Organic II- Aromatic Chemistry Flashcards
Describe the structure of benzene.
Benzene is an aromatic compound consisting of a ring of six carbon atoms, each bonded to a hydrogen atom, with a ring of delocalised electrons.
How does the bond length in benzene compare to other types of bonds?
Each bond in the benzene ring has an intermediate length between that of a double bond and a single bond.
Explain the stability of benzene compared to other molecules.
The ring structure of benzene, with delocalised electrons, makes it very stable compared to other molecules of a similar size.
Define the role of p-orbitals in benzene’s structure.
The outer electron from the p-orbital of each carbon atom is delocalised to form the central ring of benzene.
Contrast the structures of cyclohexatriene and benzene.
Cyclohexatriene has three double bonds and three single bonds, while benzene has a stable ring structure with delocalised electrons.
What was the initial understanding of benzene’s structure upon its discovery?
It was thought to resemble hexatriene, consisting of 6 carbons alternately bonded by double or single bonds, also known as Kekule structure.
Describe the predicted enthalpy change of hydrogenation for benzene.
The predicted enthalpy change of hydrogenation for benzene was -360 kJ/mol, which is three times the predicted enthalpy change of hydrogenation for cyclohexene, which only has one double bond.
What was the actual enthalpy change of hydrogenation for benzene?
The actual enthalpy change of hydrogenation for benzene was -208 kJ/mol. This is less than expected as benzene is more stable than the kekule structure. This is known as the delocalisation energy.
Define arenes.
Arenes are compounds that contain benzene as part of their structure, also known as aromatic compounds.
How do the melting and boiling points of arenes compare?
Arenes have high melting points due to the high stability of the delocalised electron ring, but low boiling points because they are non-polar molecules.
Explain why arenes often cannot be dissolved in water.
They are non-polar
Why is benzene susceptible to attack from electrophiles
It has a delocalised ring with high electron density.
Explain the process of electrophilic substitution in benzene.
An electrophile attacks the benzene ring which is partially destroyed and then restored.
Identify the products that can be formed from benzene through electrophilic substitution.
Aromatic amines and nitrobenzene can be produced from benzene through electrophilic substitution.
Define the role of the electrophile in the electrophilic substitution mechanism.
The electrophile, represented as A +, is a species that attacks the electron-rich benzene ring during electrophilic substitution.
How is the NO2+ electrophile generated for the nitration of benzene?
The NO2+ electrophile is produced in the reaction of concentrated sulfuric acid (H2SO4) with concentrated nitric acid (HNO3).
H2SO4 + HNO3 → HSO4 - + H2NO3+
H2NO3+ → H2O + NO2+
Describe the reaction that occurs when benzene is heated with concentrated sulfuric and nitric acid.
When heated with benzene, concentrated sulfuric and nitric acids lead to the substitution of the NO2+ electrophile onto the benzene ring, resulting in the removal of a hydrogen ion.
Describe the significance of temperature in the substitution reaction involving NO2 and an electrophile.
The reaction temperature must be maintained at 55°C to ensure that only one substitution occurs, which is crucial for the production of aromatic amines. At temperatures above this, multiple substitutions can happen.
Define Friedel-Crafts acylation.
The delocalized electron ring acts as a nucleophile and attacks the acyl chloride, resulting in the formation of a phenylketone.
How is a reactive intermediate produced in Friedel-Crafts acylation?
R-COCl + AlCl3 → AlCl4- + RCO+
The reactive intermediate is RCO+.
Explain the role of aluminium chloride in Friedel-Crafts acylation.
Aluminium chloride acts as a catalyst in Friedel-Crafts acylation, facilitating the formation of a reactive intermediate and being regenerated at the end of the reaction.
What is the final product of the Friedel-Crafts acylation reaction?
A phenylketone.
Identify the industrial applications of the products formed from Friedel-Crafts acylation.
Phenylketones are commonly used in the industrial production of dyes, pharmaceuticals, and explosives.
Describe the mechanism of the Friedel-Crafts acylation reaction.
The benzene ring attacks the acyl chloride , the delocalised electron ring is destroyed and then reformed with the acyl chloride substituted for a hydrogen.
