Objective of the book
The purpose of this book is to teach students how to learn
organic chemistry. Because reactions and reaction
mechanisms are the most difficult challenge, the focus of
this book is entirely on reaction mechanisms.
Pre-bonds
From experiments I performed in my class, I learned that
many students became confused in their attempts to learn
reaction mechanisms. A particular difficulty was the use of
the curved arrow convention, especially when a new bond is
formed from a covalent bond. For example, in a Markovnikov
addition reaction, the student must know the carbocation
stabilities before comprehending the meaning of the curved
arrow. The curved arrows do not tell the student what the
product must be. I found a dashed line was often used to
guide bond formation in Diels-Alder or rearrangement
reactions. I have adopted this convention and used the term
‘pre-bonds’ for them. By
adopting them, students have reported mechanisms with
‘pre-bonds’ were easier to understand.
Table of Contents and Index
The Table of Contents and
Index show the depth of coverage of
reaction types. There is also an extended index to
enable students using textbooks organized by
functional groups. The index correlates reactions by
reactions of starting materials and reactions to yield
products by functional group type.
Reaction Mechanisms
A reaction mechanism is an attempt to explain the electron
movements that lead from the starting materials to the
products. In order to predict the products of a reaction,
knowing how reactions take place is key. The mechanisms are
written to give a logical explanation that will
ultimately result in the product.
In answering student questions, I have found some students
do not wish to see a reaction mechanism. However, if those
students knew the reaction mechanism, they would not be
asking the question. The questions reveal a lack of
understanding that is common.
Example Based Teaching
For organic chemistry, I prefer examples of reactions
rather than generic descriptions with conditions and
exceptions. Many students may think that chemistry is very
black and white, right or wrong. However, there is a much
larger degree of uncertainty than many students realize.
For example, there is much confusion over SN1, SN2, E1, and
E2 reactions. Before I wrote the mechanisms for these
reactions, I first reviewed several texts. In doing so, I
was seeking to learn what common factors I could present. I
did not think that I could clarify this confusion in some
insightful paragraph. I did two things. First, I looked for
examples that produced clear-cut results and that all
textbooks would agree upon. That is, if you learned this
reaction, you could be certain that it was correct.
Secondly, I also looked for examples in which by-products
were clearly identified. This gives an indication of other
electron movements that might be competing with the
described reaction. It was my objective that as you learn
what the optimal conditions, you can begin to recognize the
limitations to those reactions and expect to encounter
another mechanisms or by-products.
Concerted Reactions
Concerted reactions are one of the largest differences in
the mechanisms in A Guide to Organic Chemistry
Mechanisms and other books. I have made a philosophic
choice to not write reaction concerted reactions unless
there was overwhelming evidence in its favor, or if no
mechanistic understanding was served in a stepwise
equivalent.
The reaction of an alkene with bromine is generally written
as a concerted reaction involving three pairs of electrons
and bromine ending up with a positive charge. I however
disagree with this single-step concerted mechanism. A
concerted reaction asks bromine to acquire a positive
charge and the concerted reaction makes no electronic
demands from an alkene. The first step for many other
alkene reactions is electron donation. In other bromine
reactions, it acts as an electron acceptor. I believe a
step-wise mechanism better illustrates this chemistry and
thus should be preferred. I would ask that you write a
stepwise reaction and thus agree with other stepwise
mechanisms (although you may also write it as a concerted
reaction if you wish). I plan to discuss this reaction
further at another time.
The addition of HCl to an alkene is known to not be a 2 + 2
addition. In writing other electrophilic reactions or
alkenes, I sought a uniform reaction mechanism. I did not
wish to require the electrons of an alkene to react
differently with other electrophiles. Accordingly, other 2
+ 2 addition reactions are written as stepwise reactions
and maintain this pattern. (Reactions of nucleophiles with
alkenes as electrophiles are a different mechanism and
occur in another chapter of the book.) A definition for a
concerted reaction is any steps that occur faster than atom
movements. Thus, if the bonds do not rotate in the
intermediate, then it can be considered concerted. I did
not feel that a slow electrophilic reaction followed by a
fast electron ring formation step contradicted a concerted
reaction.
For example, although non-concerted Diels-Alder reactions
have been suggested, no mechanistic advantage is served by
a stepwise alternative. A Guide to Organic Chemistry
Mechanisms does contain an example of a Diels-Alder
reaction in which a stepwise mechanism may help explain the
outcome of the reaction. However, the same result can also
be anticipated by the polarity of the reactants. The
example shows you how to discover the polarity and match
them to the major product.