1.Arenes Flashcards
How does benzene chemistry differ to alkenes?
- No electrophilic addition with Br2,HBr or H2O
- Will instead do electrophillic substitution
Reasons original kekule was wrong
- Bond lengths around the ring were all equal unlike his structure
- Less isomers then his structure said
- Is more stable then his structure would suggest.
- doesnt reat the sameway (eg decourlarise bromine water)
What is resonance (stabilisation) energy?
The amount by which the real structure of benzene is more stable the the proposed structure.
Describe how benzenes sresonance energy was found?
- Reacting various ring alkenes with hydrogen to fine their enthalpy of hydrogenation.
- From 1 and two bonds its predicted benzene should have a enthalpht of -360Kjmol-1
- But benze is 152Kjmol-1 more stable
Why is benzene less reactive then alkenes?
becuase its more stable
Draw the correct structure of benzene
- P orbitals laterally overlap to form a ring of 6 delocalised electrons in a pi bong
- There rest are sigma bonds.
- make sure to clearly draw the ring above and below
under normal condidition what does benzene not do?
- decolourise bromine water
- react with a strong acid
- react with halogens
why doesnt benzene do addition reactions?
it would result in the product being less stable then the original as the structure had to change
Reaction for the halogenation of benzene
Needs a halogen carrier eg FeCl3,FeBr3,AlCl3,AlBr3
mechanism for halogenation of benzene
- Create electrophile (halogen cation)
- React the electrophile
- loss of H+
- regenerate catalyst
Nitation of benzene reaction?
- Conc Nitric acid and cond sulfuric acid (sulfuric is catalyst)
- 50 degrees (must be kept lower thern this)
- In a round-bottomed flask
- In a 50 deg water bath
Mechanism for nitration of benzene?
- Create electrophile (NO2+) HNO3+H2SO4–>NO2++HSO4-+H2O
- react it
- generate H+
- regenerate sulfuric acid
why must the nitration be kept below 50 degrees?
so that multiple substutions dont take place
Whys is benzene more stable?
as the charge is shared more equally throughout the molecult.
the decloaclised electrons are evenly ditributed, and so have a lower pi electron density so theres insufficent density to polarise some moleciles eg bromine.
Reaction for makiing the diazonium ion electrophile?
- BElow 10 degress
Formation of a phenylamine?
Nitrobenzene + 6[H] —> phynlamine + 2 Water
[H]=tin +conc HCl
heat under reflux then neauralise excess acid
Coupling reaction for maiking a azodye
- Diazonium ion + Phenol + alkali condiditons
Mechanism for coupling reaction.
Same as the other electrophilic addition ones
Disassotiation order?
Carboxillic acid > Phenols > Alcohols
Due to the charge distribution of the ion being different stabilities so different chances of forming
What happens with Oh groups and sodium metal?
all groups (alcohols, phenol and carboxilic acid) preact to give hydrogen gas
What happens to OH groups with sodium hydroxide?
No reaction with alcohol
Phenol and carboxilic acid react to form a water soluble prpoduct as its an ionic salt formed from the neutralisation
what happens to OH groups with carbonates/hydrocarbonates solutions?
No reaction with phenol and alcohols
Carboxilic acids react to form salt, water and co2
Reaction for the bromination of phenol?
Room Temp
Why is phenol easier to bromante (or other electrphiles) then benzene?
- The lone pair of electron on the oxygen part of the phenol is partially delocalised into the benzene ring
- Electron density is now higher and ting is activated
- it is now dense enough to polarise bromine molecules, which are attracted more strongly
uses of phenols?
- dyes
- pharmasuticals
- resins
- explosves
- antiseptics
- detergents
