Please explain intext question 10.9

In the first reaction you are converting bromocyclohexane to Grignard reagent in the dry ether. We need to be careful here as there should not be any acidic hydrogen because Grignard reagents very susceptible to a acidic hydrogen and they get converted into hydrocarbon.This is due to negative charge on the carbon which is bonded to magnesium.Carbon with the negative charge a very strong bases accepts  hydrogen ion from even very weak acids like water or alcohol.
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In the second reaction again we are preparing Grignard reagent now if you look at the structure given after adding D2O in water in  the Grignard reagent .It is very clear Br is on  the second carbon and C is the Grignard reagent where Mg and Br are there on the center carbon.Check the explanation in the picture
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In the third reaction it is clear example of wurtz synthesis where you are preparing a symmetrical alkane with two  tert-butyl group means compound is tert butyl bromide. When you add magnesium into it you get a Grignard reagent which undergoes protonation in the presence of water to give you to final compound which is 2- methylbutane

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