Bond Polarity, Molecular Polarity, Formal Charge

HONORS ORGANIC CHEM. BACKGROUND UNIT 2 TEST REVIEW Major Topics: bond polarity, molecular polarity, formal charge, resonance, thin-layer chromatography (TLC) Format: Same as previous test: mixture of multiple choice and open-ended/ show your work 1. How can you determine if a bond is: Polar? Nonpolar? Find the difference in electronegativity between the atoms in the bond  Polar: ∆ 0.4 to 2.0 Nonpolar: < 0.4 2. In a polar bond: a.) what is the partial charge on the: less electronegative atom? + more electronegative atom? ─ b.) What symbol is used to show: a partial positive charge? δ+ a partial negative charge? δ─ c.) Describe how the d.) Use the arrow points in a polar bond. Toward more electronegative atom and partial + / ─ charge symbols to represent each indicated bond below IF it is polar. H2N ─ H δ─ δ+ H3C ─ H nonpolar HO ─ CH3 δ─ δ+ Mg ─ CH3 δ+ δ─ H3C ─ I nonpolar 3. What do you need to know about a molecule to determine if it is polar or nonpolar? Its molecular shape 4. What two molecular geometries are always polar? Bent and trigonal pyramidal 5. What determines whether the other three molecular geometries will be polar or nonpolar? If the bonds are identical 6. Identify each molecule as polar or nonpolar and how you know. CH4 CH2Cl2 CS2 NP P NP H H C C S H Cl H C HSiN PH3 P H S Si CH2O P N H P H H H H BHCl2 P P O H C P F B Cl H OF2 O F Cl Cl H 7. Determine the formal charge on all atoms (except H) in the molecule shown. The overall molecule is not neutral, so the charges will not total 0. You will need to know the formal charge formula for the test (if you use it). 0 O -1 O C 0 0 0 C H S +1N N 0 O +1 0 C C 0 H H 8. Draw the three resonance structures for each molecule/ ion below. Then, use formal charges to determine which of the structures, if any contribute more to the overall resonance structure and why. b.) NCO─ (atoms connected in that order) a.) OCS (atoms connected in that order) 0 0 0 +1 0 -1 -1 0 +1 -1 0 0 0 0 -1 O C S O C S O C S N C O N C O all formal charges are 0 -2 N - formal charge on more EN atom 0 +1 C O HONORS ORGANIC CHEM. *Be sure to read and study the notes on resonance. There will be some conceptual questions about what resonance is and is not, what are and are not valid resonance structures, and rules for drawing and evaluating resonance structure. 9. In each pair of resonance structures below, use curved arrows in the structure on the LEFT to show how it becomes the structure on the RIGHT. O a.) O C H N H H N H H C C H 10. H b.) H H H C C H c.) H H H C H C O O H C C C H S H S H Draw two additional resonance structures for each ion or molecule shown below. - Use curved arrows to show how resonance structures are formed. - Do not forget to account for unshared electrons and formal charges in each structure. * Remember, when we say “the octet rule must be obeyed” that means an atom cannot exceed an octet, but an atom may (and often does) have an incomplete octet. a.) O O C C H H H C O C C H H H H C H H b.) H H N C N H H H H C H 2 N 2 C H H *For TLC, be sure you study/ know how to: - key uses of TLC - how to calculate Rf values - how polarity affects Rf values - how Rf values are used to determine what components are in a mixture (like in the lab how you compared the Rf values of your mixture components to the Rf values of the standards) H