Chapter 25 Aromatic Compounds (benzene) Flashcards
What is the molecular formula of benzene?
C6 H6
Describe the structure of the delocalised model of benzene.
Carbon, as an atom, has 4 valent electrons.
In benzene, each carbon is bonded to two other carbons and a hydrogen.
The remaining lone electron is in a p-orbital that sticks out above and below the planar ring.
The p-orbitals from each carbon in the ring overlaps to form a delocalised ring of electrons- a Pi system.
How does Kekule Model of Benzene structure differ to the delocalised model?
Kekule model still has a six membered carbon ring. However, in this ring, there is alternating single and double bonds. In the delocalised model, only single bonds are present.
Why did scientists not accept Kekule’s model of benzene?
The structure of Kekule’s benzene did not match with the chemical and physical properties it portrayed.
One piece of evidence to disprove Kekule’s model of benzene was its reactivity with bromine. Explain this.
In accordance to Kekule’s model of Benzene, there are some double bonds in the structure. Hence, like alkenes, it was hypothesised that benzene would undergo electrophilic addition with bromine, and decolourise the bromine water.
However, when this was tested out, benzene does not undergo electrophilic addition and does not decolourise bromine water under normal conditions.
This suggests there is no double bonds present in benzene.
Another piece of evidence to disprove Kekule’s model of benzene was the length of carbon-carbon bonds. Explain this.
By using a technique called x-ray diffraction, it is possible to measure the length of bonds. When this was carried out on benzene, it was found that all the carbon-carbon lengths were the same, meaning they could only be one type of bond. In order for each carbon to have the correct amount of bonds, the type of bond must have been a sigma bond.
Another evidence to disprove Kekule’s model was the enthalpy of hydrogenation reactions. Explain this.
If benzene had Kekule’s structure, then the hydrogenation enthalpy would be three times than that of cyclohexene. Cyclohexene has a hydrogenation enthalpy of -120 kJ/mol. Hence, it was hypothesised that benzene would have a hydrogenation enthalpy of -360kJ/mol if it has 3 double bonds.
However, when carrying out the hydrogenation of benzene, it was seen that the enthalpy was -208 kJ/mol. Since the enthalpies did not match up, it also disproves that benzene had Kekule’s structure.
What are arenes?
Arenes are compounds containing a benzene ring.
In nomenclature, where is the parent compound found in the name of the whole compound?
The parent compound is found at the end of the name.
In what cases is benzene not the parent compound, but as the substituent?
If there is an alkyl chain bonded to the benzene ring and this alkyl chain contains a functional group.
OR
If the alkyl chain bonded to the benzene is made of 7 or more carbons.
What is the name of the molecule, where there is a -COOH bonded directly to benzene?
Benzoic acid.
What is the name of the molecule, where there is a -NH2 bonded directly to the benzene?
Phenylamine.
What is the name of the molecule, where there is a -CHO bonded directly to the benzene?
Benzaldehyde.
What is the name of the molecule, where there is a -OH bonded directly to the benzene?
Phenol.
What type of mechanisms is benzene involved in (for halogenation, nitration, acylation, alkylation)?
Electrophilic substitution.
What are the reagents and products of a nitration reaction of benzene?
Benzene + nitric acid > Nitrobenzene + water
What are the conditions needed for the nitration reaction of benzene?
A sulfuric acid catalyst.
Temperature of 50 degrees Celsius.
What happens when the temperature exceeds 50 degrees Celsius in the nitration of benzene?
Multiple substitutions could occur, resulting in dinitrobenzene.
What could nitrobenzene be used in?
Dyes for clothing
Explosives
Pharmaceutical- Preparation for paracetamol
State the steps in the electrophilic substitution mechanism of the nitration of benzene.
Step 1:
The electrophile is formed.
This is done by reacting the nitric acid with sulfuric acid, to produce the nitronium ion (the electrophile), HSO4- ion and water.
Step 2:
The nitronium ion is attracted to the electron dense Pi system, and the Pi system gives its electrons to the nitronium ion; it has formed a bond with it.
Now, one of the carbons in the benzene ring has 4 bonds (to 2 other carbons, the nitronium ion, and a hydrogen). Hence, the stable state of the benzene ring has been disrupted.
To re-enter a stable state, the hydrogen that is bonded to the carbon with 4 bonds, donates its electrons back into the Pi system, and the hydrogen now becomes a free hydrogen ion.
What is left is nitrobenzene, which is stable.
Step 3:
The hydrogen ion formed in step 2, reacts with the HSO4- ion formed in step 1. Via this reaction, sulfuric acid is reformed. Due to the reformation, sulfuric aid is considered a catalyst.
State the reagents and products of the halogenation of benzene.
Benzene + halogen > halobenzene + hydrogen halide
What is the conditions needed for the halogenation of benzene? Why are these conditions needed?
For the halogenation of benzene, a halogen carrier is needed (iron bromide, aluminium bromide, iron chloride, aluminium chloride) . The halogen carrier acts as a catalyst and helps to generate the nucleophile. This nucleophile is now electron deficient enough to be attracted to the Pi system in benzene.
State the electrophilic substitution mechanism for the halogenation of benzene.
Step 1:
An electrophile needs to be generated.
A bromine molecule reacts with iron bromide, to produce a bromide ion and FeBr4-. The bromide ion produced is the electrophile. (Iron bromide acts as a catalyst. Alternatively, aluminium bromide can be used).
Step 2:
The bromide ion is attracted to the Pi system in the benzene ring. The Pi system donates a pair of electrons to the bromide ion; the benzene has now formed a bond with the bromine.
However, the benzene has now entered an unstable state. This is because the carbon that has just bonded to the bromine has 4 bonds (with 2 other carbons, the bromine and a hydrogen. In benzene, carbons normally has 3 bonds and has a lone electron in the p-orbital).
To re-enter a stable state, the hydrogen bonded to the carbon that has a bromine atom, gives its electrons to the Pi system. The hydrogen becomes a hydrogen ion.
Though, the halobenzene is now stable.
Step 3:
The hydrogen ion formed in step 2, reacts with the FeBr4- ion. This produces hydrogen bromide and FeBr3. As FeBr3 is reformed, we consider FeBr3 as a catalyst.
What is an alkyl group?
An alkyl group is a chain of carbon and hydrogen atoms ( in a chain structure).
