Ph-S-CH2-CH2-Cl reacts with water 600 times faster than CH3-CH2-CH2-Cl In this type of substitution reaction, a group of the substrate first participates in the reaction and thus influences the reaction. Due to the increase in the ngp reaction rate of many stoichiometrically folds, the product experiences an attack retention position of nucleophilic changes Aliphatic C-C or C-H bonds can lead to charge relocation if these bonds are close and antiperiplanar to the starting group. The corresponding intermediates are related to a non-classical ion, the 2-norbornyl system being the best known case. In this type of substitution reaction, a group of the substrate first participates in the reaction and thus influences the reaction. Due to the increase in the NGP reaction rate of many Stochiometric folds, the product undergoes an attack position of retention of nucleophilic changes A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a nucleophile, the reaction rate is much higher for sulfur mustard and a nucleophile than for a primary alkyl chloride without heteroatom. Neighboring group involvement (NGP) (also known as anchimolar support) in organic chemistry has been defined by IUPAC as the interaction of a reaction center with a solitary pair of electrons in an atom or electrons present in a sigma bond or Pi bond contained in the parent molecule but not conjugated with the reaction center. [1] [1] [2] [3] When the NGP is in operation, it is normal for the reaction rate to be increased. It is also possible that the stereochemistry of the reaction is abnormal (or unexpected) compared to a normal reaction. Although it is possible for neighboring groups to influence many reactions in organic chemistry (for example. B, the reaction of a diene such as 1,3-cyclohexadiene with maleic anhydride usually gives the endoisomere due to a side effect {overlap of the carbonyl group π orbitals with the transition state in the Diels-Alder reaction}), this side is limited to the neighboring group effects observed in carbocations and SN2 reactions.
The carbocationic intermediate is stabilized by resonance, in which the positive charge is distributed over several atoms. The following diagram shows this. Orbitals π of an alkene can stabilize a transition state by helping to relocate the positive carbocation charge. For example, unsaturated tosylate reacts faster (1011 times faster with aqueous solsolysis) with a nucleophile than saturated tosylate. Here is a different view of the same intermediaries. If cyclopropylmethyl chloride reacts with ethanol and water, a mixture of cyclopropylmethyl alcohol at 48%, cyclobutanol at 47% and homoallyl alcohol at 5% (but-3-enol) is obtained. Indeed, the carbocationic intermediate is delocalized into many different carbons by a reversible annular opening. If the following tosylate reacts with acetic acid in solvolysis, then instead of a simple reaction SN2 forming B, a mixture 48:48:4 of A, B (which are enantiomers) and C+D [2] [3] is obtained. Even if the alkene is further away from the center of reaction, the alkene can still act in this way. For example, the alkene in the following alkylbenzene sulfonate is able to relocate the carbocation. An aromatic ring can aid in the formation of a carbocationic intermediate called phenonium ion by relocating the positive charge. In addition, the increase in the SN2 reaction rate of allyl bromide with a nucleophile compared to the reaction of n-propyl bromide is due to the fact that the orbitals of the π bond overlap with those of the transition state.
In the allyl system, alkene orbitals overlap with orbitals of an SN2 transition state. With a benzyl halide, the reactivity is higher because the transition state SN2 has an overlapping effect similar to that of the allyl system. .