Ch_322a_8.012

Ch_322a_8.012 - gas (O 2 ) through an electric discharge in...

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Ozonolysis Ozone, O 3 , reacts rapidly with alkenes yielding unstable products called initial ozonides. These products rearrange spontaneously at low temperature to more stable ozonide compounds. C=C H H H H ozone is a dipolar compound O O O : - + fast C C O O O : : initial ozonide (very unstable) H H H H C C H H H H O : O: O : : ozonide ! H f = 37.6 kcal/mol ! H f = -47.8 kcal/mol (calculated) (calculated) fast . . . . . . . . . . . . . . The rearrangement of the unstable initial ozonide to the more stable ozonide is highly exothermic. (calculated ! H ~ -85 kcal/mol)
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Products of the Ozonolysis Reaction Even the ozonides are not isolated. They are reacted with zinc (a reducing agent) and water in the reaction solution to produce ketone and aldehyde products. C C O : O: : O : : ozonide R R' R'' R''' + Zn H 2 O C=O R' R + O=C R'' R''' + Zn(OH) 2 aldehyde and ketone products overall C=C R R' R'' R''' (i) O 3 , CH 2 Cl 2 (solvent), -78 o C (ii) Zn, H 2 O C=O R' R + O=C R'' R''' Note: Unlike oxidative cleavage with KMnO 4 , aldehyde products are isolated because of the reductive workup conditions.
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Examples of the Ozonolysis Reaction Preparation of Ozone Ozone is produced in the laboratory by flowing a stream of oxygen
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Unformatted text preview: gas (O 2 ) through an electric discharge in an apparatus called an ozonator . The exiting stream of gas contains ozone in the range 4-8% by weight. 3O 2 electric discharge 2O 3 This gas stream is bubbled into a solution of the alkene in CH 2 Cl 2 (dichloromethane) at -78 o C. Examples CH 3 C=CHCH 2 CH 3 CH 3 2-methyl-2-pentene (i) O 3 , CH 2 Cl 2 (solvent), -78 o C (ii) Zn, H 2 O CH 3 CCH 3 O = + CH 3 CH 2 CH O CH 3 1-methylcyclohexene (i) O 3 , CH 2 Cl 2 (solvent), -78 o C (ii) Zn, H 2 O CH 3 CCH 2 CH 2 CH 2 CH 2 CH O O Note: The products of ozonolysis provide structural information about the alkene. Quiz 8.012 Provide the products of the following ozonolysis reactions. CH 3 CH 2 C=CH 2 CH 3 (i) O 3 , CH 2 Cl 2 , -78 o C (ii) Zn, H 2 O = C (i) O 3 , CH 2 Cl 2 , -78 o C (ii) Zn, H 2 O CH 3 H 3 C CH 3 CH 2 CCH 3 O + CH 2 =O CH 3 CCH 3 O + C O CH 2 CH 2 C=O O=C H H...
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_8.012 - gas (O 2 ) through an electric discharge in...

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