CH223 Part 2: Arenes and Aromaticity (Conceptual) Flashcards
What was so interesting to early benzene scientists?
the fact that benzene didn’t reaction with compounds that reacted easily with alkenes and alkynes
What is so interesting about the heat of hydrogenation of benzene?
significantly less than conjugation alone can explain
How are monosubstituted benzenes names?
as derivatives of benzene
How are disubstituted benzenes named?
they are named according to the positions of their substituents (ortho, meta, para)
What does ortho mean?
1,2-disubstituted benzenes
What does meta mean?
1,3-disubstituted benzenes
What does para mean?
1,4-disubstituted benzenes
What sets the 1 position in benzene nomenclature?
the name of the benzene derivative
How are polysubstituted benzenes named?
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
How can you determine the relative stabilities of polycyclic aromatic compounds?
the more full benzene rings, the more stable due to resonance delocalization
What is pi-pi stacking?
when the parallel alignment of two rings causes one to be slightly offset as it minimizes electric repulsion
What is a common large cyclic molecule that is not more stable than benzene despite being fully conjugated?
cyclooctatetraene (COT) because it has been shown to adopt a nonplanar “tub” shape
What must a molecule be to be antiaromatic?
- planar
- fully conjugated
- cyclic
- has 4n electrons
What must a molecule be to be aromatic?
- planar
- fully conjugated
- cyclic
- has 4n + 2 electrons
Does Hückel’s Rule apply to molecules that have charges?
Yes, even charged molecules can be incredibly stable if they are aromatic
Does Hückel’s Rule apply to molecules that contain heteroatoms?
Yes, the lone pairs on the heteroatoms will only be involved if it contributes to aromaticity
When the lone pair is not involved i the aromatic system, that compound is more what?
basic (available to protonate somthing)
What is a benzylic carbon?
a carbon directly attached to the benzene ring
What is always the major product of an SN1 nucleophilic substitution on a benzylic halide?
the product that keeps the aromaticity of benzene intact
Why are benzylic halides such good substrates for SN2 reactions?
they are unable to undergo competing E2 eliminations
Like with allylic free-radical halogenation, where is the regioselective preference?
at the benzylic carbon
why are benzylic protons more acidic than others?
the phenyl groups stabalizes the anion through electron delocalization and resonance
What is the rate-determining step of all EAS reactions?
generating the arenium cation because it involves breaking the aromaticity
Can regular halogens react with benzene?
No, they must be activated with a Lewis acid
What are Friedel-Crafts alkylations limited by?
- polyalkylations
- carbocation rearrangements to most stable carbocation
Explain polyalkylations in terms of Friedel-Crafts?
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
Why do Friedel-Crafts acylations give better yields than alkylations?
- the carbonyl group on the product deactivates the aromatic cycle, making it less reactive
- the acyl cation does no rearrange
Where do EDG direct?
ortho/para
Will EDG react faster or slower compared to unsubstituted benzene?
faster; more stable carbocation
Are EDG activating or deactivating?
activating
Are EWG activating or deactivating?
deactivating
Where do EWG direct?
the meta position
Do EWG react faster or slower compared to unsubstituted benzene?
slower
EDG substituents that are in resonance with benzene are strongly activated. What are these?
NH2, OH
What do strongly activated substituents allow?
multiple substitutions to occur
Are halogens weakly activating or weakly deactivating?
weakly deactivating due to their high electronegativity
Are halogen ortho/para or meta directors?
ortho/para because the ortho/para intermediate can be stabilized through resonance with the halogen
What happens when directing effects of individual substituents oppose each other?
it is the most activating substituent that controls the regioselectivity
What happens were two positions on benzene are comparatively activated?
substitution occurs at the less hindered site
Rules for Retrosynthesis of Substituted Benzenes
- don’t ad a meta if you need a para next
- don’t add an ortho/para if you need a meta next
- use SO3H if you only want to substitute at the ortho position
- introduce alkyl groups with a Friedel-Craft acylation and reduction rather than an alkylation
- if nitration needs to occur, do it last
Do normal aryl halides usually reactive in nucleophilic substitution?
No, the carbon-hydrogen bonds are stronger and the optimal transition state is usually blocked (“backside attack blocked”)
What aryl halides will react in nucleophilic substitution/
those with electronegative substituents such as nitro groups as these activate nucleophiles
Does a meta nitro group react faster or slower than ortho/para groups in nucleophilic aromatic substitution?
much slower (ie has to be ortho/para)
Describe the trend for the reactivity of aryl halides in nucleophilic aromatic substituion?
F > Cl > Br >I
Why is aryl fluoride much more reactive in nucleophilic substitutions?
the highly electronegative fluorine pulls electrons from the ring creating a much larger partial positive charge at the site of the halogen
