University:
Boston University
Course:
CAS CH 214 | Organic Chemistry with Qualitative Analysis
Academic year:

2023
Views:

271

Pages:

4
Author:

Tara Guerrero

TP Which of the following molecules are not polar? 17% 17% 17% 17% 17% 17% 1. 2. 3. 4. 5. 6. BFCl2, whose shape is trigonal‐planar, like that of BF3 SCl2, which is a bent molecule, like H2O NH2Cl, whose shape is trigonal‐pyramidal, like that of NH3 OCS, which is a linear molecule, like CO2 All of the above are nonpolar All of the above are polar Intermolecular forces: stickiness! • Hydrogen bonding • Dipole‐dipole interaction polarity Hydrogen bond angle is 180° A molecules attract one another, because of the attraction of opposite electrical charges. Most specific and strongest is “hydrogen bonding”: X—H ∙ ∙ ∙ Y More common and intermediate strength is “dipole‐dipole attraction”: Always present and weakest is “temporary dipole attraction” “dispersion interaction” : 1 Hydrogen bonds are longer than typical bonds Which of the following can form H bonds? • Ammonia, NH3 • Methanol, CH3OH • Ethanol, CH3CH2OH • Dimethyl ether, CH3OCH3 Quiz Which of the following cannot form hydrogen bonds with themselves? 13% 13% 13% 13% 13% 13% 13% 13% 1. 2. 3. 4. 5. 6. 7. 8. Ammonia, NH3 Methanol, CH3OH Ethanol, CH3CH2OH Dimethyl ether, CH3OCH3 1 and 3 1 and 4 2 and 4 All of the above can form hydrogen bonds with themselves Dipole-dipole attraction Molecules must be “polar” What makes a molecule polar is unequal sharing of electron clouds. Sharing tendency is proportional to electronegativity difference. The greater the difference of electronegativities, the more unequal the sharing, the more polar the shared electron cloud. 2 Electronegativity χ: Relative pull The larger χ the more piggish the atom Molecular polarity • If bond dipoles cancel, molecule is nonpolar • If bond dipoles do not cancel, molecule is polar The more different the χ’s, the more polar the bond • If two bonded atoms are different, the one with larger χ gets more of the shared electron cloud • Atom with larger χ has net negative charge; atom with smaller χ has net positive charge • The larger the difference in χ, the greater the charge separation unequal sharing of electron cloud • Result is a polar bonds TP Which of the following molecules are not polar? 17% 17% 17% 17% 17% 17% 1. 2. 3. 4. 5. 6. BFCl2, whose shape is trigonal‐planar, like that of BF3 SCl2, which is a bent molecule, like H2O NH2Cl, whose shape is trigonal‐pyramidal, like that of NH3 OCS, which is a linear molecule, like CO2 All of the above are nonpolar All of the above are polar Let’s illustrate, for BF3 and BH2F 3 Quiz Which of the following statements is true about carbon dioxide, O C O? 25% 1. Carbon dioxide contains polar bonds and is a polar molecule 25% 2. Carbon dioxide contains polar bonds, but is not a polar molecule 25% 3. Carbon dioxide does not contain polar bonds and is not a polar molecule 25% 4. Carbon dioxide does not contain polar bonds but is a polar molecule 4