Destroyer Orgo Questions

[mvp0130]

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Hi. I've been studying DAT Destroyer Orgo section-2012 version. I have some questions and I would appreciate if you can help me.

#67. Rank acidity
-given (Para NitroBenzoic acid); (Benzoic acid); (3-methylBenzoic acid); and (Carboxylic acid)
-My question is that is aromatic ring electron withdrawing group or donating group? I tried to google and I was not lucky to get a right answer.

#77. Aromatic
- N-methylpyrrole is an aromatic ring.
-My question is how/why N with LP is unhybridized P orbital? I would think It's SP3 atom including the bond to CH3, 2bonds within the ring,and Lone Pair.

Thanks in advance.

 

[discensdentibus]

I can answer 67 for you right now.. It really depends if an aromatic is donating or withdrawing! I know that may seem frustrating, but it's more frustrating to assume it is one or the other 100% of the time. I think your best bet for narrowing down answer choices is to go with what you know. Phenols are typically pka ~9-10 right? Alcohols/water are generally ~15/16, and therefore, it would be safe to assume that benzoic acids will be more acidic that a carboxylic acid (pka 5 ish).

If you want specifics, I am assuming the RCOOH r was a methyl; the methyl is more donating, and a phenyl does not want to break its aromaticity to send electrons to a carbon. Even if it does send electrons its way, you are stuck with two negative charges on oxygens which are bonded to carbon; this is quite unstable.

Hope this helps, I can look into the other if you want tomorrow if no one cares to share their input

 

[biomolecmed]

N-methyl pyrolle is aromatic. The lone pair on the Nitrogen is inside the aromatic ring, therefore can resonate all around the ring. That is why it is SP2 hybridized, not sp3.

 

[ranashammas]

#67
In order for the compound to be acidic, you want more withdrawing group and less donating group. So the compound with the nitro will be the most acidic, then comes the benzoic acid alone and then the one with the donating group and finally comes the carboxylic acid. Carboxylic acid is less acidic because it's attached to an sp3 carbon whereas the benzoic acid is attached to an sp2 carbon on the ring. As you know about acidity, sp > sp2 > sp3

#77
I'm not sure how to explain this one but you always count the N as sp2 if it's not double-bonded. So if it's sp2, then you count the lone pairs making the compound to have 6 pi electrons which will leave you with an aromatic compound if you follow the rule 4n+2=#pi electrons.
If you look at the other compounds, b and d have carbons that are sp3 so they are non-aromatic. C has 4 pi electrons (counting the double bond and the lone pairs of the carbon) so it fits the rule 4n=# pi electrons so it's anti-aromatic.

Hope this helps you. I'm not great at explaining stuff in theoretical way.

 

[mvp0130]

@
discensdentibus,
biomolecmed
ranashammas

Thank you guys! I got all the questions! I really appreciate your detailed explanation!!