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Chapter 12: Problem 253

What would you predict for the general character of the chemical shifts of the protons of (a) the separate \(\mathrm{CH}_{2}\) groups of 1,4 -hexamethylenebenzene as compared with 1,2 -hexamethylenebenzene; 1,4 -hexamethylenebenzene 1,2 -hexamethylenebenzene (b) cyclodctatetraene

Short Answer

Expert verified
(a) In 1,4-hexamethylenebenzene, the separate \(\mathrm{CH}_{2}\) groups will have smaller chemical shifts compared to 1,2-hexamethylenebenzene, as they are further from the electron-withdrawing aromatic ring. (b) In cyclododecatetraene, protons will experience various chemical shifts due to the conjugated system and single \(\mathrm{CH}_{2}\) groups. Larger chemical shifts are likely for protons in the conjugated double bond system.

Step by step solution

01

Draw the structure

Here is the structure for 1,4-hexamethylenebenzene: \[ \begin{array}{c} \ce{C6H4(-CH2CH2-CH2-CH2)-} \end{array} \] 1,2-hexamethylenebenzene:
02

Draw the structure

Here is the structure for 1,2-hexamethylenebenzene: \[ \begin{array}{c} \ce{C6H4(-CH2-CH2-CH2-CH2-CH2-CH2)-} \end{array} \] Now, let's examine cyclododecatetraene:
03

Draw the structure

Here is the structure for cyclododecatetraene: \[ \begin{array}{c} \ce{-(-CH=CH-)_3-CH2-(-CH=CH-)_3-CH2-} \end{array} \]
04

Compare chemical environments

The chemical shift of a proton in an NMR spectrum is influenced by the local electronic environment. In 1,4-hexamethylenebenzene, the separate \(\mathrm{CH}_{2}\) groups are further away from the aromatic ring, whereas in 1,2-hexamethylenebenzene, the separate \(\mathrm{CH}_{2}\) groups are directly adjacent to the aromatic ring. This difference in proximity to the aromatic ring will likely result in different chemical shifts for the protons. In cyclododecatetraene, the protons are part of a complex conjugated system that alternates between single and double bonds, which exposes some of them to a different chemical environment, affecting their chemical shifts.
05

Predict chemical shifts

(a) For the \(\mathrm{CH}_{2}\) groups in 1,4-hexamethylenebenzene and 1,2-hexamethylenebenzene, protons in 1,4-hexamethylenebenzene will experience a smaller chemical shift compared to 1,2-hexamethylenebenzene because they are further from the electron-withdrawing aromatic ring. (b) For cyclododecatetraene, we can expect various chemical shifts for the protons due to the different chemical environments formed by the conjugation system and the presence of the single \(\mathrm{CH}_{2}\) groups in the structure. Protons in the conjugated double bond system will experience larger chemical shifts because of their proximity to the electron-withdrawing double bonds.

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Most popular questions from this chapter

The compound indene, \(\mathrm{C}_{9} \mathrm{H}_{8}\), found in coal tar, rapidly decolorizes \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) and dilute \(\mathrm{KMnO}_{4}\). Only one mole of hydrogen is absorbed readily to form indane, \(\mathrm{C}_{9} \mathrm{H}_{10}\). More vigorous hydrogenation yields a compound of formula \(\mathrm{C}_{9} \mathrm{H}_{16} .\) Vigorous oxidation of indene yields phthalic acid. What is the structure of indene? Of indane?

Provide the missing reactant or reaction condition for the following: (a) \(\mathrm{n}\) -heptane \(+\underline{\text { ? }}\) * toluene (b) Iodobenzene \(+\underline{?}\) biphenyl (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}+?\) \+ ethyl benzene

Outline all steps in the conversion of: (a) ethylbenzene into phenylacetylene; (b) trans-1-phenylpropene into cis-1-phenylpropene.

What products would you expect to be formed in the ozonization (with reductive workup) of the following substances (consider carefully which bonds are likely to be most reactive)? (a) o-xylene (b) naphthalene

Suggest possible routes for the synthesis of the following compounds:
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