Chapter 25 - Aromatic compounds Flashcards
Formula of benzene
C6H6
What type of hydrocarbon is benzene?
Aromatic - it is an arene
What is Kekule’s model of benzene?
The six carbon ring is joined by alternating single and double bonds
What are the three pieces of evidence to disprove Kekule’s model?
Benzene’s lack of reactivity
The length of the c-c bonds
Hydrogenation enthalpy of benzene
Why does benzene’s lack of reactivity disprove Kekule’s model?
Benzene doesn’t undergo addition reactions or decolourise bromine
Why does the length of benzene’s c-c bonds disprove kekule’s model?
The c-c bond is between the length of the single and double bond
Why does the hydrogenation enthalpy of benzene disprove kekule’s model?
Its expected enthalpy would be triple the enthalpy of cyclohexene but it doesn’t
What is the delocalised model of benzene?
Each carbon uses three of its four electrons to bond to 2 x C and 1 x H
The overlapping of unused p-orbitals creates a system of pi-bonds which spread over the six atoms
When will the prefix ‘phenol’ be used?
When the alkyl group attached to the benzene has more than 7 carbon atoms
What happens when benzene is nitrated?
One of the hydrogen atoms on the benzene ring is replaced with a NO2 group
What is used to catalyse the reaction between benzene and a nitrate group?
Sulfuric acid
How is the nitronium ion produced?
Nitric acid reacts with sulfuric acid
What must be present for a halogen to react with benzene?
A halogen carrier, e.g. AlCl3
How does benzene reacts with alkyl groups?
A hydrogen atom in the benzene ring is replaced by an alkyl group
Conditions needed for alkylation?
React benzene with a haloalkane in the presence of a halogen carrier
Why is alkylation useful?
Increases the length of the carbon chain
What is an acylation reaction?
When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst
What is produced when benzene is acylated?
An aromatic ketone
Why won’t benzene react with bromine unless a halogen carrier is present?
Benzene has delocalised electrons above and below the c-c ring
Lower electron density
When a non-polar molecule like benzene approaches, this means there are insufficient electrons to polarise the bromine
What is a phenol?
An OH group bonded to a benzene ring
What is the difference between an alcohol and a phenol?
An alcohol involves a OH group bonded to a carbon side chain, not to benzene itself
Why is phenol less soluble than alcohol?
The benzene ring is non-polar
Why is phenol a weak acid?
Phenol partially dissociates when dissolved in water
Will ethanol react with sodium hydroxide or sodium carbonate?
No
Will phenols and carboxylic acids react with NaOH?
Yes
Will carboxylic acids react with sodium carbonate?
Yes
What happens when phenol is brominated?
Bromine is added to the benzene ring - no halogen carrier needed
Why do halogens and nitric acid react more readily with phenol than benzene?
A lone pair of electrons is donated from the oxygen atom to the pi-system
This increases the electron density so it can attract electrophiles more readily
What is activation?
The aromatic ring reacts more readily with electrophiles
What is deactivation?
The aromatic ring reacts less readily with electrophiles
What type of directing is -NH2?
2 or 4
What type of directing is -NO2?
3
