Also contains definition of: twist form
https://doi.org/10.1351/goldbook.C00964
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
conformation
(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
stable
conformation
. If the cyclohexane
conformation
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
conformation
(of symmetry group C2v ) is called a
boat
form and it is generally not a
stable
form. Again, this term is also applied to structural analogues. The
conformation
of D2 symmetry passed through in the interconversion of two
boat
forms of cyclohexane is called the twist form (also known as
skew
boat
,
skew
form and stretched form).
In a five-membered ring a
conformation
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
half-chair
but is better called a twist
conformation
.
See also:
half-chair
,
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
skew
.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2202 [Terms] [Paper]