In this section, I'm going to teach you possibly the most important reaction of this entire chapter. Are you listening yet? Good, because you need to. This is called the Wittig or Wittig reaction and it's definitely going to be on your test. Let's get going. The Wittig, first of all, you pronounce it also Wittig because it's like German. I say Wittig but if someone says Wittig, don't freak out. This reaction is a special way to make new carbon-carbon bonds between aldehydes and ketones. What it's going to do is it's going to make regiospecific alkenes. What do I mean by regiospecific? Regio just talks about locations. It just means that I can make a custom alkene with whatever R groups on it I want. That's actually pretty helpful because most reactions that make alkenes have to do with elimination, and sometimes the R groups are kind of in different places. In this case, I can pick exactly where I want those R groups to be. In general, I'm going to show you an easy way to do things. I'm also going to show you the whole mechanism, of course. The easy way to think of it is that you have a reaction of a carbonyl and a molecule called an ylide. I'll define that more in a second, so an ylide. When a carbonyl and an ylide come together, obviously it's kind of a weird mechanism but the R groups wind up attaching to each other through an alkene and I get my regiospecific alkene as a product. If you were to get a question on your exam about a Wittig reaction and it's not asking for a mechanism, the answer to this question would be extremely easy because all you have to do is use the box out method. This is something that I this is like a Johnny special, clutch special. What's the box out method? The box out method says if a professor gives you a Ketone or Aldehyde plus an ylide, all you do is you take them and you face them towards each other so that the phosphorus from the ylide and the oxygen from the carbonyl are almost touching. Then what you do is you take your little combination there. You draw a box around the phosphorus and the oxygen. You start scratching out. You squint a little bit. Pretend like that's actually a big alkene in the back and that's your answer. Without a mechanism, without any crazy memorization, look at that. We get our answer which would be an H and an R in this case on one side that came from the carbonyl and then whatever R groups you had on your ylide. Really straightforward, it's one of the funnest reactions to use because it's an easy reaction to learn. However, we know that organic chemistry 2 professors are so mechanistic. They want you to know all the arrows. In case your professor is one of those, let's go through the entire mechanism. The first part is the formation of the ylide. It turns out that the ylide actually comes from an alkyl halide. That alkyl halide is going to react with a molecule called a triphenylphosphine. Tri phenylwhat? 3 benzenes phosphine. A triphenylphosphine has a very nucleophilic lone pair. That nucleophilic lone pair is actually going to be able to do an SN2 backside attack reaction. You would take your alkyl halide and you would use that as your leaving group and you would kick out the halogen. This is just an SN2 mechanism from your Orgo 1 glory days. What you wind up getting is now the X is gone. The phosphorus is there but now the phosphorus has a plus charge because it's not too happy with 4 bonds. It wants 3 bonds and a lone pair like nitrogen. Everyone cool so far? We don't have an ylide yet by the way. This is just the triphenylphosphine doing the SN 2 attack. Now keep in mind guys, everything you learned about SN 2 applies here. It's not going to work on what type of alkyl halide. Do you guys happen to remember this? Guys, if you're gonna learn anything in organic chemistry, know your backside attack. You can neve
21. Aldehydes and Ketones: Nucleophilic Addition
Wittig Reaction
21. Aldehydes and Ketones: Nucleophilic Addition
Wittig Reaction - Online Tutor, Practice Problems & Exam Prep
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concept
Box-Out Method and Full-Mechanism
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Problem
ProblemDetermine the carbonyl and ylide that formed the following product.
A
B
C
D
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PRACTICE PROBLEMS AND ACTIVITIES (6)
- Trimethylphosphine is a stronger nucleophile than triphenylphosphine, but it is rarely used to make ylides. Wh...
- The Wittig reaction is useful for placing double bonds in less stable positions. For example, the following tr...
- Show how you would accomplish the following syntheses efficiently and in good yield. You may use any necessary...
- Show how you would accomplish the following synthetic conversions efficiently and in good yield. You may use a...
- a. What two sets of reagents (each consisting of a carbonyl compound and phosphonium ylide) can be used for th...
- Suggest the appropriate carbonyl and Wittig reagent to make the following alkenes.(a) (E)-7-methylnon-4-en-3-o...