Q3P - Organic Chemistry (Mcmurry)

Question

2-Chloropropene shows signals for three kinds of protons in its 1H NMR spectrum. Explain.

Step-by-Step Solution

Answer

2-Chloropropene has three kind of protons. Proton b and proton c differ because one is cis to the chlorine and the other is trans.

Step 1: The signals shown by cis 2-Chloroprene for three kinds of protons

There are three NMR signals. The signals H_a and H_b are non-equivalent protons on the same carbon. So they are close enough to split each others NMR signals. The H_a signal is split into a doublet by H_b . The H_b signal is split into a doublet by H_a . H_c has no adjacent non equivalent protons, hence its signal is a singlet.  

                                  Cis 2-Chloroprene

Step 2: The signals shown by trans 2-Chloroprene for three kinds of protons

The signal H_a is bonded to the carbon with Cl atom, making it different form H_b  and H_c  . Of the remaining two hydrogen atoms, H_b is cis to the Cl atom and H_c is cis to the hydrogen atom, making them different. All three hydrogen atoms in this compound are different. H_a   is bonded to a carbon connected to Cl, both the H_b protons are cis to the Cl whereas both the H_c protons are cis to another hydrogen.  

 

                               Trans 2-Chloroprene

Hence, 2-Chloropropene has three kind of protons. Proton b and proton c differ because one is cis to the chlorine and the other is trans.