Drawing Compounds
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Condensed Structural Formula
Go back and review the normal, common bonding pattern for each atom. With these normal bonding patterns in mind, let’s look at how organic compounds are typically drawn.
Let’s consider the following alcohol. All atoms have a normal bonding pattern. Notice every carbon atom has four bonds, the oxygen atom has two bonds and two lone pairs and each hydrogen atom has one bond.
This drawing is perfectly acceptable. The problem with it is that it takes too long to draw in all of those hydrogen atoms. A quicker way to draw this compound is to not draw every bond to every hydrogen atom. A condensed structural formula is a drawing of a compound where the carbon-hydrogen bonds are not explicitly drawn. These may be drawn several ways.
Drawing Condensed Structural Formulas
In compound A, all of the carbon-hydrogen bonds have been removed. The hydrogen atoms are usually written on the right of the carbon atom they are attached to. The exception to drawing the hydrogen atoms on the right of the carbon is shown in compound B. The carbon atom on the very left is drawn with the hydrogen atoms on the left. This may be done as it helps to show that the first and second carbon atoms are both bonded together. Compounds C and D show other ways to draw the OH. All of these are perfectly acceptable condensed structural formulas. I show them so you don’t worry when you see subtle differences drawn.
9. Draw each of the following structures as a condensed structural formula.
Instead of drawing out every bond, we can abbreviate the condensed structural formula if we have repeating groups. For instance, in the hexane, there are four methylene, CH2, groups in the middle of the molecule. We can show this repeating group by putting it in parentheses and using a subscript to tell how many of them there are.
In this next molecule, there are two methyl, CH3, groups on the same carbon atom. We can again use parentheses and a subscript to show this. There are three repeating methylene, CH2, groups.
And, in this next molecule, there are three methyl groups, CH3, on one carbon atom. There are four repeating methylene, CH2, groups.
10. Draw the Lewis structure (every bond and atom) for the following molecules.
Line Angle Formula
Chemists are smart, so they found a way to avoid drawing in all of those hydrogen atoms. If every practicing organic chemist knows that each carbon atom has four bonds, we could just not draw the H atoms at all. If we did not draw in the hydrogen atoms, and a chemist saw that the carbon atom only had two bonds drawn in, they would know that the carbon atom needs two more bonds to get to four, so it must have two hydrogen atoms attached. Remember, carbon atoms typically have four bonds. If you ever draw five bonds on a carbon atom, you might see your professor get angry. Trust me. You don’t want to see your professor get angry. It is not pretty. It’s funny what upsets an organic chemistry professor, isn’t it?
11. Draw in all of the missing hydrogen atoms on the following carbons.
It’s no fun drawing in all of those hydrogen atoms, is it? So, make sure you get good at drawing them using the other suggestions in this chapter! In fact, why draw in carbon atoms at all? Organic chemists have found an even easier-to-write notation. This notation is the line-angle formula or the line-angle structure. In the line-angle structure, a carbon atom is assumed every time a line ends, two lines come together at an angle, or a single bond meets a double or triple bond. So, instead of the above structure, chemists may write the following to save time.
Notice that the hydrogen atom on the oxygen is still drawn in. It is customary to draw the hydrogen atoms that appear on all non-carbon atoms, also called heteroatoms. Let’s make sure you can see where all of the carbon atoms are. Carbon atoms are located at the following sites.
12. Count the number of carbon atoms in the following compounds.
There is one more timesaving feature we can use to draw this compound. If practicing chemists know the normal bonding patterns, they can also figure out how many lone pairs of electrons are on atoms like nitrogen or oxygen. Normal oxygen has two bond and two lone pairs of electrons. So, our compound may be drawn like this.
13. Fill in the requested information.
Most textbooks do not draw in the lone pairs of electrons. This is a rather unfortunate practice because many chemical reactions occur with these lone pairs. Oh well, we must remember to be diligent remembering how many lone pairs are on these heteroatoms. I like to draw them in so I do not forget they are there.
14. How many lone pairs of electrons are on the following heteroatoms?
a)
b)
c)
d)
15. Identify the number of hydrogen atoms attached to each carbon atom.
a)
b)
c)
d)
e)
16. Draw the following compounds in the shorthand, line-angle notation.
To review, these three structures are all equivalent.
There are a few other things you should know about drawing compounds. One is that single bonds on these compounds can rotate around. That means that the following drawings are all for the same compound. It does not matter which way you draw it.
But, since the bonds all contain negative electrons, they repel each other. The bonds want to get as far away from each other as possible. Therefore, you would not want to draw this compound this way.
Answers
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10.
a)
b)
c)
d)
11.
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16.
