Thionyl chloride, thiols, and why I hate tutorials

So a week ago, I had an organic chemistry tutorial which kind of ruined my confidence. It's more that whenever I'm in tutorials, I'm always willing to volunteer, but my tutor likes nitpicking my answers heavily to the point I lose faith in what I'm doing. He told us to be confident and try to solve the mechanism for the formation of an acyl chloride from a carboxylic acid, and I volunteered. 

I drew this on the board:

A crime against chemistry

And immediately he stopped me there. He didn't mind that I gave up completely when he said that. And okay, no one's at fault - that's the last thing I'm suggesting - but it did deflate me a bit. 

I suppose my mentality is that I go to uni to learn, to be questioned, and for me to be able to answer those questions to get to the right answer. Being told to stop, and feeling like a laughing stock, isn't what I imagined. But okay, I'll try and understand why what I did was such an affront instead of complaining about my uni for 1,000 words.

The tutor's reasoning as to why I should be stopped was because we were forming a C-S bond. At the time, I was confused why that should matter - don't thiols exist? - but more on those later. 

The simple reason why C-S bonds are bad is because of how diffuse their orbitals are. You might remember from my carbon blogpost that carbon has no low-lying d-orbitals, but sulphur does. So if carbon bonds with sulphur, their orbitals won't overlap the best, and we'd end up with a weak, unstable bond. Safer to not do this.

Instead, this is the mechanism for the reaction - note the nucleophile is actually the oxygen in the C=O bond, whilst the thionyl chloride is the electrophile. I'd claim that if the tutor was like "hey, are you sure that's the right nucleophile?", I'd have got there in the end. Oh well, can't change the past. 

But we can change the future! You see, C-S bonds do exist for one, and we can produce them. Just not in these kinds of reactions. I have no idea why I'm writing up this next part of the blogpost, aside from trying to prove to myself I wasn't that wrong, but here goes.

First of all, it's worth addressing thiols. A thiol is like an alcohol, but replace the oxygen in your -OH group with a sulphur. Congratulations, your functional group is -SH. The only question is how you'd produce one, and the answer is actually kind of similar to my failed mechanism from earlier. However, it's an SN2 reaction, not an addition-elimination reaction like the mechanism above:

We'd source the SH- ion from sodium hydrosulfide (NaSH), and react it with any alkyl halide - I've used a bromoalkane because why not - and we'd end up producing a thiol, alongside a sodium halide. 

We could go one step further and react our thiol with another alkyl halide and NaOH to produce a thioether (a compound with a -S- functional group):

So as you can see, we can form C-S bonds just fine. The only issue would be with how stable those C-S bonds are - indeed, the bond energy of a C-S bond is lower than for a C-O bond (259 kJ/mol compared to 358 kJ/mol at room temperature), which acts as proof that we shouldn't just go forming C-S bonds everywhere - they're far more easily broken.

But that still doesn't answer why my tutor hated my mechanism so much. I suppose the question should be: why can't thionyl chloride act as a nucleophile? The simple reason is that electronically, it's just very electron-poor. Both oxygen and chlorine are more electronegative than sulphur, so the sulphur atom will have a partial positive character. Hence, it has to settle for being an electrophile.

And I suppose now I finally know why I was wrong. That leaves me with a funny conclusion - sure, if the tutor was able to prod me in the right direction, I'd have got the mechanism in the first place. It would be a stumble, yes, but I wouldn't have been Kinskied - as in, brought off after being encouraged to give it a go. Even then, I'd have no clue why I was wrong, instead I might have written this post to tell everyone how cool thiols are. It's much better to actually understand why you're doing something than to just blindly regurgitate something.

And that's also why I hate tutorials. Having someone ask people to do mechanisms, then vaguely discuss why they work or not, isn't helpful in any way to me. I think they're different in Oxbridge, where you're actually forced to reason why something works or doesn't, but I guess I'll never know at this rate. Either way, actually learning something is far more useful for me.