Indicate the number of signals and the multiplicity of each...

Key Concepts

Proton NMR Spectroscopy

Proton NMR spectroscopy is a technique used to observe the magnetic properties of hydrogen nuclei in a molecule, providing information about the number and environment of the protons. It is fundamental in determining the structure of organic compounds by analyzing how protons generate signals based on their electronic environments and interactions with neighboring nuclei.

Chemical Equivalence

Chemical equivalence refers to the situation where two or more protons experience the same electronic environment and, as a result, give rise to the same NMR signal. Recognizing which protons are chemically equivalent is essential for predicting the number of distinct signals in an NMR spectrum.

Spin-Spin Coupling

Spin-spin coupling involves interactions between neighboring protons, leading to the splitting of NMR signals into multiplets. The coupling is typically observed as splitting patterns that inform about the number of adjacent protons and their spatial arrangement. These interactions provide valuable insights into the molecular structure.

Multiplicity (Splitting Patterns)

Multiplicity, or splitting pattern, is the division of an NMR signal into multiple peaks as a result of spin-spin coupling. The n+1 rule is a common method used to predict the multiplicity, where n is the number of adjacent protons. Analyzing multiplicity helps in deducing the structure and the connectivity of atoms within the molecule.

Transcript

00:04 This question provides us with four organic molecules and asks us to predict the number of proton nmr signals and the multiplicity of each of those signals for these four molecules.

00:20 So let's get right to it.

00:22 The first molecule is nhexane that has a pair of methyl groups at the terminal ends.

00:31 And then four methylain groups in the center between the methyl groups.

00:42 So let's first try to label these protons or the protons in this molecule to identify which protons are equivalent to each other.

00:52 This being a symmetric molecule that can have a symmetry across that axis that i just drew.

01:02 We can say that the methyl groups on the two ends are equivalent to each other and the methylene groups right after them the methyline group protons are equivalent to each other and the central methylene group protons are equivalent to each other so we can tell from this that this is going to produce three signals one for the a type protons one for the b type protons and the last for the c type protons.

01:40 However, these signals are going to split further because of the influence of the neighboring protons.

01:51 So the a type protons have the b type protons as their neighbors.

02:02 So the peak from the methyl group will split into 2 plus 1.

02:10 3, a triplet.

02:17 So a, which is the methyl group at the end, we will get a triplet from 2 plus 1, where 2 is the number of protons that are in the neighboring carbon from the methyl group.

02:40 And then for the b -type protons, we can see that they have two neighbors on two sides.

02:53 So a is going to be a neighbor of the b type protons and c is also going to be another neighbor of the b type protons.

03:06 So the signal for the b type protons is going to be a multiplicity that is influenced by both of its neighbors.

03:17 So the multiplicity of that multiple is going to be 2 plus 1 times 3 plus 1.

03:43 So none of the protons are equivalent to each other.

03:48 So we can calculate the number of protons in the methyl group add 1 to that and then consider the other other methyline group that is enabled, calculate the number of or count the number of protons from that which is 2 and then add 1 to that and then multiply the two numbers which is what we are doing here and we will get 3 times 4 12.

04:19 So a multiplicit that has a multiplicity of 12 but for sure there can be a lot of overlap between these 12 peaks which might mean that we won't see all 12 peaks in the spectrum.

04:38 And then we have the c -type protons.

04:41 Again, they also have two neighbors.

04:47 Let me raise this and write again.

04:54 Sorry, this has to be ch2.

05:07 So the c protons also have two neighbors, the b -type protons.

05:13 Actually, they only have the b -type proton neighbor because the c - protons are equivalent to each other.

05:20 Both of them have only the b protons as their neighbors.

05:26 So what we can do is count the number of b -type protons and add one to that to calculate the multiplicity coming from the c -type protons.

05:38 So that is also again a ch2 group or the protons coming from the ch2 group...