16.14 Side-chain halogenation of alkylbenzenes

Chlorination and bromination of side chains differ from one another in orientation and reactivity

benzylic hydrogen of toluene

3.3 times

tertiary hydrogen of an alkane

100 million times

hydrogen of methane

as reactive as

Side-chain halogenation of alkylbenzenes proceeds by the same mechanism as halogenation of alkanes.

benzylic hydrogens are unusually easy to abstract means that benzyl radicals are unusually easy to form

Allyl radical (88 kcal) > = benzyl radical (85 kcal) > tert-bulyl radical (91 kcal)

contains less energy and is more stable than

Orientation of chlorination shows that chlorine atoms, like bromine atoms, preferentially attack benzylic hydrogen ; but, as we see, the preference is less marked:

hydrogen 3o 2o 1o Benzylic

relative reactivities: 5 3.8 1.0 1.3

the more reactive chlorine atom is less selective than the bromine atom (on alkanes): less selective between hydrogens in a single molecule, and less selective between hydrogens in different molecules.

In the attack by the comparatively unreactive bromine atom, the transition state is reached late in the reaction process: the carbon-hydrogen bond is largely broken, and the organic group has acquired a great deal of free-radical character. The factors that stabilize the benzyl free radical stabilize the incipient benzyl free radical in the transition state. In contrast, in the attack by the highly reactive chlorine atom, the transition state is reached early in the reaction process: the carbon-hydrogen bond is only slightly broken, and the organic group has acquired little free-radical character. The factors that stabilize the benzyl radical have little effect on this transition state.

Transition state reached late, resembles products.

transition state reached early,

resembles reactants.

16.15 Resonance stabilization of the benzyl radical

104-85=19

19 kcal/mole less energy is needed to form the benzyl radical

Toluene contains the benzene ring and is therefore a hybrid of the two Kekule structures, I and II:

the benzyl radical is a hybrid of the two Kekule structures, III and IV:

we can draw three additional structures for the radical: V, VI, and VII.

A double bond between the side chain and the ring, and the odd electron is located on the carbon atoms ortho and para to the side chain.

odd electron is not localized on the side chain but is delocalized, being distributed about the ring

16.3

Contribution from the three structures, V-VII, stabilizes the radical in a way that is not possible for the molecule. Resonance thus lowers the energy content of the benzyl radical more than it lowers the energy content of toluene.

16.4

the benzyl radical is stabilized by resonance.

delocalization results from overlap of the p orbital occupied by the odd electron with the p cloud of the ring.

16.16 Triphenylmethyl: a stable free radical

Is there any direct evidence for the existence of free radicals?

We do not find bottles on the laboratory shelf labeled "benzyl radicals" or "allyl radicals."

In 1900, young Russian-born chemist Moses Gomberg, an instructor at the University of Michigan

Journal of the American Chemical Society Berichte der deutschen chemischen Gesellschaft

He had prepared tetraphenylmethane.

(a synthesis a number of eminent chemists had previously attempted, but unsuccessfully),

he tried to couple together two triphenylmethyl groups by use of a metal

- sodium - finely divided silver, mercury, or, best of all, zinc dust.

Triphenylchloromethane benzene solution

zinc dust

filtered the solution free of the metal halide

benzene was evaporated

white crystalline solid

(melted at 185)

he thought was hexaphenylethane

Then for Synthesis of hexaphenylethane:

C, H Analysis: 88% carbon and 6% hydrogen, a total of only 94%.

he carried out the analysis again

he had prepared not a hydrocarbon not hexaphenylethane but a compound containing 6% of some other element, probably oxygen.

He carried out the reaction again, this time under an atmosphere of carbon dioxide.

an entirely different substance - much more soluble in benzene than his first product - having a much lower melting point - correct composition for hexaphenylethane: 93.8'% carbon, 6.2% hydrogen.

new substance ( benzene solution) yellow solution.

air

yellow color disappeared

Finally the color disappeared

evaporation of the solvent yielded the original compound of m.p. 185o

The compound of m.p. 185 was the peroxide,

X2

Gomberg was proposing that he had prepared a stable free radical.

yellow

colorless

colorless

A solution of the dimer is yellow because of the triphenylmethyl present in the equilibrium mixture.

O2

yellow color disappears

More dimer dissociates to restore equilibrium and the yellow color reappears.

I2

it is triphenylmethyl that reacts with iodine

For nearly 70 years it was believed to be hexaphenylethane (this dimer) .

Then, in 1968, the dimer was shown to have the structure I.

Gomberg's original task is still unaccomplished:

hexaphenylethane, it seems, has never been made.

dissociation to form free radicals is the result of two factors: 1) triphenylmethyl radicals are unusually stable - there are an even larger number of structures (36 of them) that stabilize the radical but not the hydrocarbon;

- the odd electron is highly delocalized, being distributed over three aromatic rings.

2) crowding among the large aromatic rings tends to stretch and weaken the carbon-carbon bond joining the triphenylmethyl groups in the dimer

- the bulky groups make it difficult for the carbon atoms to approach each other closely enough for bond formation

so difficult, in fact, that hexaphenylethane is not formed at all, but instead dimer even with the sacrifice of aromaticity of one ring.

16.17 Stability of the benzyl cation

Is the benzyl cation, like the free radical, unusually stable?

DH = + 166 kcal

61 Kcal

Carbocation can be represented by three structures

Positive charge is located on ortho and para carbon atom.

CH2

G

CH2

G

G release electrons:

stabilizes carbocation,

activates substrate

G withdraws electrons:

destabilizes carbocation,

deactivates substrate

16.18 Nucleophilic substitution in benzylic substrates

- OCH3 - NO2

SN1 type Rate of reaction ≡ Rate of formation of carbocation

Benzyl cation 1o

As stable as secondry cation

SN2 type Rate of reaction ≡ Steric factors

Benzyl cation 1o

Offer relatively little steric hindrance to Nucleophilic attack (as fast as primery substrates)

RX = C6H5CH2X (C6H5)2CHX (C6H5)3CX

SN2 increases

SN1 increases

SN2vs.SN1

Additional Phenyl groups raise the stability of the cation And speed up its formation by SN1 They increase steric hindrance to nucleophilic attack And slow down SN2

16.19 Preparation of alkenylbenzenes. Conjugation with ring

On an industrial scale, the elimination generally involves dehydrogenation.

Friedel-Crafts reaction between two simple hydrocarbons, benzene and ethylene

In the laboratory: dehydrohalogenation or dehydration

Dehydrohalogenation of l-phenyl-2-chloropropane, or dehydration of l-phenyl-2-propanol, could yield two products:

3-phenylpropene is rapidly converted into 1-phenylpropene by treatment with hot alkali.

- orientation of elimination

- ease of elimination

16.20 Reactions of alkenylbenzenes

undergo two sets of reactions:

Substitution in the ring

addition to the double bond in the side chain

Mild condition

Catalytic hydrogenation

oxidation

Mild more vigorous

Halogenation

Both double bond and ring react with halogens by ionic mechanisms

Halogen is consumed by the double bond first, and only after the side chain is completely saturated does substitution on the ring occur.

Ring-halogenated alkenylbenzenes must be prepared, therefore, by generation of the double bond after halogen is already present on the ring. For example:

16.21 Addition to conjugated alkenylbenzenes:

addition of HBr to 1-phenylpropene

16.23 Analysis of alkylbenzenes

Aromatic Hydrocarbons with saturated side chains distinguished from alkenes

Br2/ CCl4 No decolorize

KMnO4

cold, dilute, neutral No decolorize

They are distinguished from alkanes

they are sulfonated and thus dissolve in cold fuming sulfuric acid

They are distinguished from alcohols and other oxygen-containing compounds

to dissolve immediately in cold concentrated sulfuric acid Not to give a positive chromic anhydride test

Alkylbenzenes with chloroform and aluminum chloride

orange to red colors

These colors are due to triarylmethyl cations, Ar3C+, which are probably produced by a Friedel-Crafts reaction followed by a transfer of hydride ion