Chapter 17 Concepts Flashcards
Explain the relative speed of reaction of each step in an Electrophilic Aromatic Substitution reaction.
the first step, formation of the carbocation (sigma complex) is slow because aromaticity is lost. The second step, deprotonation is fast because aromaticity is restored.
Which substituents are activating, and o/p directing?
-O, -N-R-R, -OH, -OR, -NH-CH=O. All are electron-donating.
Why are activating substituents o/p directing (except for halogens)?
Reaction will occur where there is a partial negative charge (increased electron density) which is at the ortho and para positions due to delocalization of the electrons on the substituent. o/p are also more stable carbocations - have one with a full octet.
Why are halogens deactivators, and also o/p directing?
The electronegative halogen draws electon desity from the ring, but it also provides a stable carbocation in which the postitive charge resides on the halogen, and is a carbocat. with filled octets.
Why are deactivating substituents meta directing?
deactivators delocalize partial positive charge on the o/p carbons. This leaves the meta carbons with the highest electron density, so they are more reactive.
Which substituents are deactivating and meta- directing?
groups with positive charge on the atom bonded to the ring: -SO3H, -NO2, -CN, -COR, -COOR, -NR3
When there is a conflict between substituents, which ones take precedent?
activating groups. if two of the same class; whichever is stronger/mix of products.
What are the three limitations of the Fridel-Crafts Alkylation reaction?
- Fails with deactivating systems.
- Susceptible to carbocation rearrangements.
- Susceptable to polyalkylation, which can be minimized by adding excess benzene.
What is the only limitation of Fridel-Crafts Acylation?
How does it pass rearrangement and polyacylation?
fails with strongly deactivating systems.
no rearrangement issues b/c resonance stabilized acylium ion. no polyacylation b/c product is deactivated by acyl group.
What is used to turn nitrobenzene into aniline?
A reducing agent like LiAlH4.
What about an activator makes it stronger?
Resonance - has greater effect than induction on activation strength.
What is the product expected from a reaction with nitro benzene and acyl chloride in AlCl4
No reaction - nitrobenzene is too deactivated
What different compounds can be used in Fidel crafts reactions?
An alkene and HF, Cl2-R+AlCl3, an alcohol and BF3
What is the second thing you need in acylation besides AlCl3?
H2O for removal of aluminum salt.
What is the second way of achieving acylation?
Using CO and HCl, AlCl and CuCl
