[mmman]

Hello Chemical Forums! Glad to be here. So my textbook does not tell me why alkynes that are not 1-alkynes stop reacting after adding BH₃, but 1-alkynes keep reacting and undergo "mutiple addition reactions", whatever that means. For example 2-butyne will undergo hydroboration-oxidation and form butanone, but 1-butyne will not form butanal (unless you use a branched organoborane instead of BH₃). What's up with this?

[cseil]

The substituents on the alkene with -BH2 (obtained from internal alkynes) prevent the second addition.
The minor steric hindrance of the alkenes obtained by terminal alkynes makes the multiple addition reactions possible.

http://en.wikipedia.org/wiki/Disiamylborane
This is the organoborane used instead of BH3 in reactions with terminal alkynes.