Question

Please explain what is going on in this lab for STEP 3. what are some important factors?

Multistep Synthesis Preparation of 4,4-Diphenyl-3-buten-2-one! This experiment illustrates se multistep synthesis, in which the the next. This process is very common iment illustrates several important concepts of organic synthesis. It is a synthesis, in which the product of one reaction becomes the starting material of This process is very common in industry and research, and demands careful to vields and techniques. The experiment also illustrates the use of the ethylene protecting group, used here to mask one carbonyl group from reaction while the other is permitted to react with a Grignard reagent Multistep synthesis can be challenging. You can calculate the overall vield of a sequence of steps by multiplying the yield found for each step. For example, a two step synthesis with a yield of 48% at step one and 92% at step two would have an overall yield of (0.48 x 0.92 x 100) = 44%. A protecting group masks one functional group in a molecule while another undergoes reaction. To be useful, a protecting group must be capable of being introduced under mild conditions in a very high yield reaction. It must be highly resistant to the conditions employed for subsequent reaction of the unmasked group, yet not be so obtrusive as to interfere with the desired transformation. Finally, the protecting group must be easily removed by a mild, high yield reaction that will restore the masked functionality to its original state Product 4 is the desired product for this multistep synthesis. Given the starting material on the left of the figure below (ethyl acetoacetate), simply adding a Grignard reagent shown below will not give the desired intermediate 3. This is because the ketone carbonyl group is more reactive as an electrophile than the ester carbonyl group. (Why?) Thus the use of a protecting group leads to synthetic sequences that would not have been possible otherwise. MgBr OCHCHA ОСН-СН3 U ether This product will NOTform under these conditions Therefore, this multistep synthesis must incorporate a protecting group to protect the ketone carbonyl from reaction with the nucleophilic Grignard reagent. One of the most widely accepted protecting groups is that of the ethylene ketal, used to mask a carbonyl group and to protect it from reaction. Step 1 - Protection of the carbonyl group. The present procedure involves preparation of the ethylene ketal of ethyl acetoacetate (1) by reaction of the ketoester with ethylene glycol in the presence of p-toluenesulfonic acid

Answer 1

in this lab you are trying to do a grignard addition to a keto ester. here you have to two C=O centres where Grignard addition can happen. but you want to add it only on the carbocxylic carbon of ester. but grignard reagent prefer carbonyl carbon of ketone to carboxylic carbon of ester.

so in step 1 you are in a way inhabiting the carbonyl carbon of ketone by selective protection using a diol which will lead to the formation of a ketal.

in step 2 your your grignard reagent attacks on the carboxylic carbon , the only polar carbon,

now you have done the desired reaction . but it is not your desired product. in order to get your desired product you want to get back your original ketone. for that you want to remove your protecting group or in other words you want to make the ketal back into a ketone. simple hydrolysis of ketal will give your starting ketone. that is what is happening in the third step of the lab.

ROOR HOOR H30* HO* X-X -人 R R ROH ROH R R Hemiacetal Acetal “R Aldehyde or Ketone

inorder to do this conversion you take 2 or 3 mL of your step 2 product and add 3 mL of water and 5-6 drops of Con HCl ( acid hydrolysis ) and add 15 mL of acetone as solvent. after 15 to 20 minutes stirring the hydrolysis will be over and you can monitor the reaction sing a TLC. extract with diethyl ether and dry it over sodium sulphate or magnesium sulphate ( remove water). and allow the ether to evaporate. now you will have your product ready.