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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 .
syn/anti peri/clinal. a torsion angle between 0° and ±90° is called syn (s) a torsion angle between ±90° and 180° is called anti (a) a torsion angle between 30° and 150° or between −30° and −150° is called clinal (c) a torsion angle between 0° and ±30° or ±150° and 180° is called periplanar (p)
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 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.
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]
The prefix endo is reserved for the isomer with the substituent located closest, or "syn", to the longest bridge. The prefix exo is reserved for the isomer with the substituent located farthest, or "anti", to the longest bridge. Here "longest" and "shortest" refer to the number of atoms that comprise the bridge.
The diagram takes staggered and eclipsed conformations, as well as gauche and anti interactions into account. A staggered projection appears to have the surrounding species equidistant from each other. This kind of conformation tends to experience both anti and gauche interactions. [5]
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