Benzene
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What family of organic compounds is benzene part of?
Aromatic
What does the original kekule structure contain?
Alternate single and double C-C bonds
What does benzene have that makes it more stable?
Delocalised Pi electrons make it more stable than it would be f it had 3Pi bonds between alternate single and double bonds (kekule)
What is the value for delocalisation E of benzene?
+150
The structure and bonding in benzene
• Planar with a ring of 6 carbon atoms • each C atom forms 3 single covalent bonds, to 2 carbon atoms and 1 hydrogen • the spare electrons in P orbital overlap sideways to form a Pi bond • there is a Pi electron cloud above and below the plane of carbon atoms • the Pi electrons are delocalised, benzene is more stable due to this • C-C bond lengths are all equal • C-C-C bond angle is 120
Why should you be careful using benzene?
It is a liquid at room temperature and is a carcinogen (cancer causing)
Why doesn’t benzene undergo Electrophic addition reaction?
As it is very stable so this would disrupt the delocalised Pi electrons and stability would be lost which would require lots of energy
What does benzene undergo?
Electrophilic substitution - where an electrophiles substitutes an atom e.g hydrogen. This means Pi electrons aren’t disrupted, stability is not lost, and more energy isn’t required. But very reactive electrophiles are needed
What are the conditions for nitration of benzene
Concentrated nitric and sulfuric acid 55 degrees
What is the attacking electrophile in nitration of benzene?
NO2+
What type of reactions are all benzene reactions?
Electrophilic substitution
What are the conditions for halogen action of benzene?
No light, room temp, anhydrous
What is the formula of the attacking species of halogenation of benzene?
Cl+ if using an AlCl3/FeCl3 catalyst Br+ if using Br catalyst
Why is a catalyst needed in halogenation of benzene?
As bromine and chlorine are not strong enough as electrophiles by themselves so a halogen carrier is used (catalyst)
What is the purpose of a AL/Fe catalyst in halogenation of benzene?
It polarises the halogen as well as speeding up a reaction
What does a freidal-crafts reaction involve?
Involves substitution of an alkyl group such as CH3 for a hydrogen atom on benzene ring
What is Freidal crafts also known as
Alkylation
What are the conditions for alkylation of benzene
Room temp , anhydrous A catalyst is also used e.g AlCl3
Why don’t aromatic compounds e.g chlorobutane undergo hydrolysis easily?
• when the halogen is attached to the rig it is less reactive than when it’s in a halogenoalkanes, meaning C-Cl bond is stronger in chlorobenzene • this is due to the overlap of electrons in the P orbital of chlorine with the delocalised Pi electrons of the ring, to form an extended Pi system - so the C-Cl bond has some Pi/double bond character • the overlap also reduces the delta + charge on the Carbon which is attached to chlorine, as it is spread over the delocalised System, so if the chlorine atom were subsituted it would result in the delocalised Pi system being destroyed and a loss of stability.
Comparison of 1-chlorobutane and chlorobenzene
1-chlorobutane= larger delta +, weaker C-Cl bond Chlorobenzene= smaller delta +, stronger C-Cl bond
What are the attacking species in alkylation ?
The alkyl group with a + E.g IN chloromethane = CH3+ is attacking species
Naming benzene- what is NH2?
Aminobenzene / phenylamine
Naming: what is COOH?
Benzenecarboxylic acid / benzoicacid
Naming- what is OH?
Hydroxybenzene / phenol