TPRH Finding the oxidation state of phosphorous?

[Padfoot]

On page 313, it asks for the oxidation state of the phosphorous in a phospholipid. The answer was +5, and the explanation is that they "counted the number of bonds that phosphorous has with a more electronegative atom (P has 5 bonds with Oxygens) and subtract from this number of bonds the atom has with a less electronegative atom. 5-0=+5

This really confused me because it's different than the algebraic way I'm used to. O has an oxidation state of -2, so I didn't understand why they didn't multiply the number of oxygens by -2 to figure out P.

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[Odi]

The algebraic way you were taught is useful for small inorganic molecules we deal with in general chemistry.

When you're assigning oxidation states to organic molecules it can get very complicated due to the size of the molecules and varying oxidation states of carbon depending on what it's bonded to. There is a different method involved which is taught in organic chemistry.

This method requires you to look at every bond the atom in question is making. If the bond is made to a more electronegative atom (such as O in this case; you assign a +1 to that bond). If the bond is made to a less electronegative atom (usually a H; you assign a -1 to that bond). This should make sense intuitively as it let's you know where the electrons are spending most of their time within that bond. Sum up all your charges to find the oxidation state. If there is already a formal charge on the atom in question you must include that charge in your summation as well.

 

[Padfoot]

The algebraic way you were taught is useful for small inorganic molecules we deal with in general chemistry.

When you're assigning oxidation states to organic molecules it can get very complicated due to the size of the molecules and varying oxidation states of carbon depending on what it's bonded to. There is a different method involved which is taught in organic chemistry.

This method requires you to look at every bond the atom in question is making. If the bond is made to a more electronegative atom (such as O in this case; you assign a +1 to that bond). If the bond is made to a less electronegative atom (usually a H; you assign a -1 to that bond). This should make sense intuitively as it let's you know where the electrons are spending most of their time within that bond. Sum up all your charges to find the oxidation state. If there is already a formal charge on the atom in question you must include that charge in your summation as well.

Thanks, that cleared a lot up for me. There's a negative 1 on the oxygen that the Phosphorous is attached to, but that doesn't count right?

 

[Odi]

Thanks, that cleared a lot up for me. There's a negative 1 on the oxygen that the Phosphorous is attached to, but that doesn't count right?

Correct it doesn't count. You only look at the atom in question which is phosphorous in this case.