a-b unsaturated carbonyls

[MedPR]

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EK says:

The carbocation that is produced at the carbonyl carbon when the electrons in the carbonyl double bond shift to the oxygen atom is stabilized by resonance. Additionally, the electron withdrawing carbonyl group pulls electrons from the carbon-carbon double bond and makes the beta-carbon less susceptible to attack by a nucleophile (electrophilic addition

If electrons are being withdrawn, doesn't that increase the magnitude of partial positive charge on the beta-carbon? So shouldn't it be more susceptible to nucleophilic attack?

 

[chiddler]

i think you're right

?

though drawing this, i realized that the carbonyl carbon is less reactive to nucleophiles since it has less partial positive charge than a plain carbonyl. i never realized this before.

 

[MedPR]

Yea, the alpha and beta carbons will donate electrons to a carbonyl carbocation, thus reducing the carbonyl carbon's susceptibility to nucleophilic attack. I feel like what EK typed would be too obvious of a typo for them to miss.. What am I (we?) missing?

 

[MedPR]

Bump. Anyone have an idea?

 

[JD7]

the carbonyl group pulls the electrons from the carbon, not the other way around. making the electromagnetic difference less.

 

[MedPR]

the carbonyl group pulls the electrons from the carbon, not the other way around. making the electromagnetic difference less.

If the carbonyl pulls electrons from the beta-carbon, how does it make the beta-carbon less susceptible to nucleophilic attack? Nucleophilic attack = most likely at a very positive atom. If you pull electrons away from something, you make it more positive, so it should become more susceptible to nucleophilic attack...

 

[gettheleadout]

If EK meant to say "...makes the carbonyl carbon [instead of 'beta-carbon'] less susceptible..." then I think they would be more likely to catch a typo than if they simply meant to say "...more [instead of less] susceptible..." as either would be correct.

 

[MedPR]

If EK meant to say "...makes the carbonyl carbon [instead of 'beta-carbon'] less susceptible..." then I think they would be more likely to catch a typo than if they simply meant to say "...more [instead of less] susceptible..." as either would be correct.

Ok, but we are in agreement about susceptibility to nucleophilic attack?

When the carbonyl carbon withdraws electrons from the beta-carbon:
1. The carbonyl carbon becomes less susceptible
2. The beta carbon becomes more susceptible

 

[gettheleadout]

Ok, but we are in agreement about susceptibility to nucleophilic attack?

When the carbonyl carbon withdraws electrons from the beta-carbon:
1. The carbonyl carbon becomes less susceptible
2. The beta carbon becomes more susceptible

Yes we are.

 

[MedGrl@2022]

EK says:



If electrons are being withdrawn, doesn't that increase the magnitude of partial positive charge on the beta-carbon? So shouldn't it be more susceptible to nucleophilic attack?

I was just about to ask the same question... I was so confused... damn you EK with your typos... lol... I guess it means we all have to think more... better preparation for the MCAT... so then if the B-carbon is MORE susceptible to attack by the nucleophile and less susceptible for attack by an electrophile. It would be nucleophilic addition NOT electrophilic addition. Right?

I think the following sentence may also need editing. The sentence in the EK book is "Thus, rather than the nucleophile adding to the B-carbon, it may sometimes add to the oxygen atom, forming the enol-keto tautomers." What would be the correct way to write this? A nucleophile does not attack the oxygen but an electrophile would.

So to re-write these two sentences "Additionally, the electron withdrawing carbonyl group pulls electrons from the carbon-carbon double bond and makes the B-carbon less susceptible to attack by an electrophile (electrophilic addition). Thus, rather than the electrophile adding to the B-carbon, it may sometimes add to the oxygen atom, forming the enol-keto tautomers."

In addition, the alpha-carbon becomes more susceptible to electrophilic addition due to the electron withdrawing carbonyl group pulling electrons from the carbon-carbon double bond.

Also (just to clarify) EK stated that the nucleophilic addition to the carbonyl carbon is still the major product in alpha-beta unsaturated carbonyls. Is this still correct (I am uncertain if I can trust EK... lol)?

Here is a paragraph I found about alpha-beta carbonyls from Wikipedia: http://en.wikipedia.org/wiki/Carbonyl#.CE.B1.2C.CE.B2-Unsaturated_carbonyl_compounds

I find their paragraph confusing too... because Br and Cl are nucleophiles not electrophiles right?

I don't understand why anyone is writing the correct information...???

I suppose it is up to us SDNers... 🙂

 

[MedGrl@2022]

If EK meant to say "...makes the carbonyl carbon [instead of 'beta-carbon'] less susceptible..." then I think they would be more likely to catch a typo than if they simply meant to say "...more [instead of less] susceptible..." as either would be correct.

If the carbonyl carbon is less suspectible to attack by a nucleophile the parathensis should say "(nucleophilic addition)" NOT "(electrophilic addition)"... right?

However below the first picture EK says "Of course, we know that aldehydes and ketones undergo nucleophilic addition at the carbonyl, and for many nucleophiles this carbon addition is still the major product of the above reaction." I believe it is still referring to the alpha-beta unsaturated carbonyl. Is what EK is saying in that sentence correct?

Thank you for all your help.

Verónica

 

[dariidar]

I find their paragraph confusing too... because Br and Cl are nucleophiles not electrophiles right?

Br and Cl are nucleophiles, but Br2 (gas) and HCl can participate in electrophilic substitutions.
For example, H2C = CH2 + Br2 -> BrH2C - CH2Br

Wikipedia says that "the alkene group is, therefore, deactivated towards an electrophile, such as bromine or hydrochloric acid." Which I believe means that the b carbon on the alkene is less likely to participate in an electrophilic substitution. There's less electron density there, so the beta carbon is less willing to share electrons.

On the other hand, since there's less electron density, then the beta carbon will be more susceptible to nucleophilic attack, a la http://en.wikipedia.org/wiki/Nucleophilic_conjugate_addition.

 

[ChemTutor]

the resonance structures chiddler drew above demonstrate that there is delta+ both on C of the carbonyl and on the beta-carbon. Thus, a nucleophile can hit either of those places (C of the carbonyl or beta carbon).

 

[zm7906]

Additionally the electron withdrawing carbonyl group pulls electrons from the carbon-carbon double bond and makes the carbonyl carbon less susceptible to attack by a nucleophile (electrophilic addition). Thus rather than the electrophile adding to the B - carbon it may sometimes add to the oxygen.

Is this how the sentences are supposed to read? why would the electrophile add to the B-carbon?

 

[seventeenthfloor]

Here's the transcript from the Audio Osmosis discussion of this topic. I think it clears up the frustrating typo.

"Conjugated systems may exhibit unique behaviors as well. For instance a double bond between the alpha carbon and beta carbon of a carbonyl is a conjugated system. When I add an electrophile to an alkene I get electrophilic addition to the least substituted carbon but in this case the electrophile may add to the carbonyl oxygen. If I follow with a nucleophile the nucleophile adds to the beta carbon. If I were to start with the nucleophile, rather than adding to the carbonyl like one might expect the nucleophile might add directly to the beta carbon. ...but a rule change like that would be explained in a passage on the MCAT... The MCAT tests rules, not exceptions."

So, as others have said, they should have written that an electrophile adds to the oxygen and that a nucleophile adds to the beta carbon.

 

[chemtopper]

i think you're right

Look at the three resonating structures and identify which ones are major contributors.I am sure you all will agree that first is the major and second also has more contribution than the third one.So the positive charge will be more on carbonyl carbon and alpha carbon is next to carbonyl carbon ,hence it is going to bear the electron withdrawing effect of C of the carbonyl group.That is the reason they are saying that in electrophilic addition on the double bond ,beta carbon is now less susceptible to nucleophilic attack.

 

[Czarcasm]

Hmm. So would you guys this basically sums this discussion up:

Due to resonance, the oxygen of the carbonyl group is more nucleophilic and therefore susceptible to electrophile addition (attacking an electrophile) -- while the beta carbon, having a greater positive charge (also due to resonance), is likely to be attacked by a nucleophile.

 

[Czarcasm]

Since we're discussing this topic, I thought I'd mention something that really confuses me:

I always interpreted the terms "Electrophilic Addition" and "Nucleophilic Addition" as a reference point. When studying Alkenes (our focus point), we learned that double bonds of Alkenes can act either as a nucleophile or base. As such, alkenes are likely to attack the positive partial charge of an electrophile. Hence, the term "electrophilic addition."

For Aldehydes and Ketones, we instead focus on the carbon of a carbonyl, which has a partial positive charge. This makes it susceptible to being attacked by a nucleophile and so, from this viewpoint, we refer to this as "nucleophilic addition."

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From the view point of the beta carbon which has a positive partial charge and therefore acts as an electrophile -- pay attention to this quote by EK, particularly what's in bold:

"... the electron withdrawing carbonyl group pulls electrons from the carbon-carbon double bond and makes the B-carbon less susceptible to attack by a nucleophile (electrophilic addition). Thus, rather than the nucleophile adding to the B-carbon, it may sometime add to the oxygen atom, forming the enol-keto tautomers.

Even more strange is the ability of the B-carbon to undergo nucleophilic addition directly. This is sometimes called conjugate addition."

Was that another typo? Shouldn't that instead be nucleophilic addition as well? After all, the partial positive charge of the beta carbon is what makes it susceptible to nucleophilic attack.

And what also confuses me is what I marked in red from that quote: A ketone can tautomerize into it's minor form (the Enol) when the alpha hydrogen gets deprotonated, forming a double bond and kicking up the c=o pi electrons to the oxygen (which allows it to get protonated). In this case, the proton (hydrogen) is being added to the oxygen. Is this something you would refer to as a "nucleophile adding to the Oxygen"? That seems so backwards. If they instead said, the Oxygen is more likely to act as a nucleophile, that would make more sense, although I've never encountered any situation where that happens in intro Organ Chemistry.

I'd hate to spend so much time on this and overthink things, but this is why I hate EK. Their wording is extremely confusing.

 

[Czarcasm]

I think I need a break from studying. I'm losing it.

 

[Czarcasm]

Meanwhile, TPRH explains things so eloquently:

 

[Czarcasm]

 

[Czarcasm]

(It wont let me post this without typing something -- types something here

)

 

[Czarcasm]

And here's the final attached image sample:

 

[Czarcasm]

Sorry, but wanted to add onto this if anyone looks into this. Chad said this about these types of compounds:

Strong nucleophiles that are very strong bases attack the carbonyl carbon, while strong nucleophiles that are less strong bases attack the beta carbon.

Ex. of 1,2 addition [Nucleophiles that attack Carbonyl Carbon]: Grignards (RMgBr or RLi)*, Acetylides, & Hydride Reagants (NaBH4, LiAlH4)
Ex. of 1,4 addition [Nucleophiles that attack Beta Carbon]: Hydroxide (OH-), Cyanide (CN-), and "Gillman's Reagant" aka Lithium Dialkyl Cuprate (R2CuLi), an organometallic, weaker than Grignard's and organolithium -- adds a methyl group, much like a grignard would.

* Generally applies to smaller Grignards. For really big grignards, they attack Beta as well (but not something they generally test on).

 

[chiddler]

It's so cool that people are still using our old mcat questions

<3 good luck guys

 

[sniderwes]

wow EK really ****ed up on this topic. i was confused as **** after reading the page about alpha beta-unsaturated carbonyls

 

[SKation]

And here's the final attached image sample:

View attachment 177472

Hey !

That was really helpful thank you! Do u mind telling me what the highlighted sentence said it is sorta cut off

Thank You

 

[Czarcasm]

Hey !

That was really helpful thank you! Do u mind telling me what the highlighted sentence said it is sorta cut off

Thank You

Glad you found it helpful. I'm so sorry though, I no longer have that book with me.