Notes for Chapter 11

IN this chapter there is a detailed disscussion of alcohols and ethers. Consider the water molecule and replace one hydrogen atom with an alkyl group. This forms an alcohol. the alcohol has a hydroxyl group and is therefore a protic solvent. It is also weakly acidic and can hydrogen bond to other alcohol molecules or other hydrogen-bond acceptors and donors. This gives alcohols higher boiling points than alkanes and alkyl halides of similar size and structure.

If you now replace the second hydrogen of water water with another alkyl group, you form an ether. This molecule is polar and is a hydrogen-bond acceptor for protic solvents, but ether molecules cannot hydrogen-bond to other ether molecules. This means that they will have lower boiling points than alcohols of similar structure.

The chemistry of these two functional groups is dependent on the electronegativity of oxygen relative to carbon and hydrogen. Alcohols are weakly acidic and can give up hydrogen ions to stronger bases than alkoxide ions. The unshared electron pairs on oxygen allow alcohols to act as weak bases and nucleophiles in the presence of acids and electrophiles. The hydroxyl group of alcohols can be replaced by very strong nucleophiles, because the hydroxide ion is a poor leaving group. It can be converted into a good leaving group either by protonation or conversion into a very weakly basic group like tosylates. This is important to be able to carry out reactions on alcohols, so study these reactions.

Ethers are not acidic, but are weakly basic. They can be protonated and the poor alkoxide leaving group can be converted into the good leaving group, alcohol. Thus ethers can be cleaved by acid. ordinary ethers cannot be cleaved by hydroxide but highly strained cyclic ethers can be cleaved by hydroxide ion.