Presentation on theme: "Reactions of Alcohols Oxidation R-X, Ether, and Ester Preparation Protection of Alcohols Synthesis The Logic of Mechanisms."— Presentation transcript:
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
Oxidation of 2 o Alcohols with Cr(VI)
Oxidation of 1 o Alcohols
PCC oxidizes 1 o Alcohols to Aldehydes
Pyridinium Chlorochromate (PCC) PCC is a complex of chromium trioxide, pyridine, and HCl. Oxidizes primary alcohols to aldehydes. Oxidizes secondary alcohols to ketones.
Oxidation of 1 o Alcohols to Aldehydes: PCC
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); 3 o alcohol is not oxidized, therefore no color change.
Reaction of Alcohols with HBr –OH of alcohol is protonated. –OH 2 + is good leaving group. 3° and 2° alcohols react with Br – via S N 1. 1° alcohols react via S N 2.
Step 1: Protonation. Step 2: Formation of the carbocation. Step 3: Bromide attacks the carbocation. S N 1 Mechanism
S N 1: Carbocations can Rearrange
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. The alcohol is protonated by the strong acid. This protonated secondary alcohol loses water to form a secondary carbocation. Solved Problem 2 Solution
A hydride shift transforms the secondary carbocation into a more stable tertiary cation. Attack by bromide leads to the observed product. Solved Problem 2 (Continued) Solution (Continued)
Alcohol is acidic enough to react preferentially.
Mechanism is S N 2
Protect as trimethylsilyl ether
Give the Reagents…
No Protection needed
Road Map Problem
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 H 3 O + ? If so, use it in the 1 st step. Do not create negatively charged species in acid.