Rearrangements
Course details
Course lecturer: Dr Craig Butts.
Date: February 2009.
See also: Wikipedia notes on rearrangements.
Table of contents
- Carbocations, radicals and carbanions
- Electronic structure
- Substitutent effects
- Inductive effect
- Mesomeric effect
- Carbocation stability
- Hyperconjugation
- Combined effects
- Radicals
- Carbanions
- Measuring carbocation stability
- Rearrangements
- Wagner-Meerwein shift
- Camphene hydrochloride–isobornyl chloride rearrangment
- Pinacol rearrangement
- Demyanov reaction
- Asymmetric pinacol rearrangement
- Control features
- Migratory aptitude
- Stereoelectronic factors
- Synchronicity in rearrangements
- Common rearrangements
- Migrations to electron-deficient carbon
- Wolff rearrangement
- Arndt-Eistert homologation
- Corey-Fuchs reaction
- Migrations to electron-deficient nitrogen
- Common mechanism: Curtius, Lossen, Schmidt and Hofmann
- Curtius rearrangment
- Lossen rearrangement
- Schmidt reaction
- Hofmann degradation
- Beckmann rearrangement
- Application to caprolactam synthesis for nylon production
- Migrations to electron-deficient oxygen
- Baeyer-Villiger oxidation
Carbocations |
Carbon radicals |
Carbanions |
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6 valence electrons
electron deficient |
7 valence electrons
electron deficient |
8 valence electrons
unstable due to negative charge |
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- 3 filled bonding orbitals
- 1 empty orbital
- sp2 hybridised — planar
- stabilised by electron donors
- destabilised by EWGs
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- 3 filled bonding orbitals
(6 electrons in bonding orbitals)
- 1 electron in a non-bonding orbital
- tend towards sp3 hybridisation — tetrahedral
- stabilised by electron donors
- destabilised by EWGs
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- 3 filled bonding orbitals
- 1 filled non-bonding orbital
- full octet at C−, negative charge
- sp3 hybridised — tetrahedral
- stabilised by EWGs
- destabilised by EDGs
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Nitrogen is electron donating
Nitrogen is electronegative
Both are true... two competing effects
| element |
electronegativity |
| H |
2.20 |
| C |
2.55 |
| N |
3.04 |
| O |
3.44 |
| F |
3.98 |
| Cl |
3.16 |
| Br |
2.96 |
| I |
2.66 |
| Si |
1.90 |
Electronegativities of selected elements
- Depends on the relative electronegativity
- Inductive effect
- Short range effect (1–2 bonds)
- Nitrogen also has a lone pair
- Long range if conjugated
| iminium ion |
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| allylic cation |
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| oxocarbenium ion |
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| benzylic cation |
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- Effectively 'spreads' (delocalises) the positive charge over two or more centres, effectively halving (or further decreasing) the charge per centre
- Mesomeric effects are much stronger than the inductive effect, but...
- ...the donor orbital must be conjugated with the accepting orbital, i.e. dependent on the conformation of the carbocation (so if conformation prevents conjugation, only inductive effects will be present)
- π-donation depends on the ability of the atom to donate a lone pair, and thus its electronegativity, so N > O > F
- fluorine is more inductively withdrawing than mesomerically donating
- Mesomeric effects always dominate over inductive effects except with fluorine
Carbocation stability
Relative stabilities of carbocations: tertiary C+ are more stable than secondary, etc. (3° > 2° > 1° > methyl), each by about 40 kJ mol−1, a huge difference
[R3C]+ are about 100 million times more stable than [R2CH]+
Definition of hyperconjugation: Donation of a pair of bonding electrons into an unfilled or partially filled orbital
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hyperconjugation can only occur when a C-H (or C-X) bond aligns with the carbocation's p orbital |
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because bonding electrons are donated, the effect is weaker than the π-donation of lone pairs (which are non-bonding)
inductive effects |
< |
hyperconjugation |
< |
mesomeric effects |
weak |
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medium |
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strong |
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Corollary 1: The donor bond in hyperconjugation must be able to overlap with the acceptor orbital.
Rigid structures that hold the donor orbital out of alignment are less stable.
Corollary 2: Silicon is fantastic at stabilising β-carbocations.
β-Silyl carbocations are very stable.
Because silicon is more electropositive than carbon, it is able to donate electrons into an Si-C bond, making the bond more electron rich. This is referred to as the β-silicon effect. When X = Si, the formation of a fifth bond is not unreasonable.
The α-silicon effect is destabilising relative to the β-Si effect.
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NO2
π-withdrawing
(conjugated ∴ long range)
activates C=O
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F
σ-withdrawing
(short range)
cannot activate C=O
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Radicals are stabilised by pi-donation and hyperconjugation.
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radicals are stabilised by π-donation
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radicals are stabilised by π-donation from lone pairs
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radicals are stabilised by hyperconjugation: 1° < 2° < 3°
Carbanions are stabilised by pi-withdrawal but destabilised by pi-donation and hyperconjugation.
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carbanions are stabilised by π-withdrawal
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carbanions are destabilised by adjacent electron donors
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carbanions are destabilised by hyperconjugation: 1° > 2° > 3°
You can measure the stability of a carbocation by the enthalpy change (ΔHf) of the following reaction.
The graph below shows the relationship between ΔHf for the reaction above and the rate of solvolysis of RBr (e.g. in EtOH/H+) for different RBr.
The graph is evidence that carbocations are being formed as intermediates in these solvolysis reactions, suggesting that the mechanism is SN1.
A recent literature review in the Journal of Chemical Education concludes that:
- methyl, primary and secondary alkyl compounds react only by SN2
"except under rare and predictable conditions"
- tertiary alkyl compounds react only by SN1
"Absence of SN1 Involvement in the Solvolysis of Secondary Alkyl Compounds"
T. J. Murphy
J. Chem. Ed. (2009) 86, 519–524 |
The relative rates of solvolysis of different bromides are also instructive.
tBuBr reacts ten thousand times faster than adamantyl bromide. Due to the rigid structure of adamantyl bromide, C-H bonds cannot effectively hyperconjugate with the carbocation that forms when bromide leaves.
Jmol models of the mechanism
J. Org. Chem. (1999) 64, 60–64
In the presence of a Lewis acid catalyst, camphene hydrochloride (1) rearranges to isobornyl chloride (2).
The mechanism is believed to involve the following steps:
- the Lewis acid catalyst abstracts chloride from camphene hydrochloride (1), leaving behind a tertiary carbocation (2)
- the tertiary carbocation (2) rearranges to a secondary carbocation (3) by a [1,2]-alkyl shift
- a chloride ion adds to the secondary carbocation (3), forming isoborny chloride (4)
The carbocations (2) and (3) only differ in the positions of electrons, so they are resonance forms of the same ion, and can be described as a non-classical carbocation. NMR evidence for the non-classical nature of such cations is given in March (6th ed.), pp. 1582–1583.
Application to caprolactam synthesis for nylon production
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