Determining Where Hydrogen Bonds

[Michelle_Nguyen_3F]

Hello!

In the reaction mechanism example in the course reader, we said that the pi bond between the carbons in CH3CHCH3 was broken and that H--Br bond was broken. How did we know that the the carbon was partially positive and would be the atom to which the hydrogen would bond when a collision occurred? Thank you!

[AlyssaD_2K]

The double bond between the carbon atoms contain a lot of electrons, giving that area a more negative charge. That's why the hydrogen in the HBr molecule is attracted to it, because the H has a partial positive charge and Br has a partial negative charge due to its larger electronegativity. When these molecules collide, H becomes attracted to one of the C atoms, with the electrons in the pi bond of the C-C double bond being used to create a new bond between C and H, and Br takes the electron from the former HBr bond making a Br- ion.

[Jordan Pace 2B]

Is the explanation above the reason why the curved arrows point from the pi-double bond to the H and why another curved arrow points from the single bond to the Br? I'm not completely sure what the arrows represent. Do the arrows just represent bonds being broken and the transfer of electrons?

[Miya McLaughlin 2B]

The arrow doesn't necessarily mean a bond is being broken, it just indicates where the electrons are coming from and where they end up. It's just easy for the electrons to come from bonds because they are more exposed than the electrons within the shells, thus they are easier to remove. The explanation above tells you in detail why the C-C double bond is attracted to the H, and why the electrons from the H-Br bond are attracted to Br.

[Melanie_Wong_1K]

The curved arrows are meant to model which bonds are broken or which lone pairs break off, they show which atom the electrons are moving to. An arrow with a normal head means two electrons move from the bond/lone pair while an arrow with half a head means only one electron moves.

[Vista_Farkhondeh_3O]

So if the electron is coming from the bond, make sure to place the arrow starting at the bond instead of starting at the atom.