UH CHEM 3331 - reactions of alcohols (4 pages)

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reactions of alcohols

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reactions of alcohols


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University of Houston
Chem 3331 - Organic Chemistry
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CHEM 3331 1nd Edition Lecture 24 Outline of Last Lecture I Organometallic compounds II Grignard reagents III Addition to C O bonds IV Addition of H V thiols Outline of Current Lecture I Oxidation II Biological oxidation III Formation of sulfonate esters IV Reduction of alcohols V Reaction of alcohols with HX Current Lecture I Oxidation There are plenty of reactions with alcohols dehydrogenation yields alkenes Sn2 reaction yields RX reduction yields RH oxidation yields a C O bond Oxidation in organic chemistry is the loss of hydrogen or the addition of an electronegative element O N X Reduction is the addition of hydrogen or the loss of and electronegative element When an alkane is oxidized adding O it results in an alcohol when oxidized again it yields a diol which is unstable and loses H2O forming an aldehyde oxidized one last time it yields a carboxylic acid Oxidation of secondary alcohols oxidizing agents Na2Cr2O7 or KMnO4 When an alcohol reacts with Na2Cr2O7 in H2SO4 it results in a ketone with 80 90 yield These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute The acid and oxidizing agent react together forming 2 moles of HO Cr O 2 OH When this reacts with the alcohol it loses water and the two compounds bond together forming a chromate ester Since it is in an aqueous solution water acts as a base on the chromate ester and yields H3O R2C O and Cr O O OH Oxidation of primary alcohols the alcohol reacts with the previous mixture of H2SO4 and Na2Cr2O7 to create an aldehyde which reacts with water in aqueous solution creating a diol This diol reacts with remaining Cr2O7 to yield carboxylic acid To stop this reaction at aldehyde we must not do the reaction in water We do this by using PCC Tertiary alcohols cannot be oxidized without breaking a C C bond The Swern oxidation II Biological Oxidation In nature there isn t a surplus of Na2Cr2O7 to be used to oxidize alcohols Alcohols are nucleophiles Carbon is electrpphilic but OH is a bad leaving group so alcohols act as a nucleophile R CH2 OH HBr RCH2Br H2O H2O RCH2Br III Formation of Sulfonate esters The sulfonate ester is named by its R group Tosylate is the most common one used IV Reduction of alcohols Reduction of an alcohol is done through the use of sulfonate esters Cyclopentanol reacts with TosCl to yield 1 TosO cyclopentane which we react with LiAlH4 to achieve cyclopentane We can also reduce cyclopentane using H2SO4 and high temperature to yield cyclopentene which reacts with H2 Pt to yield cyclopentane This way is much more cost effective since most of the components are cheap or reusable Whereas TosO is thrown away as waste and not reused V Reaction with HX R OH HX R O H2 X Sn1 Sn2 R X H2O R 3COH HBr R 3CBr Sn1 CH3 CH2 3OH HBr CH3 CH2 3Br Sn2 R 3COH HCl R 3CCl with 98 yield HCl works well with tertiary substrates but becomes slower with secondary and almost doesn t react with primary To speed up the reaction we use a mixture of HCl and ZnCl2 CH3 CH2 3OH HCl ZnCl2 CH3 CH2 3O H ZnCl2 Cl CH3 CH2 3Cl HOZnCl2 Secondary reactions move faster but reactions with primary substrates still move slowly Problems with Sn1 1 produce cation rearrangement 2 competes with E1 3 HCl vs ZnCl2 4 works poorly with alkyl iodides

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