Lecture 16 Date: 10/22/2024 Content: Formal Charge Example: A molecule with a carbon atom bonded to two hydrogen atoms and two oxygen atoms. Formal charge calculations are shown for each atom in the molecule. "Formal charge of atoms in neutral molecules = 0; stable molecules try to have minimal formal charge in nature (↓↓ charge separation)" "For both: charge = ENC - e-s; sum of charges = charge of molecule/ion" Shorthand for writing structures & sites of unsaturation (SOU) Site of unsaturation: A ring or a bond where an additional atom can be added (breaking a ring open, can add 2 atoms w/o losing anything) Alkane: 2n H, no SOU Alkene: 2n H, 1 SOU (π) Alkyne: 2n H, 2 SOU (π) C added, but also appropriate H & double bonds; for every C added, 2H As # of carbons expands, so does # of hydrogens by double if SOU = 0 For every SOU, 2 hydrogens are lost (1 double bond or ring) Examples: Several organic molecules are shown with their corresponding molecular formulas and the number of sites of unsaturation (SOU) calculated. Nitrogens/Oxygens: "no effect on SOU" But H accompanies N Cyclic Hydrocarbons All isomers have the same SOU. Small rings w/ only σ bonds: H on C forced to be eclipsed. → strain bc ideal bond angle is 109° for sp³ 88° further from 109° than 90° but puckering relieves eclipsed hydrogens and minimizes C-H bond repulsion. 2 x 90° and 2 x 90° All bond angles can be ~90° Slight rotation on C-C bonds, few eclipsed interactions Cyclohexane: all bond angles are 109° and no hydrogens are eclipsed → any staggered Chair conformation: Allows 109° instead of 120° Minimizes eclipsing interactions Not planar Axial H: perpendicular to plane (parallel to axis; up-and-down) Equatorial H: on equator of axis Can rotate σ bonds (w/o breaking) to switch axial ↔ equatorial H Conformational isomer: same molecule, diff. geometry Isomers (Same Molecular Formula) Constitutional Isomers (Structural): Molecules differing in atom connectivity (sequence). Example: alkane vs. alkene Stereoisomers (geometrical): Molecules differing in geometry but not connectivity. Conformational Isomers: Same molecule but single bond rotation (ex. eclipsed vs. staggered, where lone pair of N inverting in different directions) - treated as identical. Configurational Isomers: A bond must be broken to get from one to another (ex. π bond broken from cis-trans). Different traits/molecules. Diastereomers: Stereoisomers that aren't enantiomers/mirror images of each other. Enantiomers: Optical isomers; isomers that are mirror images of each other and not superimposable.