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Macroscale and Microscale Organic Experiments 6th Edition



Macroscale and Microscale Organic Experiments 6th Edition

Author: Kenneth L. Williamson

Publisher: Cengage Learning

Genres:

Publish Date: May 3, 2010

ISBN-10: 0538733330

Pages: Pages

File Type: PDF

Language: English

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Book Preface

The main goal of the laboratory course is for you to learn and carry out techniques for the synthesis, isolation, purification, and analysis of organic compounds, thus experiencing the experimental nature of organic chemistry. We want you to enjoy your laboratory experience and ask you to remember that safety always comes first.

E X P E R I M E N TA L O R G A N I C C H E M I S T R Y

You are probably not a chemistry major. The vast majority of students in this laboratory course are majoring in the life sciences. Although you may never use the exact same techniques taught in this course, you will undoubtedly apply the skills taught here to whatever problem or question your ultimate career may present.
Application of the scientific method involves the following steps:
1. Designing an experiment, therapy, or approach to solve a problem.
2. Executing the plan or experiment.
3. Observing the outcome to verify that you obtained the desired results.
4. Recording the findings to communicate them both orally and in writing.
The teaching lab is more controlled than the real world. In this laboratory environment, you will be guided more than you would be on the job. Nevertheless, the experiments in this text are designed to be sufficiently challenging to give you a taste of experimental problem-solving methods practiced by professional scientists.
We earnestly hope that you will find the techniques, the apparatus, and the experiments to be of just the right complexity, not too easy but not too hard, so that you can learn at a satisfying pace.

Macroscale and Microscale Experiments
This laboratory text presents a unique approach for carrying out organic experiments; they can be conducted on either a macroscale or a microscale. Macroscale was the traditional way of teaching the principles of experimental organic chemistry and is the basis for all the experiments in this book, a book that traces its history to 1934 when the late Louis Fieser, an outstanding organic chemist and professor at

Harvard University, was its author. Macroscale experiments typically involve the use of a few grams of starting material, the chief reagent used in the reaction. Most teaching institutions are equipped to carry out traditional macroscale experiments. Instructors are familiar with these techniques and experiments, and much research in industry and academe is carried out on this scale. For these reasons, this book has macroscale versions of most experiments.

For reasons primarily related to safety and cost, there is a growing trend toward carrying out microscale laboratory work, on a scale one-tenth to onethousandth of that previously used. Using smaller quantities of chemicals exposes the laboratory worker to smaller amounts of toxic, flammable, explosive, carcinogenic, and teratogenic material. Microscale experiments can be carried out more rapidly than macroscale experiments because of rapid heat transfer, filtration, and drying. Because the apparatus advocated by the authors is inexpensive, more than one reaction may be set up at once. The cost of chemicals is, of course, greatly reduced. A principal advantage of microscale experimentation is that the quantity of waste is one-tenth to one-thousandth of that formerly produced. To allow maximum flexibility in the conduct of organic experiments, this book presents both macroscale and microscale procedures for the vast majority of the experiments. As will be seen, some of the equipment and techniques differ. Acareful reading of both the microscale and macroscale procedures will reveal which changes and precautions must be employed in going from one scale to the other.

Synthesis and Analysis

The typical sequence of activity in synthetic organic chemistry involves the following steps:
1. Designing the experiment based on knowledge of chemical reactivity, the equipment and techniques available, and full awareness of all safety issues.
2. Setting up and running the reaction.
3. Isolating the reaction product.
4. Purifying the crude product, if necessary.
5. Analyzing the product using chromatography or spectroscopy to verify purity and structure.
6. Disposing of unwanted chemicals in a safe manner.
1. Designing the Experiment
Because the first step of experimental design often requires considerable experience, this part has already been done for you for most of the experiments in this introductory level book. Synthetic experimental design becomes increasingly important in an advanced course and in graduate research programs. Safety is paramount, and therefore it is important to be aware of all possible personal and environmental hazards before running any reaction.

2. Running the Reaction

The rational synthesis of an organic compound, whether it involves the transformation of one functional group into another or a carbon-carbon bond-forming reaction, starts with a reaction. Organic reactions usually take place in the liquid phase and are homogeneous—the reactants are entirely in one phase. The reactants can be solids and/or liquids dissolved in an appropriate solvent to mediate the reaction. Some reactions are heterogeneous—that is, one of the reactants is a solid and requires stirring or shaking to bring it in contact with another reactant. Afew heterogeneous reactions involve the reaction of a gas, such as oxygen, carbon dioxide, or hydrogen, with material in solution.

An exothermic reaction evolves heat. If it is highly exothermic with a low activation energy, one reactant is added slowly to the other, and heat is removed by external cooling. Most organic reactions are, however, mildly endothermic, which means the reaction mixture must be heated to overcome the activation barrier and to increase the rate of the reaction. A very useful rule of thumb is that the rate of an organic reaction doubles with a 10°C rise in temperature. Louis Fieser introduced the idea of changing the traditional solvents of many reactions to highboiling solvents to reduce reaction times. Throughout this book we will use solvents such as triethylene glycol, with a boiling point (bp) of 290°C, to replace ethanol (bp 78°C), and triethylene glycol dimethyl ether (bp 222°C) to replace dimethoxyethane (bp 85°C). Using these high-boiling solvents can greatly The progress of a reaction can be followed by observing: a change in color or pH, the evolution of a gas, or the separation of a solid product or a liquid layer. Quite often, the extent of the reaction can be determined by withdrawing tiny samples at certain time intervals and analyzing them by thin-layer chromatography or gas chromatography to measure the amount of starting material remaining and/or the amount of product formed.

The next step, product isolation, should not be carried out until one is confident that the desired amount of product has been formed.


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