benzyne

Problem # 616

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.

Problem Details

MendelSet practice problem # 616 submitted by Matt on July 10, 2011.

Subject: Organic Chemistry

Subject Topic(s): Nucleophilic Aromatic Substitution and Benzyne

Question Type: Mechanism (show a mechanism using curved arrows..)

Keyword(s): benzyne, Grignard

Problem # 615

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.

Solution

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.