Benzene (Chapter 15)

Question 1

1,2 Disubstitution

Answer 1

Ortho-

o–

Question 2

1,3 Disubstitution

Answer 2

Meta-

m–

Question 3

1,4 Disubstitution

Answer 3

Para-

p–

Question 4

Answer 4

Phenol

Question 5

Answer 5

Aniline

Question 6

Answer 6

Benzaldehyde

Question 7

Answer 7

Benzoic Acid

Question 8

Answer 8

Styrene

Question 9

Benzene

Answer 9

A six-carbon cyclic compound consisting of six equally overlapping p orbitals and six equal-length C—C bonds.

All carbons within the benzene ring are sp²-hybridized.

Question 10

Benzene vs. 1,3,5–Hexatriene

Stabiilty Comparison

Answer 10

Benzene is more stable than 1,3,5-Hexatriene due to the “closure” of the six-membered carbon compound.

Six-membered cyclic π systems are more stable than acyclic π systems.

Question 11

Aromatic System

Huckel’s Rule

Answer 11

A cyclic planar system of sp²-hybridized atoms containing (4n + 2)π electrons.

Aromatic systems are highly stabilized.

Question 12

Antiaromatic System

Huckel’s Rule

Answer 12

A cyclic planar system of sp²-hybridized atoms containing (4n)π electrons.

Antiaromatic systems are highly destabilized (i.e. less stable than alkenes or dienes).

Question 13

Nonaromatic System

Answer 13

• Non-planar
• Acyclic
• sp³-Hybridized Atoms

Nonaromatic systems are comparible in stability to alkenes and dienes.

Question 14

1,3–Cyclobutadiene

Answer 14

• Highly Unstable (Antiaromatic + Ring Strain)
• Ability to Act as Alkene and Diene
• Alternating Single + Double Bonds
• Two Distinct Isomers (Rapid Isomerization through Symmetrical Transition State)

The two isomers of 1,3–Cyclobutadiene are not resonance structures (because atoms are moved during the isomerization process).

Question 15

1,3,5,7–Cyclooctatetraene

Answer 15

• Non-Aromatic (Tub-Shaped)
• Non-Conjugated π Bonds
• Acts an Simple Alkene

Question 16

Reactivity of Benzene

Answer 16

• Benzene is unreactive in electrophilic addition mechanisms.
• Benzene is reactive in electrophilic substitution mechanisms.

Question 17

Why are certain cationic/anionic cyclic structures highly stable?

Answer 17

Cationic/anionic cyclic structures containing (4n +2)π electrons are highly stabilized due to aromatic stabilization.

• Anionic cyclic structures containing (4n +2)π electrons have highly acidic conjugate acids.
• Cationic cyclic structures containing (4n +2)π electrons serve as effective bases.

Question 18

Aromaticity of Polycyclic Hydrocarbons

PCHC = Polycyclic Hydrocarbon

Answer 18

• If the PCHC contains (4n +2)π electrons, it is aromatic.
• If all individual rings of the PCHC are aromatic, the PCHC is aromatic .
• If the PCHC consists of one (or more) aromatic ring and no antiaromatic rings (i.e. solely nonaromatic rings), the PCHC is aromatic.

Question 19

Antiaromaticity of Polycyclic Hydrocarbons

PCHC = Polycyclic Hydrocarbon

Answer 19

• If the PCHC consists of one (or more) antiaromatic rings, the PCHC is antiaromatic.
• If the PCHC contains (4n)π electrons, it is antiaromatic.

Question 20

Electrophile of EAS Reactions

EAS = Electrophilic Aromatic Substitution

Answer 20

A strong (highly partial-positive) electrophile is required for EAS reactions to occur (due to the low reactivity of the benzene ring).

Since the C—E bond formation must be highly thermodynamically favorable (to overcome the thermodynamic instability of the loss of aromaticity) for EAS to occur, the electrophile must be highly electronegative.

Question 21

RDS of EAS Reactions

RDS = Rate-Determining Step

Answer 21

Creation of the Arenium Ion Intermediate

The creation of the arenium ion intermediate results in the loss of aromaticity, which destabilizes the cyclic compound.

Question 22

Why is EAS of Benzene thermodynically favored?

Answer 22

The C—E bond formed during EAS is stronger/shorter (i.e. more stable) than the C—H bond that is broken.

E = Electrophile

Question 23

Overalkylation

Answer 23

Since Friedel-Crafts Alkylation substitutes an electron-donating alkyl group into the benzene ring, the resulting phenyl compound is more electron-rich (i.e. more reactive).

Overalkylation results in additional alkylations of the phenyl compound at the para and ortho (if steric hindrace is insignificant) positions.

Question 24

Overfunctionalization

Answer 24

If an EAS reaction results in the substitution of an electron-donating group into the benzene rings, additional substitutions will occur at the para and ortho positions (if steric hindrance is minimal).

Question 25

Hydride Shift in Friedel-Crafts Alkylation

Answer 25

A concerted 1,2–hydride shift will occur during haloalkane activation of a primary alkyl, unless the halokane is a 1-carbon (methyl-) or 2-carbon (ethyl-) compound.

Question 26

Alkyl Shift in Friedel-Crafts Alkylation

Answer 26

A concerted 1,2–alkyl shift will during during haloalkane activation of a primary alkyl, if the activated carbon (i.e. the carbon of the C—X bond) is bonded to a quaternary carbon.