Chem 125 Lecture 3 9/11/06 Projected material This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not readily understood without reference to notes from the lecture.

Is there a Chemical Force Law? Why Different? What ARE Bonds? 19th Century Experiments led to VALENCE 1861 Different for different atoms: H(1), C(4), O(2), N(3) NH 4 Cl or 5? )

G.W. and Robert Robinson (1917) “Loop” and “Partial” Valences might “explain” trivalent and pentavalent N and amine reactivity? What does the loop mean?

Electron Discovered 1897

G.N. Lewis ( ) & his Cubic Octet

Octet "Explains" Periodicity, Electron Transfer (1902)

Octet Predicts Shared Pair Bonding ? shared edge shared face

Cubic Octet to Tetrahedral Octet to Tetrahedral Octet N N :: Tetrahedral distribution of the bonding of C had been known in organic chemistry for ~40 years!

Good Theory should be Realistic & Simple In relation to Facts it should allow: Prediction Suggestion Explanation Classification & Remembering as as possible Postdiction: Realm of Lore

From Number of Valence Electrons we would like to predict: Constitution (valence numbers for different atoms) Structure (distances & angles ) Energy Content Reactivity Charge & Polarity

Constitution (Electrons, Valence, and Unshared Pairs) H B C N O F H N H H Why Octet? Why Pairs?

+ H H H B H H H N HCN H C Tetravalent N is positive. N C H N C H N Tetravalent B is negative. NH 3 BH 3 H 3 N-BH 3 + H H H N H H H B Bookkeeping & “Formal” Charges (each atom is assigned half-interest in bonding pairs)

Lewis Explains “Pentavalent” N. It is Tetravalent, and thus Charged. N H H H H + Cl

Amine R R R N S R R Sulfide O Oxide O O oxide Sulfone

also for H,C,N,O (in all different orders) Draw Lewis Dot Structure(s) for: H N C (in the order shown) Start Problems:

Double Minimum equilibrium EQUILIBRIUM vs. RESONANCE O HCN + - HCN O + - all octets charge sepn  all octets still charge sepn  poorer site for -  N position Energy midway left shift : to eliminate charge sepn.shift : to restore octet N further left N ~midway C-O Geometric Implication? but maybe in truth…

EQUILIBRIUM vs. RESONANCE HCN O + - N position Energy midway left Single Minimum resonance all octets charge sepn  all octets still charge sepn  poorer site for -  O HCN + - N further left N ~midway C-O

Choice between Resonance and Equilibrium must be based on experimental facts (or a better theory) that can distinguish single from double minimum

Equilibrium vs. Resonance AB AB Two Real Species One Real Species Two “Reasonable” Structural Formulas Failure of Simplistic Notation Unusually Stable

Equilibrium vs. Resonance H C O O H HC O O H H C O O HC O O Two Species Two Species? H C O O HC O O One Nuclear Geometry! One Species! (Infrared) LORE (Electron Paramagnetic Resonance)

End 9/11/06