Halogenacja rodnikowa: stereochemia

Radical halogenation of achiral molecules creates a trigonal planar radical intermediate, which is achiral. The halogen can attack this intermediate from either face with equal probability, generating equal amounts of R and S enantiomers. This produces a racemic mixture even though a new chiral center is formed. The trigonal planar geometry eliminates stereochemical preference during the attack.

When radical halogenation occurs at an existing chiral center, the resulting radical intermediate loses the original molecule's configuration and becomes achiral. Despite the starting material being chiral, the trigonal planar intermediate allows halogen attack from either face equally, producing a 1:1 ratio of enantiomers and forming a racemic mixture.

Diastereomers form when radical halogenation occurs at a position other than an existing chiral center. The existing chiral center makes the trigonal planar radical intermediate chiral, causing the halogen to attack one face preferentially over the other. This unequal attack produces diastereomeric products in unequal amounts, introducing a second chiral center.

The trigonal planar geometry of radical intermediates is crucial to stereochemical outcomes. When the intermediate is achiral, both faces are equivalent, allowing equal halogen attack and producing racemic mixtures. When the intermediate is chiral due to an existing stereocenter, the two faces become diastereotopic, leading to unequal halogen attack and diastereomeric products.

No, the hydrogens on carbon-2 of n-butane are enantiotopic. Although they appear equivalent in the starting material, radical chlorination of either hydrogen produces different enantiomers. The trigonal planar radical intermediate allows chlorine attack from either face, generating R and S products in equal amounts as a racemic mixture.

An existing chiral center makes the trigonal planar radical intermediate chiral, differentiating the two faces. This creates diastereotopic faces, causing the halogen to attack one face preferentially. The unequal attack produces diastereomeric products in unequal amounts, unlike achiral intermediates where both faces are equivalent and attack occurs equally.

Enantiotopic hydrogens produce enantiomers when substituted, as seen with carbon-2 hydrogens in n-butane, which form a racemic mixture. Diastereotopic hydrogens produce diastereomers when substituted, occurring when an existing chiral center influences the radical intermediate. The distinction depends on whether the resulting products are enantiomers or diastereomers.