Ī corollary of the fact that the conjugate base is more stable is that thiolates are weaker bases than alkoxides. I can’t find a pK a value for propaneselenol but would estimate it to be about 7-8. The same holds true for selenium, which has an even larger atomic radius.
Greater volume = more diffuse charge = greater stability.Note the greater pK a of the molecule pictured above (about 10-11). In the case of sulfur, the negative charge will be borne on a larger atom (sulfur > oxygen) and that negative charge dispersed over a greater volume. Recall that acidity will be increased by any factor which stabilizes the conjugate base. When understanding acidity trends it greatly helps to think about the stability of the conjugate base Remember that pK a is logarithmic, so that means they’re about 10 5 times more acidic. Thiols are more acidic than alcohols by an average of about 5 pK a units or so ( pK a of about 11 for the thiol pictured below). We’ve seen that alcohols are relatively acidic (pK a‘s of about 16-17). Those stories were not uncommon from the people in that lab.īTW: the sulfur chemists’ best friend is bleach, which oxidizes stinky thiols to relatively odourless sulfoxides (see #5, below). Imagine getting on the city bus and having the people next to you get up from their seats and move down to the other end. These were not people you wanted to be downwind from. Thiols contribute to the smell of skunks and “skunky” beer.ĭuring my time in grad school one of the world’s leading organosulfur chemists had a lab across the hall. Hydrogen sulfide (H 2S) is responsible for the smell of rotten eggs and bad breath, among other unpleasant things. We wouldn’t want to break with this honourable tradition here at MOC. Open a textbook that discusses thiols and one of the first things they’ll mention is their vile stench. Let’s start with what’s similar between alcohols/thiols, ethers/thioethers, and then move to what’s vastly different. We won’t really talk about selenium chemistry beyond briefly mentioning its acidity, and we definitely won’t go anywhere near tellurium chemistry, following Wöhler’s advice on diethyl tellurium that “ its obnoxious and persistent smell is connected to unpleasantness, which one wouldn’t like to endure a second time“, and that “ the smell is so persistent, that one has to avoid social life for several months in order not to molest other people“. Great question! That’s the topic of today’s post, specifically thiols and thioethers. Specifically, how does the chemistry of alcohols (ROH) and ethers (ROR) compare with the chemistry of thiols (RSH) and thioethers (RSR)?. So how does what we’ve learned about the reactivity of oxygen (O) based functional groups translate to its heavier cousins sulfur (S), selenium (Se) and tellurium (Te)? Noble gases (He, Ne, Ar, Kr…) tend to be chemically inert.Halogens (F, Cl, Br) all gain an electron relatively easily to form halide ions.Alkali metals (Li, Na, K…) all shed an electron relatively easily to form monovalent cations.One of the most powerful insights from learning the periodic table is the fact elements in the same period (column) have similar chemical properties. (Advanced) References and Further Reading.Key Differences Between Thiols and Alcohols.Common Reactions of RS(–) : SN2 Reactions With Alkyl Halides.Sulfur: Less Basic, More Nucleophilic Than Oxygen.Thiols can undergo oxidation to give disulfides.Their conjugate bases, “thiolates” are excellent nucleophiles.Thiols tend to be considerably more acidic than alcohols since the sulfur atom is more polarizable.Like alcohols, they can be deprotonated with base and undergo S N2 reactions with alkyl halides to give thioethers (sulfides).If you can get beyond their foul smells, thiols have a lot of similar characteristics to alcohols! Thiols and Thioethers: Properties and Key Reactions
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