5. Benzene and Aromaticity

Aromatic Compounds The term “Aromatic” is used to refer to the

class of compounds structurally related to Benzene.

The first discovered of these compounds were fragrant substances but the term aromatic, though still used, is not applicable to the vast majority of these compounds

The common names of some substituted aromatics are so firmly entrenched in the literature that they must be memorized

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15.2 Naming Aromatic Compounds

Monosubstituted benzenes are named by first naming the substituent and following this with the word benzene

Naming Alkyl Substituted Benzenes Alkyl benzenes are named in one of two different ways: If the alkyl group contains 6 or fewer carbons, then the

compound is named as an alkyl substituted benzene If the alkyl group contains more than 6 carbons then the

compound is named as phenyl substituted alkane

Naming Benzenes With More Than Two Substituents Choose numbers to get lowest possible values List substituents alphabetically with hyphenated numbers Common names, such as “toluene” can serve as root name

Naming Disubstituted Benzenes Relative positions on a benzene ring are indicated by

the following prefixes ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4)

Also used to describe reaction patterns (“reaction occurs at the para position”)

Complete the Following Examples

Structure of Benzene

The actual structure of benzene lies somewhere between the two resonance forms pictured below

Experimental Observations That Lead To This Resonance Picture of Benzene

All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds

Electron density in all six C-C bonds is identical Structure is planar, hexagonal

Molecular Orbital Description of the Resonance in Benzene

Each C is sp2 hybridized and has a p orbital perpendicular to the plane of the six-membered ring. Each p orbital has one electron in it. This makes it impossible to identify 3 localized double bonds in benzene

.

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Consequence of Resonance Stability The resonance stability of benzene is so very

substantial that benzene shows none of the characteristic chemical behavior of other alkenes Alkene + Br2/CCl4 dibromide (addition product)

Benzene + Br2/CCl4 no reaction. Alkene + HBr Bromoalkane (addition product)

Benzene + HBr no reaction. The reason that benzene does not take part in any

electrophilic addition rxns. is that, to do so, would destroy it’s stable conjugated system. An energetically unfavorable situation.

Please Recall the General Mechanism for Aromatic Substition

+

Br -

Heats of Hydrogenation as Indicators of Resonance Stability of Benzene The addition of H2 to C=C normally gives off about

118 kJ/mol ; 3 double bonds should give off 356kJ/mol

Benzene has 3 double bonds but gives off only 206 kJ/mol on reacting with 3 H2 molecules

Therefore it is about 150 kJ more “stability” than a regular alkene having s set of three double bonds

Reactions of Aromatic Compounds Electrophilic addition reactions, so common amongst normal alkenes, do not occur

with aromatics, in spite of the fact that each aromatic ring contains three double bonds.

The reason for this is that these reactions break the double bond and this would mean that the very stable aromatic system would be disrupted.

Instead, the characteristic reactions of aromatics are electrophilic substitution reactions rather that addition because these retain the very stable cyclic aromatic system

Electrophilic Addition and Electrophilic Substitution

ElectrophilicAddition

E++ base-

base:-

Electrophilic Substitution

Aromatic Addition Compared to Aromatic Substition

Br+ Br-

All Electrophilic Aromatic Substitution Reactions take place by the same General Mechanism.

Aromatics (benzene) are less reactive towards electrophiles then are normal alkenes.

Consequently, a catalyst is usually needed to convert the “electrophile containing reagent” into a stronger electrophile.

The catalyst needed to react molecular bromine (Br2) with benzene is ferric bromide. FeBr3 basically turns the weaker electrophile, Br2, into the stronger electrophile, Br+

FeBr3 is a Lewis Acid and accepts an electron pair from Br2 and thereby puts a strong positive charge on the end Bromine atom.

A stronger electrophile than Br2

FeBr4- + Br+

Generalized Mechanism for Electrophilic Aromatic Substitution cont.

Once generated. the stronger electrophile gets attacked by the pi electrons of the aromatic system, forming an intermediate, resonance stabilized, carbocation.

Finally, the carbocation stabilizes itself by loosing a ring H+ and regenerating the stable cyclic conjugated system, with the electrophile on the ring where the H+ used to be. See next slide.

+

+

-

FeBr4- + Br+

A stronger Electrophile than Br2

Br +

+

+

-

FeBr4- + Br+

A stronger Electrophile than Br2

Br +

Aromatic Chlorination Chlorine and iodine (but not fluorine, which is too

reactive) can substitute on an aromatic ring. Each requires a special catalyst or promoter to generate a sufficiently strong electrophile

Chlorination follows the same mechanism as bromination and requires FeCl3 catalyst

Aromatic Iodination

Iodine (I2) must be oxidized with Cu+2 or peroxide to form the more powerful electrophile, I+

Aromatic Nitration

The combination of nitric acid and sulfuric acid produces the electrophile NO2

+ (nitronium ion) It reacts with benzene to produce nitrobenzene

HNO3

Nitroaromatics are Important for Two Reasons

Nitroaromatics are important in themselves and also the nitro group can be converted into other functional groups that couldn’t be placed on the aromatic ring directly

For example, reduction of the nitro group by stannous chloride yields the corresponding amine

Aromatic Sulfonation The combination of sulfuric acid and sulfur

trioxide (SO3) produces the electrophile HSO3

+

Its reaction with benzene produces benzenesulfonic acid

SO3

Importance of Aromatic Sulfonic Acids Aromatic Sulfonic Acids are valuable intermediates in

the preparation of dyes and pharmaceuticals. Aromatic Sulfonic Acids are the precursors needed

for the synthesis of Sulfa Drugs such as sulfanilamide.These were among the first useful antibiotics known and credited with saving countless lives during W.W.II

Aromatic Sulfonic Acids are also important for the further chemistry that they can undergo When sulfonic acids are mixed with sodium hydroxide

at elevated temperatures a net replacement of the sulfonic group by the hydroxyl group results.

This constitutes one of the few methods for preparing phenols.

16.3 Alkylation of Aromatic Rings: The Friedel–Crafts Reaction

Aromatic substitution of a R+ for an aromatic proton (H+)

Aluminum trichloride, a Lewis Acid catalyst, promotes the formation of the (R+) carbocation

Limitations of the Friedel-Crafts Alkylation Only alkyl halides can be used (F, Cl, I, Br) Aryl halides and vinylic halides do not react (their

carbocations are too hard to form) This rxn will not work with rings containing an amino group

or a strongly electron-withdrawing deactivating group

Control Problems with F/C Alkylations Unwanted multiple alkylations can occur because the first alkylation is activating. That

is to say, once the first alkyl group substitutes on the ring; the monosubstituted benzene is more reactive than benzene itself and consequently more likely to be substituted with another alkyl group

Carbocation Rearrangements During Alkylation The last problem associated with F/C Alkylation is the possible

rearrangement of the intermediate carbocation to a more stable carbocation

These rearrangements usually involve hydride (H-) or alkide (R-) shifts

16.4 Acylation of Aromatic Rings

Reaction of an acid chloride (RCOCl) in the presence of AlCl3 catalyst with an aromatic ring substitutes an acyl group, COR , on to the aromatic ring Benzene with acetyl chloride yields acetophenone

Mechanism of Friedel-Crafts Acylation Similar to alkylation Reactive electrophile: resonance-stabilized

acyl cation An acyl cation does not rearrange

Electrophilic Aromatic Substitution of a Monosubstituted Benzene

What effects does a substituent already present on a benzene ring have on the electrophilic substitution of a second group? Reactivity: Some monosubstituted benzenes are

more reactive that benzene towards further electrophilic aromatic substitution (activating substituents); some monosubstituted benzenes are less reactive (deactivating substituents)

Orientation: A substituent that is already on a benzene ring directs the position of any incoming groups

Reactivity: Activating Substituents

Activating Substituents – these activate a benzene ring towards further substitution by donating electron density into the aromatic ring. Donating electon density into the ring increases the reaction rate by stabilizing the intermediate carbocation.

Reactivity: Deactivating Substituents Deactivating Substituents – these deactivate a benzene

ring towards further substitution by withdrawing electron density from the aromatic ring. Withdrawing electon density from the ring decreases the reaction rate by destabilizing the intermediate carbocation

Orientation The second effect that the substituent of a

monosubstituted benzene can have on further electrophilic aromatic substitution is to direct incoming electrophiles to particular positions on the aromatic ring. Substituents are either ortho – para directors or they are meta directors. Combining this information with the reactivity characteristics of a substituent we find that all substituents can be classified into one of three groups; Ortho – Para Activators Meta Deactivators Ortho – Para Deactivators

Ortho-Para Activating Groups

Please recall that activating groups increase the e- density of the aromatic ring. These substituents also direct incoming groups to the ortho and para positions as only these positions afford a resonance structure for the intermediate carbocation in which the positive charge is on the ring carbon to which the e- donating group is bonded – a very stable situation. The increased stability of this resonance structure favors substitution in these positions. The electron donating substituents may stabilize the positive charge by the inductive effect or by resonance. See Next Slide for Example

+

Meta Deactivators

Recall that deactivating groups withdraw e- density from the aromatic ring. All members of this group except for the halogens direct incoming groups to the meta position for it is only in this position that resonance structures for the intermediate carbocation do not place the positive charge on the ring carbon to which the e- withdrawing group is bonded (an unstable situation). Avoiding this extremely unstable situation is what makes the meta position the most highly favored (most stable).

Ortho-Para Deactivating Groups

Recall that halogens deactivate aromatic rings by inductive withdrawal of e- density. In addition to this ability, all halogens possess nonbonded e-’s that can be used to resonance-stabilize a positive charge on an adjacent carbon. It is this ability that make halogens ortho-para directors. If the incoming group attaches to either the ortho or para position, one of the resonance structures for the intermediate carbocation places the positive charge on a ring carbon to which the halogen is bonded. This allows the halogens to resonance-stabilize the positive charge.

16.5 Substituent Effects in Aromatic Rings: Summarized

Substituents already present on an aromatic ring can cause the aromatic compound to be (much) more or (much) less reactive than benzene

Substituents also direct the orientation of incoming groups on to the aromatic ring ortho- and para-directing activators, ortho- and para-

directing deactivators, and meta-directing deactivators

16.7 Trisubstituted Benzenes: Additivity of Effects How does one predict the orientation of a third group coming in to a

disubstituted benzene If the directing effects of the two groups are the same, the result is

additive

Substituents with Opposite Effects

If the directing effects of two groups oppose each other, the more powerful activating group decides the principal outcome

Meta-Disubstituted Compounds

Substitution between two groups in a meta-disubstituted compound rarely occurs because the site is too sterically hindered

To make aromatic rings with three adjacent substituents, it is best to start with an ortho-disubstituted compound

16.10 Oxidation of Aromatic Compounds Alkyl side chains can be oxidized to CO2H by strong

reagents such as KMnO4 and Na2Cr2O7 if they have a C-H next to the ring

Converts an alkylbenzene into a benzoic acid, ArR ArCO2H

16.11 Reduction of Aromatic Compounds Aromatic rings are inert to catalytic hydrogenation

under conditions that reduce alkene double bonds Can selectively reduce an alkene double bond in the

presence of an aromatic ring Reduction of an aromatic ring requires more powerful

reducing conditions (high pressure or rhodium catalysts)

Reduction of Aryl Alkyl Ketones

Aromatic ring activates neighboring carbonyl group toward reduction

Ketone is converted into an alkylbenzene by catalytic hydrogenation over Pd catalyst

16.12 Synthesis Strategies

These syntheses require planning and consideration of alternative routes

Work through the practice problems in this section following the general guidelines for synthesis