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Chapter 11: Q27P (page 570)

When the following substituted cycloheptanol undergoes dehydration, one of the minor products has undergone a ring contraction. Propose a mechanism to show how this ring contraction occurs.

Short Answer

Expert verified

Dehydration reactions are the opposite of hydration reactions. They involve the deprival of water molecule from the reacting species. Fischer esterification is an example of dehydration reaction.

Step by step solution

01

Dehydration

Dehydration reactions are the opposite of hydration reactions. They involve the deprival of water molecule from the reacting species. Fischer esterification is an example of dehydration reaction.

02

Ring contraction

Ring contractions constitute a part of organic reactions like the cyclization reaction. Smaller and strained rings are transformed into larger rings in the ring contraction.

03

Mechanism showing the ring contraction when cycloheptanol undergoes dehydration

The dehydration of the given alcohols happens via the protonation of OH followed by the carbocation formation and stabilization. The carbocation stabilization happens via the 1, 2-methyl shift, which causes ring contraction. The mechanism can be given as:

Mechanism of ring contraction

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Most popular questions from this chapter

Write balanced equations for the three preceding reactions.

To practice working through the early parts of a multistep synthesis, devise syntheses of

(a) pentan-3-one from alcohols containing no more than three carbon atoms.

(b) 3-ethylpentan-2-one from compounds containing no more than three carbon atoms.

Both cis- and trans-2-methylcyclohexanol undergo dehydration in warm sulfuric acid to give 1-methylcyclohexene as the major alkene product. These alcohols can also be converted to alkenes by tosylation usingand pyridine, followed by elimination using KOC(CH3)3as a strong base. Under these basic conditions, the tosylate of cis-2-methylcyclohexanol eliminates to give mostly 1-methylcyclohexene, but the tosylate of trans-2-methylcyclohexanol eliminates to give only 3-methylcyclohexene. Explain how this stereochemical difference in reactants controls a regiochemical difference in the products of the basic elimination, but not in the acid-catalyzed elimination.

Neopentyl alcohol, (CH3)3CCH2OH, reacts with concentrated HBr to give 2-bromo-2-methylbutane, a rearranged product. Propose a mechanism for the formation of this product.

Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.
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