# Hindered carbon atom [closed]

In the hydration reaction that is addition of H2O , in the presence of (H.B.O/T.H.F/H2O2), where no rearrangement takes place, Why does H is placed on more hindered C atom and OH on less hindered C atom ?

• H is smaller and less cumbersome than OH. – Maurice Feb 10 at 9:25
• And what chemical exactly is "HBO"? – orthocresol Feb 10 at 10:20

1)Electronic consideration: When the borane ($$\ce{BH_3}$$) approaches the $$\pi$$ bond of the alkene, the attack of the $$\pi$$ bond to the empty p orbital of the boron triggers a hyride shift (the mechanism of this is similar to that of pericyclic reactions). There is a partial positive charge developed on the vinylic position of the alkene that triggers the hydride shift.
1. Steric considerations: In the first step of the mechanism of hydroboration, we add $$\ce{BH_2}$$ and $$\ce{H}$$ simultaneously. As the $$\ce{BH_2}$$ is bigger in size than $$\ce{H}$$, the transition state of this step would be less in energy and also less crowded if the $$\ce{BH_2}$$ is positioned at the less steric position.
Both factors suggest that the mechanism must follow anti-markovnikov addition. After the attach of $$\ce{BH_2}$$, the further mechanism will replace the $$\ce{BH_2}$$ with $$\ce{OH}$$. Here I showed why the $$\ce{H}$$ attaches to the more crowded site and follows anti-Markovnikov addition.