Alkoxides from Alcohols
Introduction
We’ve previously learned about the synthesis of alcohols. We learned about this early because alcohols are incredibly useful compounds that can react to form many different kinds of compounds. In this chapter, we will investigate many of these reactions of alcohols.
Deprotonation of alcohols
Synthesis of an alkoxide
The hydroxyl group of an alcohol compound contains a hydrogen atom on an oxygen atom. Because oxygen atoms are fairly electronegative and can carry a negative charge, it is possible to deprotonate the hydroxyl group to form alkoxides (groups that contain a negative oxygen atom) if we use a strong enough base. Usually, for alcohols, sodium metal, sodium hydride, potassium metal, or potassium hydride is used. The potassium compounds are a little more aggressive and stronger than the sodium compounds. Also, in the reaction, hydrogen gas is produced. One should be very careful adding these bases to a flask containing alcohol. The alcohol should be dry (without water around). The base should also be added in slow increments and never put into the reaction mixture all at once. It might also be a good idea to cool the reaction down using an ice bath. It is quite easy to start a fire if you are not careful performing this reaction.
Why would we want to make an alkoxide?
An alkoxide, with its negatively charged oxygen atom is a good nucleophile. It can go attack another compound. For instance, an alkoxide can perform an SN2 reaction on an alkyl halide to make an ether.
Which of the following compounds is the most acidic?
We remember that any time we try to determine relative acidities, we always, Always, ALWAYS look at the conjugate bases. In other words, what type of negative charged molecule does it become after it loses its H+?
Both compounds have a negative oxygen atom. This is pretty good. The only difference we can see is the double bonds in the phenoxide where the cyclohexyl alkoxide does not. Therefore, the negative charge on the phenoxide can be spread out through resonance while the negative charge on the cyclohexyl alkoxide is stuck on the oxygen atom alone. Since the negative charge can be spread out over four atoms in phenoxide, it is much more stable than the cyclohexyl alkoxide. That is why phenol is more acidic than cyclohexanol. In fact, it is 100 million times more acidic! In fact, phenol is so acidic, it can be deprotonated with even weaker bases. Potassium hydroxide (KOH) and sodium hydroxide (NaOH) can deprotonate phenol, whereas, the stronger alkali metals (Na, K) and hydrides (NaH, KH) must be used to deprotonate regular aliphatic alcohols.
1. Draw the ether products of the following reactions that involve alkoxides. the following reductions.
a)
b)
c)
d)
Answers
1.
