Reactions of Alcohol.pdf

Alcohols Organic chemistry Alcohols Substitution and Elimination Reactions of Alcohols Treatment of alcohols with a strong acid protonates the O converting the bad leaving group ¯OH into H2O, a good leaving group. This makes it possible to perform substitution and elimination reactions on alcohols....

Alcohols Organic chemistry Alcohols Substitution and Elimination Reactions of Alcohols Treatment of alcohols with a strong acid protonates the O converting the bad leaving group ¯OH into H2O, a good leaving group. This makes it possible to perform substitution and elimination reactions on alcohols. Reactions of Alcohols Dehydration Dehydration, like dehydrohalogenation, is a β is a elimination reaction in which the elements of OH and H are removed from the α and β carbon atoms respectively. Reactions of Alcohols Dehydration Requires Strong Acids Dehydration is typically carried out using H2SO4 and other strong acids, or phosphorus oxychloride (POCl3) in the presence of an amine base. Typical acids used for alcohol dehydration are H2SO4 OR ptoluenesulfonic acid (TsOH). Reactions of Alcohols Dehydration and Alcohol Substitution More substituted alcohols dehydrate more easily, giving rise to the following order of reactivity. Reactions of Alcohols Dehydration by E1 Mechanism 2° and 3° alcohols react by an E1 mechanism, whereas 1° alcohols react by an E2 mechanism. Reactions of Alcohols Zaitsev’s Rule(Saytzeff's rule) When an alcohol has two or three β carbons, dehydration is regioselective and follows the Zaitsev rule. The more substituted alkene is the major product when a mixture of constitutional isomers is possible. NOTE: The alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents (Zaitsev's rule). Reactions of Alcohols E2 Dehydration of 1o Alcohols Since 1° carbocations are highly unstable, their dehydration cannot occur by an E1 mechanism involving a carbocation intermediate. However, 1° alcohols undergo dehydration by way of an E2 mechanism. Reactions of Alcohols Dehydration of Alcohols Using POCl3 Some organic compounds decompose in the presence of strong acid, so other methods have been developed to convert alcohols to alkenes. A common method uses phosphorus oxychloride (POCl3) and pyridine (an amine base) in place of H2SO4 or TsOH. Reactions of Alcohols Dehydration of Alcohols Using POCl3 POCl3 serves much the same role as a strong acid does in acid-catalyzed dehydration. It converts a poor leaving group (¯OH) into a good leaving group. Dehydration then proceeds by an E2 mechanism. Reactions of Alcohols Dehydration of Alcohols Using POCl3 Phosphorus oxychloride in the amine solvent (pyridine) can lead to dehydration of secondary and tertiary alcohols at low temperatures. Reactions of Alcohols Reactions of Alcohols Conversion of Alcohols to Alkyl Halides with HX Substitution reactions do not occur with alcohols unless ¯OH is converted into a good leaving group. The reaction of alcohols with HX (X = Cl, Br, I) is a general method to prepare 1°, 2°, and 3° alkyl halides.. Reactions of Alcohols Mechanism of Reaction of Alcohols with HX More substituted alcohols usually react more rapidly with HX: This order of reactivity can be rationalized by considering the reaction mechanisms involved. The mechanism depends on the structure of the R group. Reactions of Alcohols Reactions of Alcohols Reactions of Alcohols Reactivity of Hydrogen Halides The reactivity of hydrogen halides increases with increasing acidity. Because Cl¯ is a poorer nucleophile than Br¯ or I¯, the reaction of 1o alcohols with HCl occurs only when an additional Lewis acid catalyst, usually ZnCl2, is added. Complexation of ZnCl2 with the O atom of the alcohol makes a very good leaving group that facilitates the SN2 reaction. Reactions of Alcohols Stereochemistry of Reaction of Alcohols with HX Knowing the mechanism allows us to predict the stereochemistry of the products when the reaction occurs at a stereogenic center. Reactions of Alcohols Reactions of Alcohols Reactions of Alcohols Conversion of Alcohols to Alkyl Halides with SOCl2 and PBr3 Primary and 2° alcohols can be converted to alkyl halides using SOCl2 and PBr3. SOCl2 (thionyl chloride) converts alcohols into alkyl chlorides. PBr3 (phosphorus tribromide) converts alcohols into alkyl bromides. Both reagents convert ¯OH into a good leaving group in situ—that is, directly in the reaction mixture—as well as provide the nucleophile, either Cl¯ or Br¯, to displace the leaving group. Reactions of Alcohols Conversion of Alcohols to Alkyl Chlorides with SOCl2 When a 1° or 2° alcohol is treated with SOCl2 and pyridine, an alkyl chloride is formed, with HCl and SO2 as by-products. Reactions of Alcohols Conversion of Alcohols to Alkyl Chlorides with SOCl2 The mechanism of this reaction consists of two parts: 1. Conversion of the OH group into a better leaving group. 2. Nucleophilic substitution by Cl¯ via an SN2 reaction.