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Relative conformation energy diagram of butane as a function of dihedral angle. [9] A: antiperiplanar, anti or trans. B: synclinal or gauche. C: anticlinal or eclipsed. D: synperiplanar or cis. [2] Rotating their carbon–carbon bonds, the molecules ethane and propane have three local energy minima.
Anti-periplanar geometry will put a bonding orbital and an anti-bonding orbital approximately parallel to each other, or syn-periplanar. Figure 6 is another representation of 2-chloro-2,3-dimethylbutane (Figure 5), showing the C–H bonding orbital, σ C–H, and the C–Cl anti-bonding orbital, σ* C–Cl, syn-periplanar.
The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by reflecting the stereochemistry of the products in a reaction. The type of addition that occurs depends on multiple different factors of a reaction, and is defined by the final orientation of the substituents on the parent molecule .
When the single bond between the two centres is free to rotate, cis/trans descriptors become invalid. Two widely accepted prefixes used to distinguish diastereomers on sp³-hybridised bonds in an open-chain molecule are syn and anti. Masamune proposed the descriptors which work even if the groups are not attached to adjacent carbon atoms.
Anti or syn indicates the substituents are on opposite sides or the same side, respectively. Clinal substituents are found within 30° of either side of a dihedral angle of 60° (from 30° to 90°), 120° (90°–150°), 240° (210°–270°), or 300° (270°–330°).
When talking of diastereomers, syn and anti are used to describe groups on the same or opposite sites in zigzag prijection, see Diastereomer#Syn / anti. syn and anti are always written small and italic, locants (if used) are placed in front of the word and separated by hyphens.
The disposition of the formed stereocenters is deemed syn or anti, depending if they are on the same or opposite sides of the main chain: Syn and anti products from an aldol (addition) reaction. The principal factor determining an aldol reaction's stereoselectivity is the enolizing metal counterion.
1,3-Diols are described as syn or anti depending on the relative stereochemistries of the carbon atoms bearing the hydroxyl functional groups. Zincophorin is a natural product that contains both syn and anti 1,3-diols. Zincophorin depicting syn and anti-1,3-diol configurations