Alkenes restricted rotation around

Note from Fig. 22.7 that the p orbitals on the two carbon atoms in ethylene must be lined up (parallel) to allow formation of the -tr bond. This prevents rotation of the two CH2 groups relative to each other at ordinary temperatures, in contrast to alkanes, where free rotation is possible (see Fig. 22.8). The restricted rotation around doubly bonded carbon atoms means that alkenes exhibit cis-trans isomerism. For example, there are two stereoisomers of 2-butene (Fig. 22.9). Identical substituents on the same side of the double bond are designated cis and those on opposite sides are labeled trans. 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

The sp hybridized carbon atoms of alkenes (olefins) and the atoms or groups attached to these carbons all lie in the same plane, and rotation around the double bond is restricted. As a result, stereoisomerism is possible when each carbon atom of the double bond is asymmetrically substituted. Because geometric isomers are non-superimposable, non-mirror images they... 

Rotation around the double bond is restricted, and substituted alkenes can therefore exist as cis-trans stereoisomers. The geometry of a double bond can be specified by application of the Cahn-Ingold-Prelog sequence... 

The C=C Bond and Geometric (cis-trans) Isomerism There are two major structural differences between alkenes and alkanes. First, alkanes have a tetrahedral geometry (bond angles of —109.5°) around each C atom, whereas the double-bonded C atoms in alkenes are. trigonal planar (—120°). Second, the C—C bond allows rotation of bonded groups, so the atoms in an alkane continually change their relative positions. In contrast, the -n bond of the C=C bond restricts rotation, which fixes the relative positions of the atoms bonded to it. 

Inspection of these structures makes it clear that the chemical bonds between carbon and nitrogen have a double bond character. Consequently, the rotation around these bonds is restricted as is the rotation around the double bonds in alkenes. This restricted rotation makes polypeptide chains rigid. Because of structural rigidity polypeptide chains appear in two forms the a-helix and the P-sheet. 

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