[URGENT] Chemistry OCR F324 question

Hello,
I was doing the paper Chemistry OCR F324 january 2010 and i got stuck in question 1b(i) and 5(d)(ii) and (iii).
Here's the paper:
http://www.ocr.org.uk/images/65129-question-paper-unit-f324-rings-polymers-and-analysis.pdf
Mark schemes:
http://www.ocr.org.uk/images/63346-mark-scheme-january.pdf

First 1b(i):
White precipitate? I never learned this, can you tell me more about what unit was this and what observation is this.

5(d)(ii):
Why doesn't NaHCO3 OR Na2CO3 react with paracetamol?

5(d)(iii):
Why Br2 doesn't react with aspirin? It should on the benzene/phenol?

Thanks in advance,

First 1b(i): Activated benzene rings such as those of phenol and phenylamine react directly with halogens to form 2,4,6 trisubstituted products. These are the white precipitates.

5(d)(ii): There is no acid group to react with

5(d)(iii): The amide group deactivates the ring by conjugation. The lone pair on the nitrogen goes between the nitrogen and the carbon of the carbonyl group while the pi electrons of the carbonyl go oto the oxygen. This makes a large conjugated system with the ring, effectively stabilising and allowing electron density to be drawn from the ring.

Thanks a lot.
but i still didn't get:
5(d)(iii):
Why Br2 doesn't react with aspirin? It should on the benzene/phenol?

for Br2 to react with apirin there needs to be a catalyst (FeBr3/ AlBr3) because its a benzene ring and not a phenol.

Can someone confirm this statement?

for Br2 to react with apirin there needs to be a catalyst (FeBr3/ AlBr3) because its a benzene ring and not a phenol.

Yes, you need a halogen carrier to halogenate a Benzene ring (NOT PHENOL) as it is not able to polarise the halogen due to its low electron density.

If this was the reason to my answer then Br2 would only go paracetamol on the 2,4,6 of the phenol.
This is right but not answering the question:
Why Br2 doesn't react with aspirin? It should on both.

In a phenol, because the oxygen atom is bonded directly to the benzene ring, oxygen's lone pair can delocalise into the ring and activate it, causing a greater electron density and easier polarisation of molecules. However, aspirin is NOT a phenol because it has no alcohol group attached directly to the ring, so the ring is just as unreactive as it would be had it been simply Benzene.

Because of this, the bonding of aspirin's ring is still delocalised across six carbons and hence it has poor electron density. It cannot polarise the Bromine molecules itself and that's why it needs the halogen carrier.

This is not the reason for it because why can the Br be substituted in any part of the phenol in the aspirin, if your explanation was true than Br could only be substituted in carbon 2,4 or 6!

Aspirin is NOT a phenol.

Paracetamol is NOT a phenol...

Perhaps I phrased it poorly. I meant: anything with an alcohol group attached directly to a benzene ring can be considered to have a phenol group, which causes the ease of polarisation I described above. Paracetamol does, so it will react with bromine.

Aspirin has the carboxyl group, therefore will not react with bromine.

If your explanation was true than Br could only be substituted in carbon 2,4 or 6 of the phenol in paracetamol. But mark schemes says Br can be substituted anywhere.

Well your original question was why Br2 didn't react with Aspirin; you know now.

In the case of paracetamol, it can't substitute on 4 because that bond is already occupied by the nitrogen in the amide bond. You're allowed the Br anywhere and more than once (but not more than 3 I suppose) because as you say it would normally triply substitute. The phenol part of paracetamol is symmetrical - if you triply substitute in this case you could have 2,3,5 or 2,3,6 (as 4 is occupied, naturally). Because you always take the lowest numbers, to use 2,4,5 would be the same as using 2,3,6 if you count round the other side from 1 starting at the OH carbon.

I appreciate that was a convoluted explanation, if it makes no sense I'll try again.

Sorry but i didn't get, can you explain again.

Basically it can't substitute on 4 as it's occupied by a nitrogen atom so it can choose any of the remaining four carbons. I assume the mark scheme allows multisubstitution as that is the normal course of reaction for phenol; that's all you really need to know in this question, I don't know why I launched into an explanation of its isomerism :L

Just remember to be classed as having a phenol group the alcohol group must be directly attached to the ring.
For example, 4-nitrophenol can be considered to have a phenol group but phenyl ethanol can't as the OH group is on a side chain.


Posted from TSR Mobile

Thanks,
Another question I never get when to use "benzene", "phenyl", "phenol"
in naming the the aromatic compounds. Can you help me.
E.g NitroBenzene, phenyl ethanol and 4-nitrophenol.

Use nitro when there is a NO2 group attached directly to the ring. You don't usually have to put numbers in the name unless there's another thing attached to the ring.
You use phenyl when the benzene ring is tagged on to the end of a larger molecule. To use your example, phenyl ethanol would be HOCH2CH2C6H5. One of the hydrogen atoms of regular ethanol is just replaced with a benzene ring.
For anything to have phenol in its name it must have an OH group attached to the ring directly, and you treated that carbon as Carbon 1. In your example, you'd have a NO2 group directly opposite your OH group as that would be carbon 4 counting round both ways.

Benzene... You have to remember pretty much every other prefix takes precedence over benzene. That's why phenol is phenol rather than benzanol or hydroxybenzene I guess. Only use benzene when there are alkyl groups attached to the ring, NO2 groups or halogens; it's the only ones I've seen used. There are other instances such as Benzene-1,4-dicarboxylic acid in polymerisation but they're unlikely to ask you to name monomers: they much more often ask you to draw them.


Posted from TSR Mobile