Ketone and Aldehyde Synthesis Reactions

Problem: Provide the major product for the following reaction. 

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Problem: Provide the major product for the following reaction. 

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Concept: Practice 3: Chain Lengthening

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Video Transcript

Hey guys, let's take a look at the following practice question. So here it says provide the chemical steps necessary for the following synthesis. So here these molecules look very similar to each other, they're just slightly different. Now we have to think about what are the changes that occurred? What we're going to say here initially start out with 1, 2, 3 carbon chain but now we have what? We still have have 1, 2, 3 but now we have an additional carbon somehow. An additional carbon, that is an aldehyde carbon. So we have to think to ourselves how in the world do we get this? Now there's a lot of different ways you can approach a problem like this. Now here one way we can do it is we could do this.

We could have this carboxylic acid here and what I choose to do here is I use lithium aluminum hydride on it with the acid catalyst on the bottom. Remember lithium aluminum hydride is a very strong reducing agent. So what would it do? It would reduce this carboxylic acid to a primary alcohol. Now we need to add an additional carbon somehow to our chain. Now one thing we could do here is we could do P B R 3 and remember the whole point of P B R 3, what does it do? It changes O H into a P R. If you wanted to change O H into a C L we'd use S O C L 2 which is a thionyl chloride. So here that becomes a B R because I used P B R 3. Now what I want to do next is I want to somehow add another carbon to my chain. Now what we could do here is next we could use magnesium over ether and who remembers what that does? That would change this alkyl halide into grignard's reagent. And now that we have grignard's reagent what could I do here? I can bring in formaldehyde, formaldehyde is the simplest aldehyde out there and remember we're going to say that a grignard would attack an aldehyde carbonyl carbon in order to form an alcohol. So this bond here would break, this carbonyl carbon kicked this bond up so we would get initially we're going to be connected to that carbon which is this carbon, it's going to be single bonded to an oxygen now that oxygen will be negative, it still has those two hydrogen's it's connected to and the M G B R will be positive and near the oxygen. Then what do we do? We use water to wash it off. Now all you say to yourself is what do we have at this point? All we have at this point is a primary alcohol. We made our chain a little bit longer by one to form a primary alcohol then you just say to yourself do I remember how to change a primary alcohol into an aldehyde? Well remember to change a primary alcohol in to an aldehyde, we would use a weak oxidising agent.

So here we'd say the weak oxidising agent that we used to do this is P C C which is pyridinium chlorochromate. The solvent we usually use on the bottom of this is methylene chloride. This is just a solvent, it doesn't do anything but it's just at the bottom. So now that would oxidize, weakly oxidize your primary alcohol and change it into an aldehyde. So these are the steps that we would do in order to change our initial reactant into this one carbon chain longer aldehyde. So this incorporates a lot of things that we've gone over in this chapter in terms of reduction, in terms of substituting in alcohol for a halogen, grignard synthesis as well as soft oxidation, weak oxidation at the end. So remembering these key steps and piecing them together was key to answering this question correctly.

Problem: Provide the major product for the following reaction dealing with the Grignard reagent.

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