which is more reactive

[chiddler]

which will react with SOCl2 more readily?

The answer is the Br one. "The bromine atom is electron withdrawing making the carbonyl carbon more electrophilic."

My question is why not OH? The oxygen is more electronegative. It's not really donating its lone pair. I'd expect it to be electron withdrawing rather than donating. More so than Br.

rephrasing: the alcohol in this context can only act via inductive effect. Which is why i'd expect EWG rather than EDG.

---

Members don't see this ad.

 

[MedPR]

which will react with SOCl2 more readily?

The answer is the Br one. "The bromine atom is electron withdrawing making the carbonyl carbon more electrophilic."

My question is why not OH? The oxygen is more electronegative. It's not really donating its lone pair. I'd expect it to be electron withdrawing rather than donating. More so than Br.

rephrasing: the alcohol in this context can only act via inductive effect. Which is why i'd expect EWG rather than EDG.

Your explanation seems spot on even though it is inconsistent with the book explanation.

I looked at it from the point of view that SOCl2 has a lone pair on the Sulfur, making it a nucleophile and since carbonyls typically act as electrophiles, I figured this was some kind of substitution reaction. Probably chlorination of some sort. Since there is nothing to protonate the OH groups, they won't be turned into a good leaving group (water). Since Br- is a better leaving group than Cl-, I figured the Br one was more reactive.

 

[davcro]

I think Br is more electronegative than -OH. I know that OH is electron donation, but I'm not entire sure why.

 

[MedPR]

I think Br is more electronegative than -OH. I know that OH is electron donation, but I'm not entire sure why.

I know that Br is less electronegative than O, but how would you go about determining if it is more or less than OH?

 

[thestrokes14]

I think Br is more electronegative than -OH. I know that OH is electron donation, but I'm not entire sure why.

Never underestimate the power of having a LONE PAIR of electrons. These are always donated (which amines and alcohols are considered electron donating groups).

 

[thestrokes14]

I know that Br is less electronegative than O, but how would you go about determining if it is more or less than OH?

The inductive effect is a weak effect. Doesn't compete with resonance.

 

[MedPR]

Never underestimate the power of having a LONE PAIR of electrons. These are always donated (which amines and alcohols are considered electron donating groups).

Why is the Br one more reactive?

 

[chiddler]

the electronegativity for Br is 2.96. for O is 3.44. whatever units that is in.

does this change because of the presence of H in OH?

OH is electron donating in aromatics, but here i wouldn't think so. nowhere to really donate.

 

[thestrokes14]

the electronegativity for Br is 2.96. for O is 3.44. whatever units that is in.

does this change because of the presence of H in OH?

OH is electron donating in aromatics, but here i wouldn't think so. nowhere to really donate.

Electronegativity is a scaled value (4.0 is the highest for Flurorine - scale is called the Pauling scale).

I've definitely seen this problem before (from EK?). It is a pretty poor question because the resonance is extremely weak for the OH molecule. So perhaps the stronger inductive effect of OH could win out. The point here is that this is an MCAT question and you really need to go with the clearest answer, which is that the EWG will promote carbocation formation at the carbonyl and the EDG will not (even though mechanism by which this is accomplished is unclear).

I wouldn't get caught up on this question.

 

[MedPR]

Electronegativity is a scaled value (4.0 is the highest for Flurorine - scale is called the Pauling scale).

I've definitely seen this problem before (from EK?). It is a pretty poor question because the resonance is extremely weak for the OH molecule. So perhaps the stronger inductive effect of OH could win out. The point here is that this is an MCAT question and you really need to go with the clearest answer, which is that the EWG will promote carbocation formation at the carbonyl and the EDG will not (even though mechanism by which this is accomplished is unclear).

I wouldn't get caught up on this question.

But aren't Oxygens EWG too? And they are more electronegative than Br?

 

[chiddler]

Electronegativity is a scaled value (4.0 is the highest for Flurorine - scale is called the Pauling scale).

I've definitely seen this problem before (from EK?). It is a pretty poor question because the resonance is extremely weak for the OH molecule. So perhaps the stronger inductive effect of OH could win out. The point here is that this is an MCAT question and you really need to go with the clearest answer, which is that the EWG will promote carbocation formation at the carbonyl and the EDG will not (even though mechanism by which this is accomplished is unclear).

I wouldn't get caught up on this question.

well i'm satisfied.

thanks everyone.

 

[thestrokes14]

But aren't Oxygens EWG too? And they are more electronegative than Br?

Yeah, but like I said, it doesn't compete with resonance (or the ability to donate a lone pair). Remember the definition of electronegativity. It is the general affinity of electrons within a covalent bond. The overall properties of a MOLECULE are going to be determined the donation of the lone pair, rather than the weak EWG effect. Same goes for Bromine, but Bromine doesn't have a lone pair to donate.

Remember that among all the electron withdrawing groups, which is the weakest? The halogens. Why? Because resonance withdrawing groups are much STRONGER forces than electronegativity effects.

 

[MedPR]

Yeah, but like I said, it doesn't compete with resonance (or the ability to donate a lone pair). Remember the definition of electronegativity. It is the general affinity of electrons within a covalent bond. The overall properties of a MOLECULE are going to be determined the donation of the lone pair, rather than the weak EWG effect. Same goes for Bromine, but Bromine doesn't have a lone pair to donate.

Remember that among all the electron withdrawing groups, which is the weakest? The halogens. Why? Because resonance withdrawing groups are much STRONGER forces than electronegativity effects.

Ok I guess I have a bigger problem then. I agree with everything you are saying, it all makes sense. However, doesn't that point to the Br one being less reactive?

 

[thestrokes14]

Ok I guess I have a bigger problem then. I agree with everything you are saying, it all makes sense. However, doesn't that point to the Br one being less reactive?

Keep in mind what the reaction is: a substitution reaction at the carbonyl. The more reactive molecule is the one that promotes carbocation formation. OH is not doing this (even though it is more electronegative) because of its stronger ability to donate electrons. Bromine doesn't do this, therefore Bromine withdraws, albeit relatively poorly) away from the carbonyl.

 

[MedPR]

Keep in mind what the reaction is: a substitution reaction at the carbonyl. The more reactive molecule is the one that promotes carbocation formation. OH is not doing this (even though it is more electronegative) because of its stronger ability to donate electrons. Bromine doesn't do this, therefore Bromine withdraws, albeit relatively poorly) away from the carbonyl.

So there is resonance between the three oxygens? Thus creating a cloud of electrons around the carbonyl carbon? The same concept/reasoning as why carboxylic acids do not undergo nucleophilic substitution -- electron cloud prevents the nucleophile from getting close enough?

 

[chiddler]

Keep in mind what the reaction is: a substitution reaction at the carbonyl. The more reactive molecule is the one that promotes carbocation formation. OH is not doing this (even though it is more electronegative) because of its stronger ability to donate electrons. Bromine doesn't do this, therefore Bromine withdraws, albeit relatively poorly) away from the carbonyl.

oxygen cannot donate electrons in this case! there is no possible resonance formation with the alcohol without removing an H from the carbon the alcohol is connected to. very unlikely.

what you're saying is when the substituents are involved in aromatic rings. not in such a case as this one.

 

[thestrokes14]

So there is resonance between the three oxygens? Thus creating a cloud of electrons around the carbonyl carbon? The same concept/reasoning as why carboxylic acids do not undergo nucleophilic substitution -- electron cloud prevents the nucleophile from getting close enough?

That is the part that I don't like about this question. Techically, there is no p orbital on the carbon attached to the OH group (it is sp3 hybridized). This is why chiddler and I are having trouble believing that OH is really all that strong of a donating group, as there is no pi network that links the OH electrons to the carbocation.

Like I said, don't stress out about this question. Pretty sure they are simply testing whether you understand that OH is more of an electron donating group than an electron withdrawing group. If you understand that OH is an EDG, you should be good. I never quite understood this question either.

 

[MedPR]

I guess we would have to assume that the solvent is responsible for deprotonating one of the OH groups?

 

[thestrokes14]

I guess we would have to assume that the solvent is responsible for deprotonating one of the OH groups?

No. OH doesn't need to be deprotonated to be considered electron donating (although it would certainly increase its ability to donate).

 

[MedPR]

No. OH doesn't need to be deprotonated to be considered electron donating (although it would certainly increase its ability to donate).

So how do we know that OH is an EDG? 🙂

 

[chiddler]

I guess we would have to assume that the solvent is responsible for deprotonating one of the OH groups?

nah i don't think so. eve if it tautomerizes, the double bond on that alcohol carbon would also make the carbonyl carbon double bonded. this would make it unreactive.

 

[thestrokes14]

So how do we know that OH is an EDG? 🙂

It has high electron density (or lone pairs). Anything with lone pairs is EDG (except perhaps the halogens). If you don't currently know your EDG and EWG, I highly recommend studying this and recognizing common EDG and EWG.

For example, NH2 is EDG because it has a lone pair. NO2 is electron withdrawing because it is both electronegative and, more importantly, resonance forms that can be drawn where electrons are be transferred to the oxygens of the NO2 group.

 

[MedPR]

It has high electron density (or lone pairs). Anything with lone pairs is EDG (except perhaps the halogens). If you don't currently know your EDG and EWG, I highly recommend studying this and recognizing common EDG and EWG.

For example, NH2 is EDG because it has a lone pair. NO2 is electron withdrawing because it is both electronegative and, more importantly, resonance forms that can be drawn where electrons are be transferred to the oxygens of the NO2 group.

Yea that's something I definitely have to go over. I haven't found a good source for it though..

 

[BerkReviewTeach]

which will react with SOCl2 more readily?

The answer is the Br one. "The bromine atom is electron withdrawing making the carbonyl carbon more electrophilic."

My question is why not OH? The oxygen is more electronegative. It's not really donating its lone pair. I'd expect it to be electron withdrawing rather than donating. More so than Br.

rephrasing: the alcohol in this context can only act via inductive effect. Which is why i'd expect EWG rather than EDG.

There are compelling arguments posted by a couple people as to why the alpha OH is not electron donating (having no pi-bond to donate its lone apir to negates resonace). And to be honest, I am not convinced that the Br-species is actually more reactive for the reason listed. The one factor that might come into play is the steric hindrance created by the intramolecular hydrogen bonding involving the hydroxyl oxygen and the carboxylic acid proton. That's a nearly planar five membered ring that would hinder attack at that the carbonyl carbon. It could be a small argument to consider.

But, the mechanism involves an oxygen attacking the S of SOCl2 in its first step. The hydroxyl oxygen is tied up in the ring (hydrogen-bonding ring system), so it can't attack the sulfur. So the only O that can attack the S is the carbonyl oxygen (typical when a Lewis acid such as SOCl2 is added to a carbonyl). If the OH of the acid is involved in the hydrogen bonded ring system, then it won't donate to the carbonyl carbon as readily which won't make the carbonyl oxygen as nucleophilic. With bromine in the alpha position, you don't have this problem, so the carbonyl oxygen is electron richer and can attack S more readily. If the Br-species is truly more reactive, then I'd think the hydrogen bonding of the OH group causing a reduction in the partial negative charge on the carbonyl oxygen by resonance would be the best explanation.

 

[hellocubed]

It has high electron density (or lone pairs). Anything with lone pairs is EDG (except perhaps the halogens). If you don't currently know your EDG and EWG, I highly recommend studying this and recognizing common EDG and EWG.

For example, NH2 is EDG because it has a lone pair. NO2 is electron withdrawing because it is both electronegative and, more importantly, resonance forms that can be drawn where electrons are be transferred to the oxygens of the NO2 group.

Alright this may seem like a dumb question, but I'm having trouble understanding this trivial point.


People keep noting that O has lone pairs, but never mention that Br has lone pairs.

http://www.poly.edu/sites/polyproto...lf/pressrelease/RS12614_Lewis Dots screen.jpg

Why is it that Br's 3 electron pairs are not considered lone pairs? Because if they are, it would render the above quote completely moot.

 

[BerkReviewTeach]

Alright this may seem like a dumb question, but I'm having trouble understanding this trivial point.


People keep noting that O has lone pairs, but never mention that Br has lone pairs.

http://www.poly.edu/sites/polyproto...lf/pressrelease/RS12614_Lewis Dots screen.jpg

Why is it that Br's 3 electron pairs are not considered lone pairs? Because if they are, it would render the above quote completely moot.

While Br has lone pairs as you note, they are in 4p orbitals, which don't overlap as well with carbon as the electrons in a 2p orbital like O. This makes O lone pairs better at resonance donating.

 

[hellocubed]

Oh wow, thanks~

Does this mean that F has 3 legitimate lone pairs that can be pushed into resonance? (I ask because I don't believe I've ever seen a resonance involving F)

Also, Why is Sulfur capable of resonance with Oxygen
http://www.websters-online-dictiona...-2D.png/500px-Sulfur-dioxide-resonance-2D.png
When one is 3P and the other is 2P?