Thiols and sulfides
preparation of sulfides
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- Let's look at how to prepare sulfides from thiols.
- So, over here on the left I have my Thiol
- And to that thiol I'm going to add sodium hydroxide.
- And the sodium hydroxide is going to deprotonate the thiol
- Which is then going to react with this alkyl halide in the second step of the reaction
- to produce this sulfide as my product
- Here is my sulfide right here.
- This reaction is the analogue of the Williamson ether synthesis
- which we've seen in an earlier video
- So, in that video we started out with an alcohol
- and we reacted our alcohol with as strong base in the first step and an alkyl halide in the second step.
- And we formed an ether as our product.
- We can go ahead and draw our ether in like that.
- So, the thiol is the sulfur analogue to an alcohol
- and a sulfide is the sulfur analogue to an ether.
- Let's look at the mechanism to make sulfides...
- So, if I start with my thiol right here.
- I have carbon bonded to sulfur bonded to hydrogen
- and two lone pairs of electrons on that sulfur.
- And if I think about the difference in electro-negativity between carbon and sulfur
- there's actually not much of a difference in terms of numbers.
- So, this is not a very polar bond actually.
- That's different with what we see from an alcohol.
- So, up here if we look at the alcohol, I know that oxygen is a lot more electro-negative than carbon
- This oxygen here would get a partial negative
- and this carbon on the left (R-) would get a partial positive.
- There is much more of an electro-negativity difference in alcohols.
- In thiols there's not really that much of a difference
- But thiols can still function as nucleophiles
- Because these lone pairs of electrons are located further away from the nucleus than the lone pair on the oxygen
- because sulfur is a larger atom.
- Those electrons (on sulfur) are more polarizable, and so thiols are actually excellent nucleophiles.
- If we go back here to our mechanism, we are going to add sodium hydroxide which is a base.
- So, we can go ahead and put OH- over here
- The hydroxide anion is going to function as a base
- and a lone pair of electrons are going to take this proton and leave these protons behind on the sulfur.
- Let's go ahead and draw the conjugate base to the thiol.
- We now have Carbon bonded to sulfur, and sulfur now has 3 lone pairs of electrons.
- Giving it a negative one (-1) formal charge.
- This is called a "Thiolate anion". Let me go ahead and write that.
- A thiolate anion.
- And thiolate anions are very stable.
- That negative charge of the sulfur, since sulfur is a large atom
- You can spread out that negative charge over a very large area.
- So the thiolate anion is relatively stable and that makes thiols more acidic than alcohols.
- Alcohols don't have the same type of stabilization, since alcohols are smaller.
- So, thiols are actually very acidic and that's why we can use sodium hydroxide here
- to deprotonate our thiol to form the thiolate anion.
- In the second step we add our alkyl halide
- Here's my alkyl halide.
- And the alkyl halide does have a polarized bond.
- The difference in electronegativity between a halogen and a carbon atom is fairly large.
- This halogen is going to get a partial negative charge and this carbon is going to get a partial positive charge.
- So, thiols are good nucleophiles.
- Thiolate anions are even better nucleophiles.
- So, the thiolate anion is going to function as a nucleophile.
- The partially positive carbon is going to function as the electrophile
- and we're going to get an SN2 type mechanism.
- Where our strong nucleophile attacks our electrophile
- and kicks these electrons off here, on to the halogen.
- And we can go ahead here and form our product. Right?
- This is an SN2 type mechanism.
- And we end up with the sulfur now bonded to two R groups.
- They could obviously be the same R groups or they could be different R groups.
- And so, we've formed our sulfide like that.
- Let's do an example of the preparation of sulfides.
- We're going to start with this one right here.
- We start with this molecule and to that thiol we're going to add sodium hydroxide in the first step.
- Let's go ahead and write "sodium hydroxide"
- Na+ Then OH- like that
- And in the second step we're going to add an alkyl halide.
- Let's add this as our alkyl halide: Ethyl Bromide.
- When I think about the mechanism, I know the first step is an acid/base reaction.
- The electrons on the hydroxide anion... so one of these electron pairs here...
- Are going to take this proton, that's the acidic proton on my thiol.
- Leaving these electrons behind on the sulfur
- I can go ahead and draw the resulting thiolate anion.
- I can go ahead and draw that. Let me see what we have here.
- We have our ring, and then we have our sulfur, and our sulfur now has three lone pairs of electrons around it
- Like that.
- Thiolate anions are excellent nucleophiles and when I look at my alkyl halide
- once again I know the electronegativity difference between bromine and this carbon
- is going to give bromine a partial negative charge and this carbon's going to be partially positive.
- So, the thiolate anion is going to act as a nucleophile in an SN2 type mechanism
- and a lone pair of electrons here are going to attack my electrophile
- and we can go ahead and form our product.
- Now we have our ring here, which is connected to our sulfur.
- And our sulfur now just picked up two more carbons right?
- Because these electrons here are going to kick off on to the bromine
- and we end up putting an ethyl group on to that sulfur.
- so there are two carbons, now, on the sulfur like that.
- and the sulfur has two lone pairs of electrons
- And so, we've formed our sulfide!
- Now, if I were to name this sulfide, it is a lot like naming ethers.
- I could use the common way of naming this and treat those as alkyl groups.
- If I look on the right this would be an ethyl group
- I could go ahead and start naming it.
- I could say it's "ethyl". And if I look at the alkyl group on the left side, this is a phenyl group.
- So it's ethyl and then phenyl since I'm following the alphabet rule.
- E before P.
- And then finally, I know it's a sulfide.
- So I can just go ahead and finish the nomenclature here by saying, "sulfide" here.
- So, "ethyl phenyl sulfide" is the sulfide produced in this analogue of the Williamson ether synthesis.
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