I have recently read that the number of sterioisomers in a compound having n chiral centres is $2^n$. (That is, if no meso form is present)

The below question asks for the number of sterioisomers in the product obtained and the product obtained has 3 chiral centres, so it should be $2^3$ = 8, but the answer given is two. There is no plane of symmetry.

Do I not consider the carbon attached to the benzene ring when looking for chirality?

Original question:

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  • 3
    $\begingroup$ The formula is of no use here. Simply think about where the configuration of which carbon(s) in the final product can change in the course of the reaction(by looking at the mechanism), which carbons will maintain a constant configuration, and then count manually the number of ways the configuration of the variable carbon(s) can vary $\endgroup$ Apr 6, 2020 at 22:34
  • 2
    $\begingroup$ Is the Grignard reagent truly optically active? I doubt it. $\endgroup$
    – user55119
    Apr 6, 2020 at 23:49
  • $\begingroup$ How can you calim that a Grignard reagent can be optically pure? $\endgroup$ Jun 11, 2021 at 14:05

2 Answers 2


I believe you are misreading the question. Rather than asking for the number of stereoisomers the product can have (this is where 2^n comes in), it is asking for the number of stereoisomeric products formed. Note that that the two products are diastereomers, which is a type of stereoisomeric relationship. It's not looking for any fancy calculation; just for the related products.

In a Gringard mechanims, the Gringard reagent can attack the carbonyl on either the re or si face, which in turn, can yield an R chiral center or an S center at the location of attack. This lends to the diastereotpic products.


The inversion takes place at a single carbon inbetween, so 2^1 = 2. Knowing that the mechanism takes place at the carbonyl carbon, the inversion takes place at one point only. It will not form several stereoisomers, only two at the reaction centre.


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