Electrophilic Aromatic Substitution

What does EAS stand for? :: Electrophilic Aromatic Substitution.

What happens in electrophilic aromatic substitution? :: An electrophile replaces a hydrogen on an aromatic ring.

Why does benzene undergo substitution instead of addition? :: Substitution allows benzene to regain and maintain aromaticity.

Why is maintaining aromaticity important? :: Aromatic systems are very stable.

What type of electrons in benzene attack an electrophile? :: Pi electrons.

What is an electrophile? :: An electron-deficient species that accepts electrons.

What happens to a hydrogen during EAS? :: It is replaced by an electrophile.

What is the general result of EAS? :: Benzene-H becomes Benzene-E, where E is the electrophile.

What happens in the first step of EAS? :: A benzene pi bond attacks the electrophile.

What happens to benzene's aromaticity when it first attacks the electrophile? :: Aromaticity is temporarily lost.

What intermediate is formed during EAS? :: The arenium ion.

What is an arenium ion? :: A resonance-stabilized carbocation intermediate formed after an electrophile attaches to an aromatic ring.

What type of charge does the arenium ion contain? :: A positive charge.

Is the positive charge of the arenium ion localized or delocalized? :: Delocalized.

How many major resonance structures does the arenium ion have? :: Three.

Why can the arenium ion have multiple resonance structures? :: The positive charge and pi electrons can be delocalized across the ring.

What hybridization does the carbon bonded to the new electrophile have in the arenium ion? :: sp3.

Does the sp3 carbon participate in resonance in the arenium ion? :: No.

Why is the arenium ion not aromatic? :: The sp3 carbon interrupts the fully conjugated pi system.

What happens in the second major step of EAS? :: A base removes the proton from the carbon containing the new electrophile.

What happens to the electrons from the broken C-H bond? :: They form a new pi bond.

What does formation of the new pi bond accomplish? :: It restores aromaticity.

What is restored at the end of EAS? :: The fully conjugated aromatic pi system.

Why is an addition product unfavorable for benzene? :: It would cause benzene to remain nonaromatic and at higher energy.

Why is substitution favored over addition in benzene? :: Substitution restores aromaticity and stability.

Which step of EAS is the rate-determining step? :: The first step.

Why is the first step of EAS slow? :: It temporarily breaks aromaticity.

Is the first step of EAS endothermic or exothermic? :: Endothermic.

Why is the arenium ion high in energy? :: Aromaticity has been lost.

Is the arenium ion higher or lower in energy than the reactants? :: Higher in energy.

What happens energetically when aromaticity is restored? :: The energy decreases significantly.

Is proton removal relatively favorable after the arenium ion forms? :: Yes, because it restores aromaticity.

What is halogenation of benzene? :: An EAS reaction that replaces a hydrogen on benzene with a halogen.

What example of halogenation is discussed in the video? :: Bromination of benzene.

What reagents are used for bromination of benzene? :: Br2 and FeBr3.

What is FeBr3? :: Iron(III) bromide.

What role does FeBr3 play in bromination? :: It acts as a Lewis acid catalyst.

What is a Lewis acid? :: An electron-pair acceptor.

Why is FeBr3 considered a Lewis acid? :: The iron atom accepts electron density.

What is the product of benzene bromination? :: Bromobenzene.

What byproduct forms during benzene bromination? :: HBr.

Why can't benzene easily react with Br2 by itself? :: Breaking aromaticity requires too much energy.

Why is a catalyst needed for bromination? :: It lowers the activation energy and makes the electrophile reactive enough for benzene to attack.

What does a catalyst do? :: It lowers activation energy without being consumed overall.

What happens when Br2 interacts with FeBr3? :: A catalytic complex forms.

Which atom accepts electron density when Br2 interacts with FeBr3? :: Iron.

What happens to bromine in the Br2-FeBr3 complex? :: One bromine becomes strongly electrophilic.

What charge develops on the electrophilic bromine in the complex? :: A formal positive charge.

Why can benzene attack the activated bromine more easily than Br2 alone? :: The activated bromine is a stronger electrophile.

What attacks the electrophilic bromine during bromination? :: A benzene pi bond.