6.7 C-Nucleophiles

6.7 C-Nucleophiles

• There are many different kinds of nucleophiles that can attack ketones and aldehydes.
• The first thing we started with was hydrogen nucleophiles.
• We moved on to sulfur and oxygen.
• In the previous section, we talked about nitrogen nucleophiles.

• In this section, we will talk about carbon nucleophiles.
• There are three types of carbon nucleophiles.

• The Grignard reagent is our first carbon nucleophile.
• In the first semester, you may have seen this reagent.

• This reaction works with other halides as well, as an atom of magnesium inserts itself in between the C-Cl bond.
• The magnesium atom has an electronic effect on the carbon atom.

• The carbon atom has poor electron density because of the effects of the halogen.

• magnesium is more negative than carbon.
• A lot of electron density is placed on the carbon atom, making it very d-.

• This reagent is highly reactive because carbon is not very good at stabilizing a negative charge.
• It is a very strong base.
• Let's see what happens when a Grignard reagent attacks a ketone or aldehyde.

• In the previous section, we started each mechanism by turning the ketone into a better phile.
• That doesn't happen here because the Grignard reagent has no problem attacking a carbonyl group.

• The Grignard reagent acts as a base and removes a protons from water to form a more stable hydroxide ion.
• The reaction is irreversible because the negative charge is more stable on an oxygen atom.
• You can't use a Grignard reagent to attack a compound with acidic protons.

• proton transfers are quicker than nucleophilic attacks.
• When the Grignard reagent removes a protons, it irreversibly destroys the Grignard reagent.

• There was a quick review of Grignard reagents.
• Let's see how Grignard can attack a ketone or aldehyde.

• If it can, this intermediate will attempt to re-form the carbonyl group.

• The mechanisms are similar.
• While the other reactions in this chapter were different from these two reactions, it is worth a minute to think about why they are so similar.

• Our golden rule is never to expel H- or C-.
• The carbonyl group will be unable to re-form if we attack a ketone or aldehyde with either H- or C-.
• Both of these reactions have in common.

• When we learned about LiAlH, we had to show the source as a separate step.

• After the Grignard reagent has been consumed by the reaction, the source of the protons must come.

• Let's compare the reaction with LiAlH one more time in order to add it to your synthetic transformations.

• We are reducing the ketone to alcohol in both reactions.

• At the end of this chapter, we will explore synthesis problems.

• The aldehyde is going to react with a Grignard reagent.

• The carbonyl group can't re-form because H- or C- can't be expelled.

• We need to explore two more carbon nucleophiles.
• They are not the same as the Grignard reagent.
• Both reactions involve ylides.

• An ylide is a compound with two oppositely charged atoms.
• The ylide has a high electron density on a carbon atom.
• This ylide can be used as a carbon nucleophile.
• We will see another type of ylide in a few moments.
• The ylide has a special name.
• Wittig reagent is called a Wittig reagent.
• The Wittig reaction is when a ketone or aldehyde is treated with a Wittig reagent.
• Let's look at a mechanism for the Wittig reaction.

• The Wittig reagent reacts with the electrophilic carbonyl group in the first step of the cycloaddition process.
• In the second step, the oxaphosphetane is given alkene as the product.

• This reaction can be used for synthesis.

• Whenever you learn how to interconvert two functional groups, you should always take special notice.

• Several cases like this have been seen so far.

• The reagent is a Wittig reagent.
• This reagent is not the same as the one we saw before.

• To make a Wittig reagent like this, you need to use Et-I instead of Me-I.

• If you want to watch the extra carbon atom coming along for the ride, you should draw out a mechanism for this reaction.

• We will look at another type of ylide.
• If you are responsible for the following reaction, you might want to look through your lecture notes and textbook to see if you covered sulfur ylides.

• The first thing we need to do is form a sulfur ylide.
• The alkyl halide is attacked, followed by deprotonation with a strong base to form a sulfur ylide.

• There is a very different product when a sulfur ylide attacks a ketone.

• The sulfur ylide attacks the carbonyl group, giving an intermediate that undergoes an S 2-type process to give an epoxide.

• The method for making epoxides from ketones provided by this reaction is very useful.

• In this section, we have explored three carbon nucleophiles.
• We started with Grignard reagents and then moved on to ylides.

• The sulfur ylide is used to convert ketones into epoxides.