Final answer:
To determine if a reaction is syn or anti addition, check the stereochemistry of the reaction, and consider regioselectivity, which indicates the major product based on electronic and steric factors.
Step-by-step explanation:
To determine whether a chemical reaction involves syn or anti-addition, one should examine the stereochemistry of the reaction. In stereoselective electrophilic addition reactions, for instance, a nucleophile may attack a trigonal planar carbocation from either side, leading to the formation of a racemic mixture of products when non-chiral reactants and catalysts are used. This concept applies to reactions such as the addition of halogen acids and water to alkenes and alkynes. Enzymes, on the other hand, can be enantioselective, producing only one enantiomer for asymmetric centers.
Chemical reactions that yield two constitutional isomers, but favor one over the other, are known as regioselective reactions. Understanding regioselectivity is important in predicting the major and minor products of such reactions, where sterics and electronic factors play a role. Regioselective aspects must be taken into account along with the stereochemistry to determine the outcome of the addition.