TPR Gen Chem - Passage 80 #5 - Redox/Electrochem

[lovemcat]

QUESTION:
"Respiration in humans often completely oxidizes all metabolites. Carbon dioxide and water are the ultimate oxidized by-products of carbohydrate metabolism. What is one of the ultimate oxidized products of protein metabolism?

A. H2N-(C--O)-NH2
B. NH3
C. C---O
D CH4

*-- = double bond
*--- = triple bond"

ANSWER:
Because the question tells us the metabolism of proteins goes to completion, this implies the carbon atoms which make up most of the mass of a protein will be in their highest oxidation state. The highest oxidation state for carbon is +4 so the answer is the one with the carbon in the highest oxidation state (A).


I DON'T UNDERSTAND:
1) how they got from the question the implication to focus on the oxidation state of carbon (why not oxygen or nitrogen?)

2) how they found the oxidation state for carbon (the oxidation states are +4, +3, 0 respectively for choices a, c, d.)

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[GyroGyro]

Well, in one sense, you don't really need to know anything about oxidation / reduction in this problem. They tell you, when a carbohydrate breaks down, what does it break down into? CO2 and H2O. Now, when a protein breaks down, what does it ultimately break down to? Urea (choice A). I think you should at least be able to rule out C and D as there is no nitrogen in either of those answers.

Thinking of it from an oxidation / reduction perspective, in general oxidation involves adding oxygen atoms to a carbon bond because it will give the carbon that the oxygen is bound to a higher oxidation state whereas adding hydrogen atoms will just do the opposite. I am trying to think of a good way to explain this using oxidation / reduction etc. but I am not finding a good way. Its like I can think of it in my head but I can't write it down for some reason.

 

[paul411]

QUESTION:
A. H2N-(C--O)-NH2
B. NH3
C. C---O
D CH4

*-- = double bond
*--- = triple bond"
...

2) how they found the oxidation state for carbon (the oxidation states are +4, +3, 0 respectively for choices a, c, d.)

The oxidation states you listed cannot be correct.

The carbon in option (D) CH4, methane, is definitely at -4 (not 0) because hydrogen is +1.
The carbon in option (C) C---O is at +2 because O will be at -2 (not +3).

Maybe you looked at the wrong explanation or something?

 

[lovemcat]

@GyroGyro - no worries. i think i understand how to get the answer now. thanks!

@paul411 - nope i checked again and those are the oxidation states the answer book says glad to see its probably just a typo haha thanks!