKa ~ -8 pKa ~ -2

12

Step 2 TS

Section 4.9

O

H

H

© Kay Sandberg

O

H

H+

C

Reaction Coordinate

Go

1

2

Step 2

R O H Cl

H

d+ d-

O

H Cl

H

O

Cl

H

H

•unimolecular

+ Cl

O

H

HC

C O

H

H

3

13

Partial charge

1. 2. 3. 4.

0%

98%

1%1%

Section 4.9

O

H

H

© Kay Sandberg

O

H

H+

C

Reaction Coordinate

Go

1

2

Step 2

R O H Cl

H

d+ d-

O

H Cl

H

O

Cl

H

H

•unimolecular Step 2

O

H

HC

C O

H

H

O C1. - -

2. + -

3. - +

4. + +14

Step 2 TS

Section 4.9

O

H

H

© Kay Sandberg

O

H

H+

C

Reaction Coordinate

Go

1

2

Step 2

R O H Cl

H

d+ d-

O

H Cl

H

O

Cl

H

H

•unimolecular Step 2

O

H

HC

C O

H

Hd+

d+

0 +1-1d+d-

15

Step 3 TS

Section 4.9

Cl

© Kay Sandberg

C1

2

Step 3

3

Cl

R O H Cl

H

d+ d-

O

H Cl

H

O

H

H

O

H

H

C

Reaction Coordinate

Go

C O

H

Hd+d+

Cl

Cl

16

Molecularity

1. 2.

59

%41

% 1. unimolecular

2. bimolecular

Section 4.9

Cl

© Kay Sandberg

C

2

Step 3

3

Cl

C

Reaction Coordinate

Cl

Cl

17

1. unimolecular

2. bimolecular

Section 4.9

Cl

© Kay Sandberg

C

2

Step 3

3

Cl

C

Reaction Coordinate

Cl

•bimolecular Cl

18

Partial charge

1. 2. 3. 4.

0% 0%

99%

1%

Section 4.9


Go

1

2C

Cl C

Cl

C

Cl

C

Cl

Cl

C Cl

d charges?

3

1. - -

2. + -

3. - +

4. + +

4

19

Section 4.9


Go

1

2C

Cl C

Cl

C

Cl

C

Cl

Cl

C Cl

d charges?

d-d+

3

1. - -

2. + -

3. - +

4. + +

20

All 3 TS’s

R Cld-d+

ROH, HCl

ROH2, Cl-

R+, H2O, Cl-

Section 4.11

R O

H

H

Reaction Coordinate

R O H Cl

H

RCl, H2O

d+d+

d+d-

+

© Kay Sandberg

Go

bimolecularbimolecular

unimolecular

21

ROH -> RX conversion

Section 4.9

O + Cl

H

H

O

H

H

+

© Dr. Kay Sandberg

Bronsted acid/base dissociation

Name??

O + H Cl

H

Cl

22

R

C

R

R

Cl

Step 3 start

Empty p-orbital

sp2 hybrid Cplanar bonds

23

Cld-

R

C

R

R

d+

Step 3 middle

24

Cl

R

C

R

R

C

R

RR

Cl

Step 3 end

sp3 hybrid Ctetrahedralgeometry

5

25

Lewis acid base

R

C

R

R

Section 4.10Lewis acid/base rxn

Cl

Lewis acid

e- pair acceptor

Lewis base

e- pair donor

© Kay Sandberg

“grabber”

“grabbee”

26

Lewis vs Bronsted

Section 4.10

© Kay Sandberg

Acid: proton ___________.

Base: proton ___________.

Brønsted vs Lewis

donoracceptor

Brønsted:

Acid: electron pair ___________.

Base: electron pair ___________.donor

acceptorLewis:

All acid/base reactions can be classifiedas Lewis.

Limited to H1+

27

acid/base

1. 2. 3. 4. 5. 6.

0%3%

0%

89%

2%

7%

1. 1 2 3

2. 1 3 2

3. 2 1 3

4. 2 3 1

5. 3 1 2

6. 3 2 1

All 3 stepsO + H Cl

H

Section 4.9

O + Cl

H

H

O

H

H

+

Cl

© Dr. Kay Sandberg

A) Lewis but not BronstedB) Both Lewis & BronstedC) Neither

28

Nucleophile & electrophile

R

C

R

R

Cl

Lewis baseLewis acid

electrophile

© Dr. Kay Sandberg

Greek: “like”Possesses positive charge character&is attractedto specieswith negativechargecharacter

nucleophile

Possesses negative

charge character

&is attracted

to specieswith

positivecharge

character

29

Rate determining step

R Cld-d+

R+, H2O, Cl-

Section 4.11

R O

H

HCl-

E ROH, HCl

ROH2, Cl-

Reaction Coordinate

R O H Cl

H

RCl, H2O

3 steps

d+d-

d+ d+

Eact

DE < 0+

ionic intermediates (polar)


© Kay Sandberg

ROH to RX conversion energy diagram

30

SN1

R Cld-d+

R+, H2O, Cl-

Section 4.11

SN1 mechanismR O

H

HCl-

ROH, HCl

ROH2, Cl-

R O H Cl

H

RCl, H2O

d+d-

d+ d+

+


Substitution unimolecularnucleophilic

© Kay Sandberg

bimolecularbimolecular

unimolecular

Reaction Coordinate

Go

ROH RCl

6

31

SN1 – HX reactivity

Section 4.8Preparation of alkyl halides from alcohols& hydrogen halides: building blocks


HF << HCl < HBr < HI

Reactivity of hydrogen halide

© Dr. Kay Sandberg

strongest acid

pKa: +3.2 -8 -9 -10

weakest acid

32

SN1 – ROH reactivity





© Dr. Kay Sandberg

Depends upon classification

Reactivity of alcohols

What is the classification?

1. 1o alcohol

2. 2o alcohol

3. 3o alcohol

33

1o alcohol

2o alcohol

3o alcohol

1% 0%

99%

34

SN1 – ROH reactivity





© Dr. Kay Sandberg

RCH2OH < R2CHOH < R3COH

tertiaryleast reactive most reactive

secondaryprimary


35

Tertiary ROH

Section 4.8Reactivity and reaction conditions





secondaryprimary


watertert-alkylchloride

hydrogenchloride

R3C—OH + H—Cl R3C—Cl + H —OH

tert-alcohol

25oC

high yield within minutes

© Dr. Kay Sandberg

36

Secondary and primary ROH






secondaryprimary


water2o alkylbromide

hydrogenbromide

R2CH—OH + H—Br R2CH—Br + H —OH

secondaryalcohol

80-100oC

water1o alkylbromide

hydrogenbromide

RCH2—OH + H—Br RCH2—Br + H —OH

primaryalcohol

120oC

© Dr. Kay Sandberg

7

37

All 3 ROH comparison



2o alcohol

80-100oC


1o alcohol

120oC

© Dr. Kay Sandberg

H—Cl R3C—Cl + H —OH

3o alcohol

25oC

H—Br

R3C—OH +

R3C—Br + H —OH

LQ #1) Both HCl and HBr are added to pentan-3-ol & the solution is heated to 80oC. Draw MOP.

MOP = major organic product

38

All 3 ROH comparison



2o alcohol

80-100oC


1o alcohol

120oC

© Dr. Kay Sandberg

Why can 3o alcohols react with HX at cooler temperatures than can 2o or 1o ROH?

H—Cl R3C—Cl + H —OH

3o alcohol

25oC

H—Br

R3C—OH +

R3C—Br + H —OH

LQ #1) Both HCl and HBr are added

39

reactive

1. 2. 3. 4.

0%

99%

0%1%

Which species is more reactive?

Reaction Coordinate

AGo

B

Reaction Coordinate

GoC

D

I II

I II1. A C

2. A D

3. B C

4. B D

40

SN1 E diagram`

R Cld-d+

R+

Section 4.11SN1 mechanism

ROH, HCl

ROH2, Cl-

R O H Cl

H

RCl

d+d-

R O

H

Hd+ d+

+

3o

Reaction Coordinate

Go

2o

© Dr. Kay Sandberg

Why????H2O, Cl-

41

Carbocation classificationsC

H

H

C CH3CH3CH2

Section 4.10Structure, bonding & stability of carbocations

propyl cation

1-methylpropyl cation

1-methylcycloheptyl cation

C-1 is the + charged C

© Dr. Kay Sandberg

1o

C

CH3

HCH3CH2

3o

2o

H3C

CH3C

CH3

tert-butyl cation1,1-dimethylethyl cation

42

Classification

1. 2. 3. 4. 5. 6. 7. 8. 9.

0% 0% 0% 1% 0%0%0%2%

96%

A B

A) Classification of carbocation

B) # H’s bonded to +C

Carbocation

for next

clicker ?

1. 1o 0

2. 1o 1

3. 1o 2

4. 2o 0

5. 2o 1

6. 2o 2

7. 3o 0

8. 3o 1

9. 3o 2

8

43

Classification

1. 2. 3. 4. 5. 6. 7. 8. 9.

0% 0%

98%

0% 0%0%1%1%0%

A B

A) Classification of carbocation

B) # H’s bonded to +C

1. 1o 0

2. 1o 1

3. 1o 2

4. 2o 0

5. 2o 1

6. 2o 2

7. 3o 0

8. 3o 1

9. 3o 2

44

Carbocation electronic description

H

C

H

H

Section 4.10Structure & bonding of carbocations

hybridization of C+?

HC

H H

sp2

HC

H H

electronic description

1 empty pure p orbital

planar

120o

3 C-H s bonds

“sexteted” C

© Dr. Kay Sandberg

45

Carbocation stabilitiesH

C

H

H

H

C

H3C

H

H3C

C

H3C

H

H3C

C

H3C

CH3

Section 4.10Stability of carbocations

Alkyl groups donate e- density to C+.

©Dr. Kay Sandberg

Alkyl groupis an electron pump

HC

H C

H

H

H

electrophilic

methylcarbocation

1o

carbocation2o

carbocation3o

carbocation

46

Inductive effectH

C

H

H

H

C

H3C

H

H3C

C

H3C

H

H3C

C

H3C

CH3


polarization of s bonding e-s (C-C s bonding e-s

more polarizable thanC-H s bonding e-s)

©Dr. Kay Sandberg

Alkyl groupis an electron pump

d+

d+

HC

H C

H

H

H

Inductive effect:

47

HyperconjugationHyperconjugation

HC

H C

H

H

H



HC

H C

H

H

H

HC

H C

H

H

H

d+

d+

bb

b

a

aa b

delocalization of s bonding e-s in a p -like interaction

©Dr. Kay Sandberg48

Electronic effectsHyperconjugation

HC

H C

H

H

H

©Dr. Kay Sandberg

Alkyl groupsare electron donating

(electron pumps)

d+

d+

HC

H C

H

H

H

Inductive effect

via s - bond via p - fashion

d+

d+

9

49

Carbocation stabilities


The more alkyl groups the more stable the C+.

Most stable?

H

C

H

H

H

C

H3C

H

H3C

C

H3C

H

H3C

C

H3C

CH3

•inductive effect •hyperconjugation


<<<

tertiary(most stable)

methyl(least stable)

© Dr. Kay Sandberg

50

E hill analogy

The slowest rxn has the highest hill

© Dr. Kay Sandberg

51

E hill analogy continued

The slowest rxn has the highest hill

BA

H3C

C

H3C

CH3

H3C

C

H3C

H

© Dr. Kay Sandberg

RDS of SN1

52

Step 2 analysis

Section 4.11

CH3OH < RCH2OH < R2CHOH < R3COH

Structure and reaction rate: SN1 mechanism:


H3C O

H

Hd+d+

E

Reaction Coordinate

CH3OH2

+

CH3+, H2O

O

H

Hd+

d+

RCH2

RCH2OH2

+

RCH2+, H2O

O

H

Hd+

d+

R2CH

R2CHOH2

+

R2CH+, H2O

O

H

Hd+

d+

R3C

R3COH2

+

R3C+, H2OEact Eact Eact Eact

Rate = k[alkyloxonium ion]

kkkk

© Dr. Kay Sandberg

methyl

tertiary

53

Step 2 comparison

Reaction Coordinate

Go

The species with the LOWER energy hill to cross over will be the MORE reactive species

2o

R

R

O

H

H

C

R

R

H

O

H

H

3o

R

R

O

H

HR

C

R

R

R

© Dr. Kay Sandberg

Smaller activation E =

54

SN2 E diagram O

H

HC

R

HH

Br

d+d-

Section 4.12

E

SN2 mechanismSubstitution

bimolecular

nucleophilic

O

H

HBr CH2R

O

H

HC

R

H H

Br +

RCH2OH + HBr

O H Br

H

RCH2

d+d-

methyl & 1o ROH © Dr. Kay Sandberg

HBr

10

55

Thionyl chloride

base

Section 4.13Other methods to convert

•alcohols to alkyl chlorides

RCH2—OH + SOCl2 RCH2—Cl + HCl + OSO

g

rx w/basethionyl chloride

© Dr. Kay Sandberg

DANGER! CORROSIVE. CAUSES BURNS TO ANY AREA OF

CONTACT. MAY BE FATAL IF INHALED. HARMFUL IF

SWALLOWED. VAPORS CAUSE SEVERE IRRITATION TO SKIN,

EYES AND RESPIRATORY TRACT. WATER REACTIVE.

DG = DH - TDS

http://www.jtbaker.com/msds/englishhtml/t2938.htm

56

ROH -> RCl reagents


•alcohols to alkyl chlorides

pyridine or K2CO3

R2CH—OH R2CH—ClSOCl2

R3C—OH R3C—ClHCl

1o & 2o

3o

N

© Dr. Kay Sandberg

base

typcial bases

57

Phosphorus tribromide


•alcohols to alkyl bromides

3R—OH + PBr3 3R—Br + H3PO3

watersoluble

© Dr. Kay Sandberg

1o & 2o

1o, 2o & 3o

R—OH R—BrHBr

require higher temperatures

58

Alcohol transformation wheel

R OH

Alcohol Transformation WheelSection 4.0

+ H2O

© Dr. Kay Sandberg

R Cl

+ HCl + SO2

PBr3

R Br

+ H3PO3

3o ROH

1o & 2o ROH1o & 2o ROH

LQ #2: MOP formed when 4-methylheptan-1-olis treated with thionyl chloride in base

R Cl

HCl

SOCl2base

HBr

1o , 2o, 3o ROH

+ H2O

R Br

3o RCl

1o & 2o RCl

1o , 2o, 3o RBr

1o & 2o RBr

SN1

SN1 (2o, 3o)SN2 (1o)

acid: proton donor roh to rx ta k - webassignncsu/ch221... · alkyl halide alcohol section 4.0 r x...

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