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Hopefully, the order that we use will help you appreciate the similarity between the reactions.
Before we can start, we need to mention one more feature of carbonyl groups.
Carbonyl groups are very stable.
The process of forming a carbonyl group is downhill in energy.
The formation of a carbonyl group is the driving force for a reaction.
In this chapter, we will use that argument many times, so make sure you are prepared.
The stability of carbonyl groups will be explained in this chapter.
We have seen some important characteristics so far.
There are many different kinds of nucleophiles that can attack the carbon atom.
The carbonyl group is very stable.
A carbonyl group can serve as a driving force.
A carbonyl group can be attacked by a nucleophile, and after a carbonyl group is attacked, it will try to re-form.
NaH is a very strong base, but it is not a strong nucleophile.
This is an excellent example of how basicity and nucleophilicity are different.
The first semester of organic chemistry was the reason for this.
Remember the difference between basicity and nucleophilicity.
Stableity is not the basis for nucleophilicity.
The ability of an atom or molecule to distribute its electron density is called polarizability.
Smaller atoms are less strong than larger ones and therefore less polarizable.
We can understand why H- is a strong base, but not a strong nucleophile.
It's a strong base because hydrogen doesn't stable the charge.
Hydrogen is the smallest atom, and therefore the least polarizable, when we consider the nucleophilicity of H-.
H- is not seen to function as a nucleophile.
There are many reagents that can be used to deliver H-.
If we look at the periodic table, we can see that boron is in Column 3A and has three electrons.
It can form three bonds.
It has a negative formal charge because it must be using one extra electron, so we can ignore the Na+ and treat it as a counter ion.
This reaction never really exists by itself.
H- is delivered from one place to another.
That's a good thing, because H- wouldn't serve as a nucleophile.
Boron is not very polarizable because it is not so large.
NaBH is a tame nucleophile.
We will soon see that NaBH isselective in its reactivity.
There is a reagent that is very similar to sodium hydride, but it is much more reactive.
The reagent is very similar to NaBH because aluminum is in Column 3A of the periodic table.
The aluminum atom has four bonds, which is why it has a negative charge.
LiAlH is a source of H-.
LiAlH is a better nucleophile than NaBH because it is more polarizable.
It will be very important that LiAlH is more alert than NaBH.
If you are familiar with any other hydrogen nucleophiles, you should look through your textbook and lecture notes.
Let's take a closer look at what can happen after a hydrogen nucleophile attacks a carbonyl group.
Two important rules that govern the behavior of a carbonyl group were covered in the beginning of the chapter.
Carbon only has four orbitals and that would be impossible.
Unless you are dealing with one of the rare exceptions, do not expel H- or C- when considering which groups can function as leaving groups.
A general rule has just been learned.
Let's see if we can apply this rule to determine the outcome when a ketone or aldehyde is treated with a hydrogen nucleophile.
A leaving group must be expelled in order for the carbonyl group to re-form.
There are no leaving groups in this case.
The reaction is complete and waiting for a source of protons to be introduced to work up the reaction.
The product of the reaction will be an alcohol regardless of the identity of the source of the protons.
It is not possible that LiAlH and H O are present at the same time.
H2O common protons include MeOH and water.
We did not show it as two separate steps.
Useful reagents are LiAlH and NaBH.
Many synthesis problems involve the conversion between alcohols and ketones.
You need to be able to do these two things at a moment's notice.
The starting compound is an aldehyde.
The carbonyl group will not be able to re-form because there is no leaving group.
The hydride ion is delivered by LiAlH.
Each of the following transformations has a mechanism to it.
The problems will probably seem easy, but just do them.
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