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Chemistry 125: Lecture 34December 1, 2010
Cyclohexane & the Conformation of Cycloalkanes
When Baeyer suggested strain-induced reactivity due to distorting bond angles away from
those in an ideal tetrahedron, he assumed that the cyclohexane ring is flat. He was soon
corrected by clever young Hermann Sachse, but Sachse’s weakness in rhetoric led to a
quarter-century of confusion. Understanding conformational relationships makes it easy to
draw idealized chair structures for cyclohexane and to visualize axial-equatorial
interconversion. The conformational energy of cyclic alkanes illustrates the use of molecular
mechanics, a useful, but highly empirical scheme for reckoning conformational energy.
For copyright notice see final page of this file
Sachse (1890)
relatively easy, for example sufficient evidence is that there is a single isomer of hexahydrobenzoic acid [i.e. only one cyclohexane carboxylic
acid].
Baeyer (1890)A further proposal is that the atoms in hexamethylene are arranged as in Kekulé's model, that is that the arrangement of the atoms in space is the one with a minimum distortion of the valence directions.
Thus the 6 carbon atoms must lie in one plane and 6 hydrogen atoms lie in each of two equidistant parallel planes. Further each of the 12 hydrogen atoms must have the same position relative to the other 17 atoms.
The experimental test of the correctness of this assumption isrelatively easy,
Meanwhile, as long as our knowledge in this field is so incomplete, we must be satisfied that the above assumption is the most likely, and no known fact contradicts it.
Sachse (1892)41 pp. in Zeitschrift fürPhysikalische Chemie
edited by Ostwald, who did not believe in atoms and wrote disparagingly of his
successor in Riga,
"Scientifically he had been brought up in the narrow circle of contemporary organic chemistry, and to him the arrangement in space of the atoms of organic compounds was the foremost of all conceivable problems."
Sachse (1893)34 pp. in Zeitschrift fürPhysikalische Chemie
died at age 31 in 1893
Baeyer (1905)
"Sachse…disagreed with my opinion that larger rings are planar. He is certainly right from a mathematical point of view; yet in reality, strangely enough, my theory appears to be correct. The reason is not clear…"
What important lesson should we all take from the tale
of poor Sachse?
Write for your readership!
Sachse (1890)
Bragg & Bragg (1913)Diamond Structure by X-ray
Ernst Mohr Illustrations (1918)confirm Sachse’s 1890 insight.
Ernst Mohr Illustrations (1918)
“chair” “boat”
bowsprit
flagpole
“ring flip” by 60° counter-rotation
of two parallel bonds
Red bonds rotate in & up. Blue bonds rotate in & down.
invertedchair
ww
w. d
ownu
nder
chic
ago.
com
/pic
s/ol
-la-
lafu
ma-
recl
iner
--pa
dded
--la
.jpg
http://beothic.blogspot.com/2007/01/dory_13.htm
l
Ernst Mohr Illustrations (1918)
Drawing “Ideal” Chair Cyclohexane:
opposite C-C bonds are parallel
axial bonds are parallel to 3-fold axis
equatorial bonds are (anti)parallelto next-adjacent C-C bonds
Whato’clock?
??
??Z
Cholic Acid(a Steroid)
Glucose(a Carbohydrate)
For such problems D.H.R. Barton Invents Conformational Analysis (1950)
Baeyer observed only one c-Hexyl-COOH, but in theseepimers, and OH groups have different reactivity!
“up” ; “down” (for molecule in conventional orientation,
old-fashioned configuration notation, like cis / trans)
Barton redraws Ring A
Intermediates in steroid hormone synthesis
A B
C D
(configurationally diastereotopic)
(e) “equatorial”
(p) “polar” (now axial)
Cf. ~1950 Stereochemistry:Bijvoet, Newman, CIP,(Molecular Mechanics)
(Nobel Prize 1969 for “development of the concept of conformation and its application in chemistry”)
ERRORS? “up” ; “down”
(for molecule in conventional orientation, old-fashioned configuration notation, like cis / trans)
3-fold axis
For such problems D.H.R. Barton Invents Conformational Analysis (1950)
)
Ring Flip?
Ernst Mohr Illustrations (1918)
Ring flip impossible for trans decalin!
N.B. During ring flip equatorials become axials
and vice versa.
anti
gauche
gauche, but not anti, is OK for the second ring of decalin.
fused chairs in "decalin"(decahydronaphthalene)
Try with modelsif you’re skeptical.
Mol4D(CMBI Radboud University, Nijmegen, NL)
Click for INDEX or go to http://cheminf.cmbi.ru.nl/wetche/organic/index.html
Conformational Jmol Animations
(see Wiki to install Jmol)
Mol4D(CMBI Radboud University, Nijmegen, NL)
Ethane Click to Animate or go to http://cheminf.cmbi.ru.nl/wetche/organic/nalkanesconf/ethane/jmindex.html
Eclipsed barrier ~5.2 kJ/mol 0.239 = 1.24 kcal/molShould be ~2.9 kcal/mol. Caveat emptor!
Step Keys
Click Points
Staggered
Mol4D(CMBI Radboud University, Nijmegen, NL)
Propane Click to Animate or go to http://cheminf.cmbi.ru.nl/wetche/organic/nalkanesconf/propane/jmproprot.html
Staggered
Eclipsed3.3 kcal/mol
Anti Gauche+
1013 10 -3/4 3.4 = 10 10.5 /sec
Mol4D(CMBI Radboud University, Nijmegen, NL)
Butane (central bond) Click to Animate or go to http://cheminf.cmbi.ru.nl/wetche/organic/nalkanesconf/butane/jmindex.html
Gauche0.9 kcal/mol(tells how much)
eclipsed3.5 kcal/mol(tells how fast)
fully eclipsed~ 5.5 kcal/mol?
(experimentally irrelevant)
Anti
Gauche / Anti = 10 -3/4 0.9 = 10-0.68 = 1 / 4.7Gauche / Anti = 2 10 -3/4 0.9 = 2 10-0.68 = 1 / 2.4
+
+
Gauche-
H
H HH
H HH
H HH
H H
H
HH
H
H H
H
HH
H
H H
H
HH
H
H HH
H HH
H H
H
HH
H
H H
3 3 3
Conformational Energy of EthaneButane5.5
0.9(0.6?)
5.5
0.9(0.6?)
3.5
H
H HH
H HH
H HH
H H
H
HH
H
H H
H
HH
H
H H
H
HH
H
H HH
H HH
H H
H
HH
H
H H
CH3CH3 CH3
CH3
CH3 CH3
CH3
CH3 CH3 CH3CH3
CH3CH3
CH3
H H
3.5
0° 120° 240° 360°Torsional Angle
Ene
rgy
(kca
l/m
ole)
Mol4D(CMBI Radboud University, Nijmegen, NL)
Ring Flip of c-Hexane Click to Animate or go to http://cheminf.cmbi.ru.nl/wetche/organic/cyclohexane/jm/chxjmol.html
Flexible or Twist-Boat conformer
~5.5 kcal/mol
Barrier (Half-Chair)~ 11 kcal/mol
Chair conformer
Mol4D(CMBI Radboud University, Nijmegen, NL)
Flexible c-Hexane Click to Animate
or go to http://cheminf.cmbi.ru.nl/wetche/organic/cyclohexane/jm/twist_boat.html
Flexible or Twist-Boat Form
Barrier (Boat)~ 1 kcal/mol
The boat is not an isomer (an energy minimum), it is a barrier on the pleasantly smooth path between twist-boat isomers.
Shape,“Strain Energy”
&
Molecular Mechanics
“Hooke’s Law” for Strain Energy
Molecular Mechanics (1946)
Activation Energy for Racemizationobs. 19.5 kcal/mol
calc. 17.3 kcal/mol
At the barrier the C-C-Br angles
open by 12°.
t1/2 = 9 min at 0°C(1013 10-(3/4)*20 ~ 10-2/sec)
Question:How did having
COOH groups on the benzene rings
facilitate the experiment?
End of Lecture 34Dec. 1, 2010
Copyright © J. M. McBride 2009, 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
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