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Step
2.
Chemistry: identify the start (the C—Br bond) and
the end (the Br atom) for the next curly arrow. The end point
for this second reaction is an electronegative atom that can be thought
of as an "electron sink", and the result of the reaction is that Br-
is the "leaving group".
Computer: now draw the second arrow, using the mouse as
before. The second arrow will appear, along with the product, or in
extended examples, the next intermediate in the reaction.
Make sure that you understand what has happened in the reaction. You may also be given some extra information, which you should read; however, this may sometimes cover aspects of chemistry that you have not yet covered in your lectures. You will benefit from coming back to "Curly Arrows" from time to time, both to keep practising your arrow-drawing skills and to learn more about the background to the reactions.
Drawing reaction mechanisms for various types of reactions.
Historically, nucleophilic substitution reactions were among the first to be studied mechanistically, and much progress was made in the first half of the 20th century. The first section of this course focuses mainly on the reactions of nucleophiles with alkyl halides.
- It is important to remember exactly what a curly arrow represents, i.e. movement of electrons. The arrow always starts at a region of relatively high electron density and ends at a region of lower electron density.
- Usually you will want to make a bond between a nucleophile and an electrophile. If this involves making a bond to a carbon atom, use a curly arrow to break one of the pre-existing bonds to carbon (usually a pi-bond to oxygen or a sigma-bond to a good leaving group (a good "electron sink").
- In simple reactions such as SN2 substitution, this may lead directly to the product. In more complex examples, several mechanistic steps may be required.
- Once
you have correctly drawn the curly arrow(s) for each step, the product
or intermediate will be automatically displayed. If a structure is
complex or unfamiliar, concentrate on the basics and apply the chemistry
that you know to propose a viable mechanism for the particular reaction.
Further Reading
Peter
Sykes, A Primer to Mechanism in Organic Chemistry,
Longman, Harlow, 1995
Peter Sykes, Guidebook
to Mechanism in Organic Chemistry, 6th Edition, Prentice Hall,
1986.
Daniel P Weeks, Pushing Electrons, 3rd Edition,
Saunders, Orlando, 1997.
Jonathan
Clayden, Nick Greeves, Stuart Warren and Peter Wothers, Organic Chemistry,
OUP, Oxford, 2001; 123-133.
This quote
comes from the Preface to Clayden et al.
"All practising chemists protect themselves from being crushed by the vastness of organic chemistry by moulding it and ordering it with curly arrows. Without curly arrows, chemistry is chaos, and impossible to learn. Curly arrows unify chemistry, and are essential to the solution of problems. They allow known chemistry to be presented mechanism by mechanism and unknown chemistry to be predicted. We devote most of Chapter 5 to the technique of writing curly arrow mechanisms, and throughout the rest of the book we continually stress the use of curly arrows as the chemist's most important tool outside the laboratory."
Web Links that give more information
Curved
Arrows Tutorial
Virtual
Textbook of Organic Chemistry
Curved
Arrow Notation
Quiz
on the correct use of curly arrows
Using
Curly Arrows in Mechanisms
Introduction
to Mechanism
Molecules
in Four Dimensions
Educational materials for
organic chemistry