Ch 13 - Complex Coupling

Chapter 13: Spectroscopy

Complex Coupling Patterns

In reality, coupling patterns are often more complex than the simple n+1 rule since the neighbouring protons are often not equivalent to each other (i.e. there are different types of neighbours).
Different types of neighbours interact with differentcoupling constants, J.
In these cases, the "n+1" rule has to be refined so that each type of neighbour causes n+1 lines.
For example for a proton with two types of neighbour, then the number of lines, L = (n₁ + 1)(n₂ + 1).
Note that this is where the approximation of equivalent neighbours made previously breaks down (for example due to a lack of free rotation).
However, in many cases the lines overlap with each other and the result is further distortion from the "ideal" pattern.

Coupling patterns involving aromatic or alkene protons are often complex.

In the following example, 3-bromopropene (also known as allyl bromide), we have coupling patterns that look more complex (frankly, kind of ugly!) compared to what we have seen before. Note that the C atoms are numbering in accord with the naming to help with the spectral analysis.

How many different types of H are there in 3-bromoprop-1-ene ?

ANSWER

Let's talk it through...

The methylene H (2H, 4 ppm) on C3 looks like a doublet due to the coupling with the single H at C2. Note the chemical shift 4 ppm would agree based on proximity to -Br and the alkene.

The peaks at 5-5.5 ppm represents the 2 H (integration) at C1. The uneven spacing means it is not a triplet, it's more complex.... the 2H on C1 are not equivalent to each other which means that we are looking at two overlapping signals. Note the chemical shift of 5-6 ppm would agree with H-C=C.

The peaks at 5.8-6.3 ppm represent the H at C2. Clearly a complex pattern ! This H coupled to 3 different types of H each with different coupling constants which creates the complex pattern. Note the chemical shift of 5-6 ppm would agree with H-C=C.