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17_08_09 - 17.8 17.8 Acetal Formation Some reactions of...

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Unformatted text preview: 17.8 17.8 Acetal Formation Some reactions of aldehydes and ketones progress Some beyond the nucleophilic addition stage Acetal formation Imine formation Enamine formation Compounds related to imines The Wittig reaction Recall Hydration of Aldehydes and Ketones Recall R C R' HOH R HO •• •• O• • •• C R' O •• •• H Alcohols Under Analogous Reaction Alcohols with Aldehydes and Ketones R C R' R"OH R R"O •• •• O• • •• C R' O •• •• H Product is called a hemiacetal. hemiacetal Hemiacetal reacts further in acid to yield an acetal R R"O •• •• C R' O •• •• H Product is called an acetal. acetal ROH, H+ R R"O •• •• C R' O •• •• H Product is called Product a hemiacetal. hemiacetal Example O CH + 2CH3CH2OH HCl HCl CH(OCH2CH3)2 CH(OCH + H 2O Benzaldehyde diethyl acetal (66%) Diols Form Cyclic Acetals O CH3(CH2)5CH + HOCH2CH2OH benzene p-toluenesulfonic acid H 2C O C (81%) H CH2 O + H 2O (CH2)5CH3 In general: In Position of equilibrium is usually unfavorable for acetal formation from ketones. Important exception: Cyclic acetals can be prepared from ketones. Example O C6H5CH2CCH3 + HOCH2CH2OH benzene p-toluenesulfonic acid H 2C O (78%) C6H5CH2 C CH2 O CH3 + H 2O Mechanism of Acetal Formation First stage is analogous to hydration and First leads to hemiacetal acid -catalyzed nucleophilic addition of alcohol to C=O Mechanism Mechanism R • O• •• H C O• • •• H O• • + R H Mechanism Mechanism R •O• •• H C H O •• + •O • •• H R Mechanism Mechanism R •O• •• H C O •• + H Mechanism Mechanism R H O + •• C O• • H •• Mechanism Mechanism R H •• O + •• C O• • H •• O R •• H Mechanism Mechanism R O •• •• C O• • H •• H + O R •• H Mechanism of Acetal Formation Second stage is hemiacetal-to-acetal conversion Second involves carbocation chemistry Hemiacetal-to-acetal Stage Hemiacetal R O •• •• H C O• • H •• H O• • + R Hemiacetal-to-acetal Stage Hemiacetal R O •• •• H C O+ H •• H •O • •• R Hemiacetal-to-acetal Stage Hemiacetal R O •• •• H C O+ H •• Hemiacetal-to-acetal Stage Hemiacetal R O •• •• H C+ •O • H •• Hemiacetal-to-acetal Stage Hemiacetal R O •• •• R C+ + O •• C Carbocation is stabilized by delocalization of unshared electron pair of oxygen Hemiacetal-to-acetal Stage Hemiacetal R O •• •• R C+ •O • H •• Hemiacetal-to-acetal Stage Hemiacetal R O •• •• R C O+ H •• Hemiacetal-to-acetal Stage Hemiacetal R O •• •• R C O+ H •• R •O • H •• Hemiacetal-to-acetal Stage Hemiacetal R O •• •• R C O •• •• R H O + H •• Hydrolysis of Acetals OR" R C R' + H2O O C R + R' 2R"OH OR" mechanism: reverse of acetal formation; hemiacetal is intermediate application: aldehydes and ketones can be "protected" as acetals. 17.9 17.9 Acetals as Protecting Groups Example The conversion shown cannot be carried out The directly... O CH3CCH2CH2C CH 1. NaNH2 2. CH3I O CH3CCH2CH2C CCH3 because the carbonyl group and the because carbanion are incompatible functional groups. O CH3CCH2CH2C C: – Strategy 1)) protect C=O 1 protect C=O 2) alkylate 2) alkylate 3) restore C=O 3) restore C=O Example: Protect O CH3CCH2CH2C CH + HOCH2CH2OH benzene p-toluenesulfonic acid H 2C O C CH3 CH2 O CH2CH2C CH Example: Alkylate H 2C O C CH3 CH2CH2C CCH3 H 2C 1. NaNH2 2. CH3I O C CH3 CH2CH2C CH CH2 O CH2 O Example: Deprotect H 2C O C CH3 CH2CH2C CCH3 O HOCH2CH2OH + CH3CCH2CH2C (96%) CCH3 CH2 O H 2O HCl ...
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