Quiz 3 Flashcards
Arenes
class of compounds with a fully conjugated pi system in a closed ring
What is the parent compound for a benzene ring + less than 6 carbon chain
Benzene will be the parent
What is the subtiuent name for benznes?
phenyl/ Ph
Ortho
Meta
Para
Ortho: no carbons between substituents
Meta: one carbon between subtiuents
Para: two carbons between subtituents
Why is benzene so stable?
Bc of its 6 overlapping p orbitals + resonance too
What are the two requirments to be an aromatic
- overlapping p orbitals in a conjugated ring
- odd number of pi bonds (huckles rule)
Huckles rule
a compound can be aromatic only if it has 2,3,10,14…. pi electrons
4n+2
True or false?
In a benzene ring three electrons are in anti bonding MOs and three are in bonding MOs
false, all 6 are in bonding MOs
Frost Circles
A way to estimate the number of MOs in a ring
True or False?
You want all of your electrons in non bonding and bonding MOs
false, you want them in bonding, not non bonding which are unstable MOs
Anti aromatic
- smaller than 5 atoms w/ no conjugation or not enough p orbitals
- most unstable
Non aromatic
- 6 atoms or larger than can change shape to overlap orbitals but not enough p e-
- unstable but not as unstable as anti aromatic compounds
carbocations open up p orbitals, carbanions —-
fill p orbitals
If a N has no double bonds attatched it, can its lone pair participate in resonance and be counted towards the number of pi e-?
Yes!
How can aromaticity affect acidity?
Aromaticity can stabilize which makes weak bases/ strong acids
If you undergo a proton transfer that causes a molecule to go from aromatic to non aromatic have you gained or lost stability?
Lost stability
Why is the benzylic position so good for reactions?
bc it is resonance stabilized
What benzylic reaction can extend the conjugation?
Elimination
True or False?
You dont need a benzylic proton to oxidze a alkyl chain on a benzene?
False, with no proton it will not oxidize
What reaction can selectively reduce pi bonds on a benzylic chain?
hydrogenation reaction
Electrophilic Aromatic Substituion (EAS)
replacement of one of the aromatic protons with an electrophile
ex: sulfonation, halogenation, nitration
What does it mean to say aromatic moitey preserved?
It starts aromatic and ends aromatic even if aromaticity is changed intermediatley
mechanism of Halogenation by EAS
- nuc attack
- pt
- FeBr3/AlBr3 + Br2
What are FeBr3 and AlBr3 classified as?
Lewis acids that make the electrophile more electrophilic
What is unique about the proton transfer step in halogenation by EAS
the arrow starts at a bond, not a lp
sigma complex
resonance stabilized carbocation
Sulfonation by EAS
- nuc attack
- pt
- pt
- using conc. H2SO4 + SO3—-SO3H
extra pt step unique to sulfonation
Why is SO3 so electrophilic?
Bc its pi bonds have inefficent overlap
True or False
concentrated SO3 promotes sulfonation
false, concentrated H2SO4 promotes sulfonation
Nitration by EAS
- nuc attack
- pt
- HNO3 + H2SO4—-NO2
What is used to reduce a nitro group to an amino group on a benzene?
HCl + Zn/Fe
Friedel Crafts- Alkylation
addition of an akyl group to a benzene ring using an akly halide + lewis acid + benzene ring
What is the preferred conjugation of aklyl halides for friedel crafts alkyklation?
2 or 3 to prevent rearrangments
True or false?
FC alkylation can occur without the lewis acid under very harsh conditions
False, with out the lewis acid there will be no reation
What happens if you have a primary halide for a FC alkylation?
there will be rearrangements and a mixture of products
What are the three complications with FC alkylation?
- polyalkylation can occur
- some rings are too deactivated to be added to
- If the halide on the akyl chain is attatched to an sp2 carbon there will be no reaction
Friedel Crafts Acylation
addition of an acyl group to a benzen ring using an acid halide + lewis acid + benzene ring
Acyl group
C=O bond connected to an alkyl chain or aryl group
Acylation is — prone to polyacylation and rearrangements
NOT
Clemmensen reduction
using HCl + Zn to reduce the product of FC acylation so it is only an akyl chain
