Aromatic Compounds
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What is the formula of benzene?
C6H6
Describe and explain the structure of a benzene molecule.
6x sigma bonds and a continuous ring shaped pi-bonding system above and below the plane of the ring. This is formed due to the overlap of the p-orbitals of each of the six carbon atoms.
Why is benzene considered to be stable?
The electrons in the pi-bond are delocalised across the six carbon atoms, thus the -ve charge is stabilised.
What evidence is there for benzene’s structure?
X-ray crystallography: all bond-lengths are the same (intermediate between C-c and C=C) Bromine water: NOT decolourised by benzene so must have insufficient e- density Enthalpy of hydrogenation: NOT -360kJmol-1 as Kekule’s model would have implied, but -208kJmol-1 where the difference in energy is the delocalisation energy. This quantifies the extra stability of benzene
What is the systematic name for the Kekule model?
Cyclohexa-1,3,5-triene
Describe the naming conventions for benzene.
Shorter groups are given as prefixes to benzene. Where the group has chain length >=7 or it includes a functional group, we instead use the phenyl- prefix.
What type of reaction does benzene undergo typically?
Electrophilic Substitution
Why does benzene undergo substitution rather than addition?
The delocalised pi bonding system has insufficient e- density to induce a dipole in an attacking molecule. Such a reaction would also disrupt the stable pi-bonding system.
Describe a typical Electrophilic substitution.
• Formation of Electrophile E+ • Electrophile gains 2x electrons from pi-bond to form dative bond to carbon • Existing hydrogen atom donates 2x electrons to re-complete pi bond, releasing H+ which bonds with -ve ion formed originally
How many electrons in contained within the intermediate of the Electrophilic substitution of benzene?
4x pi-electrons
How do we form the Electrophile for the nitration of benzene?
Formed in situ by reacting conc. H2SO4 and HNO3, to form NO2+. This is formed in situ due to its’ instability
What are the conditions + reagents for the nitration of benzene?
Reagents: HNO3 Conditions: H2SO4 catalyst (reformed due to reaction of H+, HSO4- at end) and 50 degrees Celsius
Why must we use a temperature of 50 degrees Celsius when nitrating benzene?
This temperature ensures the mono=-substitution of NO2+
How can we halogenate benzene molecules?
Since Benzene itself doesn’t have sufficient e- density to induce an electrophile from Br2/Cl2, we use AlBr3/AlCl3/etc to form a +ve species. This mono-substitutes.
Describe Friedel-Krafts acylation and alkylation.
Alkylation: CH3Cl + ALCl3 catalyst to add CH3 group Acylation: ClCOCH3 + AlCl3 catalyst to add COCH3 group
Why do benzene and Alkenes react differently?
Alkenes have higher electorn density in their pi-bonding system since the electrons are localised over 2x carbons rather than being delocalised over six. They can more readily induce a dipole in an incoming a molecule.
What reactant and conditions are used to reduce nitrobenzene into phenylamine?
[H+ such as NaBH4] + Sn/HCl (conc)
What typically defines a group as 2/4 directing?
Presence of lone pairs. This makes the group in question electron-donating, so it activates the pi bonding system and directs attacking Electrophile to 2/4 positions.
What typically defines a group as 3-directing?
Presence of double or triple bonds. These group are typically electron withdrawing so deactivate the benzene ring. This directs electrophiles to the 3 position.
What is a phenol?
An organic compound containing a benzene ring, with a hydroxyl group directly attached to it.
Why are phenols more reactive than aromatic alcohols and benzenes alike?
The -OH group donates a pair of electrons to the pi-bonding system, increasing e- density and making the molecule more susceptible to Electrophilic attack.
What are some typical comparisons we make between the reactions of phenols and benzene?
• Lower temperatures • Less concentrated reactants • No need for a catalyst/carrier/etc
How does phenol’s increased e- density increase reactivity?
Incoming molecules are polarised more strongly.
Describe the reaction between phenol and bromine water.
3x bromine atoms bond onto phenol in the 2, 4, and 6 positions. This can happen because of the increased e- density and the strong 2,4,6 directing effect of the -OH group promoting multiple substitution.