2-EASHLS

2-EASHLS - Electrophilic Aromatic Substitution: Bromination...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
Electrophilic Aromatic Substitution: Bromination of Acetanilide and Nitration of Methyl Benzoate Reading Assignment: a) Fessenden, R.J., Fessenden, J.S., Feist, P. Organic Laboratory Techniques , 3 rd ed.; Brooks/Cole: Pacific Grove, 2001, pgs 23-31 (recrystallization, vacuum filtration) and 156-157 . b) Loudon, G.M. Organic Chemistry , 4 th ed.; Oxford University Press: New York, 2002, pgs 704-730. Introduction Aromatic molecules are remarkably stable and are inert to addition reactions under conditions in which simple alkenes react readily (Figure 1). DCl Cl 2 D Cl Cl Cl DCl Cl 2 No Reaction No Reaction Figure 1. In the presence of a catalyst aromatic compounds undergo substitution reactions preferentially over addition reactions (Figure 2). In the reaction below one of the aromatic hydrogens is substituted with a chlorine atom. Cl 2, FeCl 3 heat Cl + HCl Figure 2. Other common and useful substitution reactions observed with benzene and other aromatic compounds include bromination, sulfonation, nitration, alkylation, and acylation. The reagents required for each of these substitution reactions of aromatic rings are highly electrophilic and hence the name Electrophilic Aromatic Substitution (EAS).
Background image of page 1

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

View Full DocumentRight Arrow Icon
Electrophilic aromatic substitution occurs in three mechanistic steps (figure 3): 1) Generation of a highly reactive electrophile – often a Lewis acid like FeBr 3 is employed. 2) The π electrons of the aromatic ring attack the electrophile to form a sigma bond and a resonance stabilized carbocation. Because this step requires breaking up the aromaticity the activation energy is large and this step is rate determining. 3) Abstraction or loss of a proton from the only sp 3 hybridized carbon in the ring to reform the aromatic compound. Br Br FeBr 3 H Br H + ! + ! " Br Br Fe Br Br Br H H Br Fe Br Br Br Br + HBr + FeBr 3 Figure 3. Mechanism and energy diagram of the electrophilic aromatic bromination of benzene. If the aromatic ring is substituted by one or more non-hydrogen groups [for example toluene(methylbenzene) or aniline(aminobenzene)], these substituents can activate or deactivate the ring towards electrophilic aromatic substitution. Electron withdrawing groups such as nitro groups, aldehydes, ketones, and carboxylic acids are deactivating. Electron donating substituents such as amines, ethers, and alcohols are activating.
Background image of page 2
Along with the ability to activate or deactivate the ring, substituents have directing effects. In other words, they can influence where the ring is substituted. These groups fall into two categories: ortho- para directors and meta directors. Directing Group para ortho ortho meta meta Most activating groups are ortho-para directors and most deactivating groups are meta directors. A notable exception is the halogens, which are weakly deactivating, but ortho-para directors. The directing effect of a particular substituent can be explained by a careful examination of all the resonance structures of the carbocationic intermediate that is formed.
Background image of page 3

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

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 11

2-EASHLS - Electrophilic Aromatic Substitution: Bromination...

This preview shows document pages 1 - 4. Sign up to view the full document.

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