[blueblueblue]

Thank you all for all the help you've given me before.  
I am having a little difficulty with the following problem, and would really appreciate hearing any feedback or insight into the following problem.


These are highly regioselective, and I need to come up with the product from the reactions.  I am supposed to focus on the intermediate formed.

a) Both carbons at the double bond are secondary carbons, but one of them has fluorine attached to it.  F is very electronegative and makes the carbon very unstable, so Bromine will attach to the more stable carbon to which F is not attached.

b) Oxygen is highly electronegative, thus it makes the adjacent carbon very unstable.  So bromine will attach to the more stable carbon.

What do you think?  Am I on the right track?

[MissPhosgene]

Can an oxygen atom stabilize a carbocation on the carbon atom to which it is bound by resonance perhaps?

[Grundalizer]

Bromine forms the bridge structure over each double bond, and adds to the carbon that is of higher order (substituted with more carbons).  In a- it adds to the carbon that is bonded to two carbons, rather than 1 methyl and 1 fluorine, and in the furan, it adds to the carbon that is connected to 2 carbons, rather than 1 carbon and 1 oxygen.

[MissPhosgene]

Bridging occurs with addition of X2 where X is a halogen or something which acts in a similar manner such as some mercury species.

For B, draw out the reaction by using resonance and look at the answer. What makes sense?

[blueblueblue]

Thanks for all your *delete me*

So, oxygen induces more electrons if i attach hydrogen to the secondary carbon which is farther than from oxygen.  Then, the carbon right next to oxygen will have a positive charge.  I thought the other way first, but oxygen actually makes the carbocation more stable then?  Is anyone able to explain to me the mechanism?  Because oxygen has a greater electronegativity, will it make the carbocation more positively charged? 

Also, it seems that grundalizer described a) as I drew it...and no one has mentioned it.  Does that mean my a) is correct as it is?

[macman104]

In each case you form the more stable carbocation then the bromine adds, so I agree with a, because the fluorine would destablize the carbocation.

I agree with your reasoning so far blue, the carbon next to the oxygen will have a positive charge, but like phosgene said, can you draw a resonance form for that carbocation with oxygen?

[orgopete]

I don't agree with a) either. While fluorine has been reported as the more electronegative, it definitely is not the most electron withdrawing, see Hammett sigma values for an overall perspective, though you could infer this from the lower acidity of HF or the basicity of F^-. If HBr gives a 2,2-dibromide upon addition to 2-butyne, then surely HBr will add to the intermediate fluoride in the same manner.

[macman104]

I think it would be incorrect to infer electron withdrawing ability based on acidity of HF, as it experiences the hydrogen bonding phenomenon that the other acid halides do not.  I can accept the precedent for 2-butyne, but how would you then explain this outcome then from the perspective of forming the most stable carbocation?  I don't think you could argue that inductively fluorine stabilizes a carbocation better than an alkyl group.

[blueblueblue]

Thank you all for all the great posts.  But, I am still feeling a bit lost on b)

Before attaching hydrogen from HBr, if I consider the resonance structure from a lone pair of oxygen then that will make a double bond next to oxygen which will shift the preexisting double bond to the adjacent carbon leaving a negative charge on that carbon.

I am having a hard time understanding the positive charge on oxygen.  Is this a regular thing which occurs?  I thought oxygen was very electronegative.  Which would prefer to have a negative charge, instead of a positive one.

Is the attached figure what you mean for resonance for carbocation withoxygen?

[blueblueblue]

Is this what you meant?  Is this even possible?

[orgopete]

@blue, the resonance structure is correct. The second structure, the bromooxonium ion, needs to have curved arrows (electron movements) consistent with its formation. If that was the reaction with HBr, that would give H^- as the leaving group. That is not consistent with the ionization of HBr.

Protonation of your resonance structure should now make sense. You then have two choices for the product, addition of the bromide to the oxygen stabilized carbocation or an S_N2 opening of the ring. I don't know what might have been inferred in your class, so pick the reaction consistent with it. (I predict the bromoaldehyde is the actual product though).

@macman, granted that carbon is a better inductive electron donor than fluorine, but fluorine is a better resonance donor and resonance effects generally are stronger.

If fluoride can hydrogen bond, that simply indicates its electrons are available to hydrogen bond. That would be consistent with an increasing bond strength from oxygen to nitrogen and carbon (carbanion) with hydrogen bonds becoming covalent bonds. Hydrogen bonding effects are much harder to find in the third row because the greater nuclear charge increases the electron withdrawing character of those atoms and an increasing HX acidity. Therefore, while fluorine can hydrogen bond, hydrogen bonding virtually disappears for Cl, Br, etc.

I offered acidity, hydrogen bonding, and Hammett sigma values that show the other halogens are more electron withdrawing than fluorine. (I personally think acidity is perhaps the best method to determine heteronuclear ionization values. It is well studied, widely applied, water is virtually a universal solvent for a very large number of acids, quite consistent within the ionization range to which it can be applied.) Perhaps you could give me an example that shows fluorine more electron withdrawing than bromine (though perhaps in another thread or message as such a discussion would highjack the original posters question).

[jake.n]

I think (a) is more straightforward than we are making it.  Both possible cations have the same number of donating alkyl groups.  The only difference between these cations is the extra fluorine, which is a resonance contributor.  I don't have any rigorous experience with fluorine chemistry, but I know its presence adds a possible resonance structure in this case (probably a very minor contributor, though).

[MissPhosgene]

Attached file is a little bit redundant, but I hope the arrow pushing can help.

I think everything is right. I haven't slept in about 52 hours, so please correct me.

Best. 

[NTUstudent]

you wrote a) correct!

But b) must be vice versa, i mean bromine attaches to the carbon next to the oxygen.
because positive charge on carbon will be delocalized by oxygen(lone pair)

 

[MissPhosgene]

you wrote a) correct!

But b) must be vice versa, i mean bromine attaches to the carbon next to the oxygen.
because positive charge on carbon will be delocalized by oxygen(lone pair)

 

Yeah, I knew the answer several days ago. The post was to try to help blue. I thought i would show something that doesn't get to the correct product instead of simply providing the answer.