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lecture 16.2 - Organic Chemistry William H Brown...

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16- 16- 1 1 Organic  Organic  Chemistry Chemistry William H. Brown William H. Brown Christopher S. Foote Christopher S. Foote Brent L. Iverson Brent L. Iverson
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16- 16- 2 2 Aldehydes  Aldehydes  Ketones Ketones   Chapter 16 Chapter 16
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16- 16- 3 3 Addition of HCN Addition of HCN HCN adds to the C=O group of an aldehyde or  ketone to give a cyanohydrin Cyanohydrin: Cyanohydrin:   a molecule containing an -OH  group and a -CN group bonded to the same  carbon 2-Hydroxypropanenitrile (Acetaldehyde cyanohydrin) + HC N CH 3 C-C N CH 3 CH OH H O
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16- 16- 4 4 Addition of HCN Addition of HCN Mechanism of cyanohydrin formation Step 1: nucleophilic addition of cyanide to the  carbonyl carbon Step 2: proton transfer from HCN gives the  cyanohydrin and regenerates cyanide ion - + H 3 C C H 3 C O C O - H 3 C H 3 C C N N C C O - H 3 C H 3 C C N N H C C C H 3 C H 3 C O-H N C N + + -
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16- 16- 5 5 Cyanohydrins Cyanohydrins The value of cyanohydrins acid-catalyzed dehydration of alcohol gives an alkene catalytic reduction of the cyano group gives a 1 °   amine Propenenitrile (Acrylonitrile) + acid catalyst 2-Hydroxypropanenitrile (Acetaldehyde cyanohydrin) CH 3 CHC N N CH 2 =CHC H 2 O OH CHC OH N 2H 2 Ni OH CHCH 2 NH 2 2-Amino-1-phenylethanol (racemic) + Benzaldehyde cyanohydrin (racemic)
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16- 16- 6 6 Wittig Reaction Wittig Reaction The Wittig reaction is a very versatile synthetic  method for the synthesis of alkenes from  aldehydes and ketones   Triphenyl- phosphine oxide Methylene- cyclohexane A phosphonium ylide + + - + CH 2 Ph 3 P= O Ph 3 P- CH 2 Cyclohexanone O
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16- 16- 7 7 Phosphonium Ylides Phosphonium Ylides Phosphonium ylides   are formed in two steps: Step 1: nucleophilic displacement of iodine by  triphenylphosphine Step 2: treatment of the phosphonium salt with a very  strong base, most commonly BuLi, NaH, or NaNH 2 Ph 3 P CH 3 -I Ph 3 P-CH 3 I Triphenylphosphine + + S N 2 Methyltriphenylphosphonium iodide (an alkyltriphenylphosphine salt) CH 3 CH 2 CH 2 CH 2 - Li + H-CH 2 -PPh 3 I - - CH 2 -PPh 3 CH 3 CH 2 CH 2 CH 3 LiI A phosphonium ylide Butane Butyllithium + + + + +
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16- 16- 8 8 Wittig Reaction Wittig Reaction Phosphonium ylides react with the C=O group of  an aldehyde or ketone to give an alkene
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