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www.clutchprep.com ORGANIC - BRUICE 8E CH. 18 - REACTIONS OF BENZENE AND SUBSTITUTED BENZENES
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ORGANIC - BRUICE 8E
CH. 18 - REACTIONS OF BENZENE AND SUBSTITUTED BENZENES
CONCEPT: AROMATICTY – INTRODUCTION
Aromatic compounds display an unusual stability for their high level of electron density.
● Their high level of unsaturation should make them extremely reactive, however they are difficult to react with.
EXAMPLE: Three typical addition reactions with cyclohexene vs. benzene
What is responsible for this crazy level of stability? ___________________________
Categories of Aromatics:
● _______________________________: These compounds possess an unusually ________ level of stability
● _______________________________: These compounds do not possess any unique level of stability or instability
● _______________________________: These compounds possess an unusually _____ level of stability. Very reactive!
EXAMPLE: Differing aromaticity of conjugated trienes
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CONCEPT: FOUR TESTS OF AROMATICTY
For a compound to qualify as aromatic, it must meet 4 distinct tests. These are called Huckel’s Rule compounds.
1. Cyclic:
2. Fully Conjugated:
3. Planar:
4. Huckel’s Rule: (4n + 2) number of π electrons
● Any compound that _________ one or more of these tests is considered ____________________________
● Any compound that meets all these conditions, but only has (4n) π electrons is __________________________
□ These compounds are said to follow Breslow’s Rule
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CONCEPT: COUNTING PI ELECTRONS
When counting π-electrons, we are trying to identify the number of electrons that are freely available to circulate through
conjugated p-orbitals.
● Double Bond/Anion = ________
● Radical = ________
● Cation = ________
EXAMPLE: Count the number of π-electrons present in all of these molecules:
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CONCEPT: AROMATICITY OF HYDROCARBONS
We can use our knowledge of the Four Tests of Aromaticity to confirm aromaticity
● Huckel’s Rule = Aromatic (4n + 2) π electron numbers: ______, _______, _______, _______, etc.
● Breslow’s Rule = Anti-aromatic (4n) π electron numbers: ______, _______, _______, _______, etc.
● Non-aromatic = FAILS one or more test (including odd number of π electrons)
EXAMPLE: Determine the aromaticity of the following molecules
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CONCEPT: AROMATICTY OF ANNULENES
An annulene, sometimes referred to as a polyolefin, is the name given to a fully conjugated monocyclic hydrocarbon.
● Due to their simple structure, rings of different sizes can be named as [n]annulenes, where n = number of carbons
□ As annulenes get bigger, the challenge becomes predicting planarity.
Predicting Annulene Planarity:
Pertaining to All-cis annulenes, EXAMPLE: Cyclooctatetrene vs. Cyclooctatetraene dianion
● If 4n + 2 π electrons
□ 10+ = Non-aromatic
□ 9 or less = Aromatic
● If 4n π electrons
□ 8+ = Non-aromatic
□ 7 or less = Antiaromatic
EXAMPLE: Determine if the following annulenes display any form of aromaticity.
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CONCEPT: AROMATICITY OF HETEROCYCLES
Heterocycles are cyclic structures that contain a ______________________ within the ring.
● Heteroatoms can choose to donate up to one lone pair each only if:
1. They are already sp3 hybridized
2. It will help to create aromaticity
EXAMPLE: Determine the aromaticity of the following heterocycles. Will any lone pairs be donated to the ring?
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CONCEPT: INSCRIBED POLYGON METHOD
Also known as the polygon-in-circle method, or Frost Circle, this helps us visualize the identities of π electrons and
molecular orbitals in a ring.
EXAMPLE: Use the polygon-in-circle method to predict stability of the following molecules.
Step 1: Draw polygon with one corner facing down.
Step 2: Draw molecular orbitals on all corners of ring
Step 3: Draw a line that splits the polygon down the middle
Step 4: Insert π electrons into orbitals starting from lowest energy and working up (Aufbau Principle).
● Filled molecular orbitals contribute to unique stability (aromaticity)
● Partially filled molecular orbitals contribute to unique instability (antiaromaticity)
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PRACTICE: Apply the polygon circle method to the following compound. Does it show any special stability? If yes, why?
Tropyllium cation
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CONCEPT: ACIDITY OF AROMATIC HYDROCARBONS
Aromatic hydrocarbons are not naturally acidic. In fact, the pKa of benzene is ______
● If a hydrocarbon can become aromatic by donating a proton, it will be uniquely acidic. i.e. cyclopentadiene
● If a hydrocarbon becomes anitaromatic by donating a proton, it will be uniquely non-acidic. i.e. cycloheptatriene
EXAMPLE: Would the following hydrocarbon be expected to display unusual acidity? Explain your reasoning.
EXAMPLE: Would the following two hydrocarbons be expected to have similar acidities? Explain your reasoning.
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CONCEPT: BASICITY OF AROMATIC HETEROCYCLES
Heterocycles often have multiple lone pairs available to react with acids. The question is which lone pair do we react?
EXAMPLE: Draw the product of the following acid/base reaction with imidazole.
● Acids can only react with lone pairs that are not necessary for aromaticity.
□ sp2-hybridized lone pairs are basic.
EXAMPLE: Draw the product of the following acid/base reaction.
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CONCEPT: IONIZATION OF AROMATICS
Double bonds can be viewed as a loose pair of electrons that can undergo resonance movement and ionization if that helps
to create an aromatic compound.
Resonance of Fulvalenes:
Fulvalenes are hydrocarbons composed of two fully conjugated rings joined by an exocyclic double bond.
● Which atom would you expect to most readily react with an electrophile (E+)?
● Does this molecule possess a net dipole? If so, indicate the direction.
Resonance of Azulene:
Azulene is a polycyclic aromatic molecule with a distinctive blue color.
.
● Which atom would you expect to most readily react with an electrophile (E+)?
● Does this molecule possess a net dipole? If so, indicate the direction.
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PRACTICE: Which carbon in the following compound is most likely to react with an electrophile?
CH2
(heptafulvene)
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CONCEPT: BENZENE NOMENCLATURE
Benzene was one of the first organic molecules to be identified (1825), so common names predominate.
Common Benzene Derivatives:
___________ __________________ _________________
__________________ ___________________ _________________
_______________ _________________________ _________________
Monosubstituted Benzene: Disubstituted Benzene: Multisubstituted Benzene:
● No location necessary ● No numerical locations ● Numerical locations necessary
□ 1,2 = __________ (o-) □ Do not use -o, -m, -p
□ 1,3 = __________ (m-)
□ 1,4 = __________ (p-)
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EXAMPLE: Correctly name the following benzene derivatives.
1.
2.
3.
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CONCEPT: ELECTROPHILIC AROMATIC SUBSTITUTION – GENERAL MECHANISM
Benzene reacts with very few reagents. It DOES NOT undergo typical addition reactions. Why?
If we can get benzene to react in a substitution reaction, this preserves aromaticity.
Very strong electrophiles can temporarily disrupt aromaticity of benzene to create a substitution product.
● We call this electrophilic aromatic substitution or __________. This is the most important mechanism of benzene.
EAS: General Mechanism
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CONCEPT: ELECTROPHILIC AROMATIC SUBSTITUTION – REACTIONS
EAS reactions require strong electrophiles to take place. Some of these will require catalysts.
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CONCEPT: GENERATING ELECTROPHILES – EAS HALOGENATION
EAS Bromination and Chlorination both require complexing with a Lewis Acid Catalyst before the reaction can begin.
General Reaction:
Mechanism:
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CONCEPT: GENERATING ELECTROPHILES – EAS NITRATION
EAS Nitration requires nitric acid to react with a catalytic acid to generate a strong nitronium ion electrophile.
General Reaction:
Mechanism:
Reduction of Nitro Groups:
Nitro groups can be reduced in the presence of many reducing agents to aniline. More on this in your amines chapter.
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CONCEPT: BLOCKING GROUPS – SULFONIC ACID
As the only reversible EAS reaction, sulfonation is used to __________ the para position and _________ o-substitution.
● Sometimes called a blocking group because it is not found in the final product.
EXAMPLE: Predict the product of the following multistep synthesis.
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PRACTICE: Beginning from Benzene, synthesize the following compound.
Cl
(the only isomer)
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CONCEPT: GENERATING ELECTROPHILES – FRIEDEL-CRAFTS ALKYATION
Friedel-Crafts Alkyation requires an alkyl halide to complex with a Lewis Acid Catalyst before the reaction can begin.
● Active electrophile is a carbocation
□ Watch out for ________________________________ General Reaction:
Mechanism:
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CONCEPT: GENERATING ELECTROPHILES – FRIEDEL-CRAFTS ACYLATION
Friedel-Crafts Acylation requires an acyl halide to complex with a Lewis Acid Catalyst before the reaction can begin.
● Active electrophile is an acylium ion General Reaction:
Mechanism:
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CONCEPT: GENERATING ELECTROPHILES – ANY CARBOCATION
Popular carbocations include those catalyzed by hydrofluoric acid and promoted by boron trifluoride.
● Watch out for ________________________________
General Reaction:
Mechanisms:
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CONCEPT: EAS – MONOSUBSTITUTED BENZENE
Substituents alter the electron density of benzene rings, affecting reactivity toward subsequent EAS in two ways:
1. Activity Effects
● Electron Donating Groups EDG’s ________________________ the ring toward reactions
● Electron Withdrawing Groups EWG’s _____________________ the ring toward reactions
2. Directing Effects
● Electron Donating Groups EDG’s tend to be _____________, ____________ directors
● Electron Withdrawing Groups EWG’s tend to be ____________ directors
Badass EAS Activity Chart
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PRACTICE: Predict the major products of the following EAS reaction.
O
NH Cl2
cat. FeCl3
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PRACTICE: Predict the product of the following multi-step synthesis.
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CONCEPT: EAS-O,P-MAJOR PRODUCTS
In general, we refer to the products of an EAS o,p-director as a mixture – but there are some patterns we can learn.
● The positions compete with number vs. steric hindrance
● In most cases, steric hindrance wins.
If asked to supply only one major product, assume the para-product predominates:
There is only one major exception to this assumption, and that is if the final product can _____ - _____________ with itself.
EXAMPLE: EAS Nitration of Phenol
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CONCEPT: EAS – PROTECTION OF ANILINE DERIVATIVES
Strongly activated rings like aniline can open the ring up to unwanted reactions.
To avoid this, we can reversibly acetylate (protect) the amino group to make it moderately activating.
EXAMPLE: Synthesize the target molecule from nitrobenzene and any other reagents.
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CONCEPT: LIMITATIONS OF FRIEDEL-CRAFTS ALKYLATION
Friedel-Crafts Alkylation has several limitations that render it almost useless in the lab.
1. It does not react with vinyl or aryl halides. Their carbocations are far too unstable.
● Solution: Avoid vinyl or aryl halides
2. Aniline derivatives ruin the Lewis Acid Catalyst
● Solution: Avoid aniline derivatives or protect (reversibly acetylate) the amino group.
3. Alkylation reactions _________________ the ring further reactions
● Solution: Excess benzene or acylate instead
4. Alkylation reactions are susceptible to carbocation rearrangements
● Solution: Acylate instead
EXAMPLE: FC Alkylation vs. FC Acylation of benzene
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PRACTICE: Provide the major product and the correct mechanism for the following reaction.
PRACTICE: Provide the major product and the correct mechanism for the following reaction.
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CONCEPT: ADVANTAGES OF FRIEDEL-CRAFTS ACYLATION
Friedel-Crafts Acylation has several advantages that make it much more synthetically useful than alkylation.
1. Acylation reactions ______________________ the ring further reactions, promoting monosubstitution.
2. Acylation reactions are not susceptible to carbocation rearrangements.
3. Acylation products can be converted to alkylbenzenes with a zinc amalgam using Clemmenson Reduction.
● The mechanism for this reduction is still unknown, but you need to memorize the reagents.
EXAMPLE: Sample preparation of n-propylbenzene
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CONCEPT: EAS – POLYSUBSTITUTED BENZENE
When two or more substituents are already on benzene, there are multiple new factors we must take into account.
1. Steric Effects
● Crowded sites will not be reactive towards subsequent EAS reactions
2. Synergistic Groups
● When multiple directing groups direct toward the same position, yields of that product will be high
3. Competitive Groups
● When multiple directing groups disagree on where to substitute, mixed products will result
□ The strongest ______________________ will determine the major product of the reaction
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PRACTICE: Predict the major products of the following EAS reaction.
O Br2
cat. FeBr3
PRACTICE: Predict the major products of the following EAS reaction.
O
O
conc. H2SO4
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CONCEPT: EAS – SEQUENCE GROUPS
Sequence groups are groups that have the ability to alter the sequence of an aromatic synthesis.
● These are groups that can be easily transformed into another type of director
1. Reduction of Nitro Groups
2. Clemmenson Reduction
3. Side-Chain Oxidation
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CONCEPT: EAS — PROPOSING AROMATIC SYNTHESIS
You may be asked to propose an aromatic synthesis starting only from benzene or other benzene derivatives.
● We must use our knowledge sequence groups to plan synthetic steps in the correct order
EXAMPLE: Synthesize the target molecule from acetophenone and any other reagents.
EXAMPLE: Synthesize the target molecule from ethylbenzene and any other reagents.
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PRACTICE: Provide the product for each of the following reaction steps
Br
OH
OH
O
Br
OH
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PRACTICE: Beginning from Benzene, synthesize the following compound.
Br
PRACTICE: Beginning from Benzene, synthesize the following compound.
1-Phenylethanol
PRACTICE: Beginning from Benzene, synthesize the following compound.
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CONCEPT: DIAZONIUM COMPOUNDS – REPLACEMENT REACTIONS
Aniline reacts with nitrous acid to form a diazo functional group in a reaction called a diazotization.
● Aryl diazonium salts participate in multiple replacement reactions
List of Diazo Replacement Reactions:
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CONCEPT: ACIDITY OF PHENOLS
Phenols are substantially more acidic than typical alcohols due to the _________________ effect.
● Recall, the more we can stabilize the conjugate base, the more acidic a compound will be.
Donating and Withdrawing Groups:
EXAMPLE: Predict which of the following would be the most acidic phenol.
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O,P-Directors vs. Meta-Directors
The __________ position has a much lessor effect on acidity than the _________ and __________ positions.
● This is due to the resonance structures that are able to be produced by different positions
EXAMPLE: Predict which of the following would be the most acidic phenol.
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EXAMPLE: Predict which of the following would be the most acidic phenol.
EXAMPLE: Predict which of the following would be the most acidic phenol.
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PRACTICE: Rank the following phenols in order of increasing acidity.
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CONCEPT: DIAZO COMPOUNDS – SEQUENCE GROUPS
Sequence Groups are groups that have the ability to alter the sequence of an aromatic synthesis. These are groups that
can be easily transformed into another type of director:
Blocking Groups are groups that act only to direct other reactions and are then completely removed.
● Hypophosphorous acid (H3PO2) is used to __________ the para position and _________ o-substitution.
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CONCEPT: DIAZONIUM COMPOUNDS – PROPOSING AROMATIC SYNTHESIS
You may be asked to propose an aromatic synthesis starting only from benzene or diazobenzene derivatives.
● We must use our knowledge sequence and blocking groups to plan synthetic steps in the correct order
EXAMPLE: Synthesize the target molecule from toluene and any other reagents.
EXAMPLE: Synthesize the target molecule from the following precursor and any other reagents.
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CONCEPT: SNAr ADDITION-ELIMINATION MECHANISM
Unlike EAS, where addition is initiated by the presence of a strong electrophile, addition-elimination can also be initiated by
a strong nucleophile in the presence of a good aryl leaving group.
● Reaction has similarities to SN2 but it is not _____________________
● Known as Addition-Elimination Nucleophilic Aromatic Substitution, SNAr or ipso-substitution.
An early method of preparing phenol called the Dow Process used chlorobenzene, NaOH and high heat to force SNAr.
.
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CONCEPT: THE MEISENHEIMER COMPLEX
The Dow Process, a typical SNAr reaction, requires tons of heat and pressure to proceed forward.
● This is due to the instability of the anionic sigma-complex
● Withdrawing groups or Heteroatoms to the Ortho or Para positions (WHOP) stabilize the intermediate
□ A classical trinitrobenzene Meisenheimer Complex can proceed in room temperature
EXAMPLE: Use resonance structures to determine which of the following ipso-substitutions is more favored.
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EXAMPLE: Which of the following compounds will most readily undergo nucleophilic aromatic substitution in the addition-
elimination pathway?
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PRACTICE: Provide the structure and name of the intermediate formed from the reaction of 1-bromo-2,4,6-
trinitrobenzene with one equivalent of sodium methoxide.
PRACTICE: Provide the major organic product for the following reaction.
PRACTICE: Provide the major organic product for the following reaction.
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PRACTICE: Which of the following compounds is most likely to undergo nucleophilic aromatic substitution via the
addition-elimination Pathway?
PRACTICE: Which of the following compounds is most likely to undergo nucleophilic aromatic substitution via the
addition-elimination Pathway?
N
Cl
N
Cl
NN
N+ O
-O
Cl
NN
Cl
N+
O-
O
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CONCEPT: BENZYNE PATHWAY – GENERAL MECHANISM
Benzene can also undergo Nucleophilic Aromatic Substitution via an Elimination-Addition pathway to make aniline.
● This mechanism requires the formation of a highly unstable aryne (C6H4) intermediate.
Benzyne Amination Mechanism:
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CONCEPT: BENZYNE PATHWAY – REGIOSPECIFIC PRODUCTS
MIT chemist John D. Roberts proposed that we could use donating and withdrawing groups to favor ortho vs. meta products
● Donating Groups favor the _____________ position
● Withdrawing Groups favor the _____________ position
EXAMPLE: Predict the product of the reaction. Use your knowledge of activating and deactivating groups to determine what
the final product is. Show the full mechanism for the benzyne pathway.
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PRACTICE: Provide the major product(s) from the following reaction.
OCH3
Br
NaNH2 NH3
PRACTICE: Provide the major product(s) from the following reaction.
NaNH2 NH3
CH3
Cl
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CONCEPT: SIDE-CHAIN HALOGENATION
The alkyl group directly attached to benzene is known as an alkyl side-chain.
● The benzylic position of a side-chain is one of the most stable locations for radicals due to conjugation.
● Recall that benzylic radicals are some of the most stable radical intermediates possible
Benzylic Resonance Structures:
Specific Side-Chain Halogenations:
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CONCEPT: SIDE-CHAIN OXIDATION
The alkyl group directly attached to benzene is known as an alkyl side-chain.
Regardless of the length of an alkylbenzene side-chain, it can be oxidized to benzoic acid using hot KMnO4.
● However, there must be at least one benzylic hydrogen present for oxidation to occur.
EXAMPLE: Which of the following alkylbenzenes would not yield benzoic acid when treated with hot KMnO4?
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