[nextpauling]

I have a list of questions I've been mulling over and I'd like to see what you guys think.  I know it's alot of questions that are not simple yes or no answers but any insight you guys can provide will be greatly appreciated...

1) Why does increasing the substitution of a carbon-carbon double bond increase its stability?  I rationalize this as the electron donating groups cause increased e- density in the pi orbitals forming the double bond.  More e- density between the nuclei--> stronger bond.

2) Can we generalize halohydrin formation from alkenes (alkene + dihalide + water --> halohydrin) to include general nucleophiles?  For instance can we use alkene + Br2 + HCC- --> adjacent Br and ethynyl groups.

3) How can we shift the alkene hydration/alcohol dehydration in each direction to form our desired product?  Since entropy favors dehydration but enthalpy favors hydration, I would expect dehydration products (alkene) to dominate at high T and hydration products (alcohol) to dominated at low T.

4) In the hydration of an alkyne, does the resulting enol form from syn or anti addition?  i.e.  does the nucleophilic water attack in the plane of the vinyl carbocation from either side?  Since the p orbital of the pi bond is perpendicular to the molecular plane I would expect the vacant p orbital to be in the plane of the carbocation with the accessibility on each side depending on the subsituents on the positive carbon.

5) Can vinyl carbocations rearrange via hydride/alkyl shift reactions?  I think I read in the Carey-Saunberg text that the bridged vinyl structure is much higher in E than the linear structures and therefore presents a high activation energy transition state for the shift reaction.

6) How does the Hg2+ catalyst function in the hydration of terminal alkynes?  I would expect the mercury complexes one of the pi bonds of the alkene in facilitating addition but can anybody give me any more insight?

Thanks for your help.

[azmanam]

some thoughts...

3) Le Chatlier.  Remove water (drying agent or Dean Stark trap) to drive to dehydration product.  Flood with water to drive to hydrated product.

4) Doesn't matter.  Enol will tautomerize to carbonyl compound.  But initial addition of water is unselective (see addition of HBr for analogy)

[Yggdrasil]

1) Why does increasing the substitution of a carbon-carbon double bond increase its stability?  I rationalize this as the electron donating groups cause increased e- density in the pi orbitals forming the double bond.  More e- density between the nuclei--> stronger bond.

I believe this is due to hyper conjugation.

[vhpk]

) Can we generalize halohydrin formation from alkenes (alkene + dihalide + water --> halohydrin) to include general nucleophiles?  For instance can we use alkene + Br2 + HCC- --> adjacent Br and ethynyl groups.

Yes:
C2H4 + Cl2 + H2O -> HOCH2CH2Cl + HCl

6) How does the Hg2+ catalyst function in the hydration of terminal alkynes?  I would expect the mercury complexes one of the pi bonds of the alkene in facilitating addition but can anybody give me any more insight?

It should be Hg(OAc)2. The reaction is carried out in THF, you're right, the first step is to form a carbocation, then H2O with the lone pair of electron on Oxygen atom will attack to C(+), then reduce the product by NaBH4 in alkaline. So, can you write the mechanism 

[macman104]

) Can we generalize halohydrin formation from alkenes (alkene + dihalide + water --> halohydrin) to include general nucleophiles?  For instance can we use alkene + Br2 + HCC- --> adjacent Br and ethynyl groups.

Yes:
C2H4 + Cl2 + H2O -> HOCH2CH2Cl + HCl

I don't think that's what the OP meant.  I believe they meant to have the HCC^- attack similar to the way the H2O does.  To the OP yes, in theory you could generalize like that.  The mechanism would be the exact same, except you'd have the HCC^- attacking instead of the H2O

[nextpauling]

Yggdrasil...so the C-H bonds of the adjacent R groups can overlap with the p orbitals of the double bond?  I would visualize this as the electrons in the pi bond becoming instantaneously localized on one carbon of the double bond while the adjacent C-H bond can hyperconjugate with the electron deficient carbon of the double bond.  The net result would be a kind of electron delocalization among the electrons in the adjacent C-H bond and those in the pi bond orbitals...i think that sounds rational enough, thanks. 

azmanam...that makes sense...the relative abundance of water should help in determining equilibrium composition. 

Thank you all for your responses.