CH223 Part 2: Arenes and Aromaticity (Conceptual)

Question 1

What was so interesting to early benzene scientists?

Answer 1

the fact that benzene didn’t reaction with compounds that reacted easily with alkenes and alkynes

Question 2

What is so interesting about the heat of hydrogenation of benzene?

Answer 2

significantly less than conjugation alone can explain

Question 3

How are monosubstituted benzenes names?

Answer 3

as derivatives of benzene

Question 4

How are disubstituted benzenes named?

Answer 4

they are named according to the positions of their substituents (ortho, meta, para)

Question 5

What does ortho mean?

Answer 5

1,2-disubstituted benzenes

Question 6

What does meta mean?

Answer 6

1,3-disubstituted benzenes

Question 7

What does para mean?

Answer 7

1,4-disubstituted benzenes

Question 8

What sets the 1 position in benzene nomenclature?

Answer 8

the name of the benzene derivative

Question 9

How are polysubstituted benzenes named?

Answer 9

these are named with numbers
- benzene sets the 1 position
- the smallest number is given to the first point of different
- the substituents are listed in alphabetical order

Question 10

How can you determine the relative stabilities of polycyclic aromatic compounds?

Answer 10

the more full benzene rings, the more stable due to resonance delocalization

Question 11

What is pi-pi stacking?

Answer 11

when the parallel alignment of two rings causes one to be slightly offset as it minimizes electric repulsion

Question 12

What is a common large cyclic molecule that is not more stable than benzene despite being fully conjugated?

Answer 12

cyclooctatetraene (COT) because it has been shown to adopt a nonplanar “tub” shape

Question 13

What must a molecule be to be antiaromatic?

Answer 13

• planar
• fully conjugated
• cyclic
• has 4n electrons

Question 14

What must a molecule be to be aromatic?

Answer 14

• planar
• fully conjugated
• cyclic
• has 4n + 2 electrons

Question 15

Does Hückel’s Rule apply to molecules that have charges?

Answer 15

Yes, even charged molecules can be incredibly stable if they are aromatic

Question 16

Does Hückel’s Rule apply to molecules that contain heteroatoms?

Answer 16

Yes, the lone pairs on the heteroatoms will only be involved if it contributes to aromaticity

Question 17

When the lone pair is not involved i the aromatic system, that compound is more what?

Answer 17

basic (available to protonate somthing)

Question 18

What is a benzylic carbon?

Answer 18

a carbon directly attached to the benzene ring

Question 19

What is always the major product of an SN1 nucleophilic substitution on a benzylic halide?

Answer 19

the product that keeps the aromaticity of benzene intact

Question 20

Why are benzylic halides such good substrates for SN2 reactions?

Answer 20

they are unable to undergo competing E2 eliminations

Question 21

Like with allylic free-radical halogenation, where is the regioselective preference?

Answer 21

at the benzylic carbon

Question 22

why are benzylic protons more acidic than others?

Answer 22

the phenyl groups stabalizes the anion through electron delocalization and resonance

Question 23

What is the rate-determining step of all EAS reactions?

Answer 23

generating the arenium cation because it involves breaking the aromaticity

Question 24

Can regular halogens react with benzene?

Answer 24

No, they must be activated with a Lewis acid

Question 25

What are Friedel-Crafts alkylations limited by?

Answer 25

- polyalkylations - carbocation rearrangements to most stable carbocation

Question 26

Explain polyalkylations in terms of Friedel-Crafts?

Answer 26

once alkylated, the aromatic ring is even more prone to electrophilic additions so the reaction continues; EDG makes carbon more electron rich and thus more nucleophilic

Question 27

Why do Friedel-Crafts acylations give better yields than alkylations?

Answer 27

- the carbonyl group on the product deactivates the aromatic cycle, making it less reactive - the acyl cation does no rearrange

Question 28

Where do EDG direct?

Answer 28

ortho/para

Question 29

Will EDG react faster or slower compared to unsubstituted benzene?

Answer 29

faster; more stable carbocation

Question 30

Are EDG activating or deactivating?

Answer 30

activating

Question 31

Are EWG activating or deactivating?

Answer 31

deactivating

Question 32

Where do EWG direct?

Answer 32

the meta position

Question 33

Do EWG react faster or slower compared to unsubstituted benzene?

Answer 33

slower

Question 34

EDG substituents that are in resonance with benzene are strongly activated. What are these?

Answer 34

NH2, OH

Question 35

What do strongly activated substituents allow?

Answer 35

multiple substitutions to occur

Question 36

Are halogens weakly activating or weakly deactivating?

Answer 36

weakly deactivating due to their high electronegativity

Question 37

Are halogen ortho/para or meta directors?

Answer 37

ortho/para because the ortho/para intermediate can be stabilized through resonance with the halogen

Question 38

What happens when directing effects of individual substituents oppose each other?

Answer 38

it is the most activating substituent that controls the regioselectivity

Question 39

What happens were two positions on benzene are comparatively activated?

Answer 39

substitution occurs at the less hindered site

Question 40

Rules for Retrosynthesis of Substituted Benzenes

Answer 40

1. don't ad a meta if you need a para next 2. don't add an ortho/para if you need a meta next 3. use SO3H if you only want to substitute at the ortho position 4. introduce alkyl groups with a Friedel-Craft acylation and reduction rather than an alkylation 5. if nitration needs to occur, do it last

Question 41

Do normal aryl halides usually reactive in nucleophilic substitution?

Answer 41

No, the carbon-hydrogen bonds are stronger and the optimal transition state is usually blocked ("backside attack blocked")

Question 42

What aryl halides will react in nucleophilic substitution/

Answer 42

those with electronegative substituents such as nitro groups as these activate nucleophiles

Question 43

Does a meta nitro group react faster or slower than ortho/para groups in nucleophilic aromatic substitution?

Answer 43

much slower (ie has to be ortho/para)

Question 44

Describe the trend for the reactivity of aryl halides in nucleophilic aromatic substituion?

Answer 44

F > Cl > Br >I

Question 45

Why is aryl fluoride much more reactive in nucleophilic substitutions?

Answer 45

the highly electronegative fluorine pulls electrons from the ring creating a much larger partial positive charge at the site of the halogen