Also contains definition of: twist form
If carbon atoms 1, 2, 4 and 5 of cyclohexane occupy coplanar positions and when carbon atoms 3 and 6 are on opposite sides of the plane the @[email protected] (of symmetry group D3d) is called a chair form. The same term is applied to similar conformations of analogous saturated six-membered ring structures containing hetero-atoms and/or bearing substituent groups, but these conformations may be distorted from the exact D3d symmetry. For cyclohexane and most such analogues, the chair form is the most @[email protected] @[email protected] If the cyclohexane @[email protected] has no centre of symmetry but possesses two planes of symmetry, one of them bisecting the bonds between atoms 1 and 2 and between 4 and 5 and the other plane passing through atoms 3 and 6 (which lie out of the plane and on the same side of the plane containing 1, 2, 4 and 5), that @[email protected] (of symmetry group C2v ) is called a @[email protected] form and it is generally not a @[email protected] form. Again, this term is also applied to structural analogues. The @[email protected] of D2 symmetry passed through in the interconversion of two @[email protected] forms of cyclohexane is called the twist form (also known as @[email protected] @[email protected], @[email protected] form and stretched form).
In a five-membered ring a @[email protected] in which two adjacent atoms are maximally displaced, in opposite directions, relative to the plane containing the other three carbon atoms has been called a @[email protected] but is better called a twist @[email protected]
See also:
envelope conformation
In carbohydrate chemistry the term twist refers to a five-membered ring and the D2 symmetry six-membered ring is referred to as @[email protected]
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2202 [Terms] [Paper]