Jones M., Fleming S.A. Organic Chemistry

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11.12 Additional Problems 509

NBS, CCl

OCH

PROBLEM 11.47 Write a detailed mechanism for the bromina-

tion of ethyl phenylacetate (1) with NBS.

PROBLEM 11.48 Draw each of the resonance structures for the

radical intermediate formed in Problem 11.47. Focus on the

radical part.

PROBLEM 11.49 Predict the major product(s) for the reaction

between NBS (allylic halogenation) and the following alkenes:

(a) propene

(b) 1-butene

(d) cyclopentene

(e) 1-methylcyclohexene

PROBLEM 11.50 Write a detailed mechanism for the

photochlorination of 1 to give 2. Why is compound 1 reactive

even though it contains no π bond?

PROBLEM 11.51 Here is a more complicated photochlorina-

tion of a compound containing a three-membered ring.

Explain the formation of the following products from

spiropentane (1).

PROBLEM 11.52

Photolysis of iodine in the presence of

cis-2-butene leads to isomerization of the alkene to a mixture

of cis- and trans-2-butene. Explain.

hν

PROBLEM 11.53

Because of their high-potency, low-mammalian

toxicity, and photostability, certain esters of halovinylcyclo-

propanecarboxylic acids (pyrethroids) are promising insecticides

(see Chapter 10, p. 456). An important precursor to this class of

insecticides is the carboxylic acid (1), prepared as follows:

(a) Write a mechanism for the formation of 2. Be sure to

account for the observed regiochemistry. Hint: The CuCl

can abstract a chlorine atom.

(b) What transformations must be accomplished in the 21

conversion? Don’t worry about mechanisms.

bond in 1, how many stereoisomers of 1 are possible? The

most potent insecticides are derived from the (1R,3S)

isomer of 1. Draw this isomer.

COOH

CCCl

COOCH

two steps

CCCl

COOCH

CuCl

PCl

ROOR

85 ⬚C

PROBLEM 11.54 Write a mechanism for the following reaction

of 1-octene:

510 CHAPTER 11 Radical Reactions

PROBLEM 11.55 Provide mechanisms for the formation of the

following products:

PROBLEM 11.56 Provide mechanisms for the formation of the

following products:

77 ⬚C

(CH

)

(CH

)

(CH

)

(CH

)

CCl

ROOR,

77 ⬚C

(CH

)

(CH

)

CHCCl

CCH

CCl

PROBLEM 11.57 Propose a mechanism for the following reac-

tion. Be sure to rationalize the preferred formation of diene 1.

PROBLEM 11.58 Write a mechanism for the formation of

compound 1.

(major product)

ROOR,

500 ⬚C

ROOR, CCl

CCl

Use Organic Reaction Animations (ORA) to answer the

following questions:

PROBLEM 11.59 There are three radical processes included

in ORA. All three are on the second page of reactions.

Select the “Radical alkene hydrohalogenation” reaction. The

animation shows initiation steps and propagation steps for

the overall process. Write out the steps of the reaction and

indicate which are initiation steps and which are propagation

steps.

PROBLEM 11.60 No termination steps are shown in the

animation. Write out possible termination steps for the

reaction.

PROBLEM 11.61 The “Alkane halogenation” reaction is also

a radical process. Select this reaction and watch it several

times. What is the hybridization of the central carbon of

the intermediate? Select the SOMO (singly occupied

molecular orbital) track. Why has the calculation come up

with this hybridization? Is the hybridization fixed or does

it change?

PROBLEM 11.62 A third radical animation is “Benzylic

oxidation.”This reaction is shown using oxygen (O

) in the

initiation step. Why can oxygen remove a hydrogen from

isopropylbenzene? Is the benzylic hydrogen acidic? Is

oxygen basic? What is the hybridization of the benzylic

carbon in the intermediate? Is the hybridization of the

benzylic carbon fixed or does it change?

Dienes and the Allyl System:

2p Orbitals in Conjugation

511

12.1 Preview

12.2 Allenes

12.3 Related Systems: Ketenes and

Cumulenes

12.4 Allenes as Intermediates in the

Isomerization of Alkynes

12.5 Conjugated Dienes

12.6 The Physical Consequences

of Conjugation

12.7 Molecular Orbitals and

Ultraviolet Spectroscopy

12.8 Polyenes and Vision

12.9 The Chemical Consequences

of Conjugation: Addition

Reactions of Conjugated

Dienes

12.10 Thermodynamic and Kinetic

Control of Addition Reactions

12.11 The Allyl System: Three

Overlapping 2p Orbitals

12.12 The Diels–Alder Reaction

of Conjugated Dienes

12.13 Special Topic: Biosynthesis

of Terpenes

12.14 Special Topic: Steroid

Biosynthesis

12.15 Summary

12.16 Additional Problems

CONJUGATED ALKENES The different colors in fruits and vegetables arise from

the different conjugated alkenes that are produced by plants.

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

WEB 3D

Unconjugated dienes

Conjugated dienes

(E)-1,5-Decadiene

1,3-Butadiene 2-Methyl-1,3-butadiene

(isoprene)

1,3-Cyclohexadiene

1,4-Pentadiene

(E,E

)-2,4-Hexadiene

1,4-Cyclohexadiene1,4-Cyclooctadiene

FIGURE 12.1 Some conjugated and unconjugated double bonds.

Since chemists are not physicians, we shall scarcely benefit by their art,

except by making the physician a chemist.

—GEORGE REES

12.1 Preview

Sometimes the whole really is greater than, or at least different from, the sum of the

parts. In this chapter,we see just such a situation. One cannot always generalize from

the properties of alkenes to those of molecules with multiple double bonds. Dienes,

containing two double bonds separated only by a single bond (Fig. 12.1), often have

properties that are quite different from those of isolated, individual alkenes. This

difference depends on a phenomenon called conjugation. Conjugated double bonds

are alkenes connected by a single bond. A molecule containing widely separated

double bonds will generally react as would the individual alkenes. Double bonds

in conjugation react quite differently. A conjugated diene constitutes a separate

functional group. Cumulated alkenes (cumulenes) are molecules that have three or

more atoms attached only by double bonds.

A summary structure

for the all

l radical

The allyl radical

FIGURE 12.2 Allyl has a 2p orbital on

each carbon atom.

First we will cover the cumulenes, then we will generalize our discussion of the

phenomenon of conjugation and its effect on chemical reactions. Next we will develop

further the chemistry of a molecule we already encountered,allyl.Figure 12.2 shows

the neutral allyl radical, a system of three parallel 2p orbitals.

George Owen Rees was a nineteenth-century clinician who specialized in the analysis of blood and urine.

A student of MJ’s, Jordan M. Cummins, found this nice quote in the book by Stanley J. Reiser, Medicine and

the Reign of Technology, Cambridge University Press, Cambridge, 1978.

12.2 Allenes 513

Finally, we will see the outlines of how large molecules are constructed in Nature

from a rather simple building block, the diene isoprene (2-methyl-1,3-butadiene),

shown in Figure 12.1.

There is a chance for confusion here between the system of three parallel 2p orbitals called “allyl”and the “allene”

in which two double bonds are attached to one carbon. Be careful; these species are very different from each other!

WEB 3D

CCH

C C(CH

)

CHCH

CCHCH

3-Methyl-1,2-butadiene

(1,1-dimethylallene)

2,3-Pentadiene

(1,3-dimethylallene)

1,2-Cyclononadiene

Propadiene

(allene)

FIGURE 12.3 Some allenes.

The first problem is to work out a structure for allene.The key position in this

cumulated diene is certainly the central carbon atom that is shared by the two

double bonds, and the bonding involved has important structural consequences.

The central carbon in any allene is attached to only two groups and, therefore, sp

hybridization is appropriate.The central carbon uses two sp hybrid orbitals to form

a pair of σ bonds with the flanking methylene groups, leaving the central carbon’s

remaining two p orbitals available for π bonding (Fig. 12.4).

CCH

This carbon is attached to

only two other groups

(sp hybridization is appropriate)

2p Orbitals

FIGURE 12.4 The central carbon of

allene is sp hybridized.

ESSENTIAL SKILLS AND DETAILS

1. The overall skill that you will take away from this chapter is the ability to think about

the structural and chemical consequences of conjugation. Conjugation affects reactivity

markedly, and an ability to see how and why is essential before progressing to the

following chapters.

2. The difference between kinetic control and thermodynamic control must be understood.

Kinetic control means that the products are determined by the relative heights of the

transition states connecting starting materials and products, not by the energies of

the products themselves. Under conditions that supply enough energy to go over all

the transition states, thermodynamic control will operate. Under these conditions, the

product mixture is determined by the relative energies of the products themselves.

3. The Diels–Alder reaction has been characterized as the most important synthetic reaction

in organic chemistry. It is surely among the most important, because it makes the very

common six-membered rings in a single step. Every cyclohexene is the potential product

of a Diels–Alder reaction and the potential source of a diene and a dienophile through a

reverse Diels–Alder reaction. Be sure you can play the game of “find the cyclohexene”!

4. The number of sp hybridized carbons in an allene or cumulene has stereochemical

consequences: An even number of such carbons leads to cis/trans isomerism, whereas

an odd number produces a pair of enantiomers.

5. Allyl resonance is important in reactions of cations, radicals, and anions. Be sure you

can manipulate the allyl system easily.

12.2 Allenes

The simplest diene possible is propadiene,almost always known as allene (Fig.12.3).

Allene is the general term for any molecule containing two double bonds that share

a carbon atom. Allenes are systematically named as dienes or commonly named as

derivatives of allene (Fig. 12.3).

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

CCH

Notice that the

π bonds

are in perpendicular planes

sp sp

FIGURE 12.5 An orbital picture of

allene. Notice the perpendicular π

systems.

Each end carbon, though, is attached to three groups and is therefore hybridized

.This easy approach leads immediately to a structure for allene (shown in Fig. 12.5).

Notice how each 2p orbital of the sp

hybridized end carbons overlaps perfectly with

one of the 2p orbitals of the central,sp hybridized carbon.The important thing is not

to be panicked by the odd central carbon and to keep in mind how we dealt with new

structures before.Assign an appropriate hybridization,be sure to pay attention to the

geometrical requirements of the hybrid, and the structure will always work out.

WORKED PROBLEM 12.1 It would appear that there could be cis and trans isomers

of 1,3-dimethylallene, but there aren’t. Explain.

cis ???

trans ???

2,3-Pentadiene

(

1,3-dimeth

lallene

)

ANSWER The two-dimensional paper will fool us all the time if we do not think

in three dimensions. The end groups of an allene are not in the same plane, but

lie in perpendicular planes. The words cis (same side) and trans (opposite sides)

have absolutely no meaning for an allene.

CCH =CHCH

The planes containing the two

units are perpendicular

We have to be careful and pay attention to detail. Finding a reasonable structure

for allene is a classic example of a problem asked at this point in almost every course

in organic chemistry.

12.3 Related Systems: Ketenes and Cumulenes 515

WORKED PROBLEM 12.2 Two isomers of 1,3-dimethylallene do exist. What are they?

ANSWER There are two isomers of 1,3-dimethylallene, because it is a chiral mol-

ecule.The mirror image of 1,3-dimethylallene is not superimposable on the original;

therefore a pair of enantiomers exists.

Mirror

CCC

Allenes are high-energy molecules, just as are alkynes. The sp hybridization of

the central carbon of this functional group is not an efficient way of using orbitals

to make bonds, because π bonds are much weaker than σ bonds. The π bond energy

of about 66 kcal/mol is substantially lower than most σ bond energies. This ineffi-

ciency is reflected in relatively positive heats of formation ( values).Table 12.1

gives a few heats of formation for allenes and alkynes. Remember that heats of

formation are strictly comparable only among isomers. As you can see, allenes are

roughly of the same energy as alkynes. This similarity is not surprising because the

two molecules contain the same number of π bonds.

¢H

TABLE 12.1 Heats of Formation for Some Small Hydrocarbons

Alkynes (kcal/mol) Allenes (kcal/mol)

Propyne 44.6 Propadiene 47.7

1-Butyne 39.5 1,2-Butadiene 38.8

2-Butyne 34.7 1,2-Pentadiene 33.6

2,3-Pentadiene 31.8

≤H °

12.3 Related Systems: Ketenes and Cumulenes

In allenes, the double bonds share a central carbon atom. There are other cumulated

molecules in which heteroatoms (noncarbon atoms such as oxygen or nitrogen) take

the place of one or more carbons of an allene and/or in which there are more than

two double bonds in a row. When one end carbon of allene is replaced with an

oxygen, the compound is called ketene (Fig. 12.6). If both end atoms are oxygens,

the molecule is carbon dioxide. Cumulated hydrocarbons with three double bonds

attached in a row are called cumulenes, although they can also be named as

derivatives of 1,2,3-butatrienes (Fig. 12.6). Cumulenes are very reactive and most

difficult to handle.

OCO

C O

Carbon dioxide Ketenes Cumulenes

(butatrienes)

WEB 3D WEB 3D

FIGURE 12.6 Some molecules

containing cumulated double bonds.

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

Let’s look at this isomerization in detail. We do not begin with an allene, rather

we encounter one as an intermediate in this reaction.When 2-butyne is treated with

a very strong base such as an amide ion ( ), a proton can be removed from one

of the methyl groups to give a resonance-stabilized carbanion (Fig. 12.8).

PROBLEM 12.3 Draw Lewis structures for the molecules in Figure 12.6 as well as for

the related molecules: carbon disulfide ( ),isocyanates

carbodiimides ( ), and carbon suboxide ( ).

PROBLEM 12.4 Unlike 1,3-dimethylallene, 2,3,4-hexatriene (1,4-dimethylbuta-

triene) exists as a pair of cis and trans isomers. Explain.

PROBLEM 12.5 Demonstrate that 2,3,4-hexatriene is achiral.

ORN

O),S

1. K

–

2. H

/ NH

CHC

2-Butyne 1-Butyne

FIGURE 12.7 The isomerization of

2-butyne to 1-butyne can be carried

out in strong base.

–

C CH

CCC CH

Resonance-stabilized carbanion

–

FIGURE 12.8 Deprotonation of

2-butyne to give a resonance-

stabilized anion.

Ammonia and the carbon–hydrogen bonds of the methyl groups of 2-butyne

both have pK

values of about 38. Therefore, we can expect reasonable amounts of

the carbanion to be produced on reaction with the amide ion. 2-Butyne is much more

acidic than the parent hydrocarbon butane because the anion formed from 2-butyne

is resonance stabilized, and that from butane is not (Fig. 12.8).

What can happen to this carbanion? It can be reprotonated by ammonia to regen-

erate 2-butyne, but this reaction surely gets us nowhere, even though it is reasonable and

must happen frequently. Suppose, however, that the anion reprotonates at the other car-

bon sharing the negative charge? Now the product is 1,2-butadiene,an allene (Fig.12.9).

CCC CH

–

CCC CH

–

CCCH

1,2-Butadiene

(meth

lallene)

–

FIGURE 12.9 This resonance-stabilized anion can reprotonate in two ways. Reattachment

of a proton at the end re-forms 2-butyne, but protonation at the other carbon sharing the

negative charge produces an allene.

12.4 Allenes as Intermediates in the Isomerization

of Alkynes

Now that we have the structures of allenes and cumulenes under control, it is time

to look at reactivity. Our brief look at allenes permits us to understand one of the

most astonishing reactions of alkynes: the base-catalyzed isomerization of an inter-

nal triple bond to the terminal position (Fig. 12.7).

12.4 Allenes as Intermediates in the Isomerization of Alkynes 517

negative charge regenerates the allene, but reprotonation at the other carbon yields

a new, terminal alkyne, 1-butyne (Fig. 12.11).

–

CCC

–

Resonance-stabilized anion

FIGURE 12.10 Deprotonation of the

allene at the terminal position leads

to a new resonance-stabilized anion.

CCC

CCH

–

....

–

....

1-Butyne

1,2-Butadiene

–

FIGURE 12.11 Reprotonation either

re-forms the allene or gives the

isomerized alkyne, 1-butyne.

CCH

–

CCH

CHC

–

....

1-Butyne

CCH

An acetylide

when water is added

CCH

–

= 38

= 25

FIGURE 12.12 1-Butyne is by far the

strongest acid in this equilibrating

system. Removal of the acetylenic

hydrogen gives an acetylide, the

thermodynamic energy minimum in

this reaction. When the reaction

mixture is quenched by the addition

of water, the acetylide protonates

to give 1-butyne. It is critical to

understand why it is not possible to

re-enter the equilibrium at this point.

After acidification there is no base

present strong enough to remove the

acetylenic hydrogen!

The pK

of the allene at the terminal methylene position is also about 38, so the

amide ion is strong enough to remove the terminal proton to generate a new, reso-

nance-stabilized anion (Fig. 12.10).Reprotonation at one of the carbons sharing the

1-Butyne (pK

 25) is a much stronger acid than any other molecule present in the

mixture and will be rapidly and, in a practical sense,irreversibly deprotonated to give the

acetylide (Section 3.14).This deprotonation is the key step in this reaction (Fig. 12.12).

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

–

....

CCH

–

CCC

–

CCH

–

....

–

....

–

....

–

....

–

....

FIGURE 12.13 The full mechanism for the isomerization of 2-butyne to 1-butyne in strong base.

No matter where the equilibria in the earlier steps of this reaction settle out,the ther-

modynamic stability of the acetylide anion will drag the overall equilibrium far to

the right. When water is added to neutralize the solution and end the reaction, the

acetylide will be protonated and 1-butyne can be isolated. The overall reaction is

summarized in Figure 12.13.

–

There are no hydrogens on this

carbon to remove and carry the

isomerization further!

–

CCC

PROBLEM 12.6 Write a mechanism for the isomerization of 3-hexyne to 1-hexyne

in the presence of sodium amide (NaNH

) and NH

WORKED PROBLEM 12.7 Di-sec-butylacetylene (3,6-dimethyl-4-octyne) fails to

isomerize to another alkyne when treated with NaNH

/NH

. Explain.

ANSWER Isomerization is blocked by the alkyl groups. An allene can be formed,

but the reaction can go no further because there is no allenic hydrogen that can

be removed to produce a new resonance-stabilized anion.