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Reactions of Alcohols. Oxidation R-X, Ether, and Ester Preparation Protection of Alcohols Synthesis The Logic of Mechanisms. Alcohols are Synthetically Versatile. Oxidation levels of oxygen- halogen- and nitrogen- containing molecules. Oxidation - Reduction.

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reactions of alcohols

Reactions of Alcohols

Oxidation

R-X, Ether, and Ester Preparation

Protection of Alcohols

Synthesis

The Logic of Mechanisms

pyridinium chlorochromate pcc
Pyridinium Chlorochromate (PCC)
  • PCC is a complex of chromium trioxide, pyridine, and HCl.
  • Oxidizes primary alcohols to aldehydes.
  • Oxidizes secondary alcohols to ketones.
3 alcohols cannot be oxidized
3° Alcohols Cannot Be Oxidized
  • Carbon does not have hydrogen, so oxidation is difficult and involves the breakage of a C—C bond.
  • Chromic acid test is for primary and secondary alcohols because tertiary alcohols do not react. Orange color of Cr(VII) turns green - Cr(III); 3o alcohol is not oxidized, therefore no color change.
sodium hypochlorite naocl
Sodium Hypochlorite (NaOCl)
  • Sodium hypochlorite (household bleach) can oxidize alcohols without heavy metals or generating hazardous waste.
  • This is a much better option for acid-sensitive compounds.
swern oxidation
Swern Oxidation
  • Dimethylsulfoxide (DMSO), with oxalyl chloride and hindered base, oxidizes 2 alcohols to ketones and 1 alcohols to aldehydes (same as PCC).
conversion of alcohol into a leaving group
Conversion of Alcohol into a Leaving Group
  • Form Tosylate (p-TsCl, pyridine)
  • Use strong acid (H3O+)
  • Convert to Alkyl Halide (HX, SOCl2, PBr3)
reduction of alcohols
Reduction of Alcohols
  • Dehydrate with concentrated H2SO4, then add H2.
  • Make a tosylate, then reduce it with LiAlH4.
reaction of alcohols with acids
Reaction of Alcohols with Acids
  • The hydroxyl group is protonated by an acid to convert it into a good leaving group (H2O).
  • Once the alcohol is protonated, a substitution or elimination reaction can take place.
reaction of alcohols with hbr
Reaction of Alcohols with HBr
  • –OH of alcohol is protonated.
  • –OH2+ is good leaving group.
  • 3° and 2° alcohols react with Br– via SN1.
  • 1° alcohols react via SN2.
s n 1 mechanism
SN1 Mechanism

Step 1: Protonation.

Step 2: Formation of the carbocation.

Step 3: Bromide attacks the carbocation.

slide29

Solved Problem 2

When 3-methyl-2-butanol is treated with concentrated HBr, the major product is 2-bromo-2-methylbutane. Propose a mechanism for the formation of this product.

Solution

The alcohol is protonated by the strong acid. This protonated secondary alcohol loses water to form a secondary carbocation.

slide30

Solved Problem 2 (Continued)

Solution (Continued)

A hydride shift transforms the secondary carbocation into a more stable tertiary cation. Attack by bromide leads to the observed product.

s n 2 reaction with the lucas reagent
SN2 Reaction with the Lucas Reagent
  • Primary alcohols react with the Lucas reagent (HCl and ZnCl2) by the SN2 mechanism.
  • Reaction is very slow. The reaction can take from several minutes to several days.
dehydration of alcohols
Dehydration of Alcohols
  • Alcohol dehydration generally takes place through the E1 mechanism.
  • Rearrangements are possible.
  • The rate of the reaction follows the same rate as the ease of formation of carbocations: 3o > 2o > 1o.
  • Primary alcohols rearrange, so this is not a good reaction for converting 1° alcohols into alkenes.
esterification
Esterification
  • Fischer: Alcohol + carboxylic acid
  • Tosylate esters
  • Sulfate esters
  • Nitrate esters
  • Phosphate esters
fischer esterification
Fischer Esterification
  • Reaction of an alcohol and a carboxylic acid produces an ester.
  • Sulfuric acid is a catalyst.
  • The reaction is an equilibrium between starting materials and products, and for this reason the Fischer esterification is seldom used to prepare esters.
nitrate esters
Nitrate Esters
  • The best-known nitrate ester is nitroglycerine, whose systematic name is glyceryl trinitrate.
  • Glyceryl nitrate results from the reaction of glycerol (1,2,3-propanetriol) with three molecules of nitric acid.
mechanisms thinking logically
Mechanisms Thinking Logically
  • Do not use reagents that are not given.
  • Is the product a result of a rearrangement? Only intermediates can rearrange.
  • Is one of the reagents H3O+? If so, use it in the 1st step. Do not create negatively charged species in acid.