Tehniques labs for microscale and macroscale organic chemistry.pdf

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Licensed to: iChapters User

Licensed to: iChapters User

Techniques Labs for Macroscale and

Microscale Organic Experiments,

Sixth Edition

Kenneth L. Williamson, Katherine M. Masters

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CHAPTER

Introduction

When you see this icon, sign

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to access

videos, Pre-Lab Exercises, and other

online resources.

PRELAB EXERCISE: Study the glassware diagrams presented in this chap-

ter and be prepared to identify the reaction tube, the fractionating column,

the distilling head, the filter adapter, and the Hirsch funnel.

Welcome to the organic chemistry laboratory! Here, the reactions that you learned

in your organic lectures and studied in your textbook will come to life. 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 experi-

encing the experimental nature of organic chemistry. We want you to enjoy your

laboratory experience and ask you to remember that safety always comes first.

EXPERIMENTAL ORGANIC CHEMISTRY

You are probably not a chemistry major. The vast majority of students in this labo-

ratory 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:

Designing an experiment, therapy, or approach to solve a problem.

Executing the plan or experiment.

Observing the outcome to verify that you obtained the desired results.

Recording the findings to communicate them both orally and in writing.

The teaching lab is more controlled than the real world. In this laboratory envi-

ronment, 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 scien-

tists. 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 experi-

ments; 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

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Techniques Labs for Macroscale and Microscale Organic Experiments

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 one-

thousandth of that previously used. Using smaller quantities of chemicals exposes

the laboratory worker to smaller amounts of toxic, flammable, explosive, carcino-

genic, 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 maxi-

mum 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. A careful reading of both

the microscale and macroscale procedures will reveal which changes and precau-

tions 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:

Designing the experiment based on knowledge of chemical reactivity, the

equipment and techniques available, and full awareness of all safety issues.

Setting up and running the reaction.

Isolating the reaction product.

Purifying the crude product, if necessary.

Analyzing the product using chromatography or spectroscopy to verify purity

and structure.

Disposing of unwanted chemicals in a safe manner.

1. Designing the Experiment

Because the first step of experimental design often requires considerable experi-

ence, 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 para-

mount, and therefore it is important to be aware of all possible personal and envi-

ronmental hazards before running any reaction.

2. Running the Reaction

The rational synthesis of an organic compound, whether it involves the transfor-

mation of one functional group into another or a carbon-carbon bond-forming reac-

tion, starts with a

reaction.

Organic reactions usually take place in the liquid phase

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Chapter 1

■

Introduction

Effect of temperature

Chapters 8–10:

Chromatography

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. A few heterogeneous

reactions involve the reaction of a gas, such as oxygen, carbon dioxide, or hydro-

gen, with material in solution.

exothermic

reaction evolves heat. If it is highly exothermic with a low acti-

vation 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 intro-

duced the idea of changing the traditional solvents of many reactions to high-

boiling 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

increase the rates of many reactions.

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 sam-

ples at certain time intervals and analyzing them by

thin-layer chromatography

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 confi-

dent that the desired amount of product has been formed.

3. Product Isolation: Workup of the Reaction

Running an organic reaction is usually the easiest part of a synthesis. The real chal-

lenge lies in isolating and purifying the product from the reaction because organic

reactions seldom give quantitative yields of a single pure substance.

In some cases the solvent and concentrations of reactants are chosen so that

after the reaction mixture has been cooled, the product will

crystallize

precipitate

if it is a solid. The product is then collected by

filtration,

and the crystals are washed

with an appropriate solvent. If sufficiently pure at that point, the product is dried

and collected; otherwise, it is purified by the process of

recrystallization

or, less com-

monly, by

sublimation.

More typically, the product of a reaction does not crystallize from the reaction

mixture and is often isolated by the process of

liquid/liquid extraction.

This process involves two liquids, a water-insoluble organic liquid such as

dichloromethane and a neutral, acidic, or basic aqueous solution. The two liquids

do not mix, but when shaken together, the organic materials and inorganic byprod-

ucts go into the liquid layer that they are the most soluble in, either organic or aque-

ous. After shaking, two layers again form and can be separated. Most organic

products remain in the organic liquid and can be isolated by evaporation of the

organic solvent.

If the product is a liquid, it is isolated by

distillation,

usually after extraction.

Occasionally, an extraction is not necessary and the product can be isolated by the

process of

steam distillation

from the reaction mixture.

Chapter 4:

Recrystallization

Chapter 7:

Liquid/Liquid Extraction

Chapter 5:

Distillation

Chapter 6:

Steam Distillation and

Vacuum Distillation

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