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Free keywords:
2-bromo-3-methylbutyric acid; 2-bromo-3-methylbutanoic acid; 2-bromoisovaleric acid;
halogenated carboxylic acid; hydrogen bonding; chirality; absolute configuration; racemate; crystal
structure; X-ray crystallography
Abstract:
Halogenated carboxylic acids have been important compounds in chemical synthesis
and indispensable research tools in biochemical studies for decades. Nevertheless, the number of
structurally characterized simple α-brominated monocarboxylic acids is still limited. We herein
report the crystallization and structural elucidation of (R)- and rac-2-bromo-3-methylbutyric acid
(2-bromo-3-methylbutanoic acid, 1) to shed light on intermolecular interactions, in particular hydrogen
bonding motifs, packing modes and preferred conformations in the solid-state. The crystal structures of
(R)- and rac-1 are revealed by X-ray crystallography. Both compounds crystallize in the triclinic crystal
system with Z = 2; (R)-1 exhibits two crystallographically distinct molecules. In the crystal, (R)-1 forms
homochiral O–H···O hydrogen-bonded carboxylic acid dimers with approximate non-crystallographic
C2 symmetry. In contrast, rac-1 features centrosymmetric heterochiral dimers with the same carboxy
syn···syn homosynthon. The crystal packing of centrosymmetric rac-1 is denser than that of its
enantiopure counterpart (R)-1. The molecules in both crystal structures adopt a virtually identical
staggered conformation, despite different crystal environments, which indicates a preferred molecular
structure of 1. Intermolecular interactions apart from classical O–H···O hydrogen bonds do not appear
to have a crucial bearing on the solid-state structures of (R)- and rac-1.
