ch. 10 - the chemistry of aromatic...

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CH. 10 - THE CHEMISTRY OF AROMATIC COMPOUNDS

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: 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: 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: 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 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: 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 – 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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O Br2

cat. FeBr3


O

O

conc. H2SO4

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ch. 10 - the chemistry of aromatic...

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