In all the reactions i had Seen I pointed out that aldehyde groups of a compound like glucose are easy to oxidize than the hydroxyl groups. I had also read some similar answers but I was unable to find any reason. Can anyone help?

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    $\begingroup$ Can you help us out with a few more details? Which reactions have you seen and which answers are you referring to? What exactly do you mean by "easy"? $\endgroup$ Nov 27 '21 at 20:38
  • $\begingroup$ It is an experimental result. One big drop of benzaldehyde on a ceramics plate gets oxidized simply by contact with air. After one or two minutes, you will see solid needles of benzoic acid at the surface of the drop. Nothing similar is formed with corresponding alcohol. $\endgroup$
    – Maurice
    Nov 27 '21 at 21:30
  • $\begingroup$ Are you sure it is the aldehyde group and not the hemiacetal that is being oxidized? What is the oxidant? $\endgroup$
    – user55119
    Nov 28 '21 at 0:31

Yeah, that's a good question. I like your reasoning. It's sort of like the question, "why is the $\mathrm{p}K_\mathrm{a}$ of a carboxylic acid $(\mathrm{p}K_\mathrm{a} \approx 3)$ lower than a primary alcohol $(\mathrm{p}K_\mathrm{a} \approx 15)$?" The answer is simple. Let's compare the structures of aldehydes, carboxylic acids, and primary alcohols.

Organic Compounds

Notice that and aldehyde and carboxylic acid have a double bond on the oxygen. This is what's called a "carbonyl" and these chemical groups love taking electrons (that's why they're called "electrophiles"). Even as these groups are in a molecule, they like taking electrons and you'll often find more electron density in that bond than in other chemical bonds. What this means is that poor hydrogen on an aldehyde (or that alcohol on the carboxylic acid) has fewer electrons on average because they're all hanging out in the carbonyl double bond! This means that hydrogen in aldehyde is far more reactive because that bond has fewer electrons in it and can therefore be broken easier. This isn't the case for just a simple hydroxyl group (as in your glucose example). A primary or secondary alcohol has all the electrons it wants and isn't going to react as readily as its carboxylic acid counterparts. In short, it's the delocalization of electron density caused by the carbonyl group that makes an aldehyde more reactive.

  • $\begingroup$ If what you say were correct, then anhydrous reagents such as PCC and PDC would oxidize primary alcohols to carboxylic acids. This said, it is more likely that it is the anomeric hydroxyl that is capable of facile oxidation over the other hydroxyls present in glucose. At least in the case of chromium-based oxidants, the rate-limiting step is the breaking of the C-H bond, which should be facilitated by the presence of the ring oxygen. All of this being said, the OP might have offered more details.@user118909 $\endgroup$
    – user55119
    Dec 7 '21 at 19:20
  • $\begingroup$ Correct and I've gone ahead and updated the answer to be more accurate. As far as offering more details, yes, that would've been ideal but since they were discussing glucose, I assumed that what they were referring to wasn't the anomeric carbon but was rather the cyclization of glucose since it is the glucose aldehyde that participates in that reaction. A cyclized glucose (the form of glucose that is far more abundant) doesn't have an aldehyde. $\endgroup$ Dec 7 '21 at 22:48

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