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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.
The gauche effect is very sensitive to solvent effects, due to the large difference in polarity between the two conformers.For example, 2,3-dinitro-2,3-dimethylbutane, which in the solid state exists only in the gauche conformation, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride, it prefers the anti conformer by a ratio of 58:42. [9]
Consequently, the elimination product is always syn and rarely occurs with 6-membered rings. ( Rings with 5 or 7 or more members undergo the reaction just fine.) [ 6 ] [ 7 ] [ 8 ] This organic reaction is closely related to the Hofmann elimination , [2] but the base is a part of the leaving group .
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The use of syn and anti to indicate the configuration of double bonds is nowadays obsolete, especially in case of aldoximes and hydrazones derived from aldehydes. Here, the compounds were designated as syn configured when the aldehyde H and the O (of the oxime) or the N (of the hydrazone) were cis aligned.