17.8 Acetal Formation ditions of acid catalysis. The expected product of nucleophilic addition of the alcohol to the carbonyl group is called a hemiacetal. The product actually isolated, however, cor- ponds to reaction of one mole of the aldehyde with two moles of alcohol to give gem- diethers known as acetals ROH.HT R'OH. H RCH RCH + H2o Alde Acetal Water CH 2CH CH OH HCT CH(OCH, CH3)2 Benzaldehyde Ethanol Benzaldehyde diethyl acetal(66%) The overall reaction proceeds in two stages. The hemiacetal is formed in the first stage by nucleophilic addition of the alcohol to the carbonyl group. The mechanism of hemiacetal formation is exactly analogous to that of acid-catalyzed hydration of alde hydes and ketones(Section 17.6 O OH RCH RCH Hemiacetal Under the acidic conditions of its formation the hemiacetal is converted to an acetal b way of a carbocation intermediate R RCH RCH Ha OR Hemiacetal Carbocation Wa This carbocation is stabilized by electron release from its oxygen substituent: A particularly stable esonance form both carbon and oxygen have octets of electrons Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
ditions of acid catalysis. The expected product of nucleophilic addition of the alcohol to the carbonyl group is called a hemiacetal. The product actually isolated, however, corresponds to reaction of one mole of the aldehyde with two moles of alcohol to give geminal diethers known as acetals: The overall reaction proceeds in two stages. The hemiacetal is formed in the first stage by nucleophilic addition of the alcohol to the carbonyl group. The mechanism of hemiacetal formation is exactly analogous to that of acid-catalyzed hydration of aldehydes and ketones (Section 17.6): Under the acidic conditions of its formation, the hemiacetal is converted to an acetal by way of a carbocation intermediate: This carbocation is stabilized by electron release from its oxygen substituent: R H C OR R H C OR A particularly stable resonance form; both carbon and oxygen have octets of electrons. H, fast slow H H RCH O OR OR RCH OH Hemiacetal R H C OR Carbocation Water H2O RCH O Aldehyde H H ROH RCH OH O H R RCH OH OR RCH OH Hemiacetal RCH O Aldehyde ROH, H ROH, H RCH OH OR Hemiacetal RCH OR OR Acetal H2O Water CH O Benzaldehyde 2CH3CH2OH Ethanol HCl CH(OCH2CH3)2 Benzaldehyde diethyl acetal (66%) 17.8 Acetal Formation 669 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
670 CHAPTER SEVENTEEN Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group Nucleophilic capture of the carbocation intermediate by an alcohol molecule leads to an CO RCH H OR' OR′ Alcohol Acetal PROBLEM 17.7 Write a stepwise mechanism for the formation of benzaldehyde diethyl acetal from benzaldehyde and ethanol under conditions of acid catalysis Acetal formation is reversible in acid. An equilibrium is established between the reac- to designate the products of tants, that is, the carbonyl compound and the alcohol, and the acetal product. The position dition of alcohols to k of equilibrium is favorable for acetal formation from most aldehydes, especially when excess keta/s. This term h een dropped from the alcohol is present as the reaction solvent. For most ketones the position of equilibrium is IUPAC system of nomencla- unfavorable, and other methods must be used for the preparation of acetals from ketones ure and the term acetal is applied to the adducts Diols that bear two hydroxyl groups in a 1, 2 or 1, 3 relationship to each other yield of both aldehydes and cyclic acetals on reaction with either aldehydes or ketones. The five-membered cyclic acetals derived from ethylene glycol are the most commonly encountered examples Often the position of equilibrium is made more favorable by removing the water formed in the reaction by azeotropic distillation with benzene or toluene CH(CH,)s CH HOCH,CH,OH 2-Hexyl-1 3-dioxolane C6HS CH, CCH3 HOCH, CH,OH C6HsCH Benzyl methyl 2-Benzyl-2-methyl-1, 3-dioxolane PROBLEM 17.8 Write the structures of the cyclic acetals derived from each of he foll (a)Cyclohexanone and ethylene glycol (b)Benzaldehyde and 1, 3-propanediol (c)Isobutyl methyl ketone and ethylene glycol yl methyl ketone and 2, 2-dimethy l-1, 3-pro SAMPLE SOLUTION (a) The cyclic acetals derived from ethylene glycol contain a Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Nucleophilic capture of the carbocation intermediate by an alcohol molecule leads to an acetal: PROBLEM 17.7 Write a stepwise mechanism for the formation of benzaldehyde diethyl acetal from benzaldehyde and ethanol under conditions of acid catalysis. Acetal formation is reversible in acid. An equilibrium is established between the reactants, that is, the carbonyl compound and the alcohol, and the acetal product. The position of equilibrium is favorable for acetal formation from most aldehydes, especially when excess alcohol is present as the reaction solvent. For most ketones the position of equilibrium is unfavorable, and other methods must be used for the preparation of acetals from ketones. Diols that bear two hydroxyl groups in a 1,2 or 1,3 relationship to each other yield cyclic acetals on reaction with either aldehydes or ketones. The five-membered cyclic acetals derived from ethylene glycol are the most commonly encountered examples. Often the position of equilibrium is made more favorable by removing the water formed in the reaction by azeotropic distillation with benzene or toluene: PROBLEM 17.8 Write the structures of the cyclic acetals derived from each of the following. (a) Cyclohexanone and ethylene glycol (b) Benzaldehyde and 1,3-propanediol (c) Isobutyl methyl ketone and ethylene glycol (d) Isobutyl methyl ketone and 2,2-dimethyl-1,3-propanediol SAMPLE SOLUTION (a) The cyclic acetals derived from ethylene glycol contain a five-membered 1,3-dioxolane ring. CH3(CH2)5CH O Heptanal HOCH2CH2OH Ethylene glycol (1,2-ethanediol) p-toluenesulfonic acid benzene O O H (CH2)5CH3 2-Hexyl-1,3-dioxolane (81%) p-toluenesulfonic acid benzene C6H5CH2CCH3 O Benzyl methyl ketone HOCH2CH2OH Ethylene glycol (1,2-ethanediol) 2-Benzyl-2-methyl-1,3-dioxolane (78%) C6H5CH2 CH3 O O R H O Alcohol H R RCH O OR H, fast OR RCH OR Acetal fast R H C OR 670 CHAPTER SEVENTEEN Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group At one time it was customary to designate the products of addition of alcohols to ketones as ketals. This term has been dropped from the IUPAC system of nomenclature, and the term acetal is now applied to the adducts of both aldehydes and ketones. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website