Sigma bonds and bond rotation
Sigma bonds and bond rotation
Groups bonded by only a sigma bond can undergo rotation about that bond with respect to each other. the temporary molecular shapes that result from rotation of groups about single bonds are called conformations of a molecule. each ossible structure is called a conformer. An analysis of the energy changes associated with a molecule undergoing rotation about single bonds is called conformational analysis When we do conformational analysis, we will find that certain types of structural formulas are especially convenient to use. One of these types is called a newman projection formula and another ty pe is a sawhorse formula Sawhorse formula are much like other three-dimensional formulas we have used so far In conformational analyses, we will make substantial use of Newman projections
• Groups bonded by only a sigma bond can undergo rotation about that bond with respect to each other. The temporary molecular shapes that result from rotation of groups about single bonds are called conformations of a molecule. Each possible structure is called a conformer. An analysis of the energy changes associated with a molecule undergoing rotation about single bonds is called conformational analysis. • When we do conformational analysis, we will find that certain types of structural formulas are especially convenient to use. One of these types is called a Newman projection formula and another type is a sawhorse formula. Sawhorse formula are much like other three-dimensional formulas we have used so far. In conformational analyses, we will make substantial use of Newman projections
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To write a Newman pro jection formula, we imagine ourselves taking a view from one atom(usually a carbon) directly along a selected bond axis to the next atom(also usually a carbon atom). The front carbon and its other bonds are represented as and those of the back carbon as o
• To write a Newman projection formula, we imagine ourselves taking a view from one atom (usually a carbon) directly along a selected bond axis to the next atom (also usually a carbon atom). The front carbon and its other bonds are represented as and those of the back carbon as
The rotation around the sing le bond in ethane while not obvious ly hindered does generate conformational isomers having ferent potential energies. As shown bellow, as the dihedral angle between the ethane hydrogen atoms changes from 60(a staggered conformation) to 120(an eclipsed conformation), the potentia energy, of the molecule increases by about 3 kcal/mole. As the methyl gi roup continues to rotate towards 180, the potential energy again drops and rises again as the next eclipsed structure is formed
• The rotation around the single bond in ethane, while not obviously hindered, does generate conformational isomers having different potential energies. As shown bellow, as the dihedral angle between the ethane hydrogen atoms changes from 60 (a staggered conformation) to 120 (an eclipsed conformation), the potential energy of the molecule increases by about 3 kcal/mole. As the methyl group continues to rotate towards 180, the potential energy again drops and rises again as the next eclipsed structure is formed
120 =3 kcal/mol 180° 300
This can be contrasted however with rotation around the central carbon-carbon bond in butane shown below, in which two methyl groups clearly overla luring a singl rotation (the van der Waals radii of the methyl hydrogen atoms clearly overlal
• This can be contrasted, however, with rotation around the central carbon-carbon bond in butane, shown below, in which two methyl groups clearly overlap during a single rotation (the van der Waals radii of the methyl hydrogen atoms clearly overlap)
eclipsed eclipsed eclipsed eclipsed gauche 360 120 s kcal 5.5 kcal 300° 80.9 kcal 180 ache an
●●●●● ●●●● The effect of rotation on the potential energy of ●●0 butane around the central carbon -carbon bond is ●●● ●●●● more significant, as shown above. The structure shown at o is fully eclipsed, that is, both methyl groups are aligned and are interacting maximally As che front methyl group is rotated 60, a gauche conformation is produced in which the methyl group is nestled between the back methyl and the adjacent hydrogen atom. Another 60 rotation produces an approximately 2. 4 kcall mole less stable. At 180. the eclipsed version of the gauche conformation which anti conformation is formed in which the two meth groups are on opposite faces of the molecule and no groups are eclipsed. This is the most stable conformers and it differs from the fully eclipsed conformers by about 5 kcal mole in potential energy Further rotations regenerate an equivalent eclipse gauche conformer(at 240), another gauche form 300)and finally, the eclipsed form at 360
⚫ The effect of rotation on the potential energy of butane around the central carbon-carbon bond is more significant, as shown above. The structure shown at 0 is fully eclipsed, that is, both methyl groups are aligned and are interacting maximally. As the front methyl group is rotated 60, a gauche conformation is produced in which the methyl group is nestled between the back methyl and the adjacent hydrogen atom. Another 60 rotation produces an eclipsed version of the gauche conformation which is approximately 2.4 kcal/mole less stable. At 180, the anti conformation is formed in which the two methyl groups are on opposite faces of the molecule and no groups are eclipsed. This is the most stable conformers and it differs from the fully eclipsed conformers by about 5 kcal/mole in potential energy. Further rotations regenerate an equivalent eclipsed gauche conformer (at 240), another gauche form (300) and finally, the eclipsed form at 360
Rotations such as these are not possible in cycloalkanes, where the ring constrains the movements around the carbon-carbon single bonds. Cyclopropane rings are generally flat and have little conformational flexibility. The flexibility of four- and five- membered rings is signif icantly greater and these molecules exist as a dynamic equilibrium among various puckered conformations, as shown below
• Rotations such as these are not possible in cycloalkanes, where the ring constrains the movements around the carbon-carbon single bonds. Cyclopropane rings are generally flat and have little conformational flexibility. The flexibility of four- and five-membered rings is significantly greater and these molecules exist as a dynamic equilibrium among various "puckered" conformations, as shown below
