Transcript Lecture 23 timed ppt

Chemistry 125: Lecture 23
October 27, 2010
Valence & Constitutional Structure (1858)
Formulae, Models, and Isomers (1860s)
Youthful chemists Couper and Kekulé replaced both radical and type theories with a
new approach involving atomic valence and molecular structure, and rooted in the
tetravalence and self-linking of carbon. Constitutional valence structures offered the first
clear explanation for isomerism, and led to the invention of nomenclature, notation, and
molecular models closely related to those in use today. 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 conceivble, 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
Theory and Types
p. 106 - on the French Type Theory
Should the principle which is therein
adopted be applied to the common events
of life, it will be found that it is simply
absurd. Suppose that some one were to
systematize the formation of letters into
words that formed the contents of a book.
p. 106 - on the French Type Theory
Were he to begin by saying that he had
discovered a certain word which would
serve as a type, and from which by
substitution and double decomposition
all the others are to be derived, - that he
by this means not only could form new
words, but new books, and books almost
an infinitum, - he would state certainly
an empirical truth.
p. 106 - on the French Type Theory
At the same time, however, his method
would, judged by the light of common
sense, be an absurdity. But a principle
which common sense brands with
absurdity, is philosophically false and a
scientific blunder.
p. 107 - on the German Radical Theory
I can only remark that it is not merely an
unprofitable figure of language, but is
injurious to science, inasmuch as it tends
to arrest scientific inquiry by adopting the
notion that these quasi elements contain
some unknown and ultimate power which
it is impossible to explain.
p. 107 - on the German Radical Theory
It stifles inquiry at the very point where
an explanation is demanded, by putting
the seal of elements, of ultimate powers,
on bodies which are known to be anything
but this.
Absolutely true
but stated undiplomatically,
especially for a newcomer.
p. 108 - Look to the Elements
Science demands the strict adherence to
a principle in direct contradiction to this
view. That first principle, without which
research cannot advance a step, dare not
be ignored; namely, that a whole is
simply a derivative of its parts. [?]
p. 108 - Look to the Elements
As a consequence of this, it follows that it
is absolutely necessary to scientific unity
and research to consider these bodies as
entirely derivative, and as containing no
secret ultimate power whatever, and that
the properties which these so-called quasi
elements possess are a direct consequence
of the properties of the individual elements
of which they are made up.
p. 109 - Focus on Carbon
In applying this method, I propose at
present to consider the single element
carbon. This body is found to have two
highly distinguishing characteristics: -
1. It combines with equal numbers of
hydrogen, chlorine, oxygen, sulfur, &c.
2. It enters into chemical union with itself.
These two properties, in my opinion, explain
all that is characteristic of organic chemistry.
1858
Tetravalence
and Self-Linking
of Carbon
C2 as nucleus
H=1
C=6
O=8
Cl = 35.5
(Cf. Wöhler/S.C.H.Windler hoax)
"Structural"
Formulae
CH3-OH
CH3CH2-OH
Ethyl Ether
..
.
There is a Printer’s Error.
(This was unfamiliar territory.)
CH3CH2-O-CH2CH3
Couper C  12
Glycerine
CH(OH)
OH 2
CH2
2
CH2OH
guessed
a bit
wrong
CH(OH)
OH 2
CH2
2
CO2H
CH
OH
2
English Glucose
CHOH
CHOH
CHOH
guessed
right for
hydrate!
CHOH
=O + H2 2O
H CH(OH)
Devise a 2- or 3-step HOMO /
LUMO scheme for H2O +
French version
of same paper.
Lines for
C-C Bonds
CH
OH
2
English Glucose
CHOH
CHOH
CHOH
CHOH
CH2OH
(Typo: Adds H2, not H2O, to C=O)
Old Aisle Cemetery
Kirkintilloch
Oct. 28, 2008
Special thanks
to our agent
Susan Frew
from K. Hafner
Angew. Chem. Int. Ed.
18, 641 (1979)
Drawing by Kekulé age 13
from K. Hafner
Angew. Chem. Int. Ed.
18, 641 (1979)
By Kekulé age 18
Giessen for architeture
Liebig's advice to Paris-bound
graduate August Kekulé (1851)
There you will broaden your horizons,
there you will learn a new language,
there you will learn to know
the life of a great city,
but there you will not learn chemistry!
Kekulé in Heidelberg (1856-58)
Performed Research on (CH3)2As
(Prof. Bunsen's cacodyl radical)
in his apartment's kitchen!
H
H
1857 - Proposed a new type, "Marsh Gas" C
H
(tetravalence)
H
1858 - Proposed self-linking of Carbon
Kekulé Claims Priority (1858)
Observations on Mr. Couper's New Chemical Theory
"In fact, in two memoirs which have appeared in
Liebig's Annalen...I have put forward different views,
which, in my opinion, should furnish a clearer insight
into the constitution of chemical compounds.
I may be allowed to indicate that [my first paper] lays
down the principle…which I have called the basicity of
atoms [we would say 'valence']. If Mr. Couper thinks he has
discovered the cause of this difference of basicity in the
existence of a special kind of affinity, I am the first to
admit that I have no right to contest his priority in this."
Kekulé
Ghent (1865) Structure of Benzene
Kekulé
Bonn (1872)
Facts, Ideas, and Words
Nomenclature

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 SHOULD
a Formula Show?
Composition:
Elements & Number of Atoms
Berzelius formulae are adequate for this, but now we also need to show
Constitution:
Nature & Sequence of Bonds
Isomers
Lohschmidt Acetic Acid (1861)
Crum-Brown (1861-1866)
benzene
reaction
isomers
Kekulé's "Preferable" Notation (1865)
How Many Isomers?
What do you think Kekulé thought?
WRONG
Constitution
same as 28!
-
3
3
-
=
O
OH
Did he think
these were
pictures
Kekulé
obviously of molecules;
CH CHOH
CH CCH
CH CHCH
CH
thought
that
his
what you would seenotation
in a “microscope”?
says
CH3CH2CH2OH
3
3
these are different.
3
3
Molecular
Models
Another
be
to
Now,
if apupil
classmay
thusthen
instructed
be at
The intelligent
educator
willasked
see
give
the proximate
analysis
the
bench,
and
onefor
of them
at arecitation
glance,
that
in this
chemical
carbonate
lime,
the cubes
is
requested
to
form
carbonic
acid,
he
alphabet
heofhas
theand
means
of at
once
before
him
will
indicate
the
will
walk
quickly
to theoftable
in front,
arresting
the
attention
his answer
class,
and
it in
hisiswhat
memory:
“Itplace
is on
where
the
box
open,
and
and fix
converting
has
hitherto
composed
of onetask
equivalent
of 6,
one
shelves
a cube
beenofanthe
irksome
intomarked
a pleasant
carbonic
acid,
one
of
lime,
and
two marked
8 –and
carbon,
one,
and
recreation.
The(22)
atomic
theory,
?of be
(28).”
Another
requested
oxygen,
two. Ifmay
requested
to form
the characteristics
affinity,
the to
give
the
ultimate
analysis,
and
the
lime,
hemultiple
will
place
on the shelf
a cube
law of
proportions,
theby
same
means
heisomerism,
is furnished
with the
marked
20, (calcium,)
and? &c.,
another
nomenclature,
answer:
“Carbonic
acid
is the
composed
marked
(oxygen,)
these
being
the
may all 8,
be
learned in
half
time
of
one atom
of carbon
and
two being
ofso
elements
of lime.
These
cubes
required
heretofore,
and
learned
oxygen,
and
the lime
isforgotten.
composed
all
placed
in contact,
and
the pupil of
as not
likely
ever
to be
one atom
of calcium
and one
of
being
asked,
“What have
we now?”
oxygen.” “Carbonate of lime.”
answers,
Gaines Models
James Dewar's
Brass 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"
4-center bond?
Prepared ~100 years
later, and called
"Dewar Benzene"
Hofmann Croquet-Ball Models (1865)
Established the “standard” colors we still use!
“CPK” Model
of Alanine
an amino acid
Hofmann Croquet-Ball Models (1865)
Hofmann Croquet-Ball Models (1865)
[NH ClO (AP)
••
is a component of
O
_
_
explosives]
•• •• ••
•military
•“Chlorox”
Gay-Lussac’s
Explosive
O
Oxidizing
bleachAgent
O
Cl
O
•• •• ••
••
O
••
••
••
••
••
••
••
••
••
••
4
4
Hofmann Croquet-Ball Models (1865)
Constitutional
Models
Which Hofmann
models
Showdid
Nature
& Sequence
Hofmann
prefer?of
Bonds NOT Position
Neither! in Space.
Science Museum, London by permission
Successive Substitution Products
Science Museum, London by permission
Isomers?
Hofmann Croquet-Ball Models (1865)
Reactivity!
Cl2
1,2-dichloroethane
(not 1,1-)
Actually, as we’ve seen: H2C=CH2  ClH2C-CH2Cl
Cl2
addition
not
substitution
“Unfinished” or “Non-Saturated”
(now called “unsaturated”)
Genealogy Top
Benzene & Molecular Structure
Simplest of the "aromatic" compounds
1825
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.
1833s
Mitscherlich names it benzine
from
gum
benzoin
(via benzoic acid)
1836
Laurent in France names it phène”
from
luban
jawiI bring light)
fain
phaino,
of Java”,it Arabic)
He calls C H (“frankincense
phenyl (we abbreviate
Ph or )
6
5
[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)
Kekulé’s Isomer Counting
1865
Isomer Count
Elukek (arabic for hexagon)
1866
End of Lecture 23
Oct. 27, 2010
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