Chemistry 125: Lecture 48February 7, 2011

Alkenes: Stability andAddition MechanismsElectrophilic Addition

This

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C6H12 Alkene Isomer Stability

2.8

3.62.5

2.6

7.7

-12

-10

-14

-16

-18

-20

-22

Hf (

kcal

/mol

)

Correct for strain by subtractingMolecular Mechanics energy to leave “intrinsic” bonding energy

sp2 C-H sp2 C-C(+hyperconjugation?)~2.5 kcal/mol

cis trans~1.5 kcal/mol

Hf from Pedley (1994)

321 4# of C-C= bonds

Cf. Table 10.1, p. 413

H-Cl H2C=CH2 H-CH2-CH2-Cl+

Alkene / HCl Thermochemistrysingle + double three singles

146

1039983 81

= 249 = 263

favorable by 14 kcal/mol

12.5-22.0Hf

(NIST)

-26.0

Problem : Use the pKa values of HCl and H2O to estimate this shift in water.

H-Cl H2C=CH2 H-CH2-CH2-Cl+

-14.5Hgas

OH-

+

Alkene / HCl Thermochemistry

HOH + Cl- 19.0

+4.5

Addition in Acid

Eliminationin Base

single + double three singles

difference in proton affinities

(NIST)

Addition Mechanism?

H H*LUMO

H HHOMO

orthogonal

Addition to AlkenesSOMO : Radical Chain H-Br

Br•C C C C•

Br Br•C C

BrHH Br

HOMO/LUMO : Concerted

C C

H H

C CH H

C CC C*LUMOHOMO

HOMO/LUMO : Stepwise “Electrophilic”

Stepwise works with Pt/C Catalyst! (e.g. J&F 10.2, 410ff)

(e.g. J&F 11.4-11.5, 481-488) (Regioselective)

Br-HC C C C

HBrBrC C

H+

for any H-Hal (e.g. J&F Ch. 3, Ch. 9)

“Markovnikov”

HOMO-HOMO repulsive empty

C C BrH

C C Br

H

“anti-Markovnikov”

Pt

-22 kcal/mole

+17 kcal/mole

Surface Potential

(energy of proton on van der Waals

surface)

Electrostatics is important in positioning fragments, but new bonding requires orbital mixing.

Electrophilic Addition to Alkenes.

(e.g. J&F. 3.16-3.19)

HX addnvia R+

Regiochemistry(Markovnikov)

HydrationR+ + H2O

34:08-42:50

Section 9.1-9.7

Addition of Hydrogen Halides

9.3 148 Regioselectivity

9.5 374 Intermediate Cations

9.6 366;10.6 445 Cation Stability

9.9 386-9 Cation Rearrangement

Cation Stability e.g. Loudon 4.7C pp. 151-4

Compared to What? (Evidence) :

Bond Energy: sp2C-H vs. sp2

C-C

Hyperconjugation: C H vs. C C H

+ +C C

H+

Starting Alkene C C or Halide C X (ease of formation)

+HProduct or Transition State C :Nu (reactivity)+

Rationale (Theory) :

e.g Loudon 4.7C 151-4 Cation Stabilitye.g. J&F Table 10.2 p. 445

Compared to What? different numbers of atoms

in their standard states

not relative to each other, nor

to their respective starting materials

or products.

IRRELEVANT

(source?)

1° to 2° = 21

kcal/molevs. 2.5 kcal/mole

for n-alkenes

Bond Energy: sp2C-H vs. sp2

C-C

Hyperconjugation: C H vs. C C H

+ +

Intramolecular “Solvation” (Polarizability)

(In EtOH/H2O SN1 of t-Bu-Br is only ~5 kcal/mol easier than i-Pr-Br)

Cl

CH3 C CH

CH3

CH3

83% of product

CH3

H-Cl

2° Cation

+CH3 C CH

CH3

CH3

3° Cation(more stable)

CH3CH3 C CH

CH3

CH3

+CH3

CH3 C CH CH2

CH3

CH3

CH3 C CH

CH3

CH3

CH3

Cl

17% of product

Cl

H

Cl-

CH3 C CH

CH3

CH3

CH3

OH HO

CH3 C CH

CH3

CH3

CH3

H2O -H+

H-Catalyzed Hydration Subject to Rearrangement +

Rearrangement Competes with Reaction

Markovnikov Regiochemistry

+HClCH3 C CH CH3 C CH2

Cl- CH3 C CH2

Cl

unstable vinyl cation100-1000 times slower than alkene

HCl (excess)

CH3 C CH3

Cl

Cl

56%

44%

HBrCH3 C C CH3 CH3 C CH2

Br

Br

CH3

60%Markovnikov Regiochemistry

CH3 C CH3

Cl

+

Is halogen favorable or unfavorable for cation?

Second Step SlowBUT Markovnikov?

Yes!Hard to make cation because of electron withdrawal by halogen.

But if you’re going to make it, make it where you can get help.

Influence of Halogens

End of Lecture 48February 7, 2011

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