ORGANIC - CLUTCH CH. 17 -...
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ORGANIC - CLUTCH
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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