Bond Line Drwaigns

Bond-line drawings
Carbon and its compounds
PC4012

Carbon and its compounds
PC4012

Name: ________________________________________________
List #: ___________ Group: ___________

To do well in Organic Chemistry, you must first learn to interpret the drawings that organic chemists use. When you see a drawing of a molecule, it is absolutely critical that you can read all of theinformation contained in that drawing. Without this skill, it will be impossible to master even the most basic reactions and concepts.

Molecules can be drawn in many ways:



Without a doubt, the last structure (bond-line drawing) is the quickest to draw, the quickest to read, and the best way to communicate. Most students will gain a familiarity with these drawings over time, notrealizing how absolutely critical it is to be able to read these drawings fluently.

1.1 HOW TO READ BOND LINE DRAWINGS

Bond line drawings show the carbon skeleton (the connections of all the carbon atoms that build up that backbone, or skeleton, of the molecule) with any functional groups that are attached, such as –OH or –Br. Lines are drawn in a zigzag format, so that the end of every linerepresents a carbon atom. For example, the following compound has 6 carbon atoms:

It is a common mistake to forget that the ends of lines represent carbon atoms as well. For example, the following molecule has six carbon atoms (make sure you can count them):

Double bonds are shown with two lines, and triple bonds are shown with three lines:



When drawing triple bonds, be sure todraw them in a straight line rather that zigzag, because triple bonds are linear. This can be quite confusing at first, because it can get hard to see just how many carbon atoms are in a triple bond, so let’s make it clear:

so this compound has 6 carbon atoms
so this compound has 6 carbon atoms
is the same as
is the same as


Don’t let triple bonds confuse you. The two carbonatoms of the triple bond and the two carbons connected to them are drawn in a straight line. All other bonds are drawn as zigzag:

is drawn like this
is drawn like this


BUT
BUT

is drawn like this
is drawn like this


EXERCISE 1.1 Count the number of carbon atoms in each of the following drawings:



Now that we know how to count carbon atoms, we must learn how tocount the hydrogen atoms in a bond-line drawing of a molecule. The hydrogen atoms are not shown, and this is why it is so easy and fast to draw bond-line drawings. Here is the rule for determining how many hydrogen atoms there are on each carbon atom: neutral carbon atoms always have a total of four bonds. In the following drawing, the highlighted carbon is showing only two bonds:

Therefore, itis assumed that there are two more bonds to hydrogen atoms (to give a total of four bonds). This is what allows us to avoid drawing the hydrogen atoms and to save so much time when drawing molecules. It is assumed that the average person knows how to count to four, and therefore it is capable of determining the number of hydrogen atoms even though they are not shown.
So you only need to countthe number of bonds that you can see on a carbon atom, and then you know that there should be enough hydrogen atoms to give a total of four bonds to the carbon atom. After doing this many times, you will get to a point where you do not need to count anymore. You will simply get accustomed to seeing these types of drawings, and you will be able to instantly “see” all of the hydrogen atoms withoutcounting them.

EXERCISE 1.2 The following molecule has 14 carbon atoms. Count the number of hydrogen atoms connected to each carbon atom:

Now we can understand why we save so much time by using bond-line drawings. Of course, we save time by not drawing every C and H. But, there is an even larger benefit to using these drawings. Not only they are easier to draw, but they are easier...