Ring flip
Encyclopedia
Ring flipping is a phenomenon involving the interconversion (by rotation
) about single bonds
of cyclic conformers having equivalent ring shapes but not necessarily equivalent spatial positions of substituent atom
s.
The six-membered ring of the alkane
, cyclohexane
, has the 'chair' as its preferred conformation and through proton
NMR
it would be expected that the produced spectrum would show the different sorts of protons (axial
and equatorial
) resonating at different frequencies
, and so two signals would be seen, due to the protons being in slightly different chemical environments. However, only one signal can be seen as the two isomers are rapidly interconverting. This also occurs with monosubstituted cyclohexanes. This changes with the use of low temperature
, as the two isomers visible interconvert much more slowly than at room temperature.
When cyclohexane undergoes a ring inversion, all axial bonds become equatorial, or vice-versa. The half-chair conformation is the energy maximum when proceeding from the 'chair' to the 'twisted boat' conformation. The 'true boat' conformation is the energy maximum for the interchanging of the two mirror image
twist boat conformers, the second of which is converted to the other chair confirmation through another half-chair.
However, the chemical reactivity of cyclohexane, is inconsistent with two types of hydrogens in a stable form of the molecule (for example, there is only one monochlorocyclohexane, not two, as would be predicted if axial and equatorial hydrogens could be replaced independently), but this is due to the presence of rapid ring inversion as explained above. In substituted cyclohexanes, steric repulsion can be minimised by the substituents occupying positions along the equatorial plane
rather than the axial plane, and thus would also lead to greater stability in the molecule.
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
) about single bonds
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
of cyclic conformers having equivalent ring shapes but not necessarily equivalent spatial positions of substituent atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s.
The six-membered ring of the alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
, cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
, has the 'chair' as its preferred conformation and through proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
it would be expected that the produced spectrum would show the different sorts of protons (axial
Axial
Axial may mean:* Along the same line as an axis of rotation in geometry* A type of modal frame in music* One of several anatomical directions in an animal body* Axial age, the period from 800 to 200 BC in China, India and the western world...
and equatorial
Equatorial
Equatorial may refer to:* Equator of the Earth* Equatorial climate in meteorology* The ring-shaped outer boundary of the cross-section of a round three dimensional shape or object in geometry* The equatorial bond of a molecule in chemistry...
) resonating at different frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, and so two signals would be seen, due to the protons being in slightly different chemical environments. However, only one signal can be seen as the two isomers are rapidly interconverting. This also occurs with monosubstituted cyclohexanes. This changes with the use of low temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, as the two isomers visible interconvert much more slowly than at room temperature.
When cyclohexane undergoes a ring inversion, all axial bonds become equatorial, or vice-versa. The half-chair conformation is the energy maximum when proceeding from the 'chair' to the 'twisted boat' conformation. The 'true boat' conformation is the energy maximum for the interchanging of the two mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...
twist boat conformers, the second of which is converted to the other chair confirmation through another half-chair.
However, the chemical reactivity of cyclohexane, is inconsistent with two types of hydrogens in a stable form of the molecule (for example, there is only one monochlorocyclohexane, not two, as would be predicted if axial and equatorial hydrogens could be replaced independently), but this is due to the presence of rapid ring inversion as explained above. In substituted cyclohexanes, steric repulsion can be minimised by the substituents occupying positions along the equatorial plane
Plane (mathematics)
In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...
rather than the axial plane, and thus would also lead to greater stability in the molecule.