Chapter 16- Aromatic Compounds

Question 1

Each sp2 hybridized C in the Benzene ring has what? How are the six pi electrons oriented?

Answer 1

An unhybridized p orbital perpendicular to the ring that overlaps around the ring

They are delocalized over the 6 carbons

Question 2

Describe the physical properties of aromtic compounds. (MP, BP, Density, Solubility)

Answer 2

MP- more sym than corresponding alkane, pack better into crystals, so HIGHER melting points

BP- depends on dipole moment. Ortho>meta>para for disubstituted benzenes

Density- MORE dense than nonaromatic compounds, LESS dense than water.

Solubility- generally INSOLUBLE in water

Question 3

What are the resonance energies of cyclohexene, 1,3- cyclohexadiene, and “cyclohexatriene?”

Answer 3

Cyclohexene: -119.7 kj/mol

1,3 cyclohexadiene: -239.4 kj/mol (predicted) and
-232.0 kj mol (experimental)

Cyclohexatriene: -359.1 kj/mol (predicted) and -208.4 kj/mol (expiremental)

Question 4

What are the various positions on a substitued Benzene?

Answer 4

Ortho- 1,2
Meta- 1,3
Para- 1,4

Question 5

Why is the resonance energy difference between the predicted and expiremental calculations of cyclohexatriene so great? (150.7 kj/ mol)

Answer 5

“Cyclohexatriene” does not exist, the compound is called Benzene. The great energy difference between the predicted and expiremental means that Benzene is more stable than expected.

Question 6

What are the three rules of aromaticity?

Answer 6

1. Continuous cyclic cloud of pi electrons (p orbital on each carbon of the ring)
2. Cyclic molecule must be planar
3. 4n+2 pi electrons (huckle number)

Question 7

How does deprotonating cyclopentadiene allow it to be aromatic?

Answer 7

When deprotonated, a pair of electrons is left in one of the sp3 orbitals, which can then rehybridize to a p orbital. The 6 electrons in the p orbitals can now resonate around the 5 carbon atoms, making the molecule aromatic.

Question 8

Why are planar molecules with an uninterrupted pi electron cloud and 4n+2 pi electrons so stable?

Answer 8

If you look at MO diagram, all the bonding pi orbitals are filled with electrons (closed shell), which as an extremely stable arrangement.

Question 9

What makes a molecule nonaromatic?

Answer 9

If either of the first two rules for aromaticity is violated

Question 10

What makes a molecule antiaromatic?

Answer 10

If the first two aromaticity rules are followed, but the third rule is violated

Question 11

Anitaromatic compounds always have what?

Answer 11

Unfilled bonding molecular orbitals

Question 12

Describe the MO diagram of antiaromatic cyclobutadiene.

Answer 12

Two of the four electrons are in SEPARATE nonbonding mos. This diradical is reactive and unstable.

Question 13

Benzyne (and alkynes in general) are good what?

Answer 13

Dienophiles

Question 14

For a hydrocarbon, azulene has an extremely large dipole moment. Why?

Answer 14

It is aromatic. It can resonate to create a negative charge on the cyclopentadienyl anion and a positive charge on the tropylium cation.

Question 15

What are annulenes?

Answer 15

Conjugated hydrocarbons

Question 16

What are the four annulenes?

Answer 16

[4]annulene (Cyclobutadiene)
[6]annulene (Benzene)
[8]annulene (Cyclooctatetraene)
[10]annulene (Cyclodecapentaene)

Question 17

Describe annulenes in terms of aromaticity.

Answer 17

Cyclobutadiene- antiaromatic
Cyclooctatetraene- nonaromatic b/c nonplanar
Cyclodecapentaene- aromatic except isomers that are nonplanar
Larger 4N annulenes are antiaromatic bc they are flexible enough to become nonplanar

Question 18

What are the three forms of [10]annulene? Which one is aromatic?

Answer 18

1. All-cis- nonaromatic
2. Two trans- nonaromatic
3. Naphthalene- aromatic

Question 19

Why can’t the all-cis and two trans isomers of [10] annulene adopt planar conformations to become aromatic?

Answer 19

All-cis- planar conformation requires an excessive amount of angle strain

Two trans- the two hydrogen atoms interfere with each other

Question 20

What are the five allotropes of carbon? Which ones are the new ones?

Answer 20

1. Amorphous
2. Diamond
3. Graphite

New:

4. Fullerenes
5. Nanotubes

Question 21

Which allotrope is small particles of graphite like charcoal, soot, coal, and carbon black?

Answer 21

Amorphous

Question 22

Which allotrope is:

A lattice of tetrahedral carbons
One giant molecule
Composed of sigma bonds
Electrical insulator

Answer 22

Diamond

Question 23

Which allotrope:

Is layers of fused benzene (aromatic) rings
Has planar layered structure
Has only van der Waals forces b/w layers
Conducts electrical current parallel to layers

Answer 23

Graphite

Single layer of graphite -> graphene

Question 24

Which allotrope is composed of five or six- memebered rings arranged to form a “soccer ball” structure?

Answer 24

Fullerenes

Ex. Buckyball

Question 25

Which allotrope is half of a C60 sphere fused to a cylinder of fused aromatic rings?

Answer 25

Nanotubes

Question 26

How do anisotopic effects make chemical shifts so different?

Answer 26

Magnetic anisotopy means that magnetic properties differ with respect to location in 3-D space.