Substitution and -Elimination Reactions at C sp

It is not obvious at first sight whether the ring O s in the product come from the diol or the acetal. Draw the by-products You will see that two equivalents of MeOH are obtained. This information suggests that the Me—O bonds of the acetal do not break and therefore that the ring O s in the product come from the diol. The first two steps of the reaction must be protonation of a MeO group and loss of MeOH to give a very stable carbocation. 

Problem 3.3. Draw mechanisms for each of the following substitution reactions. Remember that in each case, loss of the leaving group is preceded by coordination of the leaving group to a Lewis acid such as H+. 

Problem 3.4. Alkenyl and aryl halides do not undergo substitution reactions under acidic conditions. Why not  

When SnI substitution occurs at a stereogenic center, loss of configurational purity is usually observed. The carbocationic intermediate is planar, and there is no bias for approach of the nucleophile from one face or the other. Occasionally the loss of configurational purity is only partial for reasons that need not concern you here. 

Problem 3.5. Draw a reasonable mechanism for the following substitution reaction. Which O is protonated first  

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