As stated in the syllabus, the point of organic chemistry
is synthesis, the construction of more complex molecules from simpler
ones. By the end of the semester, you will have to solve synthesis
problems in which you put together a sequence of several reactions to
convert a starting molecule into a product molecule or product molecules.
How do you study to prepare yourself for these problems? I believe the
best way to review/learn organic chemistry reactions is through the creation
of a so-called roadmap.
A roadmap is a large piece of paper, usually the size of a poster, in which
the essential information (i.e. reagents used, stereochemistry, regiochemistry,
key mechanistic steps) about each reaction we study is recorded. To make a
1) Write the names of the key functional groups we study (alkanes,
alkyl halides, vicinal dihalides, alkenes, alkynes, alcohols, ketones/aldehydes,
carboxylic acids, carboxylic acid derivatives, amines, thiols, ethers,
epoxides, various aromatic compounds we will study this semester)
spread out over the paper. I have prepared a template for 310M/318M.
As an additional aid I have prepared
chart summary that lists the important reactions from the entire
semester of 310M/318M . You
can click here for a pdf
version of the 310M reaction summary chart.
2) List all reactions as arrows that point from a starting functional
group to a product functional group.
3) On the top of the arrow is written the reagents required to carry
out the reaction. For example, you would write an arrow from alkenes
to alkanes that has H2 / Pt,
Pd, or Ni written on top of it to describe catalytic hydrogenation.
4) Just under the arrow write the important features of the mechanism
of the reaction, listing the stereochemistry (syn or anti), regiochemistry
(Markovinikov, non-Markovnikov), and key mechanistic steps ( i.e. free
radical process, E2 elimination, occurs on metal surface, etc.). Note
that not all reactions have stereochemistry or regiochemistry considerations
to keep track of, but many do and these issues were discussed at the
time the reactions were introduced to you. Recall that knowing the exact
stereochemistry or regiochemistry of a reaction, if relevant, will tell
you exactly which isomer, either which stereoisomer or regioisomer,
of a given product is produced (i.e. cis or trans, R or S configuration,
primary or secondary alcohol, etc.)
5) Keep track of those few precious reactions that make or break carbon-carbon
bonds. I will make a big deal of these in class.
For example, I would have the following written on an arrow from alkenes to
alkanes, describing transition metal catalyzed hydrogenation of alkenes:
Click here to see
the suggested arrangement of functional groups for a first semester
roadmap (310M/318M), with some representative examples filled in. This
arrangement was chosen to minimize the number of arrows that must
cross each other, although some will. Write the functional groups
in this arrangement on your large piece of paper, leaving plenty of
space between functional groups.
Make sure you use a VERY LARGE sheet of paper, and leave plenty of
room between functional groups.
I recommend you view the map as being roughly analogous to the state
of Texas, with the I-35 corridor represented by alkanes (like San Antonio
in the south), followed by alkyl halides (San Marcos), alkenes (Austin),
vicinal or geminal dihalides (Waco) and alkynes (Dallas). You will find
that as you convert different functional groups in synthesis problems,
you will often travel along this path. Even more, you will notice that
the alkenes, aka Austin, is the most versatile functional group covered
in 310M/318M and it is the center of the "action", as Austin
is in real life, of course. Most syntheses involve alkenes in one step
or another (just like Texans generally find some reason to go through
Austin whenever traveling within the state).
By the time you have completed this task for all the reactions we learn,
you should be struck by how alkenes (aka Austin) really is at the center
of the chemistry you learned in 310M/318M. That is, alkenes have the
most arrows pointing toward them and away from them. You should now
be ready to study for synthesis. For example, you should be able to
see that if you were asked to synthesize an alkene from a corresponding
alkane, you cannot do it directly. Rather, you would break it into
two steps (alkane to alkyl halide using X2 and light or heat, followed
by an E2 elimination of the alkyl halide using base such as KOtBu making
sure to follow Zaitsev's rule). I call this a roadmap, because synthesis
comes down to moving from one location (i.e. functional group)
on the roadmap, to a different location, and the arrows (i.e. roads)
tell you how you must travel. For 310M/318M, "I-35" is the most
often traveled route in synthesis problems!
A final comment. Many students find it helpful to keep the large
roadmaps posted on their wall. This will not only reinforce the material
several times a day as you glance at it, it will also greatly impress
your friends, parents, and roommates who do not know organic chemistry.
As you go through the semester in 310M starting with Chapter 6, add
new reactions to the roadmap after each lecture. It will help provide
a big picture understanding of the chemistry you are learning!