A-level Chemistry Revision Squad!

Colour changes for methyl orange and phenolphtalein ?

Colour changes for methyl orange

and phenolphtalein ?

Is this everything or have I missed something? ASMechanisms.pdf

I know that phenolphthalein goes from colourless to pink when alkaline is added.

Orangey/red to yellow.



Colourless to pink.

^

In acidic conditions, I think.

Could someone explain why NO2 groups are electron withdrawing? Im struggling to understand the chemistry behind it.

Thank you in advance

Draw resonance forms. You can push an electron from N=O to O, etc.

Im not sure i fully understand. The quote something i read online.

"Electron withdrawing groups (EWG) with p bonds to electronegative atoms (e.g. -C=O, -NO₂) adjacent to the p system deactivate the aromatic ring by decreasing the electron density on the ring through a resonance withdrawing effect. The resonance only decreases the electron density at the ortho- and para- positions. Hence these sites are less nucleophilic, and so the system tends to react with electrophiles at the meta sites."

The underlined is what i dont seem to understand? How can removing electron density from benzene ring make it less nucleophilic rather than less electrophillic?

Do you know what a resonance form is?

Ive drawn resonance structures for CO3 2- but i havent fully understood it

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So, usually when we draw the structures of molecules the drawings do not perfectly represent electron density in the system (which is more accurately described by molecular orbital theory).

Resonance forms help to suggest the electron density (and hence reactivity) of different sites in a molecule without the need for lots of calculations. For most molecules there are many resonance forms you can draw but you want to look out for forms which would be expecially stable, such as those with negative charges localised on oxygens as these will have more of their 'character' in the real molecule. When you draw the resonance form of something like nitrobenzene, the most stable resonance forms are those where you push an electron from the π system to the neutral oxygen of the nitro substituent. Using the Kekulé structure of benzene this gives postivie charges at the ortho and para postitions, and hence these positions will be less likely to donate electrons.

So when they say its a better electrophile as electrons are withdrawn they're talking the functional group -NO2 ?

Yes. Electrons are withdrawn from the π system by the nitro substituent.

But how how does this allow for the polynitration of nitrobenzene?

What is the mechanism for addition of nitro groups to an aromatic ring?

electrophillic substitution

But isnt there going to be delta+ in ortho and para positions on the benzene molecule thus making those areas less electrophilic as a result of the NO2 group becoming more electrollic. Why would NO2 substitute the Hydrogens in those positions?

So, you know ortho and para are less likely to have electrophillic substitution occur, which means... ?

it adopts a meta position?