benzene reactions Flashcards
What is the difference in reactivity between aromatic compounds and alkenes and why?
Aromatic compounds, like benzene, are less reactive and more stable than alkenes due to their pi electrons being spread out over a larger area, while alkenes have a high region of electron density in a small space.
What type of reaction do alkenes undergo with halogens?
Alkenes react with halogens in electrophilic addition reactions, where the double bond breaks and the halogen is added to the carbons of the double bond, forming an alkane.
Can aromatic compounds undergo electrophilic addition reactions?
No, aromatic compounds cannot undergo electrophilic addition reactions; they can only react with halogens in electrophilic substitution reactions.
What occurs during electrophilic substitution reactions in aromatic compounds?
Electrophilic substitution involves breaking the delocalised pi system, forming a positively charged intermediate, and then losing a hydrogen to reform the delocalised system.
What is required to add a halogen atom onto benzene?
The presence of a halogen carrier catalyst is required to add a halogen atom onto benzene.
What are examples of halogen carrier catalysts?
Examples include aluminium chloride (AlCl3) and iron (III) bromide (FeBr3).
These are not strictly catalysts as they are permanently changed during the reaction.
What are Friedel-Crafts reactions?
Friedel-Crafts reactions are a type of electrophilic substitution where carbon-containing groups are added to the aromatic ring, useful for forming carbon-carbon bonds.
What is acylation in Friedel-Crafts reactions?
Acylation involves refluxing an acyl chloride with benzene in the presence of a halogen carrier to form phenylketones and hydrogen chloride.
What is alkylation in Friedel-Crafts reactions?
Alkylation involves refluxing benzene with a haloalkane (e.g., chloromethane) in the presence of a halogen carrier, substituting the alkyl group for hydrogen.
What is nitration in the context of electrophilic substitution?
Nitration is a type of electrophilic substitution where a nitronium ion (NO2+) is substituted for a hydrogen atom on the benzene ring.
What reagents are required for nitration?
Concentrated nitric acid as a source of the electrophile and concentrated sulfuric acid to act as a catalyst.
How does sulfuric acid act as a catalyst in nitration?
H2SO4 generates the NO2+ electrophile and reacts with the hydrogen ion produced after substitution to regenerate H2SO4, leaving it unchanged by the end of the reaction.
What is the temperature condition for mononitration?
Mononitration occurs at temperatures below 55°C; higher temperatures lead to multiple substitutions.
What does the nitronium ion act as in electrophilic substitution?
An electrophile
