Given below are two statements : In the light of the above statements, choose the most appropriate answer from the options given below.
- Statement I is incorrect but Statement II is correct
- Both Statement I and Statement II are correct
- Both Statement I and Statement II are incorrect
- Statement I is correct but Statement II is incorrect
Answer
(1) Statement I is incorrect but Statement II is correct
Solution
Statement-I: Br2 in CCl4 adds to an alkene by anti addition (through the cyclic bromonium ion). Anti addition to *trans*-but-2-ene gives meso-2,3-dibromobutane.
In the meso compound the two halves are mirror images, so in the conformation where the two Br atoms are anti, the two CH3 groups and the two H atoms are also anti (centre of symmetry):
[figure]
The projection given in Statement-I has Br anti, but its two CH3 groups (and two H atoms) are gauche (60 apart). That conformation belongs to the chiral (±)-2,3-dibromobutane, which is formed from *cis*-but-2-ene. Hence Statement-I is incorrect.
Statement-II: Cold, dilute alkaline KMnO4 (Baeyer's reagent) adds two OH groups by syn addition (through a cyclic manganate ester). Syn addition to *cis*-but-2-ene gives meso-butane-2,3-diol.
Test the given projection: rotate the rear carbon by 60 . Then OH lies behind OH, CH3 behind CH3 and H behind H. This eclipsed form has a plane of symmetry, so the projection shows the meso compound. Hence Statement-II is correct.
Statement-I is incorrect but Statement-II is correct.
Answer: (1)