Table Of ContentKINETICS OF ELIMINATION REACTIONS OF DIHALOETHYLENES
AND THE MECHANISM OF TRANS ELIMINATION
by
Sidney I. Miller
Dissertation submitted in partial fulfillment of the requirements
for the Degree of Doctor of Philosophy in the Faculty of Pure
Science, Columbia University.
New York, N. Y.
1951
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DEDICATED
to
Mom, Deb, and Laura,
three very patient womeno
i
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A cknowledgement s
I am deeply indebted to Professor ft. M. Noyes whose guidance,
encouragement, and provocative discussion led to the completion
of this work.
I wish also to acknowledge the help given in discussions
on specific problems by Professors W. E. Doering, ft. Halford,
and D. Curtin.
I appreciate also the invaluable assistance of Mr. K.
Schumann and members of the Chemistry Shop who helped to make
this research possible.
Finally,invaluable "sessions” with fellow graduate students
did much to enrich my understanding of this research problem.
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TABLE OF CONTENTS
Survey of the Literature
1. The Michael Rule........................................................................ 1
2. Related Eliminations................................................................5
3. Mechanism of Elimination..................................................... 7
4. The I" Elimination.............................................*................. 16
5. Stoichiometry and Side Reactions.................................. 16
' Experimental............................................................. 20
1. Preparation of Materials......................................................20
2. Analytical Methods................................................................. 27
3. Kinetic Experiments.............................................................. 29
Results and Discussion.......................................................................... 33
1. Experimental Data................................................................... 33
2. Discussion................................................................................... 76
3 . Summary.....................; .................................................................. 89
References...................................................................................................... 90
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Survey of the Literature
In spite of a considerable number of qualitative studies
of bimolecular elimination reactions, it is not yet clear why
groups located in trans positions are removed more easily than
those in cis positions. No exceptions to this "trans elimination”
rule are yet available. The chief purpose of this work is to
provide quantitative kinetic data on some representative reactions
The relative rates of elimination of the isomers CgHgC^j
and with methoxide, and of with iodide, are reported
V'e can represent such reactions rather generally
R y Y
0^=0 + B --------> BY + R-f-C = CR^ + X
X r 2
where B and X are generally ionic.
The Michael Rule
Historically, the rule of trans elimination was enunciated
1 2
by Arthur Michael. ’ It states that, when X and Y are eliminated
from each of a pair of geometrical isomers, the isomer in which
X and Y are trans to one another will react more rapidly than
the other isomer.
In Table 1 are a number of examples of ethylenic compounds
which "obey" the Michael Rule. The greater rates invariably
refer to the isomers which lose groups trans to one another
yet which may be named "cis" or "trans" for other reasons.
Several comments on this table are pertinent. Several
of the compounds have been assigned a cis or trans configuration .
chiefly on the basis of their reactivity, for example, the
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2
Table 1
I
Survey of Elimination Reactions of 1,2-Substituted-ethylenes
Groups Relative Refer
No. Compound Base Removed Rates ences
1 HOOCCBr = CBrCOOH Zn Br2 fa st:slow 3
2 C^OOCCBr = G3rCOOC2H5 Zn Br2 4:1 4
3 CH^CBr = CBrCH^ Zn Br2 fa st:slow 10
4 AgOOCCBr = CCH-jCOOAg heat + HN03 AgBr--co2 2:1 5
5 AgOOCCBr = CHCOOAg heat + HN03 AgBr'-co2 3:1 5
6 CH^CCl = CHCOOAg heat AgCl'-co2 1.5:1 4
7 NaOOCCCH^ = CCI^Br heat NaBr'-co2 fast:slow 11
8 CH^CH = CHC1 KOH HC1 2:1 9
9 C2H5CH = CHC1 KOH HOI 2.9:1 24
10 CH^CCl = CHCH^ KOH HC1 2.5:1 24
11 KOH HC1 20:1 15
C2H2C12
12 CH^CCl = CHCl NaOCgHj HCl 25:1 27
13 ClCH = CHCH20H NaOH HC1 fa s t:slow 20
14 ch3cci = chch2oh NaOH HCl 3:1 21
15 H00CCC1 = CHCOOH NaOH HCl 10:1 29
16 0CH = CClCOOH (0=C6H5') KOH HCl 7:1 12
17 0CC1 = CHCOOH KOH HCl 11:1 13
IS CH-jCH = CClCOOH KOH HCl fa st:slow 9
19 CH^CCl = CHCOOH NaOH HCl 4:1 6
20 CHCl = CHBr KOH HBr 39:1 19
21 CH^CH » CHBr KOH HBr 7:1 22
22 C2H5CH = CHBr KOH HBr 8:1 23
23 CH3CH = C3rCH3 KOH HBr 4.5:1 23 .
24 C2H2Br2 KOH HBr 125:1 17
25 0CH = CHBr KOH HBr fa s t:slow 25
26 p-N02.C6H^.GH = CHBr NaOH HBr 2300:1 26
27 0CH = CBr0 KOH HBr 6:1 10
2$ . HOOCCBr = CHCOOH NaOH HBr 10:1 29
29 CH3CBr = CHCOOH NaOH HBr fa s t: slov; 7
30 CH3CH = CBrCOOH KOH HBr fa st:slow 7
31 0CH = CBrCOOH KOH HBr 3.5:1 8
32 0CH = CBrCOOH KOH HBr 55:1 14
33 0CBr = CHCOOH KOH HBr 400:1 ■14
34 0CBr = CHCOOCH3 KOH HBr fa s t:slow 14
35 {DC H = CHCl KOH HI 2 pi 18
36 G2H2I2 KOH HI 200:1 16
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I 3
pair I-icCl = and cannot but obey the Michael rule. The
thermal decompositions of the silver salts in HNO^ are perhaps
the worst examples of the rule, since the rates in water are
in the reverse order. In most cases relative rates were found
from product yields or by comparing the percent reaction under
similar conditions; a correct comparison requires that the times
for a constant percentage reaction be compared. 'Moreover, since
these rates were not determined kinetically, a check on the
purity of the isomers and on the reaction mechanism was lacking.
A small amount of reaction of any compound may only indicate
that the more reactive isomer is present in small amount as
impurity. For example, the trans isomers of C2^2I^2* ^ 2 ^ 2 ^ r 2 * ■
in Table 1 are claimed to exhibit a small amount of
reaction under the conditions where we have not observed appreci
able reaction of these isomers. Entry number 32, the result
of a kinetic study, shows how much entry 31, a batch result,
can be improved.
In entries 16, 17» 32, and 33> rough kinetic data from
the chloro- and bromo-cinnarnic acid salts represent first attempts
12 13 1 L
at a quantitative approach to trans elimination. ’ * * Their
value is limited since each compound was usually run only once
at one temperature.
Four entries, 12, 15, 26, and 2$, represent more recent
and more satisfactory kinetic studies. Y.’e shall have occasion
to discuss this work in more detail in a later section; a few
comments, however, are of interest here. The basic elimination
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2 6
reactions of the w-bromo-p-nitrostyrenes show the greatest
relative difference in rate, but this ratio is similar to the
values obtained in this study.
27
Entry 12 is taken from some recent 'work of Smith and King.
They usel CgH^O” in CgH^OH to study the rates of displacement
of Cl" from all possible mono- and di-chloropropenes. In the
case of 1,2-dichloropropene-l and 1,1-dichloropropene-l they sug
gest that the formation of Cl” may result from the elimination
of HCl rather than from direct displacement; certainly the
removal of HCl from a vinylic chloride has more precedent than
the displacement of Cl . In the other chloropropenes the acti
vation energies for unambiguous displacement vary from 20 to
26 kcal, while the activation energies for the reactions of
u
these compounds vary from 31 to 40 kcal.'7 In any case, the
1 ,2-dichloropropene-l, with the K and Cl trans to one another,
yields Cl" twenty-five times faster than its geometrical isomer;
the 1,1-dichloropropene-l has no geometrical isomer but the
reported activation energy of 40.6 kcal. will be useful for
later comparisons.
The use of the Michael rule in differentiating between
geometrical isomers has been implied. V/hen interatomic distances
are unknown, and dipole moment data cannot be interpreted, the
"This point is more fully discussed in the "Stoichiometry"
section.
^Activation energies for the elimination of HCl from C9H,,C1~
exceed 29 kcal.
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5
rule is especially valuable. Other generalizations, like the
higher melting .point of trans, the higher boiling point of cis,
the lower molar refractivity of cis, the higher density of cis,
the higher Raman double bond frequency in the trans, have their
10 11
exceptions. Moreover, in compounds like = ^ 3^4 >
where three or four substituents may differ, the accuracy of the
preceding generalizations is further decreased; but the trans
5|*
elimination rule, when applicable, has always appeared to hold.
Related Elimination Reactions
In this section we shall turn to 1,2-type eliminations
in saturated compounds. For vicinal dibromides, rather complete
kinetic data are available for the reaction: ^ ^
Br^1^2C^Br^3^4 + --------* ‘*■3” + 2^r” + ^1^2^ = ^ 3^ 4
It is first order in both 1“ and in organic dibromide. The Br
atoms are always removed trans to one another but the rate of
the reaction is strongly influenced by the other substituents.
Thus meso dibromotartaric acid gives fumaric acid more rapidly
than the dl acid gives maleic acid. In the meso acid both the
Br and COOH groups can be trans simultaneously, while in the
dl acid the COOH groups are cis when the Br atoms are trans.
vBergmann, in the last ten years, has perhaps been the most
emphatic critic of the u tility of trans elimination in the de
termination of structure. Apparently unaware of the re
interpretation of his evidence, he continued to quote erroneous
data;" for example, he refers to Chavanne’s paper of 1912 on the
CpHjClp isomers without realizing that„Chavanne reversed his
assigned structures in a later paper.1:5 •
I
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In an early paper Young et al. concluded that both the
rate constant is higher and the activation energy is' lower for
trans elimination from meso compounds than for trans elimination
from dl compounds.^ Later, when more diastereoisomers were
studied, this conclusion was modified to exclude activation
1Q
energies as a criterion of reactivity.
Debromiriations with Zn of diastereoisomeric dibromides,
analogous to those in Table 1, follow the pattern of the I~
IO
elimination reaction just discussed.
The configuration of the four known hexachlorocyclohexanes
^nine possible) was confirmed by Cristol on the basis of the
relative rates of removal of HCl with b a se .^ The so-called
(3-isomer, in which all Cl are cis to H, showed the lowest
reactivity.
Using a similar reaction in the cholesterol series, Barton
and Miller confirmed the structure of cis (5a, 6a~) and trans
(5a, 6(3)-dichlorochblestan-3p-yl benzoate, the latter reacting
more slowly with base. By assuming a greater reactivity with
I™ of the (5a, 6(3-) over the (5P, 6a.)-dibromocholestan-3(3-yl
benzoate (Br atoms are cis in the latter), they could also dis
tinguish between these isomers. A reference to the diagram on
page 14, will this clarify the point.
* I
Similarly acting on various decalyl toluene sulfonates
and on menthyl chlorides appears^1^ to give octalin and menthene
isomers favored by trans elimination.
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