Question

Use the electronegativity values to rank the following bonds from least polar to most polar: H3C-Li, H3C-K, H3C-F, H3C-MgBr, H3C-OH. I know it's H3C-OH < H3C-MgBr < H3C-Li = H3C-F < H3C-K. I don't understand the OH and MgBr bonds. Why do we calculate it with the carbon and the more electronegative in the OH, and the less electronegative in the MgBr? I don't get it...

Answer 1

First off, draw a lewis structure or even look at a periodic table to see which elements are open for bonding. Oxygen can bond to two, hydrogen can bond to one, so you know that the molecule has to look like this:

It's not that you're choosing the ones that are more electronegative or not to bond the carbon to the oxygen or hydrogen, it's the shape. We're looking at how the Methyl group bonds to the others since it's listed above as H3C-X.

In the case of MgBr you're looking at who is bonding with carbon, not which one is more/less electronegative. You are only using the electronegativities to calculate the bond lengths.

Answer 2

If I wasn't very clear please tell me what I should clarify so that you understand. When it comes to chemistry often so much to know that in order to help someone you have to assume they know some things and not others and hope you guess that you're showing the person what they need to see! XD