Classification of Organic Reactions Study Guide

CHAPTER 11 1. Classification of organic reactions o Oxidation—increases number of bonds to O, N, halogens or decreases number of bonds to hydrogen. o Reduction—addition of H2 (or H-), loss of O or O2, loss of X2 o Neither—elimination or addition of HX, H2O (X=halogen) Gain C—H bond (reduction) and C-Cl (oxidation). Neither oxidation nor reduction. 2. Oxidation of Alcohols A) Oxidation of secondary alcohols How it works: B) Oxidation of primary alcohols (difference—can either go to acid or aldehyde) C) Oxidation of Tertiary Alcohols Generally cannot be oxidized D) Biological oxidation of alcohols Why can drink ethanol and NOT methanol? –ethanol gets oxidized to relatively non-toxic compounds in human body will not need to know in Organic 1 Get acetaldehyde, then another NAD+ oxidizes acetaldehyde to CH3CO2H—not toxic. If someone drinks CH3OH, product of oxidation is , causes blindness. 3. Tosylates and Alcohols as Electrophiles We have learned to use alcohols as nucleophiles, but they can be used also as electrophiles To use alcohol as an electrophile need to convert –OH to a good leaving group. One way is protonation: How else can we convert OH into a good leaving group? 4. Alcohol conversion to alkyl halides A. Tosylates to Halides One way is to convert alcohol to tosylate and then displace with halide. This is used sometimes but there are methods that do this transformation in one step. B. Formation of alkyl halides from 3o alcohols C. For 1o, 2o halides just HCl is not good, need to use mixture of HCl with ZnCl2 Reaction mech is SN1 with 2o alcohols, SN2 with 1o alcohols Problems: 1. Cannot make allyl iodides this way. 2. Elimination is common 3. Rearrangements D. With phosphorous halides 1. Make bromides with PBr3—can use only 1o, 2o alcohols 2. PCl3 reacts exactly the same way as does PCl5 3. PI3 is unstable, so use P/I2 mixture E. SOCl2 (thionyl chloride)—byproduct (HCl, SO2) are gases. DO NOT memorize mechanism, just remember when you have inversion and when retention. 1. If use SOCl2 in dioxane, get retention of configuration 2. Second possibility is to use amine instead of dioxane – get inversion 5. Dehydration of alcohols A. Acid catalyzed—we have already talked about it. o o Mechanism: E1 Intermediate carbocation formed gives most stable alkane (Saytzeff’s rule). Problem with primary alcohols: o Do not dehydrate well since they cannot form carbocation. Therefore, rearrangements will occur. B. Use POCl3 (phosphorous oxychloride). Can use it also for dehydration of 2o and 3o alcohols. 6. Formation of ethers/esters Ethers have two C’s attached to O. Examples: A. Acid Catalysis. Cannot make unsymmetrical ethers this way! : B. Unsymmetrical ethers can be made using Williamson ether synthesis Can use only 1o halides! No 2-3o—will eliminate For alkoxide (ROK) can use 1o, 2 Otherwise 3o will eliminate (3o can only use if primary halide): C. Ester formation Esters are compounds containing carboxylic acid and alcohol A. Fisher esterification (intermolecular alcohol dehydration). Mechanism will be covered in Org II. It is in equilibrium, so need to remove one of the products to drive reaction to the right (water usually removed). B. Can make ester also by reaction of alcohol with acid chloride C. Similarly form tosylate esters 7. Unique reaction of 1,2 diols Generally diols react just like alcohols, but in a few cases they behave differently. A. Periodic acid cleavage B. Pinacol Rearrangement 8. Summary 1. Oxidation of alcohols a. Secondary: b. Primary: 2. Conversion to alkyl halides 3. Dehydration 4. Ether/Ester formation 5. Unique diol reactions