Carboxylic Acids and Reduction/Oxidation

• Addition of H (or “H-”) Reduction 2 • Loss of O2 or O

carboxylic aldehyde alcohol alkane acid (or ketone)

• Loss of H 2 Oxidation • Addition of O2 or O

+ Neither oxidation nor reduction: Addition or loss of H , H2O, or HX

Carboxylic acids are the most oxidized functional group of carbon.

Carboxylic Acids via Oxidation

Chromate oxidation of primary alcohols:

unavoidably Na2Cr2O7

H2SO4 aldehyde carboxylic alcohol acid

Oxidation of aldehydes with Tollens’ reagent:

1. Ag2O, NH3 2. H O+ 3 Selective for aldehydes; will not oxidize alcohols.

aldehyde carboxylic acid Carboxylic Acids via Oxidation

Oxidative cleavage of alkynes:

1. O3

2. H2O

internal alkyne

Oxidation of benzylic carbons:

KMnO4

Purifying Carboxylic Acids by Acid-Base Extraction

Acidic and basic organic molecules can be separated from other substances by manipulating their protonation state and solubility.

Let’s say we run a reaction.

Na2Cr2O7 + unreacted

H2SO4 + Na+ 3+ We want this… + Cr(H2O)6 + organic + HSO - …but not these. side products 4

How do we isolate our desired product from the other materials, without using distillation or chromatography? Acid-Base Extraction

Step 1: Remove inorganics from organics via differential solubility.

a separatory

(sept) funnel - Na+ HSO4 Cr(H O) 3+ H2O 2 6

CHCl3 + organic side densities: products (H2O) = 1 g/mL (CHCl3) = 1.48 g/mL

A large density difference ensures the two liquids separate completely and quickly. (Much faster than, say, oil and water.)

Step 2: Add basic water to deprotonate carboxylic acid, and transfer it to the aqueous phase.

- Na+ HSO4 3+ discard Cr(H2O)6 H2O layer

+ organic + organic side side products products

add discard NaOH/ H2O CHCl3 layer

+ organic side products Step 3: Re-acidify carboxylate to return it to organic solution. Remove solvent in vacuo to yield pure acid.

add

HCl/ H2O, fresh CHCl3 discard

H2O layer

remove solvent in rotary evaporator

Chemistry student, removing solvent in a “rotovap”

Nucleophilic Addition to Carbonyls

Heteroatom electron donors add reversibly to C=O groups.

Equilibrium driven + to the left by entropy.

Also true for Cl , HS , NH3 nucleophiles.

Hydrogen anions (“hydride”) and carbon anions add irreversibly to C=O groups.

+ Reaction driven to the right by instability of - - charge on H , RCH2 . +