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TitleAdvanced Organic Chemistry: Part A: Structure and Mechanisms
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Page 2

Advanced Organic
Ch m • t SECOND e IS ry EDITION

Part A: Structure and Mechanisms

Page 368

352

CHAPTER 6
POLAR ADDITION
AND ELIMINATION
REACTIONS

R,CHX

B (more-substituted olefin)

Fig. 6.5. Product-determining step forE I elimination.

fL. H~CHCH2CH2CH3 ._._... CH3<;HCH2 CH2CH3 ~ CH3CH~HCH2CH3
J. H B


HzC=CHCH2CH2CH3 J.

CH3CH=CHCH2CH3

The experimental evidence that has been gathered concerning direction of elimina-
tion from substrates reacting by the El mechanism indicates that the relative stability
of the product alkene is a major factor in determining direction of elimination. The
direction of elimination will be determined by the relative energies of the transition
states A' and B' (Fig. 6.5). It is known that the order of stability of the olefins is
B more stable than A. The experimental results indicate that the transition states
A' and B' resemble A and B sufficiently that the same relative order of stability
holds. Since the activation energy for proton removal from a carbonium ion is low,
the transition states presumably resemble the intermediate carbonium to a great
extent; states A' and B' should therefore be considerably closer in energy than A
and B. Thus, El eliminations are often rather low in selectivity, giving rise to a
mixture of all possible olefins. The composition, however, roughly reflects the relative
thermodynamic stabilities of the olefin, in that the most-substituted olefin is formed
in the greatest amount. The precise product composition is governed by a number
of factors. In some cases, the product composition is a function of the leaving group,
indicating that the carbonium ion cannot be completely free of the counterion when
deprotonation occurs. In nondissociating solvents, ion pairs are probably key inter-
mediates, and the counterion may act as the proton acceptor. 65

65. D. J. Cram and M. R. V. Sahyun, J. Am. Chern. Soc. 85, 1257 (1963); P. S. Skell and W. L. Hall,
J. Am. Chern. Soc. 85, 2851 (1963).

Page 369

In the Elcb mechanism, the direction of elimination is governed by the kinetic
acidity of the individual f3 protons, which in turn is determined by the inductive
and resonance effects of nearby substituents and by the degree of steric hindrance
to approach of base to the proton. Alkyl substituents will tend to retard proton
abstraction both electronically and sterically. Preferential proton abstraction from
unhindered positions leads to the formation of less-substituted alkenes.

The preferred direction of elimination via an E2 mechanism depends on the
precise nature of the transition state. The two extreme transition states for the E2
elimination will resemble the El and Elcb mechanisms in their orientation effects.
At the "Elcb-like" end of the E2 spectrum, a highly developed bond is present
between the proton being abstracted and the base. The leaving group remains tightly
bonded, and there is little development of the carbon-carbon double bonds. At the
"El-like" end of the spectrum, the transition state is characterized by extensive
cleavage of the bond to the leaving group and a largely intact C-H bond. In a
synchronous E2 reaction, the new double bond is substantially formed at the
transition state at the expense of partial cleavage of the C-H and C-X bonds. E2
eliminations that proceed through transition states with high double-bond character
give mainly the more substituted alkene because the stability of the alkene is reflected
in the transition state. When the transition states have extensive Elcb character,
the direction of elimination is governed by ease of proton removal. In this case, the
less-substituted alkene usually dominates.

Prior to development of the mechanistic ideas outlined above, it was recognized
by experience that some types of elimination reactions gave the more-substituted
possible alkene as the major product. Such eliminations were said to follow the
"Saytzeff rule." This behavior was observed for eliminations that would now be
recognized as proceeding by the El mechanism, and for eliminations by the E2
mechanism when halides and sulfonate ions or other good leaving groups were
involved. Reactions involving the E2 mechanism, but with poor leaving groups,
especially elimination of tertiary amines from quaternary ammonium salts, were
observed to give predominantly the less-substituted olefin, and were said to
follow the "Hofmann rule." In these reactions, there is little development of the
carbon-carbon double bond at the transition state; i.e., the transition state is
"Elcb-like."

The data recorded in Table 6.4 for the 2-hexyl system illustrate two general
trends that have been recognized in other systems as well. First, poorer leaving
groups favor elimination according to the "Hofmann rule," as shown, for example,
by the increasing amount of terminal olefin in the halogen series as the leaving group

I

is changed from iodide to fluoride. Poorer leaving groups move the transition state in
the Elcb direction. A higher negative charge must be built up on the f3 carbon to
induce loss of the leaving group. This buildup is accomplished by more complete
proton abstraction. A more electronegative leaving group, such as fluoride, increases
the acidity of the f3 protons, making the transition state more "Elcb-like" and
increasing the proportion of the less-substituted alkene.

Comparison of the data for methoxide with t-butoxide in Table 6.4 illustrates
the second general trend: Stronger bases favor formation of the less-substituted

353

SECTION 6.7.
ORIENTATION

EFFECTS IN
ELIMINATION

REACTIONS

Page 736

stereochemistry (cont.)
of photochemical electrocyclic

reactions 589-590
of sigma tropic rearrangements 548-551

stereoelectronic effects
anomeric effect 132
in carbonium ion rearrangements 299
definition 100
in reactions of tetrahedral

intermediates 429-430
in stereoselective reactions of

cyclohexanones 152
stereoselective reactions, definition 78

examples (scheme) 82
stereospecific reactions, definition 78

examples (scheme) 80-81
steric effects

in Friedel-Crafts reaction 514,517
on kinetic acidity of ketones 384-385
in nucleophilic substitution 274-276
in reactions of bicyclo[2.2.1 ]heptenes !53

Stern-Volmer relationship 587
stilbene, photochemical cis-trans

isomerization 606-607
strain energy

of alicyclic compounds (table) 142
components of 100-101
of cycloalkanes (table) 124
definition 99
effect on reactivity 142-146
relation to hybridization 7

structure-reactivity relationships in free radical
reactions 647-655

substituent constants
j' 186
3t 186
a 181
a· 185
a1 187
aR 187
a~ 187
table of 183

substituent effects 179-190
on acetal hydrolysis 407 -4ll
on acidity

of carboxylic acids 16
of hydrocarbons 376-379
of phenols 210
of substituted benzoic acids 179-184

on addition reactions of alkenes 331-332, 337
on alkane conformations 105-106
on bond-dissociation energies 652-655
on bromination of alkenes 333-337
on carbanion ion stability 383-390
on carbonium ion stability 251-256
on cyclohexane conformations 113-119
on the Diels-Alder reaction 563-565

in electrophilic aromatic substitution 214-218,
489-503

on elimination reactions 349-351
on enolization 390-393
on ester hydrolysis 422-424
field effects 182
on free-radical chlorination 656-658
on free-radical rearrangements 6 77
on gas phase reactions 209-212
on hydration of alkenes 330
on hydration of carbonyl compounds 404-405
inductive effects 182
and linear free-energy relationships 179-190
on norbornyl cation 308-309
in nucleophilic substitution 277-278
polar, on free radical reactions 655-658
on radical stability 647-655
on rate

of nucleophilic substitution reactions 239,
242

of racemization of optically active biaryls 85
of reduction of aldehydes and

ketones 418-419
on stability

of methyl anion 23
of methyl cation 23
of radicals 647-655

on structure of alkyl radicals 638
sulfonate esters, in nucleophilic substitution 245,

272,279
sulfones, deprotonation of 389-390
sulfoxides

allylic, sigmatropic rearrangements 556-557
chiral 63
racemization of 84
specification of configuration 67

sulfur substituents, stabilization of
carbanions 387-388

sulfur ylides 388-390, 556-557
super acid media, carbonium ions in 257-261, 298,

302-303, 309-3ll
suprafacial, definition 544
Swain-Lupton equation 186
symmetry elements

of orbitals in concerted reactions 531-536,
560-561

as a test of chirality 63, 72
syn and anti descriptors 78

tartaric acid, stereoisomers 71
tautomerism, definition 220
telomer formation, in addition of hydrogen

chloride to alkenes 665
tetrafluoroethylene, cycloaddition reactions

of 571
tetrahedral intermediates 403

in amide hydrolysis 431-432

725
INDEX

Page 737

726
INDEX

tetrahedral intermediate (cont.)
in ester aminolysis 427-430
in ester hydrolysis 421-426
in reactions of carbonyl compounds with

amines 411-417
stereoelectronic effects on reactivity

of 429-430
tetrahedrane, derivatives 143-144
tetrahydropyran

anomeric effect in 130-132
conformation of 128

tetramethyllead 626
thermodynamic control, definition 212-213
thermodynamic equations 161-179
thiols, free-radical addition to alkenes 670-671
thionyl chloride, reaction with alcohols 287
L-threonine, configuration of 70
threo, as descriptor for diastereomers 70
threose, stereochemistry of 70
tin, organa- compounds (see stannanes)
toluene

acidity of 376-377
bromination of 655
electrophile selectivity for 498, 507, 509, 511,

513,514,516
torsional angle, definition I 0 I
torsional strain

in butane 103
definition I 0 I
effects on reactivity 150-151
in reactions of cyclic ketones 150-151, 419

transannular processes
hydride shifts in carbonium ions 302
neighboring group participation 292-293
in radical addition reactions 675-676

transition states
aromaticity and antiaromaticity in concerted

reactions 537-538
theory for kinetics 173-176

transmission characteristics of photolysis
equipment 583-584

triafulvene and derivatives 480-481
trifluoroacetic anhydride, in acylation

reactions 436
trifluoroacetyl hypohalites, halogenation

with 507
trifluoromethanesulfonate, as leaving group 272
triphenylmethyl cation 249
triphenylmethyl radical 625-626
triphenyltin hydride, as hydrogen atom

donor 677
triplet state, definition 584-585
tropane; cycloaddition reactions of 571

tropylium cation (see cycloheptatrienyl cation)
twist conformation of cyclohexane 112
type-] photochemical reactions of ketones 596
type- II photochemical reactions of ketones 597

ultraviolet-visible spectroscopy
absorption characteristics of organic molecules

(table) 584
detection of intermediates by 195, 249-250, 374

unsaturated ketones
a,/3-unsaturated

conformations of Ill
photochemistry of 598-600

f3;y-unsaturated, photochemical cleavage
of 600

valence bond theory 2-10
valence number 2
valence tautomerism

of bullvalene 554
of cycloheptatrienes 539-540
definition 539
of vinylcyclopropanes 552-555

van der Waals forces
attractive, in 11-propyl chloride I 05-106
as factor in conformation of 11-butane I 03-104
repulsive, of axial tert-butyl-groups 119, 129
repulsive, in dienes I 09

van der Waals radii of atoms (table) 103
variable transition state theory for elimination

reactions 348-351
vinyl cations 262, 343-344
vinyl radicals 641
vinylcyclopropanes, rearrangements of 552-555
vitamin O,, sigmatropic rearrangements in

549-550

Westheimer method (see molecular mechanics)
Winstein-Grunwald equation 269
Woodward-Hoffmann rules

for cycloaddition reactions
for eiectrocyclic reactions
for photochemical reactions

562
530-544

588-593
for sigmatropic rearrangements 547

X-ray spectroscopy (see ESCA)

ylides, formation and structure 388-389
Yukawa-Tsuno equation 185-186
Y value, as a measure of solvent ionizing

power 204

zero point energy 191

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