Explain symmetry elements of 1,3,5,7 tetra methyl cyclo octatetraene
Answers
Symmetry in Organic Chemistry
The symmetry of a molecule is determined by the existence of symmetry operations performed with respect to symmetry elements. A symmetry element is a line, a plane or a point in or through an object, about which a rotation or reflection leaves the object in an orientation indistinguishable from the original. A plane of symmetry is designated by the symbol σ (or sometimes s), and the reflection operation is the coincidence of atoms on one side of the plane with corresponding atoms on the other side, as though reflected in a mirror. A center or point of symmetry is labeled i, and the inversion operation demonstrates coincidence of each atom with an identical one on a line passing through and an equal distance from the inversion point (see chair cyclohexane). Finally, a rotational axis is designated Cn, where the degrees of rotation that restore the object is 360/n (C2= 180º rotation, C3= 120º rotation, C4= 90º rotation, C5= 72º rotation). C1 is called the identity operation E because it returns the original orientation.
An object having no symmetry elements other than E is called asymmetric. Such an object is necessarily chiral. Since a plane or point of symmetry involves a reflection operation, the presence of such an element makes an object achiral. One or more rotational axes of symmetry may exist in both chiral, dissymmetric, and achiral objects.
Three dimensional models illustrating these symmetry elements will be displayed on the right by clicking one of the following names. The forth and seventh of these are dissymmetric. The others are achiral.
Examplescis-1,2-Dichloroethene
trans-1,2-Dichloroethene
cis-1,2-Dimethylcyclopropane
trans-1,2-Dimethylcyclopropane
Cyclohexane (chair conformer)
Cyclohexane (boat conformer)
Cyclohexane (twist boat conformer)
Allene
1,3,5,7-Tetrafluoro-1,3,5,7-Cyclooctatetraene
One more symmetry operation must be defined. Both trans-dimethylcyclopropane and 1,3,5,7-tetrafluoro-1,3,5,7-cyclooctatetraene have a C2 axis, and both lack a plane or center of symmetry. The former is chiral, but the latter is achiral because it has a S4 improper rotational axis (sometimes called an alternating axis). An improper axis, Sn, consists of a n-fold rotation followed by reflection through a mirror plane perpendicular to the rotation axis (n is always 3 or larger because S1 = σ and S2 = i). This is equivalent to saying that a n-fold rotation converts an object into its mirror image.
The S4 element in 1,3,5,7-tetrafluoro-1,3,5,7-cyclooctatetraene
Answer:
16 c2 and c8 major axis . c8 perpendicular to c2 d8h poient group