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Chapter 21: Problem 47

When warmed in dilute sulfuric acid, 1-phenyl-1,2-propanediol undergoes dehydration and rearrangement to give 2-phenylpropanal. (a) Propose a mechanism for this example of a pinacol rearrangement (Section 10.7). (b) Account for the fact that 2-phenylpropanal is formed rather than its constitutional isomer, 1-phenyl-1-propanone.

Short Answer

Expert verified
Answer: The pinacol rearrangement mechanism involves three main steps: 1) Protonation of the 1,2-diol, 2) Rearrangement of the cation intermediate, and 3) Deprotonation of the rearranged cation to form the carbonyl compound. In the case of 1-phenyl-1,2-propanediol, the phenyl group migration during the rearrangement step leads to a more stable carbocation due to resonance with the phenyl ring. Thus, the formation of 2-phenylpropanal is favored over 1-phenyl-1-propanone.

Step by step solution

01

Understand the Pinacol Rearrangement Mechanism

The pinacol rearrangement is an organic reaction that involves the acid-catalyzed rearrangement of a 1,2-diol (such as 1-phenyl-1,2-propanediol) to give a carbonyl compound (in this case, 2-phenylpropanal). The general mechanism involves three main steps: 1. Protonation of the 1,2-diol by the acidic conditions 2. Rearrangement of the cation intermediate 3. Deprotonation of the rearranged cation to give the final carbonyl compound
02

Apply the Pinacol Rearrangement Mechanism to This Example

Now let's apply this general mechanism to the specific case of 1-phenyl-1,2-propanediol: 1. Protonation: The oxygen atom on the 1-phenyl-1,2-propanediol molecule gets protonated by the available sulfuric acid, forming an oxonium ion. 2. Rearrangement: A 1,2-migration occurs where the bond between the carbon and the substituent (phenyl group) migrates to the adjacent carbocation carbon, creating a new positive charge on the phenyl-substituted carbon. 3. Deprotonation: Finally, a water molecule deprotonates the protonated carbonyl carbon, resulting in the formation of the final pinacol rearrangement product, 2-phenylpropanal.
03

Explain the Formation of 2-Phenylpropanal over 1-Phenyl-1-Propanone

During the rearrangement step of the mechanism, the migration of the phenyl group leads to the formation of a new carbocation, which is stabilized by resonance with the phenyl ring. This rearrangement is preferred because the resulting carbocation is more stable compared to the alternative migration of the methyl group. If the methyl group had migrated, the carbocation would not have the stabilization effect provided by resonance with the phenyl ring, and therefore, the resulting carbocation would be less stable. Thus, based on the stability of the intermediate carbocations, the formation of 2-phenylpropanal is favored over the formation of its constitutional isomer, 1-phenyl-1-propanone.

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Most popular questions from this chapter

Estrogens are female sex hormones, the most potent of which is \(\beta\)-estradiol. In recent years, chemists have focused on designing and synthesizing molecules that bind to estrogen receptors. One target of this research has been nonsteroidal estrogen antagonists, compounds that interact with estrogen receptors and block the effects of both endogenous and exogenous estrogens. A feature common to one type of nonsteroidal estrogen antagonist is the presence of a 1,2 -diphenylethylene with one of the benzene rings bearing a dialkylaminoethoxyl substituent. The first nonsteroidal estrogen antagonist of this type to achieve clinical importance was tamoxifen, now an important drug in the treatment of breast cancer. Tamoxifen has the \(Z\) configuration as shown here. Propose reagents for the conversion of A to tamoxifen. Note: The final step in this synthesis gives a mixture of \(E\) and \(Z\) isomers.

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Cromolyn sodium, developed in the \(1960 \mathrm{~s}\), has been used to prevent allergic reactions primarily affecting the lungs, as for example exercise- induced emphysema. It. is thought to block the release of histamine, which prevents the sequence of events leading to swelling, itching, and constriction of bronchial tubes. Cromolyn sodium is synthesized in the following series of steps. Treatment of one mole of epichlorohydrin (Section 11.10) with two moles of 2,6-dihydroxyacetophenone in the presence of base gives I. Treatment of I with two moles of diethyl oxalate in the presence of sodium ethoxide gives a diester II. Saponification of the diester with aqueous NaOH gives cromolyn sodium. (a) Propose a mechanism for the formation of compound I. (b) Propose a structural formula for compound II and a mechanism for its formation. (c) Is cromolyn sodium chiral? If so, which of the possible stereoisomers are formed in this synthesis?

Which compound gives a signal in the \({ }^{1} \mathrm{H}-\mathrm{NMR}\) spectrum with a larger chemical shift, furan or cyclopentadiene? Explain.
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