190:
53:
103:> tertiary carbocation (if formed by migration) > secondary carbocation (if formed by migration) > methyl carbocation. {Why carbocation? Because every migratory group leaves by taking electron pair with it.} The conclusion is that the group which stabilizes the carbocation more effectively is migrated.
220:
Some of the problems during the determination of the structure are because carbon skeletal rearrangements were unknown at that time and therefore the new concept had to be found. Butlerov theory allowed the structure of carbon atoms in the molecule to rearrange and with this concept a structure for
90:
The driving force for this rearrangement step is believed to be the relative stability of the resultant oxonium ion. Although the initial carbocation is already tertiary, the oxygen can stabilize the positive charge much more favorably due to the complete octet configuration at all centers. It can
124:
of the diol plays a crucial role in deciding the major product. An alkyl group which is situated trans- to the leaving –OH group may migrate to the carbocation center, but cis- alkyl groups migrate at a very low rate. In the absence of trans- alkyl groups, ring contraction may occur as the major
111:
When a pinacol is not symmetrical, there is a choice for which hydroxyl group will leave and which alkyl shift will occur. The selectivity will be determined by the stability of the carbocations. In this case although both choices are tertiary, the phenyl groups result in significantly higher
125:
product instead, i.e. the ring carbon itself may migrate. This reveals another interesting feature of the reaction, viz. that it is largely concerted. There appears to be a connection between the migration origin and migration terminus throughout the reaction.
196:
Again Fittig was unable to assign a molecular structure to the reaction product which he assumed to be another isomer or a polymer. Contemporary chemists who had already adapted to the new atomic weight reality did not fare better. One of them,
175:
in 1860. He also wrongly believed acetone to be an alcohol which he hoped to prove by forming a metal alkoxide salt. The reaction product he obtained instead he called paraceton which he believed to be an acetone
91:
also be seen as the -OH's lone pairs pushing an alkyl group off as seen in the asymmetrical pinacol example. The migration of alkyl groups in this reaction occurs in accordance with their usual
83:
is formed. If the –OH groups are not alike (i.e. the pinacol is asymmetrical), then the one which creates a more stable carbocation participates in the reaction. Subsequently, an
363:"Stereochemical effects in the gas-phase pinacol rearrangement. 2. Ring contraction versus methyl migration in cis- and trans-1,2-dimethylcyclohexane-1,2-diol"
96:
400:
Jerome A. Berson (2002). "What Is a
Discovery? Carbon Skeletal Rearrangements as Counter-Examples to the Rule of Minimal Structural Change".
322:
De Petris, Giulia.; Giacomello, Pierluigi.; Picotti, Tito.; Pizzabiocca, Adriano.; Renzi, Gabriele.; Speranza, Maurizio. (November 1986).
402:
270:
240:
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De Petris, Giulia.; Giacomello, Pierluigi.; Pizzabiocca, Adriano.; Renzi, Gabriele.; Speranza, Maurizio. (February 1988).
533:
474:
324:"Stereochemical effects in the gas-phase pinacol rearrangement of cis- and trans-1,2-dimethylcyclopentane-1,2-diol"
295:
128:
Moreover, if the migrating alkyl group has a chiral center as its key atom, the configuration at this center is
209:. Finally Butlerov in 1873 came up with the correct structures after he independently synthesised the compound
39:
takes place under acidic conditions. The name of the rearrangement reaction comes from the rearrangement of
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In cyclic systems, the reaction presents more features of interest. In these reactions, the
469:
8:
243:, in which the leaving group is a diazo (from amine) rather than oxonium (from hydroxyl)
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172:
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for acetone, the result of a long-standing atomic weight debate finally settled at the
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Although Fittig first published about the pinacol rearrangement, it was not Fittig but
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In an 1859 publication
Wilhelm Rudolph Fittig described the reaction of
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group from the adjacent carbon migrates to the carbocation center.
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265:(2nd ed.). New York: Oxford University Press. p. 945.
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180:. In his second publication in 1860 he reacted paraceton with
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24:
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Clayden, Jonathan; Greeves, Nick; Warren, Stuart (2012).
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who correctly identified the reaction products involved.
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stabilization of the positive charge through resonance.
439:"Ueber einige Metamorphosen des Acetons der Essigsäure"
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which
Friedel had obtained earlier by oxidizing with a
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tetramethylethylene oxide in analogy with reactions of
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285:
260:
79:, protonation of one of the –OH groups occurs and a
492:
470:"Recherches sur les acétones et sur les aldéhydes"
16:Rearrangement of compound by charge rearrangement.
525:
399:
502:Justus Liebigs Annalen der Chemie und Pharmacie
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107:Example of asymmetrical pinacol rearrangement
436:
254:
201:, believed the reaction product to be the
367:Journal of the American Chemical Society
328:Journal of the American Chemical Society
403:Angewandte Chemie International Edition
526:
59:This reaction was first described by
184:(the actual pinacol rearrangement).
116:Stereochemistry of the rearrangement
13:
291:"Ăśber einige Derivate des Acetons"
132:even after migration takes place.
14:
545:
475:Annales de chimie et de physique
443:Annalen der Chemie und Pharmacie
296:Annalen der Chemie und Pharmacie
241:Tiffeneau–Demjanov rearrangement
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155:metal. Fittig wrongly assumed a
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21:pinacol–pinacolone rearrangement
486:
461:
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393:
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211:trimethylacetic (pivalic) acid
1:
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23:is a method for converting a
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7:
498:"Ueber Trimethylessigsäure"
231:Benzilic acid rearrangement
224:
221:pinacolone could be found.
10:
550:
135:
236:Semipinacol rearrangement
514:10.1002/jlac.18731700114
468:Charles Friedel (1869).
455:10.1002/jlac.18591100104
309:10.1002/jlac.18601140107
534:Rearrangement reactions
416:10.1002/anie.200290007
287:Wilhelm Rudolph Fittig
75:In the course of this
61:Wilhelm Rudolph Fittig
437:W. R. Fittig (1859).
379:10.1021/ja00212a017
340:10.1021/ja00284a009
494:Aleksandr Butlerov
173:Karlsruhe Congress
142:Aleksandr Butlerov
97:phenyl carbocation
93:migratory aptitude
334:(24): 7491–7495.
272:978-0-19-927029-3
263:Organic chemistry
157:molecular formula
37:1,2-rearrangement
33:organic chemistry
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508:(1–2): 151–162.
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373:(4): 1098–1103.
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77:organic reaction
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207:ethylene glycol
199:Charles Friedel
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122:stereochemistry
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67:fame) in 1860.
65:Fittig reaction
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303:(1): 54–63.
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31:compound in
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478:. SĂ©rie 4.
81:carbocation
248:References
215:dichromate
45:pinacolone
387:0002-7863
348:0002-7863
153:potassium
71:Mechanism
528:Category
496:(1873).
424:12481317
289:(1860).
225:See also
130:retained
29:carbonyl
25:1,2-diol
203:epoxide
149:acetone
136:History
101:hydride
41:pinacol
482:: 310.
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385:
346:
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95:, i.e.
35:. The
178:dimer
159:of (C
151:with
99:>
85:alkyl
27:to a
420:PMID
383:ISSN
344:ISSN
267:ISBN
63:(of
19:The
510:doi
506:170
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167:O)
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169:n
165:3
163:H
161:3
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