[Big-Daddy]

Is there a straightforward, reliable way of being given a reaction (reactants and products, no intermediates) and deducing whether the reactants underwent: a) hydrolysis; b) dehydration; c) oxidation; d) reduction; e) reduction with addition of an alkyl nucleophile; or f) condensation?

I know the basic definitions of each, but nonetheless it is often not obvious to me which change has occurred. Is there any reliable way to look at the structures and find out?

[Dan]

Can you post some specific examples? If you know the definitions it should be fairly straightforward to categorise the reaction by analysis of what changes have occurred.

[Big-Daddy]

Can you post some specific examples? If you know the definitions it should be fairly straightforward to categorise the reaction by analysis of what changes have occurred.

Hmm the definitions I have are the ones I've been told but I'm not entirely sure how comprehensive they are - that's the problem with using them.

For example, oxidation is "gain of O or loss of H" (in organic terms), including (I am led to think), forming new bonds to O (e.g. making a single bond a double bond). But I have an example where a N atom attached by a single bond to a C atom is then attached by a triple bond to that C atom, and the same for an S atom (it goes from single-bonded to double-bonded) - and this apparently is also oxidation. What about if the molecule gains a halogen atom - is that oxidation? These issues confuse me.

From all the examples I have for reduction I might be able to come up with a universal trend for reduction: simply put, you will gain 1 or more H atoms during reduction to your overall molecule. The general definition also says "loss of O" is reduction, but in organic terms every example I look at seems to involve H atoms being added (often accompanied with dropping from a triple or double bond to a highly electronegative element, e.g. O, S or N, to a double or single bond). Meanwhile, reduction with addition of an alkyl nucleophile is the same, but instead of H atoms being added, I look for signs that the bonds to highly electronegative elements have dropped in number and that a new carbon chain is present substituted on the C which had been (maybe still is) attached to the highly electronegative element(s). Is this correct? If so, then that means d) and e) are dealt with.

Hydrolysis, dehydration and condensation are the major concerns since I don't know how to spot them generally. I could try providing some examples but I'd rather not have example specific answers (I can get the specific answers myself, it's the principles I'm after). But also in the case of oxidation, as you can see above, I'm unclear.

Thanks.

[Babcock_Hall]

One definition of oxidation of carbon is that it is an increase in the number of bonds to an element that is greater in electronegativity (O, N, Cl, etc.) or a decrease in the number of bonds to an element that is lesser in electronegativity (principally H or Si).

[Big-Daddy]

One definition of oxidation of carbon is that it is an increase in the number of bonds to an element that is greater in electronegativity (O, N, Cl, etc.) or a decrease in the number of bonds to an element that is lesser in electronegativity (principally H or Si).

Thanks, this helps a lot! The problem is then you could describe elimination as reduction ...(Since, e.g. if the leaving group is a halide X, you are reducing the number of bonds to an element that is greater in electronegativity.)

[Big-Daddy]

Perhaps, for it to be oxidation, you must have both an increase in the number of bonds to more electronegative elements and a decrease in the number of bonds to less electronegative elements, in the same molecule, for it to be oxidation? Whereas for reduction you must have both an increase in the number of bonds to less electronegative elements and a decrease in the number of bonds to more electronegative elements?

(And then we might say, for reduction with an alkyl nucleophile, that you have a decrease in the number of bonds to more electronegative elements accompanied by the addition of a new alkyl chain where previously we would have been adding just an H.)

Does this work?

Edit: Does it count as oxidation to go from, say, C=N to C=O? What about C≡N to C=O (this would fit in fact my definition of reduction, assuming the extra bond is filled in by something less electronegative than C, e.g. H)? And does the reverse of these count as reduction? Because these don't really fit the definition I just wrote up for oxidation and reduction ...

[Babcock_Hall]

One definition of oxidation of carbon is that it is an increase in the number of bonds to an element that is greater in electronegativity (O, N, Cl, etc.) or a decrease in the number of bonds to an element that is lesser in electronegativity (principally H or Si).

Thanks, this helps a lot! The problem is then you could describe elimination as reduction ...(Since, e.g. if the leaving group is a halide X, you are reducing the number of bonds to an element that is greater in electronegativity.)

Ah, that is an interesting question.  If H-X gets eliminated, then one carbon is oxidized and the other is reduced.  Therefore there is no net oxidation or reduction.  With respect to one of your other questions, I would not say a change from a C-N to a C-O bond is a net oxidation, because the more electronegative atom "owns" the electrons completely in either case.

[Big-Daddy]

One definition of oxidation of carbon is that it is an increase in the number of bonds to an element that is greater in electronegativity (O, N, Cl, etc.) or a decrease in the number of bonds to an element that is lesser in electronegativity (principally H or Si).

Thanks, this helps a lot! The problem is then you could describe elimination as reduction ...(Since, e.g. if the leaving group is a halide X, you are reducing the number of bonds to an element that is greater in electronegativity.)

Ah, that is an interesting question.  If H-X gets eliminated, then one carbon is oxidized and the other is reduced.  Therefore there is no net oxidation or reduction.  With respect to one of your other questions, I would not say a change from a C-N to a C-O bond is a net oxidation, because the more electronegative atom "owns" the electrons completely in either case.

I nevertheless find that in every example I can check, an oxidation involves increasing the number of bonds to a more electronegative element and decreasing the number of bonds to a less electronegative element (particularly as you confirm that C=N to C=O is not oxidation, and the reverse not reduction), and a reduction involves reducing the number of bonds to a more electronegative element and increasing the number of bonds to less electronegative elements. I'm glad to hear that whether the more electronegative element is N, O or F - or the less electronegative element is H or Li - would not make a difference to the classification. As for alkyl nucleophile reduction, it's simply the same as previous reduction, except here the "less electronegative element" is in fact an alkyl chain.

Could we move on to hydrolysis/dehydration?

[Babcock_Hall]

Yes, but some of the same issues will show up.

[Big-Daddy]

Yes, but some of the same issues will show up.

Perhaps if we work with a couple of examples that will be best.

Often the products of hydrolysis contain 2 new H atoms and 1 new O atom. If I can see that this has happened and a functional group has been changed by the addition of 1 new O atom and 2 new H atoms (if all the products are shown, I should be able to tell even if the molecule was broken up) and could thus recognize the case of hydrolysis. But it is far too much to expect that I will see these exact new atoms.

Looking at this:

 

Apparently 2 O atoms and 1 H have been added on. By my definition of reduction, the number of bonds to more electronegative elements has stayed the same (3 in reactants and 3 in products), whereas there is 1 bond to less electronegative elements (H), but this does not qualify as reduction overall. Nonetheless how was I to deduce this was hydrolysis?

Another case is this:

 

(methyl 1,2-dichloropropanoate becomes 1,2-dichloropropanoic acid)

Because we have not decreased the number of bonds to more electronegative elements we cannot call this reduction (even though we have increased the number of bonds to less electronegative elements, with a new H). But how does inspection suggest this is hydrolysis?

[Dan]

Balance the equations and you will see these are hydrolyses:

RCN + 2H₂O RCO₂H + NH₃

RCO₂Me + H₂O RCO₂H + MeOH

It is important to look at all the reagents in the reaction to get the full picture of what is going on.

[Big-Daddy]

Balance the equations and you will see these are hydrolyses:

RCN + 2H₂O RCO₂H + NH₃

RCO₂Me + H₂O RCO₂H + MeOH

It is important to look at all the reagents in the reaction to get the full picture of what is going on.

But we were not given the products other than the organic one? Particularly in the second case, I'd have a hard time suggesting the other product was CH3OH without first concluding this was hydrolysis (which is backward thinking).

Perhaps a reasonable generalization for hydrolysis is that, between all products, the number of bonds to more electronegative elements (than C) must stay the same and the number of bonds to less electronegative elements (than C) must also stay the same? I observe this is the case in both of the examples I just gave. This must also be the same for dehydration ... or not?