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Green Synthesis

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Another thing to consider when we’re designing a synthesis is how green is it how environmentally friendly is our chemistry now what are some of the things that we would want to think about when we’re designing something that is going to be a green synthesis well we would like it to be green for the environment and then there’s also times when we want to be green

For ourselves or a little bit safer for us so let’s consider how we could use less toxic reagents or solvents and here’s something that we could uh just an example that we could think of here if we’re going to do an electrophilic addition i’ll just put some substitution off of this benzene ring so we’ll choose a styrene some kind of styrene derivative and we

Would like to transform this into the alcohol and we’ll go ahead and do markovnikov addition on this because we have a few options available then there’s a couple of ways that we could do this we could use oxymercuration demercuration but that is going to involve using a mercury reagent and as you may know mercury reagents or just mercury in general is not very

Clean it’s it’s toxic right we’re also going to need to use additional steps to remove the mercury we’ll have to do something with it after we’re done with properly dispose of this it’s just not very green now there are times when we’d like to use it especially when there’s a possibility of a rearrangement and it could still be a good option especially if it

Avoids us having to do another reaction or produces a much better yield but it’s not the best green option that we have now in this case there there are no rearrangement options available and so it would be just much better to use just water and sulfuric acid those are much greener than mercury now there’s no rearrangement in this case and so we should explore

Using that and hopefully the yields are pretty good now let’s look at another example let’s add bromine to this double bond now if we do this using our standard chemistry here we could do a bromine and carbon tetrachloride but carbon tetrachloride is not a very nice solvent i mean it’s a great solvent but it’s not a very it is toxic in nature so we can have

Carbon tetrachloride and this will work but there are other options and one of those options is to use sodium bromide with this sodium bromate and you use a five to one ratio of these two reagents and you can run this in acetic acid now acetic acid is a very green solvent in fact it’s possible to consume it it’s in vinegar i would suggest diluting it before you

Would do that but you can notice just how much greener it is to have something that is really a natural product now so far we’re looking at things that are better for the environment and in a lot of cases better for us right it’s better to avoid these toxic solvents we should also consider how safe a reagent is so we could make a methyl ester using the let’s

Finish writing this out we can make our methyl ester and there’s a couple ways we could do that we could use an explosive compound right here or c h2 n2 uh diazomethane uh and like i say this is explosive so that’s not the most desirable compound now the only side product is nitrogen and so that sounds very good but working with an explosive may make us think

Is there another way we could do this and i know everybody thinks that chemistry is about explosions but it’s really nice when things don’t blow up in an uncontrolled way what we could do instead is place this in methanol and use some sulfuric acid as a catalyst and affect this transformation now you may not get as high of a yield but it is significantly safer

Now let’s also consider what other reagents or what other compounds we’re making on the course of our reactions so i mentioned that here that we are only making nitrogen that’s a very environmentally friendly byproduct but it would be nice to avoid byproducts as much as possible i mean in this case we’re only making water as a byproduct so that’s pretty good

Too let’s look at this epoxidation now if we want to epoxide or form an epoxide from this we’ll just use our mcpba now mcpba is a rather large molecule and it really only adds one oxygen to our product right to our reactant adds one oxygen there and what we’re left with is the meta benzoic acid metachlorobenzoic acid i forgot the chlorine in there so we’ve

Got this rather large byproduct here just add the one oxygen and again that might be the best way of doing this but we’ll notice that we’re throwing away quite a bit of this or we have to do something with this byproduct and this brings up an important concept atom economy right it would be greener if our atoms end up in our product in fact by that definition

This reaction right up here has 100 atom economy because we’re taking bromine and we’re adding it over this double bond and all of our reactants end up in the product down here we have a much less of our reactants ending up in the product so the formula for looking at this is the mass of the atoms in the product divided by the mass of the atoms and all of the

Reagents so essentially this is just asking how much of the reagents that we’re using ends up in the product and of course we’ll multiply by 100 percent if we do this uh for this particular example you’ll notice that we have a mass of about 86 grams per mole for this this is about 70 grams per mole and our mcpba has a mass of let’s see here i only wrote down the

Total mass well the total mass of my reagents 70 plus i’m forgetting what the mass of mcba is is 242.6 that’s the 70 plus this and then if we take our 86 that is our product mass multiply by 100 we’ll see that our atom economy is only 35.4 percent which is pretty low uh because this weighs a lot and it weighs much more than my product and so i’m i don’t have the

Best atom economy with this and sometimes it’s just unavoidable and you’ll notice that this might lead to other choices like if we choose a leaving group that is a triflate or a tosylate those are excellent leaving groups but they’re rather large they contain quite a bit of atoms and and they’re really big and while that makes it an excellent leaving group it’s

Going to lower your atom economy because fewer of those atoms end up in your product according to this formula as well if our leaving group is say an iodine versus a bromine the bromine would have better atom economy even though it’s just one atom just because the iodine weighs more i think that that distinction is a little less useful in those cases because it

Is only one atom and we just have that one leaving group there and so just because it weighs a little bit more doesn’t necessarily mean it’s bad so that’s where some of this can break down a little bit in this case though i think it is quite valid just because of the large by-product that we’re making here so these are some of the things that we would consider for a green synthesis

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Green Synthesis By Chemistry university