CHAPTER 9    Further Reactions of Alcohols and the Chemistry of Ethers

 

9-1   Reactions of Alcohols with Base: Preparation of Alkoxides

           Alcohols are weak acids and can be deprotonated with bases. The equilibrium will depend upon the relative acidity of the alcohol and the conjugate acid formed by protonation of the base.

9-2   Reactions of Alcohols with Strong Acids: Alkyloxonium Ions.

              Protonation of the oxygen of an alcohol converts the poor leaving group OH into a good leaving group, +OH2. These alkyloxonium ions can undergo first and second order reactions

9-3   Carbocation Rearrangements

              Cations formed in first order reactions often undergo rearrangements to form more stable cations (primary –> secondary, secondary–>tertiary). These rearrangements often represent a significant disadavantage of first order reactions.

9-4   Organic and Inorganic Esters from Alcohols

           The combination of a carboxylic acid with an alcohol in the presence of a strong acid as catalyst will form a carboxylic acid ester. Carboxylic acid esters can also be prepared from carboxlic acid chlorides and alcohols. Sulfonic acid chlorides react with alcohols to form sulfonate esters which are good leaving groups.

9-5   Names and Physical Properties of Ethers

9-6   Williamson Ether Synthesis

              Ethers can be prepared by the reaction of an alkoxide ion with an alkyl halide. As this is an second order substitution, the alkyl halide can not be tertiary.

 

9-7   Synthesis of Ethers: Alcohols and Mineral Acids

              Ethers can also be formed by the reaction of alcohols with strong acids. Generally, this synthesis of alcohols is used to make symmetrical ethers where the two carbon chains are the same. This reactions will follow either a first or a second order pathway depending on the substitution of the alcohol. Unsymmetrical ethers can be formed when one of the alcohols forms a particularly stable cation (tertiary) in a first order reaction.

9-8   Reactions of Ethers

              There are two reactions of ethers which primarily limit their use as solvents.

              Ethers + molecular oxygen form peroxides which are can explode when concentrated and heated.

              Ethers + strong acids such as HBr are cleaved, with the oxygen on both carbon atoms of the ether replaced by Br.

9-9   Reactions of Oxacyclopropanes

              Epoxides are compounds with a three-membered ring containing oxygen. The ring strain weakens the C–O bonds, Nucleophiles, most importantly R–Metal and amines reaction, adding the nucleophile to one of the carbons of the epoxide, cleaving the C–O bond, forming an alcohol.

9-10 Sulfur Analogs of Alcohols and Ethers

              R–S–H and R–S–R are sulfur analogs of alcohols and ethers, know as thiols and sulfides (or thiaethers). Each can be formed in second order substitution reactions with the appropriate nucleophile (–SH for thiols, –SR for sulfides). Sulfides can be oxidized with hydrogen peroxide to form first sulfoxides and then in a slower reaction, further oxidized to sulfones.

9-11 Physiological Properties and Uses of Alcohols and Ethers.