December 07, 2022, 02:08:37 AM
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Topic: Question about (basic) oxidation and reduction as seen in organic chemistry  (Read 266 times)

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Offline ajkjmspe

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Hello fellow forum members:

I have a (very basic) question about oxidation and reduction in organic chemistry that is stumping me.

Examining the reaction in the attached "Molecule 1.jpg" image file, the following seems to occur between the starting material and the product:
1) At the left carbon of the triple bond, two hydrogens are added
1a) This creates two new C-H bonds
2) At the right carbon of the triple bond, one hydrogen is removed and two oxygens are added with one on a double bond
2a) This removes one C-H bond and creates three new C-O bonds (counting the C=O double bond as two C-O single bonds)

The starting definitions for oxidation and reduction in organic chemistry presented in my textbook are as follows:
  • Oxidation results in an increase in the number of C-Z bonds or a decrease in the number of C-H bonds
  • Reduction results in a decrease in the number of C-Z bonds or an increase in the number of C-H bonds
However, this molecule is undergoing both an increase in the number of C-Z (C-O) bonds and an increase in the number of C-H bonds.

My textbook states this is an oxidation reaction with a net increase of two C-H bonds, and here is where I am confused on multiple levels.

Firstly, one C-H bond was removed from the triple bond at the terminal end of the triple bond while two C-H bonds were added at the internal end of the triple bond.  (There is one hydrogen bonded to oxygen in the product, but no mechanism is shown so I am not sure if it is the same hydrogen that was originally bonded to carbon or supplied by an unmentioned reagent, and in any case, O-H bonds are irrelevant to oxidation-reduction as per the basic definition above.)

However, my textbook specifies that there was a net increase of two new C-H bonds in this reaction.  Is it standard for these types of questions to ignore the loss of the one C-H bond at the terminal end of the triple bond?  If not, should not there only be one net new C-H bond in the product if we count both gains of the two and losses of the one?

Even if we consider the C-H bond at the terminal end of the triple bond to have migrated to the internal end of the triple bond, should not there only be one net new C-H bond?

Secondly, is the reaction classified as an oxidation reaction because there is a NET increase of C-O bonds?  (I.e., according to my textbook there are three new C-O bonds created and two new C-H bonds created.)  There is one more new C-O bond than there are new C-H bonds, and therefore, is it correct that this results in a net increase in the number of C-Z bonds?

Here is where I would especially like some clarification as I am almost certain I have been presented with similar situations with dramatically different results.

I am fairly sure I have been asked a similar question regarding a molecule that has both gained C-Z and C-H bonds and marked as correct in answering that the reaction was "neither" oxidation nor reduction as oxidation requires there to be NO increase in the number of C-H bonds and reduction requires there to be NO increase in the number of C-O bonds.

In other words, I would have previously classified this reaction as "neither" oxidation nor reduction because there was an increase in both the number of C-H and C-O bonds and be marked correct.

Yet in this question, my textbook seems to be declaring something different.

Therefore, my second question is to kindly request black-and-white ABSOLUTE clarification on this matter!

Using the above first basic simple starting definition of oxidation and reduction in organic chemistry (increase or decrease in C-Z and C-H bonds), do we declare a reaction to be one, the other, or neither if there is both an increase or a decrease in the number of C-H and C-O bonds with net gains or losses of one more than the other?

For example, in this reaction, there was one more new C-O bond created, so this molecule undergoes oxidation in this reaction (despite the fact that two new C-H bonds were also created).

Aka (rephrasing the above definitions of oxidation and reduction):
  • Oxidation occurs when the number of C-Z bonds increases more than the number of C-H bonds or when the number of C-Z bonds decreases less than the number of C-H bonds
  • Reduction occurs when the number of C-H bonds increases more than the number of C-Z bonds or when the number of C-H bonds decreases less than the number of C-Z bonds
Thank you very much in advance for considering my question.

Offline Hunter2

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Calculate the oxidation numbers of the C atoms.

In the Phenylethyne. The 1. Carbon has Oxidation number 0 and the second one -1.
In the Phenylacetic acid. The carbon with the 2 hydrogen is - 2 and the one with the carboxyl group is + 3.

The change from 0 to -2 is a reduction, decrease of Oxidation number. The other one goes from -1 to +3 is an Oxidation. Increase of Oxidation number.
In summery we have an increase  about 2 electrons at one C and a decrease of  4 electron at the other one, what gives for both C an decrease of 2 electrons, so it is an Oxidation.

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