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Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds!...

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Reactions of Aromatic Compounds Aromatic compounds are stabilized by this “aromatic stabilization” energy Due to this stabilization, normal S N 2 reactions observed with alkanes do not occur with aromatic compounds (S N 2 reactions never occur on sp 2 hybridized carbons!) In addition, the double bonds of the aromatic group do not behave similar to alkene reactions
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Page 1: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Reactions of Aromatic Compounds

Aromatic compounds are stabilized by this “aromatic stabilization” energy

Due to this stabilization, normal SN2 reactions observed with alkanes do not occur with aromatic compounds

(SN2 reactions never occur on sp2 hybridized carbons!)

In addition, the double bonds of the aromatic group do not behave similar to alkene reactions

Page 2: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Aromatic Substitution

While aromatic compounds do not react through addition reactions seen earlier

Br2

Br

BrBr2FeBr3

Br

With an appropriate catalyst, benzene will react with bromine

The product is a substitution, not an addition (the bromine has substituted for a hydrogen)

The product is still aromatic

Page 3: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Electrophilic Aromatic Substitution

Aromatic compounds react through a unique substitution type reaction

Initially an electrophile reacts with the aromatic compound to generate an arenium ion (also called sigma complex)

The arenium ion has lost aromatic stabilization (one of the carbons of the ring no longer has a conjugated p orbital)

Page 4: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Electrophilic Aromatic Substitution

In a second step, the arenium ion loses a proton to regenerate the aromatic stabilization

The product is thus a substitution (the electrophile has substituted for a hydrogen)

and is called an Electrophilic Aromatic Substitution

Page 5: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Energy Profile

Potential energy

Reaction Coordinate

Starting material

Transition states Transition

states

Intermediate

Products

EH

E

The rate-limiting step is therefore the formation of the arenium ion

Page 6: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The properties of this arenium ion therefore control electrophilic aromatic substitutions (just like any reaction consider the stability of the intermediate

formed in the rate limiting step)

1) The rate will be faster for anything that stabilizes the arenium ion

2) The regiochemistry will be controlled by the stability of the arenium ion

The properties of the arenium ion will predict the outcome of electrophilic aromatic substitution chemistry

Page 7: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Bromination

To brominate an aromatic ring need to generate an electrophilic source of bromine

In practice typically add a Lewis acid (e.g. FeBr3) to bromine

In presence of aromatic ring this electrophilic bromine source will react

Br2FeBr3 Br

HBr

Page 8: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

With unsubstituted benzene the position of reaction is arbitrary

With a monosubstituted aromatic ring, however, can obtain three possible products

How to predict which is favored?

Controlled by stability of arenium ion intermediate

Page 9: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The stability is different depending upon placement of carbocation

Page 10: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Alkyl groups are electron donating

Therefore toluene will favor electrophilic substitution at ortho/para positions (only ortho/para substitution places carbocation adjacent to alkyl group on ring)

Often obtain more para than ortho due to sterics

Page 11: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

In addition to orientational control, substituents affect reactivity

CH3 NO2

As the aromatic ring acquires more electron density, the arenium ion will be more stable

Page 12: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Toluene therefore reacts faster in an electrophilic aromatic substitution than benzene

The alkyl group is called an activating group (it activates the ring for a faster rate)

Any substituent that increases the rate for an electrophilic aromatic substitution is called an “activating” substituent

Substituents that lower the rate for an electrophilic aromatic substitution are called “deactivating” subsituents

(nitro is one example of a deactivating group) These are groups that lower the electron density of the aromatic ring

Page 13: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Factors that affect Activators/Deactivators

There are in general two mechanisms that can affect a substituent

1) Inductive

Substituents that are more electronegative than carbon will inductively pull electron density out of the ring

2) Resonance

Substituents that have a lone pair of electrons adjacent to the ring can donate electron density into the ring through resonance

O CH3O CH3

O CH3O CH3

Page 14: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Many substituents will have both inductive and resonance effects

Need to balance the effects

R

Alkyl substituents inductively donate,

no resonance effects, therefore activators

Electronegative atoms with lone pair of electrons have opposing effects

Inductively withdrawing therefore deactivating

Resonance donating, therefore activating

Z Z

• when a neutral O or N is directly bonded to a benzene ring, the resonance effect dominates and the net effect is activating

• when a halogen is bonded to a benzene ring, the inductive effect dominates and the net effect is deactivating

Page 15: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Other Deactivating Groups

There are two other main classes of deactivating groups

1)  A conjugated system where both inductive and resonance effects pull electron density from the ring

YZ

Whenever Z is more electronegative than Y as seen in structure

NO

O

ON

2) A formal positive charge is placed directly adjacent to ring

NCH3

H3C CH3

Page 16: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Activating vs. Deactivating Ability can be compared

The substituents can be compared relatively

Page 17: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Orientational Control Can Be Predicted

1) All activating groups favor ortho/para substitution

2)  Deactivating groups with a lone pair of electrons adjacent to the ring favor ortho/para substitution (halogens are in this category)

3) Other deactivating groups favor meta substitution

Page 18: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

With deactivating groups besides halogens, the favored substitution is meta

Can be predicted by stability of arenium ion

Only the meta substitution does not place a carbocation adjacent to EWG

Page 19: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

All positions with a deactivating group are slower than benzene

* The meta position is the LEAST disfavored

Consider reaction coordinate for the first, rate-determining step

Page 20: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Multiple Substituents

How to determine orientation of electrophilic aromatic substitution if there is more than one substituent?

A few rules to consider:

1) The effects are cumulative

2)  The stronger substituent according to the relative effects will be correspondingly more important for directing effects

3)  When given a choice, a new substituent typically will not go ortho to two other substituents

Page 21: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Consider some examples:

ortho/para director CH3

CN Br2FeBr3meta director

CH3CN

Br

H3C CN Br2FeBr3

ortho/para director

meta director H3C CN

Br

Br2FeBr3

H3C

OCH3

ortho/para director

ortho/para director

H3C

OCH3

Br

Stronger director wins

Stronger director wins

Page 22: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Reactivity of aromatic ring can affect amount of reaction

In reactions with strongly activated rings, polyhalogenation occurs

With either phenol or aniline, the reaction will proceed until all ortho/para positions are reacted when catalyst is used

Page 23: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

With these highly activated ring systems, catalyst is not necessary

Reaction will only proceed with strongly activated ring systems, still need catalyst for less activated aromatic rings,

but with phenol or aniline reaction of one halogen can occur with no catalyst present

Page 24: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Other Electrophiles Beside Bromine

Chlorination reaction is similar to bromination

Often use Lewis acid with chlorine substituents to avoid cross contamination (e.g. often use AlCl3 for chlorination but FeBr3 for bromination)

Also with strongly activated rings obtain polychlorination products with catalyst

Page 25: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Iodination requires a stronger oxidizing reagent

The reagents are a method to generate iodonium ion in situ

The iodonium can then react as the electrophile

Page 26: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Fluorine is much harder to add in an electrophilic addition

There is not an easy method to generate F+

Typically to add fluorine to an aromatic ring a diazonium route is used

This chemistry will be dealt in more detail in chapter 19

Page 27: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Other Common Electrophiles Besides Halogens

Nitration

Adding a nitro group to an aromatic ring is a convenient and useful reaction

Need to generate a source of nitronium ion

Page 28: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

An advantage of nitration is the nitro group can be reduced to an amine

Allows the introduction of an amine group to the aromatic ring (almost all compounds that contain a nitrogen attached to aromatic ring

occurred through a nitration)

This conversion changes the electronic properties of the ring

Nitro Deactivating/Meta Director

Amine Activating/Ortho-Para Director

Page 29: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Sulfonation

Allows the introduction of a sulfur group (many biological/medicinal uses)

The reverse step can also occur

Page 30: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The desulfonation step allows introduction of isotopes

Occurs through arenium ion where compound is sulfonated, then add D+ and desulfonate to eliminate sulfur and add deuterium

Page 31: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Friedel-Crafts Alkylation

In general carbon-carbon bonds can be made by reacting an aromatic ring with a carbocation

This reaction is called a Friedel-Crafts alkylation

Page 32: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The key is formation of electrophilic carbon

The alkyl halide reacts with the Lewis acid

With tertiary and secondary alkyl halides this generates a discrete carbocation

With primary alkyl halides the full carbocation is not formed

Page 33: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Any method that generates a carbocation can be used in a Friedel-Crafts alkylation

Two other common methods:

1) From alkenes

Often use HF as acid source since fluoride is weak nucleophile, therefore higher lifetime of carbocation to react

2) From alcohols

Page 34: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Limtations of Friedel-Crafts Alkylation

1) Reaction does not work with strongly deactivated aromatic rings

Friedel-Crafts alkylations do not work with nitro, sulfonic, or acyl substituents

2) Carbocation rearrangements occur

Because a carbocation is formed during this reaction, similar to any reaction involving carbocations the carbocation can rearrange to a more stable carbocation

Can therefore never obtain n-alkyl substituents longer than 2 carbons

Page 35: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

3) Polyalkylation often occurs with Friedel-Crafts alkylation

Because an alkyl group is an activating group, the product is more reactive than the starting material

Causes the product to react further

To prevent polyreaction the starting material (benzene is this example) is used in excess

Page 36: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Another Option: Friedel-Crafts Acylation

Instead of adding an alkyl group this reaction adds an acyl substituent

First need to generate an acid chloride

Thus any carboxylic acid can be converted into an acid chloride

Page 37: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The acid chloride can then be reacted in a Friedel-Crafts acylation

First form an acylium ion by reacting with Lewis acid

The acylium ion then reacts with aromatic ring in a typical electrophilic aromatic substitution

Page 38: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Advantages of Friedel-Crafts Acylation

1) The acyl substituent is a deactivating group

Therefore this reaction can be stopped easily at one addition (no polyacylation occurs)

2) No rearrangements occur

Since an isolated carbocation is not formed there is no rearrangement

Due to these two advantages, the Friedel-Crafts acylation is a much more convenient reaction than the Friedel-Crafts alkylation

-still will not react with strongly deactivated aromatic rings

Page 39: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

High para substitution preference

Due to the steric bulk of the Friedel-Crafts acylation reagent, often see a high preference for the para substitution with an ortho/para directing group

*often see para preference with bulky electrophiles

Page 40: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Clemmensen Reduction

The ability to reduce a carbonyl to a methylene further enhances usefullness of Friedel-Crafts acylation

In a Clemmensen reduction the conversion occurs under acidic conditions

Overall these two steps, Friedel-Crafts acylation followed by Clemmensen, allows the introduction of an n-alkyl substituent

which would not be possible with a Friedel-Crafts alkylation

Page 41: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Wolf-Kishner Reduction

Another method to reduce a carbonyl to methylene is a Wolf-Kishner reduction

Main difference is that the Wolf-Kishner occurs under basic conditions

Both Clemmensen and Wolf-Kishner require strong conditions, Clemmensen uses acidic while Wolf-Kishner uses basic conditions

Page 42: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Benzaldehyde

Adding a formyl group requires stronger conditions than an acyl group addition

Cannot isolate formyl chloride

Therefore normal Friedel-Crafts acylation conditions cannot be used

Need to generate in situ

Page 43: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Birch Reduction

In addition to adding electrophiles to aromatic ring, the aromatic ring can be reduced by adding electrons to the system

(in essence a nucleophilic addition)

The electrons need to be generated in situ

This electron is called a “solvated” electron

Page 44: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

In the presence of an aromatic ring this electron will react

Addition of one electron thus generates a radical anion

This strongly basic anion will abstract a proton from alcohol solution

e

Page 45: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The radical will then undergo the same operation a second time

The final product has thus been reduced from benzene to a 1,4-cyclohexadiene

The aromatic stabilization has been lost

Page 46: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Orientation of Birch Reduction

What happens if there is a substituent on the aromatic ring before reduction?

Which regioisomer will be obtained?

Similar to every other reaction studied need to ask yourself, “What is the stability of the intermediate structure?”

The preferred product is a result of the more stable intermediate

Page 47: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

The intermediate in a Birch reduction is the radical anion formed after addition of electron

With electron withdrawing substituent:

With electron donating substituent:

O NH3(l), NaCH3OH

O

Placing negative charge adjacent to carbonyl allows resonance

O O

NH3(l), NaCH3OHOCH3

Want negative charge as far removed from donating group as possible

OCH3 OCH3

Page 48: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Reactions of Alkyl Substituents to Aromatic Ring

Once alkyl groups are attached to aromatic rings they can undergo subsequent reactions

Permanganate Oxidation

Any carbon adjacent to an aromatic ring that contains at least one hydrogen will be oxidized with permanganate to the carboxylic acid stage

Page 49: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Benzylic Halide

As seen in chapter 4, halogenation reactions can occur with either chlorine or bromine under photolytic conditions

Reaction proceeds through a radical intermediate

The benzylic radical is more stable due to resonance with aromatic ring

CH2

Page 50: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Remember that chlorination was more reactive, bromination though occurred selectively

Cl2, h! ClCl

Br2, h!

Br

Realize reaction does not occur on aromatic ring, do not obatin radical from sp2 hybridized carbon

Page 51: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Nucleophilic Aromatic Substitution

Another type of reaction with aromatic rings is a nucleophilic addition

Instead of adding an electrophile to form an arenium ion, a nucleophile replaces a leaving group

This is NOT a SN2 reaction

Initially a carbanion is formed which subsequently loses the leaving group, unlike a SN2 reaction which is a one step reaction

Page 52: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Mechanism

Cl

O2N

NO2

NaCN

NO2

O2N

ClCN

NO2

O2N

ClCN

NO2

O2N

ClCN

The anion is stabilized by electron withdrawing groups ortho/para to leaving group

To regain aromatic stabilization, the chloride leaves to give the substituted product

NO2

O2N

ClCN

NO2

O2N

CN

Page 53: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Unique factors for Nucleophilic Aromatic Substitution

1)  Must have EWG’s ortho/para to leaving group -the more EWG’s present the faster the reaction rate

(intermediate is stabilized)

2)  The leaving group ability does not parallel SN2 reactions -bond to leaving group is not broken in rate-determining step

(fluorine for example is a good leaving group for nucleophilic aromatic substitution but is a horrible leaving group for SN2 reaction)

More electronegative atom has a faster rate F > Cl > Br > I

(polarizability is not a factor)

Page 54: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Using Nucleophilic Aromatic Substitution for Peptide Determination

React 2,4-dinitrobenzene with peptide

F

O2N

NO2

H2NO

NH

(peptide chain)

R

HN

O2N

NO2 O

NH

(peptide chain)

R

Sanger reagent

HN

O2N

NO2 O

ROH

cleave

The Sanger reagent can react with the N-terminal amine from the peptide

The resultant nucleophilic aromatic substitution product can be cleaved and thus the terminal amino acid structure determined

Page 55: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Benzyne Mechanism

A second nucleophilic aromatic substitution reaction is a benzyne mechanism

Benzyne is an extremely unstable intermediate which will react with any nucleophile present

HBr

NH2NH2

NaNH2, NH3

benzyne

Need strong base at moderate temperatures, but do not need EWG’s on ring

Page 56: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Oxidation of Phenols

Many reactions of phenols follow the same reactivity seen earlier -they are highly activating groups (ortho/para directors)

for electrophilic aromatic substitution

Phenols can also be oxidized to create quinones

Page 57: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Coenzyme Q

All oxygen-consuming organisms use this process

Page 58: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Aromatic Substitution using Organometallic Reagents

Extremely useful in forming carbon-carbon bonds with aromatic rings

Allows a much wider diversity of products than available with Friedel-Crafts reactions

Tremendous progress has been made using transition metal compounds to catalyze reactions

To understand these reactions one key is knowing what metal is needed to catalyze the reaction and also what functional groups are needed for each specific reaction to occur

Page 59: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Organocuprates

Formed by reacting organolithium compounds with cuprous iodide (CuI)

These lithium dialkyl cuprate reagents (organocuprates) are also called Gilman reagents

These Gilman reagents can react with alkyl halides to form new carbon-carbon bonds similar to SN2 reactions

Page 60: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Reactions at sp2 Hybridized Carbons

Unlike SN2 reactions, however, these organocuprates can react with sp2 hybridized aryl or vinyl halides

Page 61: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Heck Reaction

Can also accomplish reaction at alkene using a palladium catalyzed reaction (called Heck reaction)

-the halide can be either aryl or vinyl -the halogen can be either bromine or iodine

Page 62: Reactions of Aromatic Compounds Aromatic …biewerm/17.pdf · Reactions of Aromatic Compounds! Aromatic compounds are stabilized by this “aromatic ... Br 2 Br Br Br 2 FeBr 3 Br

Suzuki Coupling

Couples aryl or vinyl halide with boronic acid with palladium catalyst and base

Can use boronic acid [R-B(OH)2] or ester [R-B(OR)2] that is alkyl, vinyl or aryl


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