The ¹H NMR spectra of three isomers with molecular ...

Question

The ¹H NMR spectra of three isomers with molecular formula C₇H₁₄O are shown here. Which isomer produces which spectrum?

b.

Accepted Answer

Hello everyone. Let's do this problem. It asks which of the three isomers with the molecular formula C six H 12 0 has the following proton and M R spectrum. We are given our spectrum and three answer choices with the three isomers. Answer choice A is methyl iso butyl ketone. Answer choice B is methyl sey ketone and answer choice C is ethyl iso butyl ketone. So how can we use our proton N M R spectrum to narrow down which structure is correct? We can look at a few different characteristics of our spectrum and I like to do so in this order. So first check how many signals each structure will produce. That's an easy way to eliminate some answer choices. Then we can look at the integration of each signal. So how many hydrogens should be represented by that signal? And all of those hydrogens are equivalent right in one signal, then we will look at the multiplicity of the signal. So that tells us how many neighboring protons there are to the proton in question. And that's because multiplicity the expression is N plus one where N is the number of neighboring protons. And lastly, we can look at chemical shift, which gives us sort of a relative structure based on various groups in the compound. So for example, if we have a proton next to an electron negative atom or an electron dense group like a double bond, then we know that proton will be more DeShields, right? Because that electron withdrawing group is pulling away the electrons from that hydrogen leaving it naked and DeShields. So that's going to make it more downfield, right. So let's compare our isomers and look at these characteristics. So first structure A, how many signals will this structure produce one for these two methyl groups? Those are equivalent, right? Because they are coming off of the same carbon two for that methane proton three for that methylene group and four for that methyl group on the other side of the ketone. So that is four signals produced by structure A and as we said, our proton N M R spectrum has four signals. So that's a possible answer choice. All right, structure B will produce 12345 signals. So that cannot be a match. What about structure C, 123 and four, right? These two methyl groups are also equivalent, they're coming off of the same carbon. So that is four signals. So we narrowed it down to answer choice B and C. So now let's go to step two, the integration. So let's look at the integration of each of these signals starting with answer choice. A signal A represents six methyl protons. Signal B represents that one methane proton signal C represents those two methylene protons and signal D represents that other methyl group. So three protons. OK. Does this potentially match our spectrum 612 and three hydrogens, 612 and three hydrogens? OK. So still a potential match. Let's look at answer choice C signal A will produce a signal with an integration of three hydrogens B has two hydrogens signal C has one hydrogen in signal D also has six hydrogens. So they are still potentially a match. Let's go on to step number three, the multiplicity of each of these signals. So back to structure a answer choice A that six H signal only has one neighboring proton. So this will be a doublet signal. B has 678, neighboring proton. So that will be a multiple signal C has one neighboring proton. So that will be a doublet and D has no neighboring proton. So that will be a singlet. So let's look at our proton and our spectrum again. So six hydrogens with a represented as a doublet that matches one hydrogen represented as a multiple that matches two hydrogens represented as a doublet that matches and three hydrogens represented as a singlet that matches. Alright, let's go back to answer choice C signal A three hydrogens has two neighboring protons. So that will be a triplet answer choice or signal B rather sorry, signal B two hydrogens has three neighboring protons. So that would be a quartet, signal C has six neighboring protons. So that will be a septet and signal D has one neighboring proton. So that will be a double it. So we don't have any triplets or quartets in our spectrum. So we know that answer choice C cannot be correct, which makes answer choice a, the correct structure of this proton N M R spectrum. And let's confirm the order of these signals, right. We have to make sure their chemical shifts are correct. So the protons that are closest to the oxygen atom are going to be the most down field, right? And the less hydrogens that are shared by that carbon, the more downfield the proton will be as well. So by looking at our structure, signal C has less protons than signal D. So signal C is going to be the most downfield. And that is that two H double it. So that makes sense, then we have the three protons from signal D that is also adjacent to the carbon group. So that makes sense, then we have our singular proton from signal B that is the next closest to the carbonyl group. And lastly those six methyl protons are the furthest from the Carboni group. So they are the most upfield. So once again, that makes a the correct answer choice for this problem. That's it for this one. I hope this was helpful and I'll see you next time.

The ¹H NMR spectra of three isomers with molecular formula C₇H₁₄O are shown here. Which isomer produces which spectrum?
b. <IMAGE>

Key Concepts

1H NMR Spectroscopy

Isomerism

Integration and Peak Splitting

The 1H NMR spectra of three isomers with molecular formula C7H14O are shown here. Which isomer produces which spectrum?b. <IMAGE>

Key Concepts

1H NMR Spectroscopy

Isomerism

Integration and Peak Splitting

The ¹H NMR spectra of three isomers with molecular formula C₇H₁₄O are shown here. Which isomer produces which spectrum?
b. <IMAGE>