Orog Question: Electrophilicity

[sp0988]

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Hey Guys the answer to the question 508 is carbon 2. But I was wondering why isnt carbon 4 most electrophilic?
I reasoned that it has the most EWGs, and fewest EDGs. I think the answer could have to do with resonance stability, but it then raises the question....how do you prioritize which factors you look at when determining electronegativity?

1) Number of substituents (Br, OH)
2) EN of the substituents (Carbonyl oxygen)
3) Resonance stability.

Which of these factors is most important? And does it change with the molecule? Also could steric hinderance having anything to do with it?

 

[Jackson-Ave-Dental]

Carbon 2 can be attacked by a nucleophile, which would cause the double bond from the oxygen to break off into a single bond.

Carbon 4 has no pi bond... and the only way it can proceed is via substitution.

 

[sp0988]

that makes sense, but then whats the difference between an electrophile (which is how i think your rationalizing the answer) vs. electrophilic carbon? My interpretation (electrophilic) is to find the carbon with the highest partial positive charge and based off the picture i felt that carbon 4 had the most EN atoms thus highest positive charge. If i am missing something, please let me know.

 

[Jackson-Ave-Dental]

An electrophile by definition is an electron loving species.

Carbon 2 has a full octect, 2 single bonds and 1 double bond.
A nucleophile can attack the carbon and force the pi bond of the double bond to break off leaving the oxygen with a negative charge.
The carbon would then be able to form a new octet with the nucleophile that attacked it.
In essence, the nucleophile 'donated' a pair of electrons to the electrophile, carbon.
This is what we call, nucleophilic attack.


What makes carbon 2 a better electrophile than carbon 4?
Remember that single bonds are sigma bonds; double bonds are sigma + 1 pi bond; triple bonds are 1 sigma + 2 pi.
What is the significance of that? Well, pi bonds are a lot weaker than sigma bonds.
Carbon 4 has 4 bonds, which are all sigma bonds while carbon 2 has 3 sigmas and 1 pi.
The pi bond will more readily break off than a sigma bond any day.


The only way carbon 4 will undergo attack by a nucleophile is via substitution.
Notice that in carbon 4, the leaving group (Br) is on a primary carbon.
This is indication that SN2 is more likely to proceed.
In which case you'd need a polar aprotic solvent AND a very strong nucleophile to even initiate the reaction.

Basically, there are a lot more requirements to get the mojo going at carbon 4 than it is at carbon 2. I hope this helps somewhat lol. If my logic is flawed anywhere, please feel free to correct me!




EDIT: Oh yes to answer your question about 'finding the carbon with the highest partial positive charge'

Notice that when a nucleophile is attacking carbon 2, the pi bond COMPLETELY breaks off, therefore giving carbon 2 a FULL positive charge briefly.
If you remember the reaction mechanism for SN2, it forms a transitional state with 5 bonds... Also having electron withdrawing groups on the carbon will only give it a 'partial' positive charge to begin with. Why have partial when you can have full 🙂


Also, you are incorrect in thinking that carbon 4 has the most electronegative groups attached to it.
Carbon 2 has 1 double bond to oxygen so this actually counts as TWO bonds to oxygen.
Carbon 4 has 1 bond to oxygen and 1 bond to Br.

If you take a quick look t your periodic table you should quickly notice that oxygen is more electronegative that Br. This makes carbon 2 more "positive".

I think that's about it now. If you are having problems conceptually might I recommend CHAD's if you haven't done so? A lot of people rave about his videos in their break downs.

Personally I've decided to try a different source!

 

[sbdento]

Carbonyl carbon's are always good electrophiles. The oxygen on c 1 is also drawing electrons making the carbonyl even more electrophilic.

 

[sp0988]

JOoa0ky, thanks for the great explanation. It makes more senes to me. I only defined it by how electropositve the carbon atom was, and failed to recognize that its "ability to get attacked" is also part of electrophilicity.

you said that you used a different source that chads? Can i ask what/who?

 

[wantVCUdental]

An electrophile by definition is an electron loving species.

Carbon 2 has a full octect, 2 single bonds and 1 double bond.
A nucleophile can attack the carbon and force the pi bond of the double bond to break off leaving the oxygen with a negative charge.
The carbon would then be able to form a new octet with the nucleophile that attacked it.
In essence, the nucleophile 'donated' a pair of electrons to the electrophile, carbon.
This is what we call, nucleophilic attack.


What makes carbon 2 a better electrophile than carbon 4?
Remember that single bonds are sigma bonds; double bonds are sigma + 1 pi bond; triple bonds are 1 sigma + 2 pi.
What is the significance of that? Well, pi bonds are a lot weaker than sigma bonds.
Carbon 4 has 4 bonds, which are all sigma bonds while carbon 2 has 3 sigmas and 1 pi.
The pi bond will more readily break off than a sigma bond any day.


The only way carbon 4 will undergo attack by a nucleophile is via substitution.
Notice that in carbon 4, the leaving group (Br) is on a primary carbon.
This is indication that SN2 is more likely to proceed.
In which case you'd need a polar aprotic solvent AND a very strong nucleophile to even initiate the reaction.

Basically, there are a lot more requirements to get the mojo going at carbon 4 than it is at carbon 2. I hope this helps somewhat lol. If my logic is flawed anywhere, please feel free to correct me!




EDIT: Oh yes to answer your question about 'finding the carbon with the highest partial positive charge'

Notice that when a nucleophile is attacking carbon 2, the pi bond COMPLETELY breaks off, therefore giving carbon 2 a FULL positive charge briefly.
If you remember the reaction mechanism for SN2, it forms a transitional state with 5 bonds... Also having electron withdrawing groups on the carbon will only give it a 'partial' positive charge to begin with. Why have partial when you can have full 🙂


Also, you are incorrect in thinking that carbon 4 has the most electronegative groups attached to it.
Carbon 2 has 1 double bond to oxygen so this actually counts as TWO bonds to oxygen.
Carbon 4 has 1 bond to oxygen and 1 bond to Br.

If you take a quick look t your periodic table you should quickly notice that oxygen is more electronegative that Br. This makes carbon 2 more "positive".

I think that's about it now. If you are having problems conceptually might I recommend CHAD's if you haven't done so? A lot of people rave about his videos in their break downs.

Personally I've decided to try a different source!

I think this is a good explanation, but when i looked at the question, i also thought carbon 4 is a better electrophile. After reading your reasoning behind how C4 is hard to be displaced, one can also make the argument that the carbonyl is extremely sterically hindered. The only way to attack the carbonyl is through an Sn2 similar attack where the nucleophile approaches from slightly above the plane of the compound, where you have methoxide/methyl and 2 secondary R groups.

C4 can also undergo Sn1 reaction because the methyl is attached to hydroxyl which can have somewhat of resonance tabilization. So there are actually 2 reactions available for an nucelophilic attack on C4 making C4 the most electrophilic carbon in this molecule?

 

[sp0988]

My personal opinion is that is has less to do with the carbon's ability to be a good electrophile, and more to do with its partial positive charge (electrophilic). I know i keep saying the same thing, but i think its important. I think it helps to explain why C2 is more EN, b/c it DOES have 2 Oxygen bonds, which are both more EN than the OH, Br substituents on C4. But the reason why I initially disregarded this is because C2, has 2 R groups attached that can function to electron donate, vs. C4. which only has 1 carbon group this can electron donate. Because of this fact, I am still having some trouble understanding how EDG compare to EWG in terms of priority.

VCU, i think C2 carbonyl is planar with oxygen, thus its less sterically hindered than C4. But i get the argument your making in that C2 substituents are most sterically hindered, because they are larger groups.

 

[sp0988]

I dont know how much this helps my case, for C4, but i ran across a separate problem, and it pretty much said that OH groups are more polar than carbonyl oxygen groups. I have posted links to MCAT forms that have discussed the
polarity of alcohols, ketones, and aldehydes.


http://forums.studentdoctor.net/showthread.php?t=662131
http://forums.studentdoctor.net/showthread.php?t=657337
http://forums.studentdoctor.net/showthread.php?t=533456
http://www.elmhurst.edu/~chm/vchembook/213organicfcgp.html


I am raising this point because I understand that OH groups are more likely to hydrogen bond, thus more polar molecules, but how does this influence the dipole moment? and how does this have anything to do with pi bonds?

And based on this logic, if OH is more EN, then why isnt C4 more EN?

 

[Avery07]

Where are those questions from?

 

[Jackson-Ave-Dental]

I dont know how much this helps my case, for C4, but i ran across a separate problem, and it pretty much said that OH groups are more polar than carbonyl oxygen groups. I have posted links to MCAT forms that have discussed the
polarity of alcohols, ketones, and aldehydes.


http://forums.studentdoctor.net/showthread.php?t=662131
http://forums.studentdoctor.net/showthread.php?t=657337
http://forums.studentdoctor.net/showthread.php?t=533456
http://www.elmhurst.edu/~chm/vchembook/213organicfcgp.html


I am raising this point because I understand that OH groups are more likely to hydrogen bond, thus more polar molecules, but how does this influence the dipole moment? and how does this have anything to do with pi bonds?

And based on this logic, if OH is more EN, then why isnt C4 more EN?

You are getting a bit carried away lol.

If you look at carbon4... the substituents attached are:
1) -Br
2) -OH
3) -R alkyl
4) -H

If you look at carbon2... the substituents attached are:
1) =O
2) -R
3) -R

I will reiterate a couple of points.

The double bond to oxygen is equivalent to being bonded to oxygen twice! Since O is more electronegative that Br... carbon2 is actually the most 'positive' and electrophilic.

Also... the -OH group adds polarity but it does not make the carbon more 'positive'... what the -OH group does is make the HYDROGEN more positive so that it may hydrogen bond. In this case... you would be talking about INTERmolecular forces but that is not what we are looking at. We are currently concerned with INTRAmolecular forces.

One extra point... -R groups are NOT donating electrons. You generally don't view -R as EDGs unless you are looking at a benzene ring. In the case of straight chain molecules, we would be taking into account the inductive effect instead.


TIP: Electronegativity is a much greater determining factor than the inductive effect.

For example when you are deciding between acidity of a -H... the PRIMARY factors to look at are:
A) Atomic Size
B) Electronegativity
C) Hybridization

The SECONDARY factors are:
A) Resonance
B) Inductive effect

The primary factors are basically what affects the thing that we are looking at DIRECTLY. Notice that the secondary factors are kind of 'long-ranged' effects.

You can ask yourself this! Would an oxygen directly bonded to a carbon make it more positive or would an oxygen that is 2 carbons down make the carbon more positive? Therefore! what is directly attached to the carbon that you are looking at is more important!


Also... I don't want to reveal anything about my source until I have a solid score to back it up 😉 I don't want to be held liable for anything!


EDIT: I forgot to add... you generally only look at the primary factors when you are comparing but you will have to take into account the secondary factors if the primary factors are tied.