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This 2 page Class Notes was uploaded by Lucious Gleichner on Wednesday October 28, 2015. The Class Notes belongs to CHM7292 at Villanova University taught by JosephBausch in Fall. Since its upload, it has received 12 views. For similar materials see /class/230584/chm7292-villanova-university in Chemistry at Villanova University.
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Date Created: 10/28/15
Strategy for Writing Lewis Structures Revised These quotrulesquot will work for a large majority of organic compounds Those systems that may not work using this method include a some carbocations meaning a compound that looks like this CmHn b any compound with an odd number of valence electrons ie radicals 1 Determine the total number of valence electrons by adding the valence electrons in the constituent atoms adjust for charge if needed 2 Write the skeletal structure linking the necessary atoms with single covalent bonds This structure has the minimum number of bonding electrons 3 For each bond subtract two electrons from the total number of valence electrons to give the total number of electrons that can exist as nonbonded electrons or forms of multiple bonds if the number here is zero you can skip to step 7 4 Determine the number of electrons required to fill the octet around each except hydrogen which requires two electrons If the number equals the number calculated in Step 3 place the electrons as nonbonded electron pairs around the appropriate atoms to complete the structure 5 If the number of electrons determined in Step 3 does not provide all atoms with octets we must use multiple bonds Subtract the number from Step 3 from Step 4 and then divide by two If this number is 1 a double bond must be used If the number is 2 either two double bonds or a triple bond must be used etc etc 6 Modify the structure with the appropriate number of multiple bonds The remaining electrons are nonbonded electrons that satisfy the individual electronic requirements of each atom 7 Indicate all formal charges that are nonzero formal charge number of valence e number of nonbonding e 12 number of bonding e Also note some organic molecules contain atoms beyond the second row such as sulfur and phosphorus These types of atoms can have more than eight bonded and nonbonded electrons in a Lewis structure If you have more than one Lewis structure generated following the above rules these are known as resonance structures In order to evaluate which of these structures is the best Lewis structure follow the guidelines given on the other side of this sheet Nonequivalent Resonance Structures To decide which resonance form is more important translation which form contributes most to the resonance hybrid use the following four quotrulesquot applied with priority 1 gt 2 gt 3 gt 4 1 Lewis structures with the maximum number of Lewis octets are the most stable Avoid charge separation if possible If needed preferred structures have charges located on atoms with the most appropriate electronegativity characteristics eg negative charges are placed on more electronegative elements If charge separation is needed structures with opposite charges located on atoms with minimum separation are better than those structures with opposite charges on atoms spread farther apart Charges can be separated if Lewis octets result modified slightly from Organic Chemistry by Ouellette and Rawn