Chap 22 Textbook slides

Chap 22 Textbook slides - CHAPTER 22 CHAPTER 22 Chemistry...

Info iconThis preview shows pages 1–15. Sign up to view the full content.

View Full Document Right Arrow Icon
HAPTER 22 CHAPTER 22 Chemistry of Benzene Substituents: Alkylbenzenes, Phenols and Benzenamines
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Reactivity at the Phenylmethyl (Benzyl) Carbon: Benzylic Resonance Stabilization 22-1 The methyl C-H bonds in methylbenzene are relatively weak with respect to homolytic and heterolytic cleavage. The phenylmethyl (benzyl) group may be viewed as a benzene ring whose system overlaps with an extra p orbital on the attached alkyl carbon:
Background image of page 2
Reactivity at the Phenylmethyl (Benzyl) Carbon: Benzylic Resonance Stabilization 22-1 Benzylic radicals are reactive intermediates in the halogenation of alkylbenzenes. Benzene will not react with Cl 2 or Br 2 unless a Lewis acid is added:
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Heat or light allows attack of Cl 2 or Br 2 on methylbenzene even in e absence of a catalyst, however, attack is at the methyl group, the absence of a catalyst, however, attack is at the methyl group, not the aromatic ring. Excess halogen leads to multiple substitution.
Background image of page 4
The mechanism of benzylic halogenation proceeds through radical termediates: intermediates: The benzylic C-H bond is relatively weak (DH o =87 kcal mol -1 ) due to resonance stabilization of the intermediate radical formed.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Subsequent halogen attack is always at the benzylic position because attack at an aromatic carbon would destroy the aromatic character of the ring.
Background image of page 6
Benzylic cations delocalize the positive charge. Benzylic resonance can strongly affect the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, a primary benzylic tosylate rapidly reacts with ethanol via an S N 1 reaction:
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Delocalization of positive charge into the aromatic ring facilitates the dissociation of the starting sulfonate:
Background image of page 8
Several benzylic cations are stable enough to have been isolated. he X y structure of 2 henyl ropyl cation (as its SbF - alt) The X-ray structure of 2-phenyl-2-propyl cation (as its SbF 6 salt) was obtained in 1997.
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
A para methoxy substituent on the benzene ring allows for extra stabilization of the benzylic positive charge. In its absence, the S N 2 reaction may dominate due to the lack of steric interference and the stabilization of the S N 2 transition state y overlap with the benzene ystem. by overlap with the benzene system.
Background image of page 10
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Resonance in benzylic anions makes benzylic ydrogens relatively acidic. hydrogens relatively acidic. The anion, radical and cation adjacent to a benzene ring are all stabilized by conjugation:
Background image of page 12
The acidity of methylbenzene (pK a ~41) is considerably greater than that of ethane (pK a ~50) and comparable to that of propene (pK a ~40) which can be deprotonated to form the resonance- stabilized 2-propenyl anion. onsequently, methylbenzene can be deprotonated by Consequently, methylbenzene can be deprotonated by butyllithium to generate phenylmethyllithium:
Background image of page 13

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Benzylic Oxidations and Reductions 22-2 xidation of alkyl ubstituted benzenes leads to Oxidation of alkyl-substituted benzenes leads to aromatic ketones and acids.
Background image of page 14
Image of page 15
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 79

Chap 22 Textbook slides - CHAPTER 22 CHAPTER 22 Chemistry...

This preview shows document pages 1 - 15. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online