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75-427: The effect was first observed by Jencks and Carriuolo in 1960 in a series of chemical kinetics experiments involving the reaction of the ester p-nitrophenyl acetate with a range of nucleophiles. Regular nucleophiles such as the fluoride anion, aniline , pyridine , ethylene diamine and the phenolate ion were found to have pseudo first order reaction rates corresponding to their basicity as measured by their pK

150-670: A i n + Δ E i n t {\displaystyle Equation\ 1\colon \ \Delta E(\zeta )=\Delta E_{strain}+\Delta E_{int}} E q u a t i o n   2 :   Δ E i n t = Δ V e l s t . + Δ E p a u l i + Δ E o i + Δ E d i s p {\displaystyle Equation\ 2\colon \ \Delta E_{int}=\Delta V_{elst.}+\Delta E_{pauli}+\Delta E_{oi}+\Delta E_{disp}} The electrostatic interaction, ∆V elst ,

225-549: A t i o n   3 : d Δ E ( ζ ) d ζ = d Δ E s t r a i n ( ζ ) d ζ + d Δ E i n t ( ζ ) d ζ = 0 {\displaystyle Equation\ 3:{d\Delta E(\zeta ) \over d\zeta }={d\Delta E_{strain}(\zeta ) \over d\zeta }+{d\Delta E_{int}(\zeta ) \over d\zeta }=0} E q u

300-534: A t i o n   4 :   Δ E ‡ = Δ E s t r a i n ‡ + Δ E i n t ‡ {\displaystyle Equation\ 4:\ \Delta E^{\ddagger }=\Delta E_{strain}^{\ddagger }+\Delta E_{int}^{\ddagger }} The bimolecular elimination (E2) and substitution (S N 2) reactions are often in competition with each other because of mechanistic similarities, mainly that both benefit from

375-428: A . Other nucleophiles however reacted much faster than expected based on this criterion alone. These include hydrazine , hydroxylamine , the hypochlorite ion and the hydroperoxide anion. In 1962, Edwards and Pearson (the latter of HSAB theory ) introduced the phrase alpha effect for this anomaly. He offered the suggestion that the effect was caused by a transition state (TS) stabilization effect: on entering

450-483: A good leaving group and that the E2 reaction uses strong bases, which are often good nucleophiles for an S N 2 reaction. Bickelhaupt et. al used the activation strain model to analyze this competition between the two reactions in acidic and basic media using the 4 representative reactions below. Reactions [1] and [2] represent the E2 and S N 2 reactions, respectively, in basic conditions while reactions [3] and [4] represent

525-410: A higher transition state energy. If only the transition states are observed, it would appear that the transition state of the second representative reaction would have a higher energy due to the higher strain energy at the respective transition states. However, if one considers the entire curves for both of the reactions, it would become clear that the higher transition sate energy of the second reaction

600-399: A long time before finally attaining the equilibrium. In general terms, the free energy change (ΔG) of a reaction determines whether a chemical change will take place, but kinetics describes how fast the reaction is. A reaction can be very exothermic and have a very positive entropy change but will not happen in practice if the reaction is too slow. If a reactant can produce two products,

675-547: A model allows for the calculation of transition state energies, and hence the activation energy , of a particular reaction mechanism and allows the model to be used as a predictive tool for describing competitive mechanisms and relative preference for certain pathways. In chemistry literature, the activation strain model has been used for modeling bimolecular reactions like S N 2 and E2 reactions, transition metal mediated C-H bond activation , 1,3-dipolar cycloaddition reactions, among others. The activation strain model

750-422: A shell explodes violently. If larger pieces of aluminium are used, the reaction is slower and sparks are seen as pieces of burning metal are ejected. The reactions are due to collisions of reactant species. The frequency with which the molecules or ions collide depends upon their concentrations . The more crowded the molecules are, the more likely they are to collide and react with one another. Thus, an increase in

825-493: A very small proportion to the increase in rate of reaction. Much more important is the fact that the proportion of reactant molecules with sufficient energy to react (energy greater than activation energy : E  >  E a ) is significantly higher and is explained in detail by the Maxwell–;Boltzmann distribution of molecular energies. The effect of temperature on the reaction rate constant usually obeys

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900-487: Is a mixture of very fine powder of malic acid (a weak organic acid) and sodium hydrogen carbonate . On contact with the saliva in the mouth, these chemicals quickly dissolve and react, releasing carbon dioxide and providing for the fizzy sensation. Also, fireworks manufacturers modify the surface area of solid reactants to control the rate at which the fuels in fireworks are oxidised, using this to create diverse effects. For example, finely divided aluminium confined in

975-409: Is added to the mixture; variations on this effect are called fall-off and chemical activation . These phenomena are due to exothermic or endothermic reactions occurring faster than heat transfer, causing the reacting molecules to have non-thermal energy distributions ( non- Boltzmann distribution ). Increasing the pressure increases the heat transfer rate between the reacting molecules and the rest of

1050-551: Is because α-nucleophiles showing the α-effect have smaller HOMO(nucleophile)-LUMO(substrate) gap, in other words, high HOMO energy level that allows more orbital interaction. Examples of α-nucleophiles with α-effects are shown in Figure 4. The α-nucleophiles have smaller HOMO lobes than the parent normal nucleophile. Examples of α-nucleophiles with α-effect and inverse α-effect are shown in Figure 5. Chemical kinetics Chemical kinetics , also known as reaction kinetics ,

1125-420: Is often between 1.5 and 2.5. The kinetics of rapid reactions can be studied with the temperature jump method. This involves using a sharp rise in temperature and observing the relaxation time of the return to equilibrium. A particularly useful form of temperature jump apparatus is a shock tube , which can rapidly increase a gas's temperature by more than 1000 degrees. A catalyst is a substance that alters

1200-570: Is reached when the rates of the forward and reverse reactions are equal (the principle of dynamic equilibrium ) and the concentrations of the reactants and products no longer change. This is demonstrated by, for example, the Haber–;Bosch process for combining nitrogen and hydrogen to produce ammonia. Chemical clock reactions such as the Belousov–;Zhabotinsky reaction demonstrate that component concentrations can oscillate for

1275-409: Is satisfied. At this point along the reaction coordinate, as long as the strain and interaction energies at ζ = 0 is set to zero, the transition state energy ( Δ E ( ζ T S ) {\displaystyle \Delta E(\zeta _{TS})} ) is the activation energy ( Δ E ‡ {\displaystyle \Delta E^{\ddagger }} ) of

1350-403: Is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is different from chemical thermodynamics , which deals with the direction in which a reaction occurs but in itself tells nothing about its rate. Chemical kinetics includes investigations of how experimental conditions influence the speed of a chemical reaction and yield information about

1425-438: Is the classical repulsion and attraction between the nuclei and electron densities of the approaching reactant molecules. The Pauli repulsion term, ∆E pauli , relates to the interaction between the filled orbitals of reactant molecules. In other words, it describes steric repulsion between approaching reactants. The orbital interaction, ∆E oi , describes bond formation, HOMO-LUMO interactions, and polarization. Further, this term

1500-636: Is the difference in the rate of a chemical reaction when an atom in one of the reactants is replaced by one of its isotopes . Chemical kinetics provides information on residence time and heat transfer in a chemical reactor in chemical engineering and the molar mass distribution in polymer chemistry . It is also provides information in corrosion engineering . The mathematical models that describe chemical reaction kinetics provide chemists and chemical engineers with tools to better understand and describe chemical processes such as food decomposition, microorganism growth, stratospheric ozone decomposition, and

1575-421: Is the partial order of reaction for this reactant. The partial order for a reactant can only be determined experimentally and is often not indicated by its stoichiometric coefficient . Temperature usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more thermal energy . Although collision frequency is greater at higher temperatures, this alone contributes only

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1650-505: Is the same as y ′ = f ( x , y ) {\displaystyle y'=f(x,y)} To solve the differential equations with Euler and Runge-Kutta methods we need to have the initial values. At any point y ′ = f ( x , y ) {\displaystyle y'=f(x,y)} is the same as y ′ = d y d x {\displaystyle y'={\frac {dy}{dx}}} We can approximate

1725-503: Is the van 't Hoff wave searching for the general laws of chemical reactions and relating kinetics to thermodynamics. The second may be called the Semenov - Hinshelwood wave with emphasis on reaction mechanisms, especially for chain reactions . The third is associated with Aris and the detailed mathematical description of chemical reaction networks. The reaction rate varies depending upon what substances are reacting. Acid/base reactions,

1800-401: Is useful to represent the interaction energy in terms of molecular orbital theory. The figure below shows the lowest unoccupied molecular orbitals (LUMO)s of ethanol (basic conditions) and protonated ethanol (acidic conditions), which can be visualized as a combinations of the fragment ⋅ CH 3 {\displaystyle {\ce {*CH_3}}} radical and either

1875-530: Is usually destabilizing as it represents the distortion of a molecule from the equilibrium geometry. The interaction term, ∆E int , is generally stabilizing as it represents the electronic interactions of reactants that typically drive the reaction. The interaction energy is further decomposed based on an energy decomposition scheme from an approach by Morokuma and the Transition State Method from by Ziegler and Rauk. This decomposition breaks

1950-400: Is well complimented by group theory and MO theory as a way to describe interaction between orbitals of the correct symmetry. The last term, Δ E d i s p {\displaystyle \Delta E_{disp}} , relates to dispersion forces between the reactants. The transition states, defined as local maxima of potential energy surface, are found where equation 3

2025-484: The ⋅ CH 2 OH {\displaystyle {\ce {*CH2OH}}}  (basic conditions) or the ⋅ CH 2 OH 2 + {\displaystyle {\ce {*CH2OH2+}}}  (acidic conditions) radical. Upon protonation of the ⋅ CH 2 OH {\displaystyle {\ce {*CH2OH}}} fragment, these orbitals are lowered in energy, resulting in

2100-460: The Arrhenius equation k = A e − E a / ( R T ) {\displaystyle k=Ae^{-E_{\rm {a}}/(RT)}} , where A is the pre-exponential factor or A-factor, E a is the activation energy, R is the molar gas constant and T is the absolute temperature . At a given temperature, the chemical rate of a reaction depends on

2175-475: The Arrhenius equation and the Eyring equation . The main factors that influence the reaction rate include: the physical state of the reactants, the concentrations of the reactants, the temperature at which the reaction occurs, and whether or not any catalysts are present in the reaction. Gorban and Yablonsky have suggested that the history of chemical dynamics can be divided into three eras. The first

2250-523: The Euler method . Examples of software for chemical kinetics are i) Tenua, a Java app which simulates chemical reactions numerically and allows comparison of the simulation to real data, ii) Python coding for calculations and estimates and iii) the Kintecus software compiler to model, regress, fit and optimize reactions. -Numerical integration: for a 1st order reaction A → B The differential equation of

2325-434: The law of mass action , but the rate law of stepwise reactions has to be derived by combining the rate laws of the various elementary steps, and can become rather complex. In consecutive reactions, the rate-determining step often determines the kinetics. In consecutive first order reactions, a steady state approximation can simplify the rate law . The activation energy for a reaction is experimentally determined through

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2400-420: The rate of enzyme mediated reactions . A catalyst does not affect the position of the equilibrium, as the catalyst speeds up the backward and forward reactions equally. In certain organic molecules, specific substituents can have an influence on reaction rate in neighbouring group participation . Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing

2475-474: The reaction mechanism . The actual rate equation for a given reaction is determined experimentally and provides information about the reaction mechanism. The mathematical expression of the rate equation is often given by Here k {\displaystyle k} is the reaction rate constant , c i {\displaystyle c_{i}} is the molar concentration of reactant i and m i {\displaystyle m_{i}}

2550-438: The reaction's mechanism and transition states , as well as the construction of mathematical models that also can describe the characteristics of a chemical reaction. The pioneering work of chemical kinetics was done by German chemist Ludwig Wilhelmy in 1850. He experimentally studied the rate of inversion of sucrose and he used integrated rate law for the determination of the reaction kinetics of this reaction. His work

2625-403: The relaxation time of the return to equilibrium. The activation energy for a chemical reaction can be provided when one reactant molecule absorbs light of suitable wavelength and is promoted to an excited state . The study of reactions initiated by light is photochemistry , one prominent example being photosynthesis . The experimental determination of reaction rates involves measuring how

2700-539: The C(β)-H bond, putting electrons into this orbital should result in strengthening of this bond, dissuading its abstraction as necessary in the E2 reaction. The opposite goes for the LUMO of CH 3 CH 2 OH {\displaystyle {\ce {CH3CH2OH}}} , as donation into the orbital that is antibonding with respect to this bond will weaken the C(β)-H bond and allow it abstraction in

2775-1810: The E2 and S N 2 reactions in acidic conditions. [ 1 ]   OH −   +   CH 3 CH 2 OH   ⟶ H 2 O   +   CH 2 CH 2   +   OH − {\displaystyle {\ce {[1]\ OH^{-}\ +\ CH_{3}CH_{2}OH\ ->H_{2}O\ {+}\ CH_{2}CH_{2}\ {+}\ OH^{-}}}} [ 2 ]   OH −   +   CH 3 CH 2 OH   ⟶ CH 3 CH 2 OH   +   OH − {\displaystyle {\ce {[2]\ OH^{-}\ +\ CH_{3}CH_{2}OH\ ->CH_{3}CH_{2}OH\ +\ OH^{-}}}} [ 3 ]   H 2 O   +   CH 3 CH 2 OH 2 + ⟶ H 3 O +   +   CH 2 CH 2   +   H 2 O {\displaystyle {\ce {[3]\ H_{2}O\ +\ CH3CH2OH_{2}+->H_{3}O+\ +\ CH2CH2\ +\ H2O}}} [ 4 ]   H 2 O   +   CH 3 CH 2 OH 2 + ⟶ CH 3 CH 2 OH 2 +   +   H 2 O {\displaystyle {\ce {[4]\ H2O\ +\ CH3CH2OH2+->CH3CH2OH2+\ +\ H2O}}} Initial calculations show that, in basic media,

2850-576: The E2 reaction. This relatively intuitive comparison within MO theory shows how the increase in stabilizing interaction for the E2 mechanism arises when switching from acidic to basic conditions. An issue in the interpretation of interaction (∆E int ) and strain (∆E strain ) curves arises when only single points along the reaction coordinate are considered. Such issues become apparent when two model reactions are considered, which have identical strain energy ∆E strain curves that become more destabilizing along

2925-537: The LUMO of CH 3 CH 2 OH {\displaystyle {\ce {CH3CH2OH}}}  has antibonding character along this bond. In either the S N 2 or the E2 pathway, the HOMO of the nucleophile/base will be donating electron density into this LUMO. As the LUMO for CH 3 CH 2 OH 2 + {\displaystyle {\ce {CH3CH2OH2+}}} has bonding character along

3000-421: The S N 2 transition state which has a single electron transfer (free radical) character. Other driving forces including the tighter transition state and higher polarizability of α-nucleophiles, involvement of intramolecular catalysis also plays a role. Another in silico study did find a correlation between the alpha effect and the so-called deformation energy, which is the electronic energy required to bring

3075-453: The TS the free electron pair on the nucleophile moves away from the nucleus, causing a partial positive charge which can be stabilized by an adjacent lone pair as for instance happens in any carbocation . Over the years, many additional theories have been put forward attempting to explain the effect. The ground state destabilization theory proposes that the electron-electron repulsion between

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3150-403: The alpha lone-pair and nucleophilic electron pair destabilize each other by electronic repulsion (filled–filled orbital interaction) thereby decreasing the activation barrier by increasing the ground state energy and making it more reactive. This explains the higher reactivity of α-nucleophiles, however, this electronic mechanism should also increase the basicity and, therefore, cannot fully explain

3225-514: The basicity or lower basicity given the reactivity, were grouped as nucleophiles showing α-effect. The second group had nucleophiles with small or no deviation from the line plotted by six normal nucleophiles. Lastly, the third group had nucleophiles showing inverse α-effect, meaning that they are above the plotted line or have less reactivity considering their high basicity. Relative density functional theory, activation strain model , energy decomposition analysis, and Kohn-Sham molecular orbital analysis

3300-405: The chemistry of biological systems. These models can also be used in the design or modification of chemical reactors to optimize product yield, more efficiently separate products, and eliminate environmentally harmful by-products. When performing catalytic cracking of heavy hydrocarbons into gasoline and light gas, for example, kinetic models can be used to find the temperature and pressure at which

3375-411: The concentrations of reactants or products change over time. For example, the concentration of a reactant can be measured by spectrophotometry at a wavelength where no other reactant or product in the system absorbs light. For reactions which take at least several minutes, it is possible to start the observations after the reactants have been mixed at the temperature of interest. For faster reactions,

3450-441: The concentrations of the reactants will usually result in the corresponding increase in the reaction rate, while a decrease in the concentrations will usually have a reverse effect. For example, combustion will occur more rapidly in pure oxygen than in air (21% oxygen). The rate equation shows the detailed dependence of the reaction rate on the concentrations of reactants and other species present. The mathematical forms depend on

3525-451: The differentials as discrete increases: y ′ = d y d x ≈ Δ y Δ x = y ( x + Δ x ) − y ( x ) Δ x {\displaystyle y'={\frac {dy}{dx}}\approx {\frac {\Delta y}{\Delta x}}={\frac {y(x+\Delta x)-y(x)}{\Delta x}}} It can be shown analytically that

3600-487: The exception to the rule that homogeneous reactions take place faster than heterogeneous reactions (those in which solute and solvent are not mixed properly). In a solid, only those particles that are at the surface can be involved in a reaction. Crushing a solid into smaller parts means that more particles are present at the surface, and the frequency of collisions between these and reactant particles increases, and so reaction occurs more rapidly. For example, Sherbet (powder)

3675-525: The extraordinary services he has rendered by the discovery of the laws of chemical dynamics and osmotic pressure in solutions". After van 't Hoff, chemical kinetics dealt with the experimental determination of reaction rates from which rate laws and rate constants are derived. Relatively simple rate laws exist for zero order reactions (for which reaction rates are independent of concentration), first order reactions , and second order reactions , and can be derived for others. Elementary reactions follow

3750-426: The formation of salts , and ion exchange are usually fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be slower. The nature and strength of bonds in reactant molecules greatly influence the rate of their transformation into products. The physical state ( solid , liquid , or gas ) of a reactant is also an important factor of

3825-490: The highest yield of heavy hydrocarbons into gasoline will occur. Chemical Kinetics is frequently validated and explored through modeling in specialized packages as a function of ordinary differential equation -solving (ODE-solving) and curve-fitting . In some cases, equations are unsolvable analytically, but can be solved using numerical methods if data values are given. There are two different ways to do this, by either using software programmes or mathematical methods such as

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3900-432: The interaction energy into terms that are easily processed within the framework of Kohn-Sham molecular orbital model. These terms relate to the electrostatic interactions, steric repulsion, orbital interactions, and dispersion forces as shown in equation 2. E q u a t i o n   1 :   Δ E ( ζ ) = Δ E s t r

3975-516: The ordinate at that moment to the curve through ( x 0 , y 0 ) is given by the third-order Runge-Kutta formula. Activation strain model The activation strain model , also referred to as the distortion/interaction model , is a computational tool for modeling and understanding the potential energy curves of a chemical reaction as a function of reaction coordinate (ζ), as portrayed in reaction coordinate diagrams. The activation strain model decomposes these energy curves into 2 terms:

4050-399: The overall LUMO for each molecule having different parentage. This change in parentage in the linear combination of atomic orbitals results in the LUMO of CH 3 CH 2 OH 2 + {\displaystyle {\ce {CH3CH2OH2+}}}  having bonding character between β-carbon and the hydrogen atom abstracted in the E2 pathway while

4125-439: The rate of a chemical reaction but it remains chemically unchanged afterwards. The catalyst increases the rate of the reaction by providing a new reaction mechanism to occur with in a lower activation energy . In autocatalysis a reaction product is itself a catalyst for that reaction leading to positive feedback . Proteins that act as catalysts in biochemical reactions are called enzymes . Michaelis–Menten kinetics describe

4200-401: The rate of change. When reactants are in the same phase , as in aqueous solution , thermal motion brings them into contact. However, when they are in separate phases, the reaction is limited to the interface between the reactants. Reaction can occur only at their area of contact; in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking and stirring may be needed to bring

4275-421: The rate of reaction. This is because the activity of a gas is directly proportional to the partial pressure of the gas. This is similar to the effect of increasing the concentration of a solution. In addition to this straightforward mass-action effect, the rate coefficients themselves can change due to pressure. The rate coefficients and products of many high-temperature gas-phase reactions change if an inert gas

4350-422: The reactant A is: d [ A ] d t = − k [ A ] {\displaystyle {\frac {d{\ce {[A]}}}{dt}}=-k{\ce {[A]}}} It can also be expressed as d [ A ] d t = f ( t , [ A ] ) {\displaystyle {\frac {d{\ce {[A]}}}{dt}}=f(t,{\ce {[A]}})} which

4425-418: The reaction coordinate but have different interaction energy curves. If one of the reactions has a more stabilizing interaction energy curve with greater curvature, the transition state will be reached sooner along the reaction coordinate in order to satisfy the condition in equation 3, while a reaction with a less stabilizing interaction curve will reach the transition state later in the reaction coordinate with

4500-450: The reaction is conducted in the gas phase, leading some to conclude that it is primarily a solvation effect. However, this explanation has limitations since similar alpha effects could be found in different solvent systems and also because the solvation affects both the basicity and the nucleophilicity of the nucleophile. In the recent article published in 2021, Hansen and Vermeeren proposed the two requirements for an α-nucleophile to present

4575-414: The reaction to completion. This means that the more finely divided a solid or liquid reactant the greater its surface area per unit volume and the more contact it with the other reactant, thus the faster the reaction. To make an analogy, for example, when one starts a fire, one uses wood chips and small branches — one does not start with large logs right away. In organic chemistry, on water reactions are

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4650-442: The reaction. The activation energy can then be defined as the sum of the activation strain ( Δ E s t r a i n ‡ {\displaystyle \Delta E_{strain}^{\ddagger }} ) and the TS interaction energy ( Δ E i n t ‡ {\displaystyle \Delta E_{int}^{\ddagger }} ) as shown in equation 4. E q u

4725-551: The six normal nucleophiles (HO, CH 3 O, H 2 N, CH 3 HN, CH 3 S, HS) followed the Brønsted-type correlation. α-nucleophiles with O, HN, and S in the α position were classified into three groups according to their degree and pattern of deviation from the Brønsted-type correlation in S N 2 reactions with the substrate, ethyl chloride (C 2 H 5 Cl) (Figure 3). First, the α-nucleophiles with downward deviation, in other words, higher reactivity shown considering

4800-454: The strain of the reactant molecules as they undergo a distortion and the interaction between these reactant molecules. A particularly important aspect of this type of analysis compared others is that it describes the energetics of the reaction in terms of the original reactant molecules and describes their distortion and interaction using intuitive models such as molecular orbital theory that are capable using most quantum chemical programs. Such

4875-415: The system, reducing this effect. Condensed-phase rate coefficients can also be affected by pressure, although rather high pressures are required for a measurable effect because ions and molecules are not very compressible. This effect is often studied using diamond anvils . A reaction's kinetics can also be studied with a pressure jump approach. This involves making fast changes in pressure and observing

4950-458: The thermodynamically most stable one will form in general, except in special circumstances when the reaction is said to be under kinetic reaction control . The Curtin–Hammett principle applies when determining the product ratio for two reactants interconverting rapidly, each going to a distinct product. It is possible to make predictions about reaction rate constants for a reaction from free-energy relationships . The kinetic isotope effect

5025-483: The three groups had a distinction in HOMO lobes and HOMO-LUMO gaps. To elaborate on the first requirement, the electronegative heteroatom reduces the electron density of the atom that attacks the nucleophile making the HOMO lobe smaller. This minimizes the Pauli repulsion between the substrate and the nucleophile. Nonetheless, these small HOMO lobes don't mean less orbital interaction than the parent normal nucleophile. This

5100-411: The time required to mix the reactants and bring them to a specified temperature may be comparable or longer than the half-life of the reaction. Special methods to start fast reactions without slow mixing step include While chemical kinetics is concerned with the rate of a chemical reaction, thermodynamics determines the extent to which reactions occur. In a reversible reaction , chemical equilibrium

5175-528: The transition state energy Δ E of the E2 pathway is lower while acidic conditions favor the S N 2. Closer observation of the interaction and strain energies show that, for the E2 mechanism, upon shifting from acidic to basic media, the strain energy becomes more destabilizing, yet the interaction energy becomes more even more stabilizing, making it the driving force for the preference of the E2 pathway in basic conditions. To rationalize this increase in stabilizing interaction upon shifting to basic conditions, it

5250-430: The two reactants together in the transition state. This explanation proposes that a stable product could contribute to the alpha effect, however, this factor could not be the sole factor. The alpha effect is also dependent on solvent but not in a predictable way: it can increase or decrease with solvent mix composition or even go through a maximum. At least in some cases, the alpha effect has been observed to vanish if

5325-482: The value of the A-factor, the magnitude of the activation energy, and the concentrations of the reactants. Usually, rapid reactions require relatively small activation energies. The 'rule of thumb' that the rate of chemical reactions doubles for every 10 °C temperature rise is a common misconception. This may have been generalized from the special case of biological systems, where the α (temperature coefficient)

5400-422: The α-effect. Many explanations fall into this category. First, the secondary orbital interactions theory emphasized that electron-donating heteroatom in the α-position could contribute to increased orbital interaction with the substrate, which stabilizes the transition state (TS) and gives greater reactivity. Second, the electron transfer (ET) mechanism presents that the heteroatom in the α position could stabilize

5475-419: The α-effect. The two proposed requirements were (1) minimization of steric Pauli repulsion via small HOMO lobes on the nucleophilic center and (2) small HOMO-LUMO orbital energy gap that ensures orbital overlap with the substrate. It was proposed that both of these two requirements should be fulfilled to have an α-effect, otherwise, the nucleophiles would show no or inverse α-effect (Figure 2). In this recent work,

5550-483: Was noticed 34 years later by Wilhelm Ostwald . In 1864, Peter Waage and Cato Guldberg published the law of mass action , which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances. Van 't Hoff studied chemical dynamics and in 1884 published his famous "Études de dynamique chimique". In 1901 he was awarded the first Nobel Prize in Chemistry "in recognition of

5625-409: Was originally proposed and has been extensively developed by Bickelhaupt and coworkers. This model breaks the potential energy curve as a function of reaction coordinate, ζ, of a reaction into 2 components as shown in equation 1: the energy due to straining the original reactant molecules (∆E strain ) and the energy due to interaction between reactant molecules (∆E int ). The strain term ∆E strain

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