Organic Chemistry 307 – Drawing Structures – H. D. Roth
Structures are an integral part of what we know about molecules and reactions;
drawing them properly helps you convey what you know and understand to your fellow
students and your teachers. If you draw a structure poorly it is difficult to assess what you
know. A well-drawn structure should be simple to draw yet convey ALL relevant
information. It may be helpful to establish some rules about what kind of structures are
practical, which are acceptable and, particularly, which kind of structures are incorrect.
Wade Organic Chemistry introduces ‘condensed structure’ representations and
‘line-angle drawings’ in Section 1-10 (see Table 1-4). Let’s look at some structures that
students have drawn for neopentyl chloride:
H
H
C
H
H
C
C
C
H
H
C
HH
H
H
CH3
Cl
H3C
C
CH2
Cl
Cl
CH3
2
1
H
H2
C
(H3C)3C
3
C
Cl
4
X
5
Cl
C
6
X
C
C
C
Cl
Structures 1 – 4 are correct: they show all groups present and all bonds between
them. Structure 1, a Lewis structure, isn’t very good because you spend a lot of time
drawing it and the essential feature, the chlorine atom at a carbon next to a quaternary
carbon, gets lost in the ocean of letters. Structures 2 and 3 are better; structure 4 offers the
clearest, least “congested” representation and is most easily drawn. In such a ‘line-angle
drawing’ each corner, line crossing, or line end represents a carbon atom; the appropriate
number of hydrogen atoms at each carbon is implied. If you choose a line structure, make
Organic Chemistry 307 – Drawing Structures – H. D. Roth
sure that the C–C–Cl bond shows an angle (arrow in 4); without the angle (arrow in 5) we
can’t be sure whether you understand that there is an extra carbon or whether you simply
drew a long bond. Practicing to draw a few ‘line-angle drawing’s should be part of your
preparation for the exams.
Structure 6, written by some, is incorrect: once you write a letter (C) you MUST
show ALL atoms attached to it (as in structures 1 – 3). This is also necessary for N as in
diisopropyl amine, H–N[CH(CH3)2]2, 7; of course, the anion, 8, doesn’t have an H any
more. There are also problems with the –OH function. You know that O is divalent and
that H is monovalent. Please, attach the bond to the O (as in HO– or –OH, e.g., 9), but
never to the H (OH– or –HO, e.g., 10).
H
N
N
7
OH
8
9
X
HO
10
When drawing cyclohexanes (Chapter 3), aim for structures like those below
(bold lines for emphasis). Axial substituents are shown as vertical lines; please note that
‘down-zig-up-zag’ corners (structure 11, bold blue) should have downward substituents
and that ‘up-zig-down-zag’ corners (11, bold purple) should have upward substituents.
The axial substituents should go beyond the cyclohexane bond on the opposite ring side
(11, bold red). NEVER show an axial substituent in the wrong direction (as in 13).
X
11
12
13
Organic Chemistry 307 – Drawing Structures – H. D. Roth
Equatorial substituents should point in directions parallel to the ring bond one
bond removed (structure 12, bold blue, bold red, and in regular line weight black).
Proper placement of axial and equatorial bonds will be helpful when you draw cis- and
trans-disubstituted cyclohexanes (Chapters 3,5).
Avoid the dreaded ‘Dachshund’ cyclohexane (14) as well as distorted structures
with stunted, poorly directed bonds (15); the bonds connecting substituents to the ring
should be of similar lengths as the cyclohexane ring bonds.
Cl
H
X
X
Cl
H
14
15
The proper representation of three-dimensional structures will become important
when we begin dealing with stereochemistry (Chapter 5). This is an area presenting
problems to many students. What is the proper way to represent a tetrahedron (a tetravalent carbon atom) in three dimensions? We use ‘dash and wedge’ structures. These are
horizontal ‘line-angle drawings’ (zig zag in the plane of the paper or screen, with angles
>90°). Only the substituents at crucial carbons are shown as wedges or dashes: the wedge
signifies a substituent above (or in front of) the plane (e.g., Br in 16), whereas the dash
(H in 16) denotes a substituent below (or behind) the plane. Dashes and wedges always
point away from the chain. The angle between dash and wedge should be <45°; this is a
consequence of the carbon being tetrahedral. Check it out on models: if you place the
carbon chain in the plane, wedges and dashes are actually eclipsing (see Chapter 3).
H Br
16a
H Br
16b
H Br
16c
H Br
X
16d
Organic Chemistry 307 – Drawing Structures – H. D. Roth
Wedges are always drawn as in 16, but there are several ways to render the dash.
It is easy to draw the dash with ChemDraw as in 16a, a series of parallel lines increasing
in size with distance from the carbon bearing the substituent; this is how it is done in
WebAssign. It isn’t easy to draw this type of dash by hand on paper; therefore, you may
use a series of parallel lines (as in 16b) or simply a dashed line (as in 16c). Both are
unambiguous and recognized by the grading staff. Note that Wade Organic Chemistry
uses a ‘reverse’ dash, i.e., a series of parallel lines decreasing in size with distance from
the carbon bearing the substituent. This is an idiosyncrasy of this particular textbook; we
strongly advise against using ‘reverse’ dashes as in 16d.
There is one more simplification to consider and one pitfall to avoid. If an H is a
substituent on a wedge or dash you may omit it (remember: he correct number of
hydrogens is implied); you may draw structure 16 as shown below (16e) or, if you ‘flip’
the molecule, as 16f. Structures 16e and 16f show all essential information and are
slightly less cluttered. Concerning the pitfall, some students believe that structures 16a
and 16h are mirror images; they are not. According to the dash and wedge convention it
does not matter whether you point the wedge a little bit to the left or right; a wedge is
always above the plane (or in front of the screen): structures 16a and 16g are identical.
Br
Br
16e
16f
H Br
16a
Br H
16g
If you follow these simple rules and avoid the pitfalls, you will effectively
communicate to your graders what you know; you will make your own life a little easier
and those of your graders as well. :-)