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Short link: mendelset.com/chapters/834
Draw in the arrows to show the electron flow and resonance forms in the nucleophilic aromatic substitution reaction below.
Note: Depending on the textbook, nucleophilic aromatic substitution is referred to as NAS, SNAr, or addition-elimination.
SNAr is sort of like SN2, except the leaving group doesn't leave right away; a tetrahedral intermediate is formed first.
The trick with SNAr (or NAS, addition-elimination, etc.) is to draw resonance forms that stabilizes the negative charge that forms. That's why EWG increase the rate of SNAr (they stabilize negative charges).
Draw a mechanism for the nucleophilic aromatic substitution (SNAr) reaction below. Show all resonance forms of the intermediate.
For SNAr to work you need to have electron withdrawing groups (EWG) either ortho or para to the leaving group.
This molecule has two leaving groups (chlorines), but only one chlorine has EWG ortho/para to it. So that's the carbon where the nucleophile (NH3) will attack.
Why do EWG need to be ortho/para? Because that's the only way for the negative charge to be stabilized by resonance. Try it- if you have the NH3 attack the other carbon with a chlorine, you will not be able to draw a resonance form that places the negative charge on one of the EWG.
Let's go through a benzyne reaction (also called elimination-addition).
In the reaction below, the strong base (NaNH2) will form a benzyne intermediate, which when forms either ortho nitroaniline or meta nitroaniline.
Used curved arrows to show the formation of each intermediate and the final products.
First the -NH2 acts as a base to eliminate a leaving group and form Benzyne. Then the -NH2 acts as a nucleophile and attacks the benzyne, and the "leaving group" is the triple bond.
Rank each group of acids in order of decreasing acidity. (1 = most acidic)
Explain your reasoning. You will have to use more than one rule in your explanation (resonance, electronegativity, atomic radius, etc.).
Phenol is more acidic than cyclohexanol because the conjugate base of phenol (phenolate) has resonance while the conjugate base of cyclohexanol does not.
Thiophenol is more acidic than phenol because sulfur is larger than oxygen, and so RS- is more stable than RO-.
Let's go through another way to make benzyne.
First, let's form a Grignard reagent. Then, let's elminate to form benzyne.
Halogens get more reactive towards Grignard formation as we go down the periodic table, so Mg0 will form a Grignard with Br before F.
Grignards behave like negatively charged carbon atoms- very unstable. So even though fluorine is a poor leaving group, the negative carbon will cause it to eliminate to form benzyne.
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