CH 221 Tips on Writing Lewis Structures Revised Version

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Chemistry 221 College of the Holy Cross Prof. Gail Edwards Organic Chemistry I Tips on Writing Lewis Structures Spring 2008 Given a molecular formula with no overall charge and containing no atoms other than carbon, oxygen, nitrogen, and halogen, calculate the number of units of unsaturation , U, using the method outlined in the handout titled “Calculating Units of Unsaturation.” Calculate the number of valence electrons. Propose an arrangement of atoms which has hydrogen and halogen on the periphery of the molecule. To minimize charge and maximize bonding, always try the following first: Bind hydrogen and halogen to only one other atom. Bind oxygen to two atoms or twice to one atom. Put in three bonds to nitrogen. Put in four bonds to each carbon. Be sure there are no more than two electrons around hydrogen. Be sure there are no more than eight electrons around other atoms.
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Unformatted text preview: • Remember that boron and aluminum are exceptions to the octet rule. • After putting in all the bonds, put in non-bonded electrons, if any. • If the molecule contains one unit of unsaturation, then there must be either a double bond or a ring in the molecule. If U is greater or equal to two, a triple bond is possible. If there are at least four units of unsaturation, then a benzene ring is possible. • Once you have a good arrangement, make sure your structure contains the correct number of atoms and valence electrons. • Only when the above steps fail to give you a good Lewis structure should you consider the possibility of separation of charge within the molecule. • Keep in mind that although many electrically neutral organic molecules have partial positive and negative charges, most do not have formal charges....
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