chapter 17 aromatic compounds

criteria for aromaticity

cyclic, planar (SP2 carbons), conjugated pi system, follows huckle’s rule

Huckle’s rule

a compound is aromatic if it fulfills the criteria for aromaticity and contains 4n+2 pi electrons

of the of the four MOs of cylcobutadiene _____ are bonding MOs, ___ are nonbonding MOs, and _____ are antibonding MOs. (hint use a frost circle)

1;2;1

antiaromatic criteria

all criteria(cyclic, planar SP2 carbons, conjugated pi system) for aromaticity except the Hückel rule are met. instead, the compound must have 4n pi electrons

cyclobutadiene has 4 pi electrons so it is ______. this results in _______ which may be relieved by taking a ________ shape

antiaromatic; instability; rectangular

aromatic rings are remarkably _____ so they are usually (unreactive or reactive)

stable; unreactive

(t/f) benzene reacts on its own with Br2

(t/f) cyclooctatetraene reacts on its own with Br2

false

true

larger molecules that fulfill the requirements for antiaromaticity may _______ to avoid being antiaromatic. How does cyclooctatetraene do this?

change shape

it forms a tub shaped structure

how do you setup a frost circle?

*make sure there are never two degenerate orbitals at the lowest level

annulene

fully conjugated rings

what is the name for benzene as an annulene

[6]Annulene

what is special about [10] Annulene

it follows all the rules for aromaticity, however the two inner hydrogens bump each other creating a twist

t/f aromatic rings can contain carbanions or carbocation

true

why is cyclopentadiene acidic?

its conjugated base is aromatic

note: while the PKa is slightly higher than water, it is still a much higher PKa than expected from a hydrocarbon

draw the structure of pyridine and pyrrole

What is the difference between the electronic situations of nitrogen in pyrrole and pyridine?

because of this, which one is a better base?

The electrons on the nitrogen in pyridine are vinylic, localized, and part of the sp2 hybridized orbital. It is not part of the aromatic ring and does not contribute to the Huckel number

The electrons on the nitrogen of pyrrole are allylic, delocalized and part of the p-orbital. they are part of the aromatic ring and do contribute to the Huckel number

Pyridine is the better base as the electrons on the nitrogen may from a bond with H+ without breaking the aromaticity of the molecule

which lone pairs participate in aromaticity?

only the lone pair on the highlighted nitrogen

benzylic position

any carbon atom attached directly to a benzene ring

alkyl benzenes may be oxidized at the benzylic position by _______

What does this result in?

chromic acid (formula shown in pic)

results in a benzoic acid

What is the limitations of oxidizing the benzylic position

it may not occur at a quaternary benzylic carbon

what type of reaction is this? what reagents are used?

free-radical bromination

what type of reaction is this? what reagents are used?

what type of reaction is this? what reagents are used?

what type of reaction is this? what reagents are used?

what type of reaction is this? what reagents are used?

what type of reaction is this? what reagents are used?

oxidation of an alcohol to an aldehyde

show an efficient synthesis for this reaction

show an efficient synthesis for this reaction

show an efficient synthesis for this reaction

Ozonolysis

show an efficient synthesis for this reaction

Birch Reduction mechanism (start with benzene)

what is this reaction called?

Birch reduction

show how and what is made by a birch reduction with this

Na, CH3OH, NH3

How can you determine the Birch reaction outcome? ie, what are you looking for?

look for any electron withdrawing groups on the aromatic ring. the carbons attached to these groups will be reduced.

If there are electron donating groups, the carbons attached to these groups will not be reduced.