Sep. Guide - Lab experiment work for 322A PDF

Preview

Summary

Lab experiment work for 322A...

Description

CHEM 322a/b - Organic Chemistry Updated: Spring 2015

Instructor: Dr. Jennifer Moore University of Southern California

Guide to Separation Schemes Introduction In chemical synthesis, there is often a need to isolate a specific chemical substance, to purify it. This process is the physical separation of a chemical substance from other substances. For pharmaceutical drugs, for instance, this could be the removal of toxic byproducts or contamination. As a result, it is often the purification that can be the most challenging aspect in chemical synthesis, regardless of how easy chemical schemes look on paper. To this end, the laboratory component of CHEM 322a/b is designed to make you think critically about the molecules you are manipulating, and to apply concepts your learn in lecture, lab lecture, and through course materials (such as solubility, polarity, mechanisms, and acid/base reactions) to real chemistry. Because even if you will never be a chemist, the ability to solve problems and provide solutions to multi-layered challenges by understanding underlying concepts is vitally important. What is a Separation Scheme? A separation scheme is an informative flow chart that tells you what you have to do to separate products from byproducts. This helps overcome one of the biggest limitations for new chemists—not being able to see molecules. Because they cannot physically see the molecules as they are working with them, they must rely on their chemical intuition. In cooking, for example, a chef can easily separate pasta from water with a strainer. For a distiller, beverages like vodka are ultimately distilled from fermented grains or potatoes. In both cases, a technique (filtration or distillation) is used to remove unwanted substances (water or fermented potatoes) to isolate a product (pasta or vodka). When does a Separation Scheme Begin & End? A separation scheme begins at the end of a reaction, or for teaching purposes, when you are given an impure mixture. It ends when you have isolated a pure product that is desired. Along the way, you will utilize several different physical manipulations or techniques, such as extraction/washing, rinsing, filtration, distillation, evaporation, and drying. We will break down the type of separations each of these techniques encompass and how to build your separation scheme. I. Extraction & Washing. The prerequisites for extraction and washing are two immiscible solvents, which will form two separate layers in a separatory funnel (macroscale) or test tube (microscale). For the actual technique, there is no difference between extraction and washing. In both cases, an organic compound, byproduct, or impurity is dissolved in one layer or the other. The layers are disturbed (by shaking or bubbling air through them with a pipet). This increases the surface area between the two layers. These components will partition between the two immiscible solvents, which means they will “choose” a solvent in which it has greater solubility. Generally, this will be either the aqueous (aq.) layer, which is mostly water, or the organic (org.) layer. Let’s cover what types of aqueous layers and organic layers are most common in extraction/washing. To help you figure out which layer would be which (what is top, what is bottom), the density of organic solvents is provided. Remember that the aqueous layer typically has a density of 1g/mL, heavier solvents (>1 g/mL) will be below this layer, lighter solvents (...