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Condensed Formulas

Because organic compounds behave very predictably it is possible to condense their Lewis structures. First, all organic compounds follow the octet rule, so each atom (except hydrogen) is surrounded by eight electrons. In the simple compounds discussed in this text, carbons always have four bonds. Hydrogen has only one bond. Oxygen will have two bonds (and two lone pairs) and nitrogen will have three (with one lone pair).

Structures are condensed by leaving off the lone pairs and grouping the hydrogens with the carbon they are bonded to. Oxygens are also grouped with the carbon they are bonded to. For example, COH implies an alcohol group, and CHO, an aldehyde.

A very condensed structure might consist only of lines representing carbon–carbon bonds. A carbon is assumed at every point where the line segments join. If that does not make four bonds around the carbon, it is also assumed that carbon–hydrogen bonds make up the missing bonds. The hydrogens are not written.

>> Example

How many hydrogens and carbons are in each of the following structures?

Solution:

1. There are eight places (including each end) where line segments join. Therefore there are eight carbons.

   Each of the carbons in the middle of the chain (six of them) needs two more bonds for its total of four. Those bonds will be to hydrogen. The carbons at the end need three more bonds to hydrogen (since it is connected to only one carbon). Therefore there are 18 hydrogens.

   Another way of determining the number of hydrogens is to recognize that this is a simple alkane. Therefore the ratio of carbons and hydrogens is C_nH_{2n + 2}.

2. There are five places where line segments join. However, at one end an OH replaces the carbon. There is also a place where an oxygen replaces a carbon. Therefore this molecule only has three carbons.

   The carbon at the end will have three hydrogens. Each of the two carbons in the middle will be bonded to two hydrogens. There is also the hydrogen at the end bonded to the oxygen. That is a total of eight hydrogens.

3. This is a six-membered ring, so it has six carbons. There is also the one at the end of the line, where it says "CH₃." The CH₃ details what is at the end of that line. If it was not present, the CH₃ would still be implied. The line represents the bond between the carbon of the CH₃ and the carbon in the ring. Consequently, this molecule has a total of seven carbons.

   Except for the carbon bonded to the CH₃, the carbons in the ring are each bonded to two hydrogens. The carbon bonded to the CH₃ needs one more bond, so a bond to a hydrogen in implied. That makes 10 + 1 hydrogens in the ring and three with the CH₃, so there are a total of 14 hydrogens.

4. with the carbons in is , which makes a total of six carbons. With the carbons numbered, the structure is . Carbons numbered 1, 3, 6 have two hydrogens each. Carbons 4 and 5, with the double bond and the bond to the next carbon, have one more bonding space. So carbons 2, 4, and 5 are each bonded to one hydrogen. The oxygen at the end is bonded to one hydrogen and the nitrogen is bonded to one hydrogen. That gives a total of 3(2) + 3(1) + 1 + 1 = 11 hydrogens.

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