Reactivity–selectivity principle

In <a href="/facts/Chemistry/SrNqkGVm">chemistry</a> the reactivity–selectivity principle or RSP states that a more reactive <a href="/facts/Chemical_compound/37FXqqtv">chemical compound</a> or <a href="/facts/Reactive_intermediate/6pajSrDX">reactive intermediate</a> is less selective in chemical reactions. In this context selectivity represents the ratio of <a href="/facts/Reaction_rate/Prwvxq0y">reaction rates</a>.
This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete.
A classic example of perceived RSP found in older organic chemistry textbooks concerns the <a href="/facts/Free_radical_halogenation/cC3L0mdK">free radical halogenation</a> of simple <a href="/facts/Alkane/Qam7Hvmf">alkanes</a>. Whereas the relatively unreactive <a href="/facts/Bromine/oLfj3C7D">bromine</a> reacts with 2-methylbutane predominantly to 2-bromo-2-methylbutane, the reaction with much more reactive <a href="/facts/Chlorine/y27UwYBM">chlorine</a> <a href="/facts/Regioselectivity/m3Bs2Xzl">results in a mixture</a> of all four <a href="/facts/Regioisomer/vEQEBbWQ">regioisomers</a>.
Another example of RSP can be found in the selectivity of the reaction of certain <a href="/facts/Carbocation/vIggUxTh">carbocations</a> with <a href="/facts/Azide/AaJwyv1z">azides</a> and <a href="/facts/Water/0lkXnJPK">water</a>. The very stable triphenylmethyl carbocation derived from <a href="/facts/Solvolysis/QfdNQzSj">solvolysis</a> of the corresponding <a href="/facts/Triphenylmethyl_chloride/5LNGq9dh">triphenylmethyl chloride</a> reacts 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiary <a href="/facts/Adamantane/tlvgCxYf">adamantane</a> carbocation (as judged from diminished <a href="/facts/Reaction_rate/Prwvxq0y">rate</a> of solvolysis) this difference is only a factor of 10.
Constant or inverse relationships are just as frequent. For example, a group of 3- and 4-substituted <a href="/facts/Pyridine/X817hCch">pyridines</a> in their reactivity quantified by their <a href="/facts/PKa/t4YyRtKC">pKa</a> show the same selectivity in their reactions with a group of alkylating reagents.
The reason for the early success of RSP was that the experiments involved very reactive intermediates with reactivities close to <a href="/facts/Diffusion-controlled_reaction/FuF3WdTa">kinetic diffusion control</a> and as a result the more reactive intermediate appeared to react slower with the faster substrate.
General relationships between reactivity and selectivity in chemical reactions can successfully be explained by <a href="/facts/Hammond%2527s_postulate/BdThbWoB">Hammond's postulate</a>.

When reactivity-selectivity relationships do exist they signify different reaction modes. In one study the reactivity of two different <a href="/facts/Free_radical/qFymuVoT">free radical</a> species (A, sulfur, B carbon) towards addition to simple <a href="/facts/Alkene/RtYcS6kU">alkenes</a> such as <a href="/facts/Acrylonitrile/zvhfg050">acrylonitrile</a>, <a href="/facts/Vinyl_acetate/qVdoopEo">vinyl acetate</a> and <a href="/facts/Acrylamide/HaTTnPvV">acrylamide</a> was examined.
The sulfur radical was found to be more reactive (6*108 vs. 1*107 <a href="/facts/Molar_concentration/P2WL5b9q">M</a>−1.s−1) and less selective (selectivity ratio 76 vs 1200) than the carbon radical. In this case, the effect can be explained by extending the <a href="/facts/Bell%25E2%2580%2593Evans%25E2%2580%2593Polanyi_principle/Ube9pP6G">Bell–Evans–Polanyi principle</a> with a factor 
 
 
 
 δ
 
 
 
 {\displaystyle \delta \,}
 
 accounting for transfer of charge from the reactants to the <a href="/facts/Transition_state/34PsdqRd">transition state</a> of the reaction which can be calculated <a href="/facts/In_silico/g2Cz0ejg">in silico</a>:

 
 
 
 
 E
 
 a
 
 
 =
 
 E
 
 o
 
 
 +
 α
 Δ
 
 H
 
 r
 
 
 +
 β
 
 δ
 
 2
 
 
 
 
 
 {\displaystyle E_{a}=E_{o}+\alpha \Delta H_{r}+\beta \delta ^{2}\,}

with 
 
 
 
 
 E
 
 a
 
 
 
 
 
 {\displaystyle E_{a}\,}
 
 the <a href="/facts/Activation_energy/GqkgPvo0">activation energy</a> and 
 
 
 
 Δ
 
 H
 
 r
 
 
 
 
 
 {\displaystyle \Delta H_{r}\,}
 
 the reaction <a href="/facts/Enthalpy/bBou8lCE">enthalpy</a> change. With the <a href="/facts/Electrophile/1AHFg1zm">electrophilic</a> sulfur radical the charge transfer is largest with electron-rich alkenes such as acrylonitrile but the resulting reduction in activation energy (β is negative) is offset by a reduced enthalpy. With the <a href="/facts/Nucleophile/xYccoaJQ">nucleophilic</a> carbon radical on the other hand both enthalpy and polar effects have the same direction thus extending the activation energy range.

Reactivity–selectivity principle open-in-new

Reactivity–selectivity principle