Home > Chemistry > Chemistry 51A: Lecture 2 Notes

Chemistry 51A: Lecture 2 Notes

Overview

Today is a 51 minute lecture on resonance.  This guy is a very good ochem teacher, his pacing is quite nice.

http://www.youtube.com/playlist?list=PL6539BAED55972D64

Details

Organic chemists focus on a portion of the molecule and forget the rest.  Skeletal structures only show non-hydrogen atoms.  Carbon is always has four bonds.

For instance, pentane as M or W.

Curved arrows are very important in ochem.  They represent how electrons flow.  They are either from lone pair to bond, bond to LP, or bond to bond.

Non equivalent resonant structures do not satisfy the octet rule.

Bond length is measured in angstroms, 10^-10 meters.  Its the distance between atoms

C-H   1.1A

C-C   1.54

C=C   1.34

C_=C   1.20

All of the carbon-carbon bonds in benzene are 1.4 A.  Multiple bonds are shorter.  Bond length decreases with increasing bond order.  Bond length increases as you move down the periodic table.

C-F  1.40

C-Cl  1.79

C-Br  1.97

C-I   2.16

Bond length decreases as you move across the period.

C-C  1.54

C-N  1.47

C-O  1.43

C-F   1.40

Methylene imine is H2C=NH.  The C=N bond is 1.27 A.  H3C – NH2 is methyl amine with the C-N bond length of 1.47.

Methylene imidine H2C=N2Hhas a C=N bond length of 1.29 A.

H2C=N+H2 <-> H2C+-NH2

Major resonance structure/Minor resonance structure

Trifluoromethyl cation

CF3+.  All C-F bonds are 1.37 A.  This molecule has 3 major resonance structures and 1 minor.  The minor resonance structures have energetically unfavorable configurations.

Review

PV = nRT

Diffusion solute moves; osmosis water moves

Electronegativity – NOF Cl Kr

pKa = -log Ka

FC = V - N - {B over 2}

The strongest acid in water is H3o+, the strongest base is OH-

nucleophiles attack electrophiles

Nucleophiles like positive charge, electrophiles like negative charge

P = mv

c = wf

G = -RT ln K

Nucleophilicity:

  1. Charge
  2. Electronegativity
  3. Solvent
  4. Steric hindrance

G = H – TS

Nucleophile = Nu = A species that donates a pair of electrons to form a new covalent bond

Lewis bases donate electrons, lewis acids accept electrons

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