Draw the structures of the following aromatic compounds [6 marks]
(a) 2-methyl-3-phenylhexane (b) 2-methylbutylbenzene (c) 1-floro-2, 4-dimethylbenzene
(a) 2-methyl-3-phenylhexane:
H
|
H-C-C-C-C-C-C-H
| | |
H CH3 C6H5
(b) 2-methylbutylbenzene:
H
|
H-C-C-C-C-C-C
| | |
H CH3 C6H5
(c) 1-fluoro-2,4-dimethylbenzene:
H
|
C-C-C-H
| | |
H CH3 CH3
(a) Why do benzene and other aromatic compounds undergo electrophilic aromatic substitution
with ease while they struggle to undergo nucleophilic substitution reaction? [6 marks]
Benzene and other aromatic compounds undergo electrophilic aromatic substitution with ease due to the special stability and delocalization of electrons within the aromatic ring. This stability arises from the presence of a conjugated π system formed by the overlapping p orbitals of the carbon atoms within the ring.
Electrophilic aromatic substitution occurs when an electrophile reacts with the aromatic ring, leading to the substitution of one of the hydrogen atoms on the ring. This reaction is favored because it allows for the preservation of the stability of the aromatic ring system.
On the other hand, aromatic compounds struggle to undergo nucleophilic substitution reactions because the reactivity of nucleophiles towards the aromatic ring is generally low. This is due to the fact that the π electrons in the aromatic ring are delocalized and tightly held. As a result, nucleophiles are unable to attack the electron-deficient carbon in the ring, making nucleophilic substitution reactions difficult to occur.
The delocalized electrons in the aromatic ring interact with the incoming nucleophile, which leads to destabilization of the system. This unfavorable interaction undermines the reaction's thermodynamic feasibility, leading to the low reactivity of aromatic compounds towards nucleophilic substitution reactions.
Additionally, the presence of a strong leaving group is necessary for nucleophilic substitution reactions to occur. However, the carbon-hydrogen bonds in the aromatic ring do not easily break, making it difficult for a leaving group to form and initiate a nucleophilic substitution reaction.
Overall, the stability and delocalization of electrons in the aromatic ring render electrophilic aromatic substitution reactions favored, while the difficulty in breaking carbon-hydrogen bonds and lack of reactivity of nucleophiles towards the aromatic ring make nucleophilic substitution reactions challenging for aromatic compounds.
(b) A student proposes to make chlorobenzene by reacting chlorine gas with benzene in the
presence of ZnCl2. Discuss whether chlorobenzene can be synthesised using the proposed
reagents. [4 marks]
Yes, chlorobenzene can be synthesized by reacting benzene with chlorine gas in the presence of a catalyst such as ZnCl2. This reaction is known as electrophilic aromatic substitution.
The reaction proceeds through the formation of a sigma complex, where the benzene ring attacks the electrophilic chlorine atom, leading to the substitution of one of the hydrogen atoms on the ring with a chlorine atom. The presence of ZnCl2 as a catalyst helps facilitate this reaction by activating the chlorine molecule and increasing the electrophilicity of the chlorine atom.
The general reaction for the synthesis of chlorobenzene can be represented as follows:
C6H6 + Cl2 → C6H5Cl + HCl
Overall, the proposed reagents - benzene, chlorine gas, and ZnCl2 - are suitable for the synthesis of chlorobenzene.
(c) Using appropriate reagents show how an alkyl group can be added to a benzene ring
(ie alkylation of benzene). [7 marks]
Alkylation of benzene can be achieved by using an alkyl halide as the alkylating agent in the presence of a Lewis acid catalyst. The general reaction for the alkylation of benzene is:
Benzene + Alkyl Halide → Alkylated Benzene
Here is an example of how an alkyl group can be added to a benzene ring:
1. React benzene with an alkyl halide, such as methyl chloride (CH3Cl).
Benzene + CH3Cl → CH3C6H5 + HCl
2. Add a Lewis acid catalyst, such as AlCl3, to promote the reaction. The catalyst coordinates with the alkyl halide, making it more reactive towards the benzene ring.
Benzene + CH3Cl + AlCl3 → CH3C6H5 + HCl + AlCl3
3. The alkyl group from the alkyl halide (CH3) replaces one of the hydrogen atoms on the benzene ring, resulting in the formation of an alkylated benzene molecule (CH3C6H5).
The reaction can also occur with other alkyl halides, such as ethyl chloride, propyl bromide, etc. The choice of Lewis acid catalyst may vary depending on the specific alkyl halide used.
It is important to note that in alkylation reactions, multiple substitutions can occur, leading to the formation of polyalkylated products, and therefore, controlling the reaction conditions and reactant ratios is crucial to obtain the desired monoalkylated benzene product.