Chapter 15 Organic Chemistry Notes PDF

Preview

Summary

Download Chapter 15 Organic Chemistry Notes PDF

Description

Chapter 15 Organic Notes

 Electrophilic aromatic substitution reactions

This occurs with different chemistry than with alkenes Benzene does not undergo electrophilic addition, but it undergoes electrophilic substitution o The mechanism goes through a cyclohexadienyl cation intermediate  It is not aromatic  It is resonance stabilized  It is very acidic  Some base restores aromaticity o Benzene does NOT react with Br2 or Cl2 unless a lewis acid is present  F2 is too reactive and gives a mixture of mono-, di-, and polysubstituted products  I2 is very unreactive even with lewis acids and it usually needs an oxidizing agent (HNO3, Cu, H2O2)  Nitration of benzene o Electrophile in this case is NO2 (nitronium ion) o In the presence of water, the extra proton can be abstracted, and the aromaticity becomes restored  Sulfonation of benzene o SO3 is protonated and forms SO3H+ o This acts as an electrophile with the benzene ring to form an arenium ion o Loss of the extra proton restores aromaticity to the ring o Sulfonation and desulfonation  SO3, conc. H2SO4 (25-80 C) for sulfonation  Dil. H2SO4, H2O at 100 C for desulfonation  Friedel-Crafts Reactions o Alkylation can occur with RX in the presence of a lewis acid (AlCl 3) o Electrophiles in Friedel-Crafts alkylation  2° or 3° alkyl halides  carbocation is electrophile  1° or methyl alkyl halides  complex molecule is electrophile  “acts like” it was just a carbocation  Other pairs of reagents that generate carbocations may be used o o

o

Acylation

 

Acid chlorides (or acyl chlorides) can be prepared by SOCl 2 or PCl5 Can also be carried out with carboxylic acid anydrides

Limitations of these rxns can come from carbocations rearranging to become more stable; poor yields are usual when powerful EWG are present on the aromatic ring or when there is a nitric group; this applies o Friedel-Crafts reactions usually give poor yield when there are powerful EWG on the ring (O,N,F)  Birch reductions o Makes a 1,4 hexadiene o Can occur with any of the alkalai metals and an alcohol and NH3 o...