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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 .
In stereochemistry, the Klyne–Prelog system (named for William Klyne and Vladimir Prelog) for describing conformations about a single bond offers a more systematic means to unambiguously name complex structures, where the torsional or dihedral angles are not found to occur in 60° increments. [1]
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.
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.
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Cis and trans descriptors are not used for cases of conformational isomerism where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms "syn" and "anti" are used. According to IUPAC, "geometric isomerism" is an obsolete synonym of "cis–trans isomerism". [2]
On a single-step or immediate-execution calculator, the user presses a key for each operation, calculating all the intermediate results, before the final value is shown. [ 1 ] [ 2 ] [ 3 ] On an expression or formula calculator , one types in an expression and then presses a key, such as "=" or "Enter", to evaluate the expression.