[Destiny]

I know that on acid catalysed hydration of pent-2-ene two products can be formed i.e pentan-2-ol and pentan-3-ol but which one will be the major product? 

THANKS
 

[AWK]

http://www.personal.psu.edu/the1/addregi.htm

[orgopete]

This is a good question and I invite someone to provide experimental data. I do not have access to chemistry searching tools so it is difficult to find a good answer. Some of the answers I have found would suggest 3-pentanol should be the greater product, though I cannot estimate the magnitude.

Chlorosulfonation is 80/20 2-pentenesulfonic acid to 3- (http://pubs.acs.org/doi/abs/10.1021/ja01170a022)
Addition of HBr is excess of 3-bromo (University Chemistry, Volume 1, Murthy)

This second reference cites competing theories of inductive vs hyperconjugation (my interest).

I would believe that 3-pentanol would be the major product. I further suspect the ratio of the addition to be subject to rate by which the carbocation intermediate is trapped. A rapid quench would reduce the selectivity and give more equal ratios. I would suggest this is a factor in the addition/rearrangement of 3-methyl-2-butene with HCl. I think the tertiary carbocation should be sufficiently more stable to give a similar ratio as the addition to 2-methyl-2-butene. However, if chloride traps the intermediate secondary carbocation, possible rearrangement to the tertiary carbocation is stopped. I think that is the reason the chlorosulfonation reaction was more selective. The elimination of a proton should occur much more slowly and more likely to capture the more stable carbocation intermediate.

I am not suggesting there should be a great difference in the ratio and stability. I think there is a difference, but it may be difficult to infer from experimental data.

[Dan]

This is a good question and I invite someone to provide experimental data.

The only thing I can find is this: http://pubs.acs.org/doi/abs/10.1021/ja01324a048

Reaction of pent-2-ene with 80% sulfuric acid followed by steam distillation gives an approx. 35:65 ratio favouring 2-pentanol.

No explanation regarding the regioselectivity is provided in the paper.

[kirushanth]

Reaction of pent-2-ene with 80% sulfuric acid followed by steam distillation gives an approx. 35:65 ratio favouring 2-pentanol.

Consider the transition states of the formations of 2-pentanol and 3-pentanol. We get C-2 and C-3 to be carbocations in the formation of the two products respectively. Also, the carbocation formation is the rate determining step. It is presumable to say that the symmetric carbocation(that is, the intermediate with C-3 being cation) is more stable(due to symmetry) than asymmetric carbocation. From this, one can expect that 3-pentanol to be more predominant than 2-pentanol. What can be the possible drawback in this explanation?

[zoork34]

Reaction of pent-2-ene with 80% sulfuric acid followed by steam distillation gives an approx. 35:65 ratio favouring 2-pentanol.

Consider the transition states of the formations of 2-pentanol and 3-pentanol. We get C-2 and C-3 to be carbocations in the formation of the two products respectively. Also, the carbocation formation is the rate determining step. It is presumable to say that the symmetric carbocation(that is, the intermediate with C-3 being cation) is more stable(due to symmetry) than asymmetric carbocation. From this, one can expect that 3-pentanol to be more predominant than 2-pentanol. What can be the possible drawback in this explanation?

Make sure to use correct terminology here.  The carbocations are actually intermediates, and not transition states (this is an important distinction).  Also, I fail to see how symmetry accounts for more stability. 

[zoork34]

This is a good question and I invite someone to provide experimental data.

The only thing I can find is this: http://pubs.acs.org/doi/abs/10.1021/ja01324a048

Reaction of pent-2-ene with 80% sulfuric acid followed by steam distillation gives an approx. 35:65 ratio favouring 2-pentanol.

No explanation regarding the regioselectivity is provided in the paper.

I would (attempt) to explain this with a hyperconjucation argument.  In the 2-position, the carbocation intermediate has 5 carbon-hydrogen sigma bonds with which to undergo hyperconjugation, but in the 3-position, it only has 4 of these sigma bonds for hyperconjugation.  This could explain the excess of 2-pentanol. 

[AWK]

Link to JACS given by Dan concerns addition of sulphuric acid, not water - to pentene!
In link given by me the author wrote (no reference given) about approx. 1:1 ratio.

[Dan]

Link to JACS given by Dan concerns addition of sulphuric acid, not water - to pentene!
In link given by me the author wrote (no reference given) about approx. 1:1 ratio.

...and that link is for addition of HBr, not water.

[orgopete]

Addition of HBr is excess of 3-bromo (University Chemistry, Volume 1, Murthy)

I erred in reporting this, "2-bromopentane is formed slightly more than 3-bromopentane." This should agree with the results reported by Dan. Sorry about that.