Assigning ox numbers

[Sammy1024]

I was wondering why the OH is +1 and the H is -1. Shouldn't it be the opposite for the figure on the left? and why is the =O in the right figure +2?

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[justadream]

@Sammy1024

I'm pretty sure you are approaching this incorrectly.

In the both figures you are looking at the middle carbon. Look at what that carbon is bonded to and compare the electronegativities.

1) If the middle carbon is bonded to another carbon, the charge is 0.
2) If the middle carbon is bonded to something MORE electronegative (like an oxygen), the middle carbon get's a "+"
3) If the middle carbon is bonded to something LESS electronegative (like a hydrogen), the middle carbon gets a "-"

So looking at the left figure:
The middle carbon is bonded to 2 carbons (providing middle carbon with charge = 0), a hydrogen (providing middle carbon with charge = -1), and an oxygen (providing middle carbon with charge = +1)

Right figure:
The middle carbon is bonded to 2 carbons (providing middle carbon with charge = 0) and two oxygens (each providing middle carbon with +1 charge).

You can do the same analysis for the Oxygen in the left figure.

Oxygen is bonded to carbon (providing oxygen with a negative charge), and to a hydrogen (also providing oxygen with a negative charge). Both carbon and hydrogen provide oxygen with negative charges since O is more electronegative than both.

 

[Sammy1024]

Why is it that on the left the oxygen is -1 but on the right each bond is providing +1?

 

[justadream]

@Sammy1024

Those are not the charges for OXYGEN.

The charges provided in the figure are for calculating the charge of the middle carbon.

In both figures, the charge on oxygen is -2.

From my first post:

You can do the same analysis for the Oxygen in the left figure.

Oxygen is bonded to carbon (providing oxygen with a negative charge), and to a hydrogen (also providing oxygen with a negative charge). Both carbon and hydrogen provide oxygen with negative charges since O is more electronegative than both.

 

[Sammy1024]

I thought to find the ox # of the carbon, you assign the other elements first, so H = +1, O = -2 and then after you add them up, you get the oxidation number of C since the overall molecule is neutral?

 

[justadream]

@Sammy1024

That H = +1 and O = -2 are generalizations based on the method I outlined in my first response.

Oxygen is usually -2 and H is usually +1.

But look at O2. Here, oxygen is 0.

Or, look at H2O2, Here, oxygen is -1.

If you use the method I outlined in my first response, you can deal with any type of molecule (provided you can draw the structure).

 

[Sammy1024]

So, basically in your method, you divide each thing into its own category. C - C, C- C, C - O, C - H, and so the C-C bonds are 0, C-O is +1, C-H = -1 (since it is less EN).

Then in the second one the bonds are: C-C, C-C, C-O, C-O and so same as before, C-C is 0, C-O is more EN so +1 and since there's 2 of them it's +1 x 2?

 

[justadream]

@Sammy1024

Yes, assuming that you are looking at these bonds from the perspective of the carbon. A C-O bond would provide C with a +1 but would provide the O with a -1.

This method is outlined in TBR somewhere. That's where I picked it up.

Btw: I realized I made a mistake in my first post. That's probably why you're confused. See the bolded for where I changed.

 

[Sammy1024]

Oh haha! Thanks so much! I did get this picture from TBR, just didn't realize they were using their own method!

 

[Teleologist]

@Sammy1024

That H = +1 and O = -2 are generalizations based on the method I outlined in my first response.

Oxygen is usually -2 and H is usually +1.

But look at O2. Here, oxygen is 0.

Or look at HOF (hypofluorous acid).