Benzene Flashcards
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
Naming- what is CH3?
Methylbenzene / toluene
Give reaction equations for formation of the nitronium ion needed in nitration of benzene
1- H2SO4 + HNO3 –> H2NO3+ + HSO4-
2: H2NO3+ —-> H2O + NO2+
Define delocalisation energy
Extra stability exhibited by benzene & other aromatic compounds due to delocalised Pi electrons
How can you separate nitrobenzene? And what temp?
Fractional distillation, temp higher than 55 degrees as both are liquids at 55, at higher temps further nitration occurs
Draw mechanism for formation of nitrobenzene
..
Draw mechanism for formation of chlorobenzene using an AlCl3 catalyst
..
Give all alkyl groups
CH3 methyl
C2H5 ethyl
C3H8 propyl
C4H9 butyl
Describe a test to distinguish between aliphatic and aromatic halogen
Hydrolysis (NUCLEOPHILIC sub) add acidified silver nitrate- reflux and 1 atm
Aliphatic- PPT corresponding to halogen
Aromatic- no PPT, bond not broken too strong and no X- ions produced
If you have a structure in electrophilic sub where a chlorine is attached: and they ask why that product wouldn’t be another molecule, why?
The p orbital electrons of the chlorine in the aromatic is incorporated into the delocalised Pi electron system of the ring
Why is the Enthalpy change using a benzene ring (e.g hydrogenation) more positiv / larger than that of alkene to alkane ?
Removing a benzene ring which is a lot more stable (due to delocalisation. Energy) Than an alkene
Why is benzene subseptible to electrophilic substitution?
Because the Pi electron cloud is electron rich and is attracted to the electron deficient species
