Paul Walden, the brilliant son of a farmer succeeded in finding a concept that is now widely known as umbrella effect in S_N2 reactions. In an S_N2 reaction, the configuration at the reaction centre is inverted as a umbrella turns inside out in a strong wind. An ideal S_N1 reaction, on the other hand, yield nearly racemised product as it proceeds through planar carbocation. S_N2 reactions proceed smoothly on primary alkyl halide whereas tertiary halides are fit for S_N1 mechanism. However, evidences indicate that all the molecules of particular substances do not necessarily react by a single mechanism.After all, the intention of the molecule is very clear, they want to follow the low energy path.If both paths have nearly the same energy barrier, then follow a mixed mechanism.For example, secondary alkyl halides follow a mixed mechanism, say 60% molecule react by S_N2 and rest 40%by S_N1 Stereochemical features, as mentioned above, are noticeable only when the reaction is carried out on a particular enantiomer.Even the reaction centres, which are paths of suitably substituted rings, also exemplify the said features. There are two basic mechanisms S_N1 and S_N2 All except one of the following substances give at least one of these two reactions enthusiastically in the absence of catalyst Identify the odd out that reacts sluggish under both the mechanisms.