Alcohols, Ethers, and Epoxides Help
Nomenclature and H Bonding in Alcohols
ROH is an alcohol and ArOH is a phenol. Some alcohols have common names, usually made up of the name of the alkyl group attached to the OH and the word "alcohol"; e.g., ethyl alcohol, CH3CH2OH. In the IUPAC method the suffix -ol replaces the -e of the alkane to indicate the OH. The longest chain with the OH group is used as the parent. Under this system, CH3CH2OH is called ethanol.
Alcohols boil at a higher temperature than the corresponding hydrocarbons, due to the hydrogen bonding between molecules in the liquid. This tendency to form hydrogen bonds causes small molecular weight alcohols (four or fewer carbons) to be water-soluble. As the R group becomes larger, ROH resembles the hydrocarbon more closely.
Preparation of Alcohols
By SN2 or SN1 from alkyl halides. Alcohols can be prepared from alkyl halides using hydroxide or water as the nucleophile. The mechanism of the reaction depends on the alkyl group. Primary alkyl halides undergo SN2 reaction with hydroxide. With secondary and tertiary halides, competing elimination reactions with hydroxide are a problem. Using water as the nucleophile can minimize these side reactions.
Oxymercuration-demercuration of alkenes leads to net addition of H–OH, giving a product that follows Markovnikov's rule and is free from rearrangement.
Anti-Markovnikov addition of water can be carried out via hydroboration-oxidation of alkenes. Treatment of alkylboranes with H2O2 in –OH replaces B with OH. The net addition of H–OH to alkenes is cis, anti-Markovnikov, and free from rearrangement.
Carbonyl Compounds and Grignard Reagents. Grignard reagents, RMgX and ArMgX, are reacted with aldehydes or ketones and the intermediate salts are then hydrolyzed to alcohols. The nucleophilic Grignard reagent attacks the partially positive carbon of the carbonyl group.
Primary alcohols can be made by adding a Grignard reagent to formaldehyde (H2C=O). Secondary aldohols are made by combining Grignard reagents with an aldehyde, and tertiary alcohols are prepared by the reaction of a Grignard reagent with a ketone.
There are several limitations of Grignard reagents, due to functional groups that react with this reagent. The halide cannot possess a functional group with an acidic H, such as OH, CO2H, NH, or SH, because then the carbanion of the Grignard group would remove the acidic H. Carbonyl groups in the Grignard reagent will lead it to react inter- or intramolecularly with itself.
Reduction of Carbonyl Compounds. Alcohols can be readily formed by reacting aldehydes or ketones with sodium borohydride, NaBH4, in protic solvents such as ROH or H2O. Lithium aluminum hydride, LiAlH4, in anhydrous ether can also be used. Esters and carboxylic acids can be reduced to alcohols using LiAlH4. The initial product is a lithium alkoxide salt, which is then hydrolyzed to a 1° alcohol by addition of dilute acid. The alkoxy part of the ester is cleaved off in this reduction.
Reactions of Alcohols
The H on an alcohol OH group is weakly acidic. Sodium hydride (NaH), sodium amide (NaNH2), organolithium reagents (RLi), and Grignard reagents (RMgX) are basic enough to deprotonate the alcohol group.
Alcohols react with HX to form alkyl halides. This reaction proceeds rapidly for tertiary alcohols. Primary and secondary alcohols react more slowly, requiring prolonged heating, which often leads to side reactions. These side reactions can be avoided by using thionyl chloride (SOCl2) or phosphorus tribromide (PBr3) to make chlorides or bromides. Both of these reagents convert the OH into a better leaving group, allowing halide to act as a nucleophile in an SN2 reaction. The p-toluenesulfonate anion, (tosylate, p-CH3C6H5SO3–) is a very weak base and is therefore a good leaving group.
Upon heating alcohols with non-nucleophilic acids (H2SO4 and H3PO4), dehydration to alkenes occurs. Intermolecular dehydrations can also occur, leading to ethers, which are discussed in a later part of this chapter.
In the presence of an acid catalyst, alcohols react with carboxylic acids to yield esters. In these reactions, the alcohol acts as a nucleophile and attacks the carbonyl group of the protonated carboxylic acid.
Alcohols with at least one H on the alcohol carbon are oxidized with Jones reagent (CrO3, H2SO4) to carbonyl compounds. Primary alcohols yield aldehydes, which are further oxidized to carboxylic acids, RCO2H. To obtain aldehydes selectively, milder reagents, such as pyridinium chlorochromate (PCC), are used.
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