# Does syn-dihydroxylation form racemic mixture?

I have been asked to determine the nature of the product for the following reaction:

And as I recognized this reaction to be a case of syn-dihydroxylation, my obvious response was that the product formed will be meso.

But, in the solution to the problem, it says it makes a racemic mixture.

Shouldn't both the $$\ce{-OH}$$ be on the same side and form a meso-compound? And what does the $$(sym)$$ stand for?

• The very fact that 'syn' has been written as 'sym' should've heightened your suspicions regarding this answer, which is blatantly wrong. Cold KMnO4 will produce a meso compound in this case. This is a very standard reaction, and it is surprising that a book messed this one up. I strongly suggest you change the book/material you're studying from. – C_Lycoris Apr 17 at 13:43
• May I ask, however which book this came from? – C_Lycoris Apr 17 at 13:45
• @C_Lycoris It's not from any standard organic chemistry book. The book is called "Problems in Organic Chemistry", and is basically a chapter-wise question bank for the JEE exam. – SmartRadical Apr 17 at 13:51
• Referring to the product through its Fischer structure alone was an obvious red flag. Ditch this textbook. – TheRelentlessNucleophile Apr 18 at 8:42

Oxidation of alkene to 1,2-syn-diol by cold basic $$\ce{KMnO4}$$ is a common classification test for alkene in undergraduate organic chemistry laboratory courses. OP had correctly recognized that this reaction is a case of syn-dihydroxylation, and OP's conclusion of the product to be meso is also correct since the alkene in hand is a cis-but-2-ene. The schematic representation of the proposed mechanism is given below:
Since but-2-ene structure is planer and a prochiral molecule, it has two faces (Si- or Re-face; above or below the plane) to get attacked by a reactant, here it is $$\ce{KMnO4}$$. The above mechanism displays the attack by only one face. The oxidation gives butan-2,3-diol by syn-attack on that face. The product has two stereocenters and the stereochemistry of that product is $$(2R,3S)$$. As indicated in the diagram, the product also has a plane of symmetry, and hence, the $$\ce{C}$$2 stereocenter is the mirror image of the $$\ce{C}$$3 stereocenter. Therefore, the product is a meso-compound.
If the $$\ce{KMnO4}$$ attack is done on the opposite plane, the product would be $$(2S,3R)$$-butan-2,3-diol, if numbering has maintained the same. This is essentially the same meso-product, as we discussed above.
If the starting substrate is trans-but-2-ene, the $$\ce{KMnO4}$$ attack from one face gives you $$(2S,3S)$$-butan-2,3-diol, while the $$\ce{KMnO4}$$ attack from opposite face gives you $$(2R,3R)$$-butan-2,3-diol. Since possibility of attacking from the Si-fase and/or Re-face is $$0.5$$ (or $$50\%$$), you get essentially a racemic mixture.