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In chemistry,
conformational isomerism is a form of
stereoisomerism involving the phenomenon of molecules with the same
structural formula existing as different
conformational isomers or
conformers due to atoms
rotating about a Chemical bond.
There are three principle effects that make some conformers more stable than others:
Bond interaction with the back lobes of orbitals on adjacent atoms is possible only when atoms on adjacent atoms are staggered. This is virtually the only reason that ethane's preferred conformation is the staggered one.
Steric effects will also make some conformers more favourable than others.
Finally, Bond dipole moment of polar bonds will influence which conformers are most stable.
Different conformers can interconvert by rotation around single bonds, without breaking
chemical bonds. A simplified example is that of a butane molecule viewed in the
Newman projection shown - i.e. as if viewed down the central C-C bond with relative rotations of C² and C³ illustrated.
Rotamers are a set of conformers and the
rotation barrier is the
activation energy required to jump from one conformer to another conformer.
The population of different conformers follows a
Boltzmann distribution:
\frac{N_i}{N_j} = \frac{g_i}{g_j} \exp \left (\frac{-(E_i-E_j)}{RT} \right)
The subscripts i and j represent the highest and lowest energy. g is the number of conformations found at that particular energy, the degeneracy. N is the population of molecules in a particular conformation.
Two important forms of conformational isomerism exist:
linear alkane conformations with staggered, eclipsed and gauche conformers, and
cyclohexane conformations with chair and boat conformers.
Another example of conformational isomerism is the Folding (chemistry) of molecules, where some shapes are stable and functional, but others are not. Conformational isomerism is also found in atropisomerisms.
Consequences
If the
eclipsed conformations of an isomer have high enough potentials, they may prevent rotation of
substituents to different staggered conformations at sufficiently low energy levels. This will result in a
racemic mixture of conformations that may or may not have different
reactivity in situations such as enzyme reactions in which molecular shape is usually a key factor of operation.
Conformer dependent reactions
The
Elimination reaction relies on the base- or acid-attacked substituent being in an
antiperiplanar configuration along a bond with respect to the leaving group. This prerequisite for reaction is important in understanding organic elimination reaction pathways, especially those involving halogenated cyclic alkanes such as
cyclohexanes. Two adjacent substituents on a cyclic alkane can only undergo an E2 elimination if they are both axial to the ring and hence antiperiplanar. A combination of axial and equatorial substituents cannot react through an E2 mechanism, though ring flips (with associated reconformation) may allow reactions to occur if they are not precluded by an energy barrier or
steric lock through isopropyl or larger substituents.
Conditions
Conformational isomerism only occurs around single bonds because double or triple bonds have one or two
pi bonds that prevent rotation about the longitudinal axis. Conformers sufficiently constrained to exhibit measurable isomerism are unique from various flavours of stereoisomers in the fact that changes in stereochemistry are independent from any mechanism and instead rely only on molecular energy.
See also
- isomer
- Steric effects
- List of publications in chemistry#Organic chemistry
In
chemistry,
conformational isomerism is a form of stereoisomerism involving the phenomenon of molecules with the same
structural formula existing as different
conformational isomers or
conformers due to atoms rotating about a Chemical bond.
There are three principle effects that make some conformers more stable than others:
Bond interaction with the back lobes of orbitals on adjacent atoms is possible only when atoms on adjacent atoms are staggered. This is virtually the only reason that ethane's preferred conformation is the staggered one.
Steric effects will also make some conformers more favourable than others.
Finally, Bond dipole moment of polar bonds will influence which conformers are most stable.
Different conformers can interconvert by rotation around single bonds, without breaking
chemical bonds. A simplified example is that of a
butane molecule viewed in the Newman projection shown - i.e. as if viewed down the central C-C bond with relative rotations of C² and C³ illustrated.
Rotamers are a set of conformers and the
rotation barrier is the activation energy required to jump from one conformer to another conformer.
The population of different conformers follows a Boltzmann distribution:
\frac{N_i}{N_j} = \frac{g_i}{g_j} \exp \left (\frac{-(E_i-E_j)}{RT} \right)
The subscripts i and j represent the highest and lowest energy. g is the number of conformations found at that particular energy, the degeneracy. N is the population of molecules in a particular conformation.
Two important forms of conformational isomerism exist:
linear alkane conformations with staggered, eclipsed and gauche conformers, and
cyclohexane conformations with chair and boat conformers.
Another example of conformational isomerism is the Folding (chemistry) of molecules, where some shapes are stable and functional, but others are not. Conformational isomerism is also found in
atropisomerisms.
Consequences
If the eclipsed conformations of an isomer have high enough potentials, they may prevent rotation of substituents to different
staggered conformations at sufficiently low energy levels. This will result in a
racemic mixture of conformations that may or may not have different reactivity in situations such as enzyme reactions in which molecular shape is usually a key factor of operation.
Conformer dependent reactions
The
Elimination reaction relies on the base- or acid-attacked substituent being in an antiperiplanar configuration along a bond with respect to the leaving group. This prerequisite for reaction is important in understanding organic elimination reaction pathways, especially those involving halogenated cyclic alkanes such as
cyclohexanes. Two adjacent
substituents on a cyclic alkane can only undergo an E2 elimination if they are both axial to the ring and hence antiperiplanar. A combination of axial and equatorial substituents cannot react through an E2 mechanism, though ring flips (with associated reconformation) may allow reactions to occur if they are not precluded by an energy barrier or
steric lock through isopropyl or larger substituents.
Conditions
Conformational isomerism only occurs around single bonds because double or triple bonds have one or two
pi bonds that prevent rotation about the longitudinal axis. Conformers sufficiently constrained to exhibit measurable isomerism are unique from various flavours of stereoisomers in the fact that changes in stereochemistry are independent from any mechanism and instead rely only on molecular energy.
See also
- isomer
- Steric effects
- List of publications in chemistry#Organic chemistry
Conformational isomerism - Wikipedia, the free encyclopedia
In chemistry, conformational isomerism is a form of stereoisomerism involving the phenomenon of molecules with the same structural formula existing as different conformational ...
A Dynamic NMR Investigation of the Conformational Isomerism in Novel 1 ...
A dynamic NMR investigation of the conformational isomerism in novel 1,3,4,5-tetrahydro-2,1-benzoxazepines Tony M. Banks, Gary J. Cowin, Stephen A Glover, Gregory J.
ECHET96: Reaction Mechanisms and Conformational Analysis
Tucker and Yang Zengjia : A dynamic NMR investigation of the conformational isomerism in novel 1,3,4,5-tetrahydro-2,1-benzoxazepines. Article Received 02/May/1996: ECHET96 Contrib 107 ...
Conformational isomerism of -ketoesters. A FTIR and ab initio study
2 PER K I N J. Chem. Soc ., Perkin Trans. 2 , 2000, 221-228 221 This journal is © The Royal Society of Chemistry 2000 Conformational isomerism of -ketoesters.
Conformational analysis. Part 33.1 An NMR, solvation and theoretical ...
Part 33. 1 An NMR, solvation and theoretical investigation of conformational isomerism in N , N -dimethylfluoroacetamide and N , N -dimethyl--fluoro-propionamide Claudio F.
Conformational Isomerism Wiki resources & Conformational Isomerism ...
In chemistry, conformational isomerism is a form of stereoisomerism involving the phenomenon of molecules with the same structural formula existing as different conformational ...
INEX: Wikipedia, the free encyclopedia (Conformational isomerism)
Table of Contents. 1 Potential energies (of butane) 2 Consequences; 3 Conformer dependent reactions; 4 Conditions; 5 See also; Conformational isomerism is the phenomenon of ...
Conformational isomerism and self-association of calixarene building ...
In order to establish the conformational equilibrium of calix[6]arene derivatives, a series of phenolic building blocks differing in chain length (oligomers) have been studied in ...
www.coacs.com
2. Conformational Isomerism; Ethane and Hydrocarbons Energy of Conformations Hydrogen Bonding Conformation in the Nucleic Acids
Impact of the Solvent on the Conformational Isomerism of Calix[4 ...
Abstract: The influence of solvation on the conformational isomerism of calix[4]arene and p-tert-butylcalix[4]arene has been investigated by using the continuum model reported by ...
