[b0l0gna]

In an addition reaction involving an acid, why is it that the carbon donates it's electrons to to a hydrogen?

[Dan]

In an addition reaction involving an acid

Please be more specific, there are many different typed of addition reaction. Can you give an example of the type of reaction you are referring to?

[orgopete]

Sometimes these mechanisms are best worked backwards in order to understand what is otherwise implausible. If you were referring to a Markovnikov addition, the electrons of an alkene can seem like an unlikely donor. If you must rationalize how the product formed, then an addition of a proton to a carbon can be seen as a general event with many examples. If thought of in that way, then an addition seems more reasonable. Even so, the addition is probably a kinetic event with low concentrations of the intermediate, consistent with your question, but once formed, even in small concentrations, leads to product.