### Lewis Dot Structures 1. Arrange atoms in space how you think they're bonded 2. Add up the total number of valence electrons, including +1 electron for each negative charge and -1 for each positive charge in the formula 3. Arrange remaining electrons around atoms, starting with single bonds and adding lone pairs or double bonds to complete octets 4. Assign formal charges as necessary to satisfy octets (formal charge = valence e⁻ − nonbonding e⁻ − bonds), placing negative charges on the most electronegative element in the molecule #### Exceptions to the octet rule - Compounds with a deficiency of valence electrons - Odd number of electrons - Early second-row elements - Expanded octet - Valence shell expansion due to availability of 3d orbitals for compounds beyond the second row #### Skeletal Structure A simpler aproach to drawing organic molecules, making the following assumptions: - a carbon atom is at every junction of two lines and end of a line - hydrogen atoms fill octets for carbon atoms Additionally, - All heteroatoms and hydrogens bonded to heteroatoms are drawn - Lone pairs and bonds to hydrogen are often not drawn For large hydrocarbons, calculate **Degrees of Unsaturation**: $DoU = \frac{2C + 2 - H - \text{Halogens}+N+P}{2}=\text{\# of rings} + \text{\# of }\pi \text{ bonds}$ Oxygen doesn't change DoU because it just inserts itself between atoms. ### Resonance Structures Resonant hybrid molecules are the weighted average of contributors with delocalized negative charge. - draw a double-sided arrow between them to show that the contributors aren't in equilibrium (they don't exist) but instead contribute to the hybrid - requires a contiguous array of $p$ orbitals, i.e. roughly coplanar and sp/sp2 - to get a contributor, move a $\pi$ bond or lone pair from one atom to an adjacent one - can't move a lone pair to another lone pair (it has to become a $\pi$ bond) - curved arrow with tail at the $\pi$ bond / lone pair and head at where it ends up - formal charges move as a consequence - why resonance matters - more resonant contributors → more stable - minor contributor shows reactive sites - bond lengths and orders get averaged - partial $\pi$ character keeps geometry planar so that p orbitals are aligned Contributors are weighted according to the following ordered priorities: 1. maximize full octets, especially on electronegative atoms 2. minimize formal charges 3. negative charge on the most electronegative atom (and positive on the least) 1. carbocations are most stable when bonded to carbons or atoms with lone pairs and less stable when bonded to electron-withdrawing groups or with higher s-character bonds 2. no [texas carbons](https://www.chem.ucla.edu/~harding/IGOC/P/pentavalent_carbon.html) 4. avoid like charges on adjacent atoms