Q13-28E - Organic Chemistry (Mcmurry)

Question

Question: Identify the indicated protons in the following molecules as unrelated,

homotopic, enantiotopic, or diastereotopic.

a)

b)

Step-by-Step Solution

Answer

(a) The indicated protons in cysteine are diastereotopic

(b) The indicated protons are enantiotopic

Step 1: Topicity

Stereochemistry is the branch of chemistry deals with three-dimensional arrangement of atoms and molecules and effect of this on chemical reactions.

In stereochemistry, topicity is the stereo chemical relationship between substituents and the structure to which they are attached. Depending upon the relationship such groups can be homotopic, enantiotopic, or diastereotopic. To determine the relationship of these protons, remember that symmetry axis means homotopic, and if there is no axis, but a there is plane of symmetry then the protons are enantiotopic
                                                        

                                     
                                                                                              Stereocenter

Step 2: Dsistinguish between homotopic, enantiotopic, or diastereotopic.

To determine the relationship of these protons, remember that symmetry axis means homotopic, and if there is no axis, but a there is plane of symmetry then the protons are enantiotopic.

The most common set of non-equivalent protons are known as diastereotopic protons. If replacing two proton with a different group give a pair of diastereomers, the proton are called Diastereotopic. These are not chemically equivalent and give two NMR signals
                                                                   

                                           

                               Chemically not equivalent                               Chemically equivalent   

Step 3: Identify the indicated protons of compound (a) and (b)

(a) The indicated protons in cysteine are diastereotopic, because cysteine has a chirality center.
                                                            

                                                     

                                                                                            Cysteine

(b) The indicated protons are enantiotopic. Imagine replacing first one, then the other of the indicated protons with a substituent X. The resulting compound would be enantiomers. The proton are thus enantiotopic.

                                                     
                                                                               Enantiotropic protons