Q. Understanding Basic Organic Chemistry Concepts

[ambuj123]

well i have a really simple question about organic chemistry if someone could help me it wil be nice.
Q. why is 2-bromocyclohexanal more easily hydrated as compared to cylohexanal.
Q. Why are 3 Degree alcohols more basic the 1 degree alcohol.
Q. Why we add finely divided platinium in hydrogenation reaction

Thank you
awaiting your reply

 

[chem_tr]

Hello, a quick reply comes as I saw this when I was online.

Well, the first made me to think that a hydrogen bonding is involved in the bromo- derivative; as water molecules must combine with the molecules (if you mean this by hydration, I've also considered hydrogenation, and it doesn't change at all).

The second one might be explained by hyperconjugation; tertiary alcohols have three alkyl groups as compared to the primary ones with single group.

The last one is about kinetic issues; finely divided Pt has more surface area than a bulky one.

Regards

 

[movies]

Chem_tr is right for 2 and 3. I think that 1 is actually an inductive effect though. Since Br has a high electronegativitiy, it causes inverse hyperconjugation by polarizing the C-Br bond. This puts a partial positive charge on the C at the 2 position, which inductively draws electron density out of the bonds and aggravates the already partially positive carbonyl carbon, therefore making it more prone to attack by water.

It may be a hydrogen bonding effect too, as chem_tr said, but halogens aren't very good hydrogen bond acceptors.

 

[chem_tr]

Yes, movies. Your approach seems to be more logical, although mine has something to do in a reduced form.

Halogens are not hydrogen bond acceptors, that's correct, but the acceptor term fits hydrogen better, I think. Bromine is electronegative, thus has a tendency to pull hydrogens to itself more than the environment.

ambuj123, I hope that you are not confused. Movies and I like discussing in a somewhat more detailed form.

 

[movies]

You are absolutely right about H being the acceptor in terms of Lewis acid/Lewis base theory. Certainly Br has some affinity for protons, but I would suspect that it would be outweighed by the hydrogen bonding in the aqueous solvent of the hydration reaction.

 

[maverick280857]

Chem_tr is right for 2 and 3. I think that 1 is actually an inductive effect though. Since Br has a high electronegativitiy, it causes inverse hyperconjugation by polarizing the C-Br bond. This puts a partial positive charge on the C at the 2 position, which inductively draws electron density out of the bonds and aggravates the already partially positive carbonyl carbon, therefore making it more prone to attack by water.

It may be a hydrogen bonding effect too, as chem_tr said, but halogens aren't very good hydrogen bond acceptors.

Yes the inductive effect is responsible for the excess partial positive charge on the carbonyl carbon. The Br through -I polarizes the polarizable carbonyl group further making it more prone to attack by the nucleophilic water molecule as movies has pointed out.

There may be a secondary hydrogen bonding effect too, but that will be less important than the stronger inductive effect due to greater proximity and polarization. For completeness however, you cannot rule out the hydrogen bonding possibility.

Cheers
Vivek

 

[ambuj123]

Thanx all
now my concepts are all clear .
Cheers to physics forums and the users.