Retrosynthetic analysis of Grignard products

To use the Grignard addition in synthesis, you must be able to determine what carbonyl and Grignard components are needed to prepare a given compound—that is, you must work backwards, in the retrosynthetic direction. This involves a two-step process  

Step [1] Find the carbon bonded to the OH group in the product. 

Step [2] Break the molecule into two components One alkyl group bonded to the 

writing the reaction in the synthetic direction— that is, from starting material to product— shows whether the analysis is correct. In this example, a three-carbon aldehyde reacts with CH3CH2MgBr to form an alkoxide, which can then be protonated by H2O to form 3-pentanol, the desired alcohol. 

There is often more than one way to synthesize a 2° alcohol by Grignard addition, as shown in Sample Problem 20.2. 

Step [2] Break the molecule into two components One alkyl group bonded to the carbon with the OH group comes from the organometallic reagent. The rest of the molecule comes from the carbonyl component. 

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