hydroboration-ox question

[wall1two]

---

Members don't see this ad.

in exam crackers orgo, they ask:

3,4-dimethyl-3pentene is hydroboration oxidized, what is the product?

i know it's anti mark, and both sides of the double bond are equally substituted right? my answer is 3,4 di methyl, 3 or 4 pentanol. the book's answer is 3,4 dimethyl-2-pentanol, meaning there was a rearrangement (the double bond goes from 3-4 in the original molecule right?) chad says HB ox never rearranges. so basically my question is where did my logic break down? and no they don't provide a picture so i dont know is thier double bond location or the naming is incorrect.

 

[Sally0627]

well first of all, the naming of the reactant is not right. It should be 2,3 dimethyl 2-pentene. So the hydroboration is anti mark and product should be 2,3 dimethyl 2 or 3 pentanol. So in that case, I think you are right. I don't think it will go through rearrangement. It only goes under rearrangement if its a terminal triple bond and then the product would be a tautomer.

 

[PreDent11]

in exam crackers orgo, they ask:

3,4-dimethyl-3pentene is hydroboration oxidized, what is the product?

i know it's anti mark, and both sides of the double bond are equally substituted right? my answer is 3,4 di methyl, 3 or 4 pentanol. the book's answer is 3,4 dimethyl-2-pentanol, meaning there was a rearrangement (the double bond goes from 3-4 in the original molecule right?) chad says HB ox never rearranges. so basically my question is where did my logic break down? and no they don't provide a picture so i dont know is thier double bond location or the naming is incorrect.

It has to be that their naming is incorrect. First of all, no compound should ever be named as 3,4-dimethyl-3-pentene because you could get lower numbers if you started counting from the other side. It should be 2,3-dimethyl-2-pentene. As for the answer, it's correct that the -OH group will be on the 2nd carbon (for anti mark), but the methyls are still named wrong. When I did it, I got 2,3 dimethyl-2-pentanol. You're correct, there is no rearrangement in hydroboration because it proceeds through a transition state, not a carbocation intermediate. Therefore, there's no way the -OH group could be on a separate carbon from both methyl groups, so it must be their naming. Hope this makes sense.