Chem 125 Lecture 39/11/06

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Is there aChemical 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)NH4Cl

or 5?)

G.W. and Robert Robinson (1917)

“Loop” and “Partial” Valencesmight “explain” trivalent and

pentavalent N and amine reactivity?

What does the loop mean?

ElectronDiscovered

1897

G.N. Lewis(1875-1946)

& his

Cubic Octet

Octet "Explains" Periodicity, Electron Transfer(1902)

Octet Predicts Shared Pair Bonding

?

shared edge

shared face

Cubic Octet to Tetrahedral Octetto 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 Factsit should allow:

PredictionSuggestion

Explanation

Classification& Remembering

as as possible

Postdiction:Realm of Lore

From Number of Valence Electronswe 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••

••••

•

1 233 4 1

H•N••

••••

•HH

Why Octet?

Why Pairs?

+H•

H•

H•B•

••

H •

H•

H•N •••

••

HCNH • •

•C ••

Tetravalent N is positive.

N ••

••

•

••CH•

• ••• N••• C ••H N

Tetravalent B is negative.

NH3 BH3H3N-BH3

+

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.

NH

HHH

+Cl

Amine

R •

R•

R•N ••

••

•

SR •

R••

••

Sulfide

•

••

O ••

• •

• •Oxide

O• •

••

••

O ••

• •

• •oxideSulfone

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:

DoubleMinimum

equilibrium

EQUILIBRIUM vs. RESONANCE

OH C N••

••• • +-

H C N O••

••

••

+ -all octets charge sepn

all octets still charge sepn poorer site for -

N position

Ene

rgy

midwayleft

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

H C N O••

••

••

+ -

N position

Ene

rgy

midwayleft

SingleMinimum

resonance

all octets charge sepn

all octets still charge sepn poorer site for -

OH C N••

••• • +-

N further left

N ~midway C-O

Choice between

Resonance and Equilibrium

must be based onexperimental facts

(or a better theory)

that can distinguish singlefrom double minimum

Equilibrium vs. Resonance

A B A B

Two Real SpeciesOne Real Species

Two “Reasonable”Structural Formulas

Failure of Simplistic NotationUnusually Stable

Equilibrium vs. Resonance

H CO

O

HH C

O

O H••

••••

•• ••

••

••

••

••

H CO

O

H C

O

O

••••

•• ••

••

••

•••

•

Two Species

Two Species?

••

H C

O

O

H C

O

O

••••

•• ••

••

••

•••

•

One Nuclear Geometry!

•

•One Species!(Infrared)

LORE(Electron Paramagnetic Resonance)

End 9/11/06