Aromatic Chemistry Watch

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Silly Sally
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#1
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Hey everyone!!!

I just wanted to ask you all 3 things regarding aromatics.

1. In my notes, it says that phenol acts as an acid as "negative charge on oxygen can be delocalised to some extent around the ring" - Whats does this actually mean in standard english and how does the delocalisation cuase phenol to be acidic??? To be honest i would have thought phenol is alklaine due to the presence of the -OH group

2. Acidified KMnO4 is used to oxidise all carbon containing side chains into -COOH groups. However, apparently, the reason that benzene will not react in the same way is because "benzene contains delocalised pi - bonds." - How does this help benzene NOT to be oxidised??? Cos surely other aromatics with carbon containing side chains would also have these "delocalised pi bonds". Plus wouldn't the reason benzene does not oxidise be because it doesn't contain any side chains????

3. Why is phenol more susceptible than benzene to attack from electrophiles???

Thanks everyone!!!

Sorry if this post is written in a very confusing way!!!
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MalaysianDude
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(Original post by Silly Sally)
Hey everyone!!!

I just wanted to ask you all 3 things regarding aromatics.

1. In my notes, it says that phenol acts as an acid as "negative charge on oxygen can be delocalised to some extent around the ring" - Whats does this actually mean in standard english and how does the delocalisation cuase phenol to be acidic??? To be honest i would have thought phenol is alklaine due to the presence of the -OH group

2. Acidified KMnO4 is used to oxidise all carbon containing side chains into -COOH groups. However, apparently, the reason that benzene will not react in the same way is because "benzene contains delocalised pi - bonds." - How does this help benzene NOT to be oxidised??? Cos surely other aromatics with carbon containing side chains would also have these "delocalised pi bonds". Plus wouldn't the reason benzene does not oxidise be because it doesn't contain any side chains????

3. Why is phenol more susceptible than benzene to attack from electrophiles???

Thanks everyone!!!

Sorry if this post is written in a very confusing way!!!
1)it means that the oxygen is used to stabilise the benzene molecule
2)Benzene will try its best to preserve its struture.If oxidation occurs to it,it`ll lose its stability and break..therefore it has a high activation energy when trying to react with oxidation or addition reactions
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Silly Sally
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Report Thread starter 14 years ago
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(Original post by MalaysianDude)
1)it means that the oxygen is used to stabilise the benzene molecule
2)Benzene will try its best to preserve its struture.If oxidation occurs to it,it`ll lose its stability and break..therefore it has a high activation energy when trying to react with oxidation or addition reactions
Thanks!!!
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MalaysianDude
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whoops i mised the third question
anyways,its because phonol is much more stable compared to benzene since it has already undergone the electrophilic substitution
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cipapfatso
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(Original post by Silly Sally)
Hey everyone!!!

I just wanted to ask you all 3 things regarding aromatics.

1. In my notes, it says that phenol acts as an acid as "negative charge on oxygen can be delocalised to some extent around the ring" - Whats does this actually mean in standard english and how does the delocalisation cuase phenol to be acidic??? To be honest i would have thought phenol is alklaine due to the presence of the -OH group

2. Acidified KMnO4 is used to oxidise all carbon containing side chains into -COOH groups. However, apparently, the reason that benzene will not react in the same way is because "benzene contains delocalised pi - bonds." - How does this help benzene NOT to be oxidised??? Cos surely other aromatics with carbon containing side chains would also have these "delocalised pi bonds". Plus wouldn't the reason benzene does not oxidise be because it doesn't contain any side chains????

3. Why is phenol more susceptible than benzene to attack from electrophiles???

Thanks everyone!!!

Sorry if this post is written in a very confusing way!!!
1) The lone of pair of electrons on the oxygen atom interacts with the 6 delocalised electrons that make up the benzene ring. Because of this the electron density around the oxygen atom is reduced as the electrons help stabilise the benzene ring. Thus the hydrogen will be dissociated more willingly so the phenoxide ion forms:
C6H5OH + H2O ------> C6H5O^- + H3O^+

2) Confused on the wording here do you mean the benzene molecule itself C6H6 will not be oxidised or functional groups e.g. CH2OH attached to the benzene ring? The benzene ring itself won't as it is far too stable due to the 6 delocalised electrons therefore it will not lose electrons and become oxidised. The functional groups like CH2OH can be oxidised.

3) This would have to do with the interaction between the lone pair of electrons on the oxygen atom and the 6 delocalised electrons of the benzene ring. In phenol there is a slightly greater electron density around the benzene ring for the reason above. Therefore it will be more susceptible to electrophilic attack/ attack by an electron deficient species. Phenol has a greater dipole moment basically.

Hope that helps!
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