[orgo814]

I have a question regarding assigning oxidation #s from lewis structures. The molecule I'm dealing with is C3H7Cl.

I drew out it's skeleton... which was simple enough obviously. I know Cl would take one electron giving it the oxidation # -1. I believe the hydrogens will take one electron giving them -1 and since the carbon must share with its equally electronegative carbon neighbor it should receive back an electron after giving one up giving it +3. Well... that was my thinking anyway. But when I add all the oxidation #s up I don't get 0 so something is wrong here. What am I doing wrong?

[Hunter2]

If you deal with CH₃CH₂CH₂Cl then Cl will have -1 , all Hydrogen +1. C-C bonds get 0.  So the CH₃ will have then -3, the middle CH₂ -2 and the last CH₂ -1.

If you wouild have (CH₃)₂CHCl then for Chlorine and hydrogen the same as above. Both Methyl-groups have -3 and the middle C has 0.

[orgo814]

That cleared it up a lot.

However, I have one more question. How do you know whether the hydrogen is going to be +1 or -1. I know the usual case is +1 however I saw -1 a few times come up in homework problems. HCCl3 is an example.

[Hunter2]

Hydrogen is normaly +1. Only in metal hydrides like NaH, CaH₂ it gets -1.

In CHCl₃  H is +1, Cl^- -1 and C +2. In organic compounds H has everytime +1.

[orgo814]

Yeah, that's what I thought but HCCl3 had the hydrogen having a -1 in my textbook answers. Cl each would be -1, H -1, and Carbon would be +4. When adding these all up, you get 0 so it makes sense but I still don't get why the hydrogen in this case would be -1?

[Hunter2]

I would say its error in the text book. Hydrogen and carbon electronegative values gave every time H +1 and C -value.

[orgo814]

I think you are probably right

[orgo814]

Why would it be -1 when bonded to Si though? Si isn't a metal

[Hunter2]

Silicon stands in middle and silanes are hydrides.