Chapter 27 (Aromatic Chemistry) Flashcards
Define Aliphatic
Straight or branched chain organic substances
Define Aromatic/arene
Including one or more ring of six carbon atoms with delocalised bonding
What is the molecular formula or benzene?
C6H6
What can be inferred from benzene being a planar molecule?
C-C bonds are all the same lengthened the bond energy between C-C and C=C bonds are equal.
What is the bond angle of H-C-C?
120
Describe the electrons in the π bond?
Delocalised and spread out over the whole ring, they are not attached to a particular atom.
How is a benzene ring drawn?
Hexagon with circle inside
Define delocalisation energy?
The increase in stability connected to delocalisation
Why is the actual enthalpy of hydration of benzene less than the theoretical?
There is an increase in stability due to the delocalised electrons so benzene is more thermodynamically stable than the theoretical structure.
Describe and explain the hydration enthalpy of cyclohexane-1,3-diene
Hydrogenation would be less negative than -240 kJ mol-1
As the π bonds are closer together and overlap there is some delocalisation and extra stability.
Describe and explain the hydration enthalpy of cyclohexane-1,4-diene
Hydrogenation enthalpy would be -240 kJ mol-1
There would not be delocalisation as the π bonds are too far apart and don’t overlap.
What is the molecular formula of a phenyl group?
C6H5
When is phenol used as a name?
When the benzene ring is regarded a side group of another molecule.
Why does benzene not undergo addition reaction?
These would involve breaking up delocalised systems.
Why does benzene undergo electrophilic substitution?
A H atom can be substituted for another group of atoms.
Why is methylbenzene used instead of benzene?
It is less toxic (benzene is carcinogen) and reacts more readily that benzene
Why is methyl benzene more reactive than benzene?
The methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.
What structure did Kekule suggest benzene had?
Alternating C-C and C=C bonds
What was the problem with the shape of Kekule’s benzene?
The structure would be irregular while benzene is planar due to bond lengths.
What was the problems with the reactivity of Kekule’s benzene?
The structure would undergo reactions similar to alkenes but benzene does not undergo addition reactions readily.
What was the problems with the halogenation of Kekule’s benzene?
Would expect isomeric products but benzene only produces one product.
What was the problems with the energy of hydrogenation of Kekule’s benzene?
A ∆H of -360 kJ mol-1 but benzene has an actual value of -208 kJ mol-1.
What type of reaction is the nitration of benzene, what is the electrophile, the reagent, the conditions and the catalyst?
Reaction = Electrophilic substitution
Electrophile = nitronium ion/NO2+
Reagent = Concentrated HNO3
Conditions = 50º
Catalyst = Concentrated H2SO4
How is a nitronium ion formed?
Reacting concentrated nitric acid and concentrated sulphuric acid.
What is used to reduce a nitro group to an amine group?
Tin and HCL OR Nickel and Hydrogen
What is the equation for the formation of a nitronium electrophile?
HNO3 + 2H2SO4 > NO2+ + 2HSO4- + H3O+
OR
H2SO4 + HNO3→ HSO4– +NO2+ +H2O
What are the uses for nitro compounds?
- Explosives
* Make aromatic amines (used to make dyes)
What is Friedel Crafts Acylation?
Reaction between benzene and acyl group
What temperature is needed to add the first nitronium ion to benzene?
60º
What is the equation for the formation of the electrophile for an acyl chloride?
AlCl3 + Acyl chloride (CH3COCl) > CH3CO+ + AlCl4-
What is formed when benzene reacts with an acyl chloride?
Phenyl Ketone + HCl (+ AlCl3)
What type of reaction is between acyl chloride and benzene?
Electrophilic substitution
What are the reagents and conditions for a reaction between acyl chloride and benzene?
Heat under reflux at 50º
Anhydrous aluminium chloride catalyst
What are the reagents used for reducing an nitroarene to an amine?
Sn and HCl OR Fe and HCl OR Ni and H2
What is the equation for the reaction of nitrobenzene to a phenyl amine?
Nitrobenzene + 6[H] > Phenylamine + 2H2O
Describe and explain the difference in properties between a phenol and other alcohols?
It is more acidic and does not oxidise. Delocalisation makes the C-O bond stronger and the O-H bond weaker.
Describe and explain the difference in properties between a chlorobenzene and other chlorine haloalka/enes?
The C-Cl bond is made stronger and typical haloalkane substitution and elimination reactions will not occur as the electron rich benzene ring will repel nucleophiles.
Describe and explain the difference in properties between a phenylamine and other amines?
Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton.
