Chemistry 125: Lecture 23October 28, 2009

Constitutional Structure: Formulae, Models, and Isomers

(1860s) Describing constitutional structure (nature and sequence of bonds) required notation.

Kekulé’s advocacy of his “sausage” notation seems ludicrous in retrospect, but Crum-

Brown’s notation is strikingly modern. Half a century before direct experimental observation

became possible, most structures of organic molecules were assigned by inspired

guessing based on plausibility. Investigation of contemporary molecular models shows

how adventuresome students could begin considering the implications of arrangement of

atoms in space.

For copyright notice see final page of this file

Nomenclature

Facts, Ideas, and Words

Notation

Models

Kekulé on Notation("Benzene" Paper, 1865)

"For greater clarity I am presenting at the end of this note a table giving graphical formulae for most of the substances mentioned. The idea that these formulae are designed to express is rather well known now; so it will not be necessary to dwell upon it. I am keeping the form that I had adopted in 1859 when expressing for the first time my views on the atomic constitution of molecules. This form is nearly identical with that which M. Wurtz used in his beautiful lectures on chemical philosophy. It seems to me preferable to the modifications proposed by MM. Loschmidt and Crum-Brown."

What SHOULDa Formula Show?

Constitution:Nature & Sequence of Bonds

Composition:Elements & number of Atoms

Isomers

Berzelius is fine for this, but now we also need to show

Lohschmidt Acetic Acid (1861)

Crum-Brown (1861-1866)

isomers

benzene

reaction

What do you think Kekulé thought?

Did he think these were pictures of molecules;what you would see in a “microscope”?

How Many Isomers?

Kekulé's "Preferable" Notation (1865)

WRONGCH3CH2CH2OH

CH3CCH3

O=CH3CHOH

CH3

- CH3CHCH3

OH-

Constitution same as 28!

Kekulé obviously thought that his notation says

these are different.

MolecularModels

Gaines Models Now, if a class thus instructed be at the recitation bench, and one of them is requested to form carbonic acid, he will walk quickly to the table in front, where the box is open, and place on one of the shelves a cube marked 6, and two marked 8 – carbon, one, and oxygen, two. If requested to form lime, he will place on the shelf a cube marked 20, (calcium,) and another marked 8, (oxygen,) these being the elements of lime. These cubes being all placed in contact, and the pupil being asked, “What have we now?” answers, “Carbonate of lime.”

Another pupil may then be asked to give the proximate analysis for carbonate of lime, and the cubes before him will indicate the answer and fix it in his memory: “It is composed of one equivalent of carbonic acid, (22) and one of lime, (28).” Another may be requested to give the ultimate analysis, and by the same means he is furnished with the answer: “Carbonic acid is composed of one atom of carbon and two of oxygen, and the lime is composed of one atom of calcium and one of oxygen.”

The intelligent educator will see at a glance, that in this chemical alphabet he has the means of at once arresting the attention of his class, and converting what has hitherto been an irksome task into a pleasant recreation. The atomic theory, the characteristics of affinity, the law of multiple proportions, the nomenclature, isomerism, &c., may all be learned in half the time required heretofore, and learned so as not likely ever to be forgotten.

?

?

James Dewar'sBrass Strip

Models(1866)

"to make the combination look like an atom, a thin round disc

of blackened brass can be placed under the central nut"

Prepared ~100 yearslater, and called

"Dewar Benzene"

4-center bond?

“CPK” Model

of Alaninean amino acid

Hofmann Croquet-Ball Models (1865)

Established the “standard” colors we still use!

Hofmann Croquet-Ball Models (1865)

Hofmann Croquet-Ball Models (1865)

O• •

• •

• •

_

Cl• •

• •

• •

• • • •

• ••

•O• •

• •

• •O

O• •

• • • •

• •

• •

• •O

Oxidizing agent for Gay-Lussac

“Chlorox” bleachExplosive

_NH4ClO4 (AP)

is a component of military explosives.

Hofmann Croquet-Ball Models (1865)

Isomers?

Constitutional Models Try

to Show Nature & Sequence of Bonds NOT Position in Space.

Successive Substitution Products Sc

ienc

e M

useu

m, L

ondo

n by

per

mis

sion

Science Museum, London by permission

Hofmann Croquet-Ball Models (1865)Reactivity!

“Unfinished” or “Non-Saturated”(now called “unsaturated”)

Cl2

additionnot

substitution

ClH2C-CH2ClCl2

Actually, as we’ve seen: H2C=CH2

Genealogy Top

Benzene & Molecular Structure

Faraday isolates “bicarburet of hydrogen” ("C2H", actually C6H6) from “gas oil”,

a 1 gal. per 1000 cu. ft. by-product from preparation of illuminating gas.

1825

1833s Mitscherlich names it benzine

phaino, I bring light)1836 Laurent in France names it phène”

Simplest of the "aromatic" compounds

from gum benzoin (via benzoic acid)

from luban jawi(“frankincense of Java”, Arabic)He calls C6H5 phenyl (we abbreviate Ph or )

[incidentally, benzyl is not Ph-, but rather PhCH2-]

How did Kekulé know Benzene is Hexagonal?On the Constitution of Aromatic Compounds (1866)

1) All aromatic compounds, even the simplest, are significantly richer in carbon than analogous compounds from the class of fatty substances.

2) For aromatic compounds, as for fatty compounds, there are numerous homologous substances; i.e. those whose compositions differ by n CH2.

3) The simplest aromatic compounds contain at least six carbon atoms.4) All reaction products from aromatic substances show a certain family similarity, constituting the group of "aromatic" compounds. More vigorous reaction can remove part of the carbon, but the major product contains at least six carbon atoms... Decomposition stops at this point, unless there is complete destruction of the organic group.

Did he just take a wild guess? (based on tetravalence)

1866

Kekulé’s Isomer Counting

1865

Isomer CountElukek (arabic f or hexagon)

Isomer Count Might Distinguish Among Dewar's 2D Benzene Structures

Mono: 3

A A

B B

C C

Di: 9

A A

B B

A A

Mono: 2

Di: 6

Mono: 1 Di:

Mono: 6

Di:

54321

4

15

Isomer Numbers for“Dewar's” Benzene Structures in 3D

C

C

C

CC

C

H H

HH

H

H

also"Prismane"

or

"LadenburgBenzene"

"Dewar Benzene"

C

C

H H

C

C

C C

H HHH

C C

CC

C

CHH

H H

H H

C C

C

C

C C

H

H

H

H

H

H

C C

CC

CC

HH

H H

HH

C C

C C

CC

HH

H HHH

C

C

H H

C

C

CC

H

HH

H

TRY ISOMER COUNTS IN 3-DIMENSIONS FOR FRIDAY

planar(2D)

Structure from Synthesis (Kekulé, 1867)

3 Acetone Mesitylene

Dewar had sent Kekulé brass-strip models in 1866.

-H2O

-H2O-H2O

There should be a methyl on every

other ring carbon!

Kekulé’sLectureModel

ofBenzene

Tetrahedral(not square!)

carbons

Ghent (1865)

Kekulé

rheumatism

Koerner

End of Lecture 23Oct. 28, 2008

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