Aromatic Compounds and Substituent Effects

Flashcards covering the concepts of aromatic compounds, substituent effects on reactivity and regiochemistry in electrophilic aromatic substitution reactions.

What is the effect of electron donating groups (+M or +I effect) on the rate of electrophilic substitution (SE) reactions?

Increase the rate of SE reactions by donating electrons to the benzene ring, making it more electron-rich.

What is the effect of electron withdrawing groups (-M or -I effect) on the rate of electrophilic substitution (SE) reactions?

Decrease the rate of SE reactions by withdrawing electrons from the benzene ring, making it electron-poor.

What are ortho-para directors?

Substituents that direct incoming groups to the ortho or para positions.

What are meta directors?

Substituents that direct incoming groups to the meta position.

What positions do all activators direct incoming electrophiles to?

They direct incoming electrophiles to the ortho- and para-positions.

What positions do most deactivators direct incoming electrophiles to?

They direct incoming electrophiles to the meta position, except for halogens.

Why is a hydroxy group activating and o-, p-directing?

It has an electron-donating effect through the +M mesomeric effect, increasing electron density in the benzene ring and stabilizing the σ-complex intermediate at the ortho- and para- positions.

Why is an alkyl group in alkylbenzene activating and o-, p-directing?

They are electron-donating groups with a +I effect that activates the benzene ring toward electrophilic substitution by stabilizing the formed σ-complex.

Why is a nitro group deactivating and m-directing?

It has electron-withdrawing effects (-M and -I) that decreases the electron density of the benzene ring, deactivating it and destabilizing the σ-complex at the ortho- and para- positions.

Why are halogens o-, p-directing although they are deactivating groups?

Halogens have both -I and +M effects, but the -I effect is stronger, making them deactivating, but the +M effect still allows for o,p-direction.

What happens when the directing effects of two substituents reinforce each other?

The predicted product predominates.

What happens when the directing effects of two substituents oppose each other?

The more activating group dominates, but mixtures often result.

Where does substitution almost never occur on a benzene ring?

Substitution almost never occurs between two substituents meta to each other due to steric hindrance.

With a bulky o,p- director and/or a bulky electrophile, which substitution predominates?

Para substitution predominates due to steric hindrance.