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CH. 17 - AROMATICITY

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: 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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