Preparation of Alcohol PDF

Alcohols Organic chemistry Alcohols Mechanism of Conversion of Alcohols to Alkyl Bromides with PBr3 Alcohols Alcohols Tosylate as Leaving Group Alcohols can be converted into alkyl tosylates. An alkyl tosylate is composed of two parts: the alkyl group R, derived from an alcohol; and the tosylate (sh...

Alcohols Organic chemistry Alcohols Mechanism of Conversion of Alcohols to Alkyl Bromides with PBr3 Alcohols Alcohols Tosylate as Leaving Group Alcohols can be converted into alkyl tosylates. An alkyl tosylate is composed of two parts: the alkyl group R, derived from an alcohol; and the tosylate (short for p-toluenesulfonate), which is a good leaving group. A tosyl group, CH3C6H4SO2—, is abbreviated Ts, so an alkyl tosylate becomes ROTs. Alcohols Tosylate as Leaving Group Alcohols Formation and Use of Tosylates Alcohols are converted to tosylates by treatment with p-toluenesulfonyl chloride (TsCl) in the presence of pyridine. This process Converts a poor leaving group (—OH) into a good one (—OTs). Tosylate is a good leaving group because its conjugate acid, p-toluenesulfonic acid (CH3C6H4SO3H, TsOH) is a strong acid (pKa = -7). Alcohols Substitution and Elimination of Tosylates Because alkyl tosylates have good leaving groups, they undergo both nucleophilic substitution and β elimination, exactly as alkyl halides do. Generally, alkyl tosylates are treated with strong nucleophiles and bases, so the mechanism of substitution is SN2, and the mechanism of elimination is E2. Alcohols Substitution and Elimination of Tosylates Alcohols SN2 Inversion When Replacing Tosylates Because substitution occurs via an SN2 mechanism, inversion of configuration results when the leaving group is bonded to a stereogenic center. Alcohols Alcohols Reaction of Ethers with Strong Acid In order for ethers to undergo substitution or elimination reactions, their poor leaving group must first be converted into a good leaving group by reaction with strong acids such as HBr and HI. HBr and HI are strong acids that are also sources of good nucleophiles (Br— and l—, respectively). When ethers react with HBr or Hl, both C-O bonds are cleaved and two alkyl halides are formed as products. Alcohols Alcohols Mechanism of Ether Cleavage The mechanism of ether cleavage is SN1 or SN2, depending on the identity of R. When 2° or 3° alkyl groups are bonded to the ether oxygen, the C-O bond is cleaved by an SN1 mechanism involving a carbocation. With methyl or 1° R groups, the C-O bond is cleaved by an SN2 mechanism. Alcohols Alcohols Reactions of Epoxides Epoxides do not contain a good leaving group. Epoxides do contain a strained three-membered ring with two polar bonds. Nucleophilic attack opens the strained three-membered ring, making it a favorable process even with a poor leaving group. Alcohols Addition of Nucleophiles to Epoxides Nucleophilic addition to epoxides occurs readily with strong nucleophiles and With acids like HZ, where Z is a nucleophilic atom. Alcohols Mechanism of Epoxide Reactions Virtually all strong nucleophiles open an epoxide ring by a two-step reaction sequence: Alcohols Mechanism of Epoxide Reactions In step , the nucleophile attacks an electron-deficient carbon, by an SN2 mechanism, thus cleaving the C-O bond and relieving the strain of the three-membered ring. In step , the alkoxide is protonated with water to generate a neutral product with Mo functional groups on adjacent atoms. Common nucleophiles (Nu) that open the epoxide ring include —OH, —OR, —CN, —SR, and NH3. Alcohols Stereochemistry of Epoxide Reactions Alcohols Acidic Epoxide Ring Opening Acids HZ that contain a nucleophile Z also open epoxide rings by a two-step sequence. HCI, HBr, and HI, as well as H2O and ROH in the presence of acid, all open an epoxide ring in this manner. Alcohols Regioselectivity of Epoxide Ring Opening Ring opening of an epoxide with either a strong nucleophile or an acid HZ is regioselective because one constitutional isomer is the major or exclusive product. The site selectivity of these two reactions is exactly opposite.