conformers Flashcards
Define conformer
Conformers are different physical arrangement in space of a single molecule as the result of rotation about a sigma bond or ring puckering.
Compare a dash-and-wedge structure to a Newman projection
Both of these structure give a three dimensional perspective of a molecule.
A dash-and-wedge structure takes a view from the side of the molecule, where “normal” bond lines are in the place of the page, the wedge is “coming out” at you, and the dash is “going away” from you.
A Newman projection takes a view looking down a sigma bond from one carbon to the next. The front carbon is represented by simply where three bond lines come together, and the back carbon is represented by a larger circle.
Define torsional angle.
Torsional angle is the angle between two groups (one attached to the front carbon and the other to the back carbon) in a Newman projection. A “group” can be a single atom (e.g. H, Cl, F) or a collection of atoms (e.g. CH3, CH2CH3). Torsional angle is also known as dihedral angle.
Define staggered conformation.
A conformation in which nearby groups in a Newman projection have a torsional (dihedral) angle of 60°. A staggered conformation is always of lower energy than an eclipsed conformation.
Define eclipsed conformation.
A conformation in which groups are eclipsing each other in a Newman projection. An eclipsed conformation is always of higher energy than a staggered conformation.
Define torsional strain.
Torsional strain is the energy diffence between an eclipsed and a staggered conformation, This strain is due to the repulsion of bonding electrons of one group as they pass close to the bonding electrons of another group (i.e. bond to bond repulsion).
Define confomational analysis.
Conformational analysis involves tracking the energy states usually in 60° increments as one carbon is held stationary and the bond to the second carbon is rotated a full 360°. A conformational analysis is usually displayed as a plot of energy versus torsional (dihedral) angle.
Define steric strain.
Steric strain is the result of the “electron cloud” repulsion of two groups on adjacent carbons coming in close vicinity.
In eclipsed conformations, steric strain is the difference in energy between when two hydrogens on adjacent carbons are eclipsed and when two groups are eclipsed.
In staggered conformations, steric strain is the difference in energy between an anti-conformation and a gauche conformation.
Define anti-conformation.
Anti-conformation is a staggered conformation where the two largest groups on adjacent carbons have a torsional (dihedral) angle of 180°.
An anti-conformation will also always be the lowest energy conformation. Note that not all molecules will have a clear anti-conformation as it will not be clear which two groups to compare. For example, sighting down carbon’s 2 to 3 of butane will give an anti-conformation.
By contrast 1,1-dichloropropane does not have an anti-conformation.
Define gauche-conformation.
Gauche conformation is a staggered conformation where the two largest groups on adjacent carbons have a torsional (dihedral) angle of 60°.
Note that not all molecules will have a clear gauche-conformation as it will not be clear which two groups to compare. For example, sighting down carbon’s 2 to 3 of butane will give a gauche-conformation. But there is no gauche conformation for 1.1-dichloropropane.
Define angle strain.
Angle strain is the strain experienced when a cyclical molecule has bond angles distorted from their ideal values of 109.5° (as expected for sp3 hybridized carbons).
The energy of angle strain can be calculated by finding the enthalpy of formation for a ring structure as compared to cyclohexane (b/c cyclohexane has close to ideal bond angles).
For a disubstituted cyclohexane, which conformer would be more stable?
- two equitorial groups
- one equitorial and one axial
- two axial groups
Cyclohexane adopts a chair conformation where an equitorial position is always more stable. Therefore, for a disubstituted cyclohexane, two equitorial groups would be the most stable (i.e. the lowest energy).
The following defines what concept?
Different physical arrangement in space of a single molecule as the result of rotation about a sigma bond or ring puckering.
Conformer
The following defines what concept?
Angle between two groups (one attached to the front carbon and the other to the back carbon) in a Newman projection. A “group” can be a single atom (e.g. H, Cl, F) or a collection of atoms (e.g. CH3, CH2CH3).
Torsional angle.
The following defines what concept?
A conformation in which nearby groups in a Newman projection have a torsional (dihedral) angle of 60°.
Staggered conformation.
