Chemistry - STEREO ISOMERISM Question with Solution | TestHub
Give the names, structural formulas and stereochemical designations of the isomers of (a) bromochlorocyclobutane, (b) dichlorocyclobutane, (c) bromochlorocyclopentane, (d) diiodocyclopentane, (e) dimethylcyclohexane. Indicate chiral C's.
(a) 
(b)
(c) 
(d)
(e) 
Options:
Answer:
Solution:
Ans. (a) Bromochlorocyclobutane: 1-bromo-1-chlorocyclobutane - only one structure, no chiral carbon. 1-Bromo-2-chlorocyclobutane: cis and trans forms; both substituted carbons are chiral and, because Br and Cl differ, there is no internal symmetry: cis = (1S,2R) and trans = (1R,2R)/(1S,2S), each existing as a racemic pair. 1-Bromo-3-chlorocyclobutane: cis and trans, but C1 and C3 are not chiral (a plane perpendicular to the ring bisects them), so no enantiomers.
(b) Dichlorocyclobutane: same pattern, except cis-1,2-dichlorocyclobutane has a plane of symmetry and is meso; trans-1,2 is a racemic pair; the 1,3-isomers (cis/trans) are not chiral.
(c) Bromochlorocyclopentane: nine isomers - 1-bromo-1-chloro (one), and cis/trans 1,2- and 1,3-isomers, each having a pair of enantiomers (racemic).
(d) Diiodocyclopentane: similar, except that cis-1,2- and cis-1,3-diiodocyclopentane each have a plane of symmetry and are meso; the trans forms are enantiomeric pairs.
(e) Dimethylcyclohexane: nine isomers - 1,1 (one, achiral); 1,2: cis is meso, trans is a racemic pair; 1,3: cis is meso, trans is a racemic pair; 1,4: cis and trans, both achiral (no chiral carbons). Chiral carbons are marked with *.
