Chemistry 125: Lecture 54February 22, 2010

Linear and Cyclic Conjugation Allylic Intermediates(4n+2) Aromaticity

This

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Is There a Limit to 1 Energy for Long Chains?

8 1/8 1/8 7 7/8

4 1/4 1/4 3 3/4

Chain length

2

Normalized AO size

1/2

Overlapper bond

(AO product)

1/2

Number of

bonds

1

Total overlap

stabilization

1/2

N 1/N 1/N N-1 (N-1)/N

Yes, the limit is 1, i.e. twice the stabilization of the H2C=CH2 bond.

Similarly, the LUMO destabilization limit is twice that of the H2C=CH2 MO..

N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “” units.

N=2 ethyleneN=3 allylN=4 1,3-butadiene

:

.p

:

N=1 an isolated 2p AO

:

.

+1

-1

0

Semicircle Mnemonic for MO Energy in Conjugated Chains.

Radius of circle = 2 stabilization of H2C=CH2

Place points denoting length of chain evenly along circumference between upper and lower limit (+1 and -1).

.

.

..

.

.

.

.

.

:::

. etc.. . .All odd chains have a non-bonding MO with nodes on alternant carbons. It is the locus of the “odd” electron in the radical, and of + (-)

charge in the cation (anion).

As the conjugated chain lengthens, more and more levels are crowded

between -1 and +1, and the HOMO-LUMO gap decreases.

Color shift toward red.

MO

Ene

rgy

(uni

ts o

f 2

)

allylic stabilizationsame 2 electron stabilization

for cation, radical, anion

(vs. isolated p and )

(difference is resonance stabilization of butadiene vs. 2 isolated ethylenes)

[ limit of ±(N-1)/N ]

Allylic Intermediates:Allylic Free-Radical Bromination

Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543

NBS

Allylic Intermediates:Addition of HX to Butadiene

Sec. 12.9-12.10 pp. 534-541

+17.6

-21.4

HOMO-4HOMOLUMO+1LUMOHOMO-1LUMO+1LUMOHOMO

Butadiene Propenyl CationH+

hyperconjugated C-H

bestoverlap

bestpotential

best productbest overlap

Propenyl Cation

+152+144

+132+99

Surface Potential

bestpotential

best potential

symmetrical(but for D)

p. 1288

3.1 : 1-78°

1.6 : 125°

Cl-

DCl

rapid ion-pair collapse competes with motion

D CD2 CD3

CD3

HCH2

If Step 1 (motion) is rate-limiting, H- and D-transfer products should

form in equal amounts. (because their motions should be equally fast)

If Step 2 (atom shift) is rate-limiting, more H-transfer product should form.

kH/kD > 1 (kinetic “isotope effect”)

In a Very Viscous Solvent Can Short-Range Motion Constitute a Rate- (and Product-) Determining Step?

CH3

CH3

H3C CD3

CD3

CD3

•

•

CH3

CH3

H3C

(2) Shift D atomexothermic/easy/fast

N

N

(2) Shift H atomexothermic/easy/faster

CD3

CD3

CD3

CH3

H3C

NN

NN

Kinetic vs. Thermodynamic ControlSec. 12.10 pp. 537-540

Allylic Intermediates, Transition States:SN1 and SN2

Sec. 12.11a,b pp. 541-543

Allylic Intermediates:RH Acidity

Sec. 12.11d pp. 543-544

Cf. Benzylic Intermediates (sec. 13.12)

e.g. Ph-CH2-H pKa = 41

predicted

observed !

Conjugation worth ~30 kcal !

AROMATICITYCh. 13-14

Cf. 13.5a pp. 580-581

Bringing the ends of a conjugated chain together to form a ring gives a

lowest MO with one additional bonding interaction.

In a conjugated ring peripheral nodes must come in even numbers. e.g. cyclopropenyl

0 nodes 2 nodes

Lowest MO will have energy = -N/N = -1

2 nodesE = -1 E = +1/2 E = +1/2

Energy Shifts on “Ring Formation”

+1

-1

0

.

.

.

.

::M

O E

nerg

y (u

nits

of

2)

::

End to End Interaction

favorable

favorable

unfavorable

unfavorable

Shifts Alternate (because of node parity).

Hückel’s Rule: 4n+2 electrons is unusually favorable in a conjugated ring.

On bringing the ends of a chain together, odd-numbered MOs (1, 3, 5, etc.) decrease in energy

(favorable terminal overlap for 0,2,4… nodes), while even-numbered MOs (2, 4, 6, etc.) increase in energy

(unfavorable terminal overlap for 1,3,5… nodes).

Thus having an odd number of occupied orbitals (more odd-numbered than even-numbered)

insures overall stabilization of ring (compared to chain).

[though there may be strain in the bonds]

an odd number of e-pairs

(where n in an integer)

:

:

:

:

..

There is always an MO at -1.

Circle Mnemonic for MO Energy in Conjugated Rings.

Inscribe regular polygon with point down.

+1

-1

0

MO

Ene

rgy

(uni

ts o

f 2

)

Same radius as for open chain

Read MO energies on vertical scale.

:

4 cyclobutadiene6 benzene

3 cyclopropenyl

..

.

.

.

.

.

. .

..

reactive SOMOs !

Cation strongly stabilized(vs. allyl+)

::

:4n “Antiaromatic”!

slightly destabilized(vs. butadiene)

Stabilized(vs. hexatriene):

::

Radical less stabilized (vs. allyl•)

..

Anion de stabilized •-

open

-cha

in

ene

rgie

s fr

om s

emic

ircl

e m

nem

onic

Generalization of Aromaticity:

4n+2 StabilitySec. 13.6 pp. 582-595

Transition State “Aromaticity”

:

H

X-

Y

:

H

.

H

:

N

Pyridine

HH

H H

O

Furan

H

HH

H H

N

PyrroleHH

H

N

NImidazole

Heteroaromatic Compounds(Sec. 13.9 pp. 598-601)

::

::

.

Y-

HX

:

.

Relay for long-rangeproton transfer by enzymesN.B. Single . denotes contribution of 1 e to

system (redundant with double bond).

(occurs in amino acid histidine)

End of Lecture 54Feb. 22, 2010

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