Chemistry 125: Lecture 48February 8, 2010

Addition to Alkenesa Physical-Organic

MO Perspective This

For copyright notice see final page of this file

28:17-31:58

Butenes

Combustion of 4 Cgraphite + 8 HH-H

gives 607.5 kcal/mole

http://webbook.nist.gov/chemistry/

Use NIST Webbook to study factors influencing relative stability of butenes

For high precision convert to a common product, butane, using catalytic hydrogenation, which is fast and clean, and much less exothermic than combustion.

Compound Delta H f (gas)(+/-) DeltaH f (liquid)(+/-) MolMech Strain EnergyNIST webbook source1-Hexene -10.2 0.6 -17.5 0.6 1.6 9values2-Hexene, (E)- -12.3 0.2 -19.89 0.2 3.6 Wiberg 19812-Hexene, (Z)- -11.8 -19.15 0.2 4.9 Wiberg 19813-Hexene, (E)- -12.3 -19.6 0.2 2.7 Wiberg 19813-Hexene, (Z)- -11.2 0.2 -18.7 0.2 3.9 Wiberg 19812-Butene, 2,3-dimethyl- -16.8 0.36 -24.6 0.35 12.9 Wiberg 19912-Pentene, 3-methyl-, (Z)- -14.79 0.21 -22.48 0.2 7.2 Wiberg 19922-Pentene, 3-methyl-, (E)- -15.18 0.21 -22.69 0.2 7.5 Wiberg 1993Pentane, 3-methylene- -13.4 0.21 -20.8 0.2 6.9 Wiberg 19941-Butene, 2,3-dimethyl- -15.74 0.36 -22.73 0.35 11.6 Wiberg 19951-Pentene, 4-methyl- -11.82 0.16 -18.7 0.15 10.0 Steele 19971-Butene, 3,3-dimethyl- -21.3 0.3 17.8 Rossini 19601-Pentene, 2-methyl- -21.7 0.22 6.9 Rossini 19601-Pentene, 3-methyl- -18.88 0.29 9.1 Rossini 19602-Pentene, 2-methyl- -20.03 0.21 6.7 Kabo 19762-Pentene, 4-methyl-, (E)- -22.08 0.26 7.3 Rossini 19602-Pentene, 4-methyl-, (Z)- -21 0.18 8.7 Rossini 1960

Isomeric HexenesHf

Problem: Are these data consistent with those for the butenes?

C6H12 Alkene Isomer Stability

2.8

3.6

2.5

2.6

7.7

•

-12

-10

-14

-16

-18

-20

-22

Hf (

kcal

/mol

) Strain Energy (MM2)

sp2 C-H sp2 C-C~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

58:50-1:02:45

H H H H*LUMO HOMO

orthogonal

Addition to AlkenesSOMO : Radical Chain H-Br

Br•

C C C C•Br Br•

C CBr

HH Br

HOMO/LUMO : Concerted

C C

H H

C C

H H H HH H*LUMOHOMO

C CC C

*LUMOHOMO

HOMO/LUMO : Stepwise “Electrophilic”

(“works” with Pt/C Catalyst! Sec 10.2, 410ff)

(Section 11.4-11.5, 481-488 Regioselective)

Br-H

C C C CH

BrBrC CH

+

(for any H-Hal Ch. 3, Ch. 9)

(Markovnikov)

HOMO-HOMO repulsive empty

C C BrH

C C Br

H

11:35-19:41

-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.

Sec. 3.16-3.19

HX addnvia R+

Energetics

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

Loudon 4.7C 151-4 Cation Stability

(source?)

1° to 2° = 21

kcal/mole

vs. 2.5 kcal/molefor n-alkenes

Table 10.2 p. 445

Compared to what?

Double or ?

C C

H

C CH

C CH

MinimumSingle

HOMO-1

LUMO

HOMO

LUMO+1

“Hyperconjugation”( HOMO-LUMO mixing)

CCH

C C

H

C C

H

C C

H

C C C CC C

H2CH H2CH

H2CH

Double or ?MinimumSingle

LUMO+1

HOMO

H HH

CH3CH3

CH3CH3

CH3CH3C C C CC C

H2CH H2CH

H2CH

Double or ?MinimumSingle“ ”

LUMO+1

HOMO-7

9.9 386-9 Cation Rearrangementdo Problem 9.11(ring expansion)

End of Lecture 48Feb. 8, 2010

Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).

Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.

Materials from Wikimedia Commons are denoted by the symbol .

Third party materials may be subject to additional intellectual property notices, information, or restrictions. 

The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0