27 - Aromatic Chemistry Flashcards
What are arenes?
Hydrocarbons based on benzene.
What was the anomaly with benzenes reactions?
Despite being unsaturated it does not readily undergo addition reactions.
What was the anomaly with benzenes bond lengths?
All of its bond lengths were equal, when they should have been different sizes, due to C=C bonds being shorter than C-C bonds.
What was the anomaly with benzenes enthalpy?
The enthalpy change for the hydrogenation of cyclohexane is -120, so the expected enthalpy for benzene is -360, however the actual enthalpy is -208 so benzene is more stable than expected.
Explain the structure of benzene.
One electron of each carbon atom is in a p-orbital, all p-orbitals overlap, creating a delocalised pi system and forming a region of electron density above and below the ring, making it extremely stable.
What is the shape and bond angles of benzene?
Trigonal planar
120
What is the melting and boiling point of benzene like?
Its boiling point is similar to hexane but its melting point is much higher than hexane, as the flat molecules pack together well in the solid state.
What is the solubility of benzene like?
Only mixes with other hydrocarbons and non-polar solvents as its non-polar.(Not soluble in water).
What reaction does benzene undergo and why?
Electrophilic substitution
The area of high electron density means they are susceptible to attack by electrophiles, but the ring is very stable so the ring remains intact in most reactions.
What is an electrophile?
Electron pair acceptor
What are the reagents of nitration of benzene and their reactions?
conc. nitric acid + conc. sulphuric acid
H2SO4 + HNO3 –> H2NO3+ + HSO4-
H2NO3+ –> NO2+ + H2O
Why does the sulfuric acid in the nitration of benzene act as a catalyst?
As it can be regenerated with the H+ product formed.
What is the name of the mechanism for the nitration of benzene?
Electrophilic substitution
What are the conditions of the nitration of benzene?
Reflux at 55 degrees
What is acylation?
Substituting an acyl group.
What are the reagents for Friedel-Crafts acylation of benzene?
An acyl chloride + anhydrous aluminium chloride (catalyst)
What are the conditions of the acylation of benzene?
Reflux at 50 degrees
Dry inert solvent, as anhydrous AlCl3 is highly reactive in water and the acyl chloride will form back into a carboxylic acid.
What are the products of the acylation of benzene?
Aldehyde or ketone
What is the electrophile for the acylation of benzene?
RCO+
What is the electrophile for the nitration of benzene?
NO2+
How do electron releasing groups affect further substitution of benzene?
They make the ring more susceptible to further electrophilic substitution as the electron density of the ring increases.
What are examples of electron releasing groups?
CH3, OCH3, OH, NH2
To which positions will further substitutions go if an electron releasing group is attached to the benzene ring?
2, 4, 6
How do electron withdrawing groups affect further substitution of benzene?
Makes the ring less susceptible to electrophilic substitution as the electron density of the ring decreases.
What are examples of electron withdrawing groups?
NO2, COCL
To which positions will further substitutions go if an electron withdrawing group is attached to the benzene ring?
3, 5
