Jones M., Fleming S.A. Organic Chemistry

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12.7 Molecular Orbitals and Ultraviolet Spectroscopy 529

molecular orbitals of ethylene and 1,3-butadiene. You can see that there is a much

lower energy ππ* (HOMO–LUMO) transition possible in the conjugated

molecule 1,3-butadiene than there is in ethylene. Spectroscopy bears this out, and

1,3-butadiene absorbs at 217 nm, which is a much longer wavelength (lower energy)

than the absorption in ethylene.

Further conjugation does two things. First, it reduces the energy gap between

the HOMO and LUMO, and this pushes the position of the lowest energy absorp-

tion to a longer wavelength (lower energy). Eventually, the HOMO–LUMO gap

becomes small enough so that absorption enters the visible range (400–700 nm),

and as a result we perceive highly conjugated polyenes as colored. The first

polyene to absorb in the visible range appears yellow, because it is violet light

(400–420 nm) that is first absorbed as the HOMO–LUMO gap shrinks.

Remember that perceived color is a result of reflected light, the wavelengths of

light that are not absorbed. β-Carotene (Fig. 12.29), an important precursor to

vitamin A, absorbs blue light at 453 and 483 nm, and appears orange.

WORKED PROBLEM 12.16 How much energy is contained in light of wavelength

217 nm?

ANSWER E  Nhc/λ and the quantity Nhc  28.6  10

(p. 527). So,

E  (28.6  10

)/217  132 kcal/mol at 217 nm.

WEB 3D

Energy

π Orbitals of

ethylene

HOMO

LUMO

HOMO

π Orbitals of

1,3-butadiene

π Orbitals of

1,3,5-hexatriene

HOMO and LUMO

orbitals of

β-carotene

␤-Carotene

max

= 453 and 483 nm

FIGURE 12.29 As conjugation

increases, the HOMO–LUMO gap

decreases and the position of the

ππ* absorption shifts to longer

wavelength (lower energy).

530 CHAPTER 12 Dienes and the Allyl System: 2p Orbitals in Conjugation

The second impact of increased conjugation is that the number of possible UV

transitions increases and,therefore,the spectra become complex. Figure 12.30 shows

the spectrum of one moderately complicated conjugated molecule,which surely will

resist a simple analysis, and which would be most difficult to predict in detail.

However, many important molecules contain simple diene or triene systems, either

acyclic or contained in rings, and UV spectroscopy remains a useful tool for

structure determination in such molecules.

5.0

4.5

4.0

3.5

3.0

2.5

2.0

200 220 240 280 300 400 320 340

log ε

Wavelen

th (nm)

Solvent:

isooctane

1,5-Dimethyl-

naphthalene

FIGURE 12.30 A complicated UV

spectrum.

The position of longest wavelength absorption (λ

max

) depends in a predictable way

on the substitution pattern of these simple molecules. A set of empirical correlations

was collated by Robert Burns Woodward (1917–1979) and was known initially as

“Woodward’s rules”and now sometimes as “Woodward’s first rules.”These rules were

extended by Louis Fieser (1899–1977) and Mary Fieser (1909–1997) to include

cyclic dienes in the polycyclic compounds called steroids. Table 12.2 summarizes

Woodward’s and the Fiesers’rules.Woodward’s very different second set of rules will

appear in Chapter 20.

TABLE 12.2 Woodward’s and Fieser’s Rules for Ultraviolet Spectra of Dienes

Woodward’s Rules for Fieser’s Rules for

Acyclic Dienes (nm) Steroid Dienes (nm)

Base diene value 217 Base diene with both double

bonds in the same ring 253

Each added alkyl group 5 Base diene with double bonds

in different rings 214

Each double-bond exocyclic Each additional conjugated

in a six-membered ring 5 double bond 30

Each alkyl substituent 5

Each exocyclic double bond 5

12.7 Molecular Orbitals and Ultraviolet Spectroscopy 531

R. B.Woodward was acknowledged as the greatest American organic chemist,

and was certainly one of the greatest chemists of all time.There is remarkable agree-

ment on the subjective evaluation of Woodward’s preeminence among organic

chemists, and it’s worth taking a little time to see what set this man apart from

his merely excellent colleagues. First of all, he was devoted to the subject and

worked unbelievably hard at it. Although the main themes of his professional life

were the elucidation of structure and the synthesis of molecules of ever more

breathtaking complexity, his work is also notable for the attention he paid to

mechanistic detail, and to the broad implications of the work. Working hard is not

a matter of mere hours, but of maintaining focus as well. Woodward was able to

concentrate and either had or developed a prodigious memory for minutiae. In

another person that might amount to mere obsession, but Woodward was able to

extract from the sea of detail the important facts that allowed him time and again

to turn his work from the specific to the general. Early on he was able to propose

correct structures for such disparate molecules as penicillin, tetracycline, and fer-

rocene (Fig. 12.31).

WEB 3D

COOH

CONH

N(CH

)

Penicillin V

Ferrocene Tetrac

cline

PhOCH

FIGURE 12.31 Some complex molecules whose structures were proposed by Woodward and

his co-workers.

Woodward was able to pull obscure facts out of remote chemical hats and apply

them to the matter at hand in a way that often transformed the very nature of the

problem. The molecules he considered worthy of synthesis grew in complexity

throughout his career, but it was more than the difficulty of the target that made his

work so universally impressive.Woodward’s work led places—it had “legs.”For exam-

ple, Woodward, primarily a synthetic chemist, made the observations [with Roald

Hoffmann (b. 1937)] that led to the development and exploitation of what is arguably

the most important theoretical construct of recent times. Hoffmann and Kenichi

Fukui (1918–1998) of Kyoto University received the Nobel prize for this work in

1981. Many believe Woodward would have shared this Nobel prize had he lived (it

would have been his second; his first was awarded in 1965 for “his outstanding

achievements in the art of organic synthesis”). Some of the molecules synthesized

by Woodward and his co-workers are shown in Figure 12.32. Woodward said that

the synthesis of quinine was conceived when he was 12 years old.

Should you memorize Woodward’s rules? Frankly, we think not—although there

are those who strongly disagree, and some of these people write examination ques-

tions.These rules were an important part of structure determination but are restricted

in utility to relatively simple polyenes. You should know they exist, know what

systems they apply to, and look them up when you need them. If you wind up in

steroid research, you’ll learn them.

532 CHAPTER 12 Dienes and the Allyl System: 2p Orbitals in Conjugation

WEB 3D

Strychnine, 1954

Vitamin B

with A. Eschenmoser and a cast of thousands, 1972

Triquinacene, 1964

Reserpine, 1956

OCH

OOC

Quinine,

with W. von E. Doering, 1944

Patulin, 1950 Cholesterol, 1951

OCH

CONH

COO

COOH

–

CONH

NOC

–

Cephalosporin C, 1966

FIGURE 12.32 Some molecules synthesized by the Woodward group.

12.8 Polyenes and Vision 533

12.8 Polyenes and Vision

When we hear or read the word “spectroscopy,”we tend to imagine rooms full of expen-

sive machines measuring away. It’s quite an inhuman vision. Yet we are spectrometers

ourselves. All of our sensory experiences, most obviously vision, are examples of spectro-

scopic measurements. Those machines in our imaginations are mere extensions of our

biological,in-house equipment.When you see a red apple among a basket of green ones,

and decide that it is the ripe red one you will eat,you are acting on a spectroscopic meas-

urement. Let’s examine vision a little bit here, because it depends on polyene chemistry.

Many polyenes are important in biological processes, and as we might imagine,

polyenes that absorb in the visible spectra are important in human vision. Vitamin A

is trans-retinol,for example (Fig. 12.33).We have two enzymes,one of which, retinol

dehydrogenase, oxidizes trans-retinol to trans-retinal and another, retinal isomerase,

which isomerizes one double bond of trans-retinal to produce cis-retinal.

Vitamin A,

trans-retinol

trans -Retinal

cis-Retinal

retinal

A trans double bond

A cis double bond

Still a trans double bond

isomerase

retinol

dehydrogenase

WEB 3D WEB 3D

FIGURE 12.33 Vitamin A (trans-

retinol) is oxidized to trans-retinal

and then enzymatically isomerized to

cis-retinal.

cis-Retinal (but not trans-retinal) has the proper shape to react with a protein

called opsin to form rhodopsin (Fig. 12.34). Rhodopsin absorbs strongly in the visi-

ble spectrum, which results in isomerization of the cis double bond back to trans.

This absorption of visible light changes the shape of the molecule severely, destroy-

ing the crucial fit of molecule and protein. The attachment point is exposed, and

the retinal is detached from the protein. As this process occurs, a nerve impulse is

generated and perceived by our brains as light.The chemistry involved in the attach-

ment and detachment of retinal and opsin is covered in Chapter 16.

cis -Retinal

Opsin

visible

light

Rhodopsin

Opsin

trans -Retinal

FIGURE 12.34 cis-Retinal has the correct shape to react with opsin to form rhodopsin.The squiggly lines indicate

the rest of the molecule.

534 CHAPTER 12 Dienes and the Allyl System: 2p Orbitals in Conjugation

12.9 The Chemical Consequences of Conjugation:

Addition Reactions of Conjugated Dienes

Now we move on to reactions (at last!). Conjugation affects not only the physi-

cal properties of molecules but their reactions as well. Although the basic chem-

istry of conjugated dienes does resemble that of isolated alkenes,there are detailed,

but important differences introduced by the presence of the pair of connected

double bonds.

12.9a Addition of HBr and HCl When hydrogen chloride adds to

1,3-butadiene at 78 °C, two major products of hydrohalogenation are formed,

3-chloro-1-butene and trans-1-chloro-2-butene (Fig. 12.35). There is a very small

amount of cis-1-chloro-2-butene also formed. The main product is produced

through a pathway of 1,2-addition and the minor products by a 1,4-addition.

1,2-Addition 1,4-Addition

–78 ⬚C

trans -1-Chloro-2-buten

(24%)

3-Chloro-1-butene

(75.5%)

FIGURE 12.35 Hydrohalogenation

of 1,3-butadiene with HCl

produces the products resulting

from a 1,2- and a 1,4-addition (red

numbers not IUPAC).

BOMBYKOL

receptor for bombykol also recognizes the other stereoiso-

mers, but the proper Z, E (cis, trans) isomer is far

more active than the Z, Z (cis, cis) or E, E (trans, trans)

versions.

As you might imagine, if bombykol is to function

effectively as an attractant in the open air, reception must

be very efficient; a little bombykol must go a long way if

the silkworm moths are to have a satisfactory love life.

In fact this is so; the male is staggeringly efficient at

detecting this molecule. It is effective in doses as small

as 1  10

12

μg/mL.

For a nice introduction to pheromones, see William

C. Agosta, Chemical Communication, Scientific American

Library, New York, 1992.

Most insects (and other animals,

including humans) “talk” to each

other by releasing chemicals that

carry meaning. Such molecules

are called “pheromones.” Some

are sexual attractants, others are trail markers, and still

others carry an alarm signal. Here is bombykol, a simple

cis, trans dienol that serves as the female silkworm moth’s

attractant for the male. The female of the species, Bombyx

mori, releases bombykol from two glands located at her

abdomen. The structure of bombykol was worked out by

the German chemist Adolph Butenandt over a long

period, ending in 1956. He collected 6.4 mg of the pure

pheromone from 500,000 moth abdomens. Such work was

extraordinarily difficult in those days, as he did not have the

advantage of modern spectroscopy to help with structure

determination.

Eventually, the correct structure was determined and

the molecule was made independently. The male moth’s

1,2-Hydrohalogenation of dienes

1,4-Hydrohalogenation of dienes

12.9 The Chemical Consequences of Conjugation 535

We saw this reaction briefly in the discussion of resonance in Chapter 9

(p. 369). The source of the two pathways becomes clear as we work through the

mechanism step by step. The first reaction must be protonation of butadiene by

hydrogen chloride (Fig. 12.36). There is a choice of two cations, one of which is

a resonance-stabilized allylic cation, and the other an unstabilized primary

carbocation. The stabilized cation is much lower in energy and its formation is

greatly preferred.

C(1)

C(2)

–

Resonance-stabilized allylic cation

(more stable)

Unstabilized primary

carbocation

(very unstable)

–78 ⬚C

–

FIGURE 12.36 Formation of a

resonance-stabilized allylic cation

is preferred.

The resonance description of the allylic cation shows the source of the two prod-

ucts,because chloride can add to either of the two carbons sharing the positive charge

to give the two observed alkyl chloride products (Fig. 12.37). Hydrohalogenation of

conjugated dienes with HBr also results in 1,2- and 1,4-addition.

–

(b)(a)

path a

1,4-Addition

1,2-Addition

–

path b

–

FIGURE 12.37 Addition of the

nucleophilic chloride ion at the two

positions (a and b) sharing the

positive charge gives the products of

1,4- and 1,2-addition.

PROBLEM 12.17 When treated under S

1 conditions, the two allylic chlorides

shown here give the same pair of alcohols in the same ratio. Draw structures for

the two alcohols and show the mechanism leading to each product.

Two

alcohols

Same two

alcohols

in the same

ratio

536 CHAPTER 12 Dienes and the Allyl System: 2p Orbitals in Conjugation

ANSWER Although there is nothing wrong structurally or electronically with the

five-membered ring bromonium ion, it cannot be involved in this reaction.

Opening of this ring must lead to an alkene containing a cis double bond, and the

1,4-product is,in fact, only trans (Fig. 12.38).The technique for solving this prob-

lem is to use the five-membered bromonium ion as the intermediate, see what the

structure of the product must be, and compare that to what is actually observed.

If we pay careful attention to stereochemistry, we can see that the five-membered

ring cannot be involved.

12.9b Addition of Br

and Cl

Other addition reactions of conjugated dienes

also produce two products. Like an isolated alkene, conjugated dienes react with

bromine or chlorine to give dihalides. As in hydrohalogenation addition, products

of both 1,2- and 1,4-addition are formed (Fig. 12.38).

1,2-Addition 1,4-Addition

–75 ⬚C

trans -1,4-Dichloro-2-butene

(50%)

3,4-Dichloro-1-butene

(50%)

–15 ⬚C, CCl

trans -1,4-Dibromo-2-butene

(33%)

3,4-Dibromo-1-butene

(67%)

FIGURE 12.38 Addition of bromine

or chlorine to conjugated dienes gives

two products.

WORKED PROBLEM 12.18 One potential source of the 1,4-addition product in the

bromination of a conjugated diene is the bromonium ion shown below. Look

carefully at the products of the reaction of 1,3-butadiene and bromine in Figure

12.38 and then explain why the mechanism shown is certainly wrong.

–

1,4-Produc

–15 ⬚C, CCl

–

A cis double bond

12.10 Thermodynamic and Kinetic Control of Addition Reactions 537

12.10 Thermodynamic and Kinetic Control

of Addition Reactions

The addition reactions in the previous sections allow us to look at general properties of

chemical reactions and to address fundamental issues of reactivity. Just what are the fac-

tors that influence the direction a reaction takes? Why is one product formed more than

another? Is it because of the energies of the products themselves, or something else?

There are curious effects of temperature and time on the addition reactions of

HX and X

with conjugated dienes. If the reactions are run at low temperature, it

is the product of 1,2-addition that is generally favored. The warmer the reaction

conditions, the more product of 1,4-addition is formed.At high temperature (25 °C)

the product of 1,4-addition is favored (Fig. 12.39).

–78 ⬚C

25 ⬚C

H H

1,2-Addition 1,4-Addition

(81%) (18%)

(44%) (56%)

trans -1-Bromo-2-buten

3-Bromo-1-butene

FIGURE 12.39 The ratio of 1,2-

to 1,4-addition is temperature

dependent.

The effect of time is seen in the observation that the products shown in Figure

12.39 will equilibrate if allowed to stand in solution.As Figure 12.40 shows,the same

mixture of 1,2- and 1,4-addition products is formed from each compound, and the

product of 1,4-addition is favored.

25 ⬚C

....

1,2-Addition

product

(

100%

)

1,2-Addition

product

(

30%

)

1,4-Addition

product

(

70%

)

1,4-Addition

product

(

100%

)

ZnCl

25 ⬚C

ZnCl

FIGURE 12.40 Equilibration of the

products of 1,2- and 1,4-addition at

25 °C.

Several questions now confront us: (1) Why is the product of 1,4-addition

favored? (2) How do the products of 1,2- and 1,4-additions equilibrate? (3) Why

is the 1,2-addition product favored at low temperatures?

The first question is easy. The product of 1,4-addition contains a disubstituted

double bond,whereas the 1,2-product has only a monosubstituted double bond, and

is therefore less stable (Fig. 12.41). Remember: The more substituted an alkene, the

more stable it is (p. 115).

Monosubstituted

double bond

(less stable)

Disubstituted

double bond

(more stable)

1,2-Addition

product

1,4-Addition

product

FIGURE 12.41 The compound

containing a disubstituted double

bond is more stable than the

compound with a monosubstituted

double bond.

538 CHAPTER 12 Dienes and the Allyl System: 2p Orbitals in Conjugation

It is also not difficult to see how the two products equilibrate. For example, S

solvolysis of either of the two products in Figure 12.40 leads to the same,resonance-

stabilized allylic cation (Fig. 12.42). The chloride ion can then add at either of the

two positions sharing the positive charge and at equilibrium it will be the more sta-

ble product of 1,4-addition that dominates.

–

(a)

(b)

FIGURE 12.42 The 1,2- and

1,4-addition products equilibrate

through the formation of a

resonance-stabilized allylic cation.

The last question is tougher. Why should one product be favored at low tempera-

ture and another at higher temperature and at longer reaction times? With longer

reaction times and higher temperatures, thermodynamics takes over and the more

stable product prevails. At shorter reaction time,or under mild conditions, it is not the

more stable 1,4-compound that is favored, but the less stable 1,2-addition product.This

change can be explained if the 1,2-addition product is formed more easily.The rate of

formation of the 1,2-product is then by definition faster than that of the 1,4-product.

Therefore, the transition state for formation of the less stable 1,2-product must be lower

in energy than the transition state for formation of the more stable 1,4-product.The

lower energy product is formed by passing over the higher energy transition state and

the higher energy product is formed by passing over the lower energy transition state

(Fig. 12.43). In Chapter 8, you were warned that such cases would occur; here is one.

Reaction progress

Energy

Transition state

for 1,4-addition

Transition state

for 1,2-addition

–

1,2-Product

(less stable)

1,4-Product

(more stable)

Activation energy for

formation of the more

stable product

(formed slower)

Activation energy

for formation of the

less stable product

(formed faster)

The energy difference

between the products

FIGURE 12.43 An energy diagram

that summarizes formation of

1,2- and 1,4-addition products from

a common intermediate.