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Chapter 21: Problem 2

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

Short Answer

Expert verified
Explain your answer. Answer: Furan has a larger chemical shift in its proton NMR spectrum compared to cyclopentadiene. This is due to the presence of an electron-withdrawing oxygen atom in the furan ring, which leads to a greater downfield shift in the protons compared to the protons in cyclopentadiene, which does not have an electron-withdrawing group.

Step by step solution

01

Analyze the structure of furan and cyclopentadiene

Given are two compounds: furan and cyclopentadiene. First, let's draw their structures and identify the different types of protons in each compound: Furan: \(\displaystyle \mathrm{C_{4} H_{4} O}\) - a five-membered aromatic ring containing an oxygen atom and four carbon atoms with four protons. Cyclopentadiene: \(\displaystyle \mathrm{C_{5} H_{6}}\) - a five-membered non-aromatic ring containing five carbon atoms with alternating single and double bonds and six protons.
02

Understand the factors affecting chemical shifts in NMR spectroscopy

Now, let's examine the factors that affect the chemical shift of protons in NMR spectroscopy. The most important factor is the electronic environment around the protons. Electron-donating groups (like alkyl groups) typically lead to upfield shifts (lower chemical shift values), whereas electron-withdrawing groups (like oxygen and halogens) lead to downfield shifts (higher chemical shift values).
03

Compare the electronic environment of protons in furan and cyclopentadiene

In furan, the oxygen atom is electron-withdrawing and contributes to the ring current, thereby delocalizing the electrons in the ring. This results in a downfield shift in the ring protons. On the other hand, the protons in cyclopentadiene experience a less downfield shift because there is no electron-withdrawing group in the molecule.
04

Conclude which compound gives a larger chemical shift

Comparing the electronic environments and chemical shift factors of furan and cyclopentadiene, we can conclude that the protons in furan will give a larger chemical shift than those in cyclopentadiene due to the presence of an electron-withdrawing oxygen atom in the furan ring. This leads to a more downfield shift in furan's protons as compared to the protons in cyclopentadiene.

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Most popular questions from this chapter

Following are \({ }^{1} \mathrm{H}\)-NMR and \({ }^{13} \mathrm{C} N \mathrm{NMR}\) spectral data for compound \(\mathrm{G}\left(\mathrm{C}_{10} \mathrm{H}_{10} \mathrm{O}\right)\). From this information, deduce the structure of compound \(G\). $$ \begin{array}{lrr} { }^{1} \text { H-NMR } & \multicolumn{2}{c}{{ }^{13} \text { C-NMR }} \\ \hline 2.50(\mathrm{t}, 2 \mathrm{H}) & 210.19 & 126.82 \\ 3.05(\mathrm{t}, 2 \mathrm{H}) & 136.64 & 126.75 \\ 3.58(\mathrm{~s}, 2 \mathrm{H}) & 133.25 & 45.02 \\ 7.1-7.3(\mathrm{~m}, 4 \mathrm{H}) & 128.14 & 38.11 \\ 127.75 & & 28.34 \\ \hline \end{array} $$

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.

Following is a synthesis for albuterol (Proventil), currently one of the most widely used inhalation bronchodilators. (a) Propose a mechanism for conversion of 4-hydroxybenzaldehyde to \(\mathrm{A}\). (b) Propose reagents and experimental conditions for conversion of \(\mathrm{A}\) to \(\mathrm{B}\). (c) Propose a mechanism for the conversion of B to \(\mathrm{C}\). Hint: Think of trimethylsulfonium iodide as producing a sulfur equivalent of a Wittig reagent. (d) Propose reagents and experimental conditions for the conversion of \(C\) to \(D\). (e) Propose reagents and experimental conditions for the conversion of D to albuterol. (f) Is albuterol chiral? If so, which of the possible stereoisomers are formed in this synthesis?

Naphthalene and azulene are constitutional isomers of molecular formula \(\mathrm{C}_{10} \mathrm{H}_{8}\).

Following is a synthesis for toremifene, a nonsteroidal estrogen antagonist whose structure is closely related to that of tamoxifen. (a) This synthesis makes use of two blocking groups, the benzyl (Bn) group and the tetrahydropyranyl (THP) group. Draw a structural formula of each group, and describe the experimental conditions under which it is attached and removed. (b) Discuss the chemical logic behind the use of each blocking group in this synthesis. (c) Propose a mechanism for the conversion of D to \(E\). (d) Propose a mechanism for the conversion of \(F\) to toremifene. (e) Is toremifene chiral? If so, which of the possible stereoisomers are formed in this synthesis?
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