View attachment Screen shot 2011-09-07 at 11.26.47 AM.png
View attachment Screen shot 2011-09-07 at 11.24.04 AM.png
112. The major product for the aldol reaction is:
A. Product I, because all of the peaks are singlets.
B. Product I, because the methyl group adjacent to the carbonyl carbon appears at 5.26 ppm.
C. Product II, because all of the peaks are singlets. C is the best answer. Figure 1 shows the 1H NMR spectrum for the major product of the reaction, so we must determine whether the spectrum fits Product I or Product II. Product I shows both a triplet and quartet in its 1H NMR spectrum because of the ethyl group, with the signal for the CH3 being found near 1.00 ppm. This does not fit the spectrum in Figure 1, which eliminates both choice A and choice B. Because there are four types of hydrogens in Product II, there can be only four signals in its 1H NMR spectrum, so choice D is eliminated. In Product II, all of the hydrogens are isolated on carbons without any neighboring hydrogens. This ensures that all of its peaks are singlets, as seen in Figure 1. The best answer is C.
D. Product II, because it shows a total of five signals in its 1H NMR spectrum.
I see why II is a better answer than I, if we accept that all the H's on the benzene ring are equivalent, and I did choose C. But aren't the ortho, meta, and para hydrogens on the aromatic ring all different? That is, I expected the ortho H's to be split by the metas, the metas split by both the orthos and the para, and the para split by the metas, resulting in 3 different peaks from the aromatic H's instead of one peak for all 5. Is there something I'm missing here?
View attachment Screen shot 2011-09-07 at 11.24.04 AM.png
112. The major product for the aldol reaction is:
A. Product I, because all of the peaks are singlets.
B. Product I, because the methyl group adjacent to the carbonyl carbon appears at 5.26 ppm.
C. Product II, because all of the peaks are singlets. C is the best answer. Figure 1 shows the 1H NMR spectrum for the major product of the reaction, so we must determine whether the spectrum fits Product I or Product II. Product I shows both a triplet and quartet in its 1H NMR spectrum because of the ethyl group, with the signal for the CH3 being found near 1.00 ppm. This does not fit the spectrum in Figure 1, which eliminates both choice A and choice B. Because there are four types of hydrogens in Product II, there can be only four signals in its 1H NMR spectrum, so choice D is eliminated. In Product II, all of the hydrogens are isolated on carbons without any neighboring hydrogens. This ensures that all of its peaks are singlets, as seen in Figure 1. The best answer is C.
D. Product II, because it shows a total of five signals in its 1H NMR spectrum.
I see why II is a better answer than I, if we accept that all the H's on the benzene ring are equivalent, and I did choose C. But aren't the ortho, meta, and para hydrogens on the aromatic ring all different? That is, I expected the ortho H's to be split by the metas, the metas split by both the orthos and the para, and the para split by the metas, resulting in 3 different peaks from the aromatic H's instead of one peak for all 5. Is there something I'm missing here?
