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sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikiversity wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia discipline of organic chemistry this article has multiple issues please help improve it or discuss these issues on the talk page learn how and when to remove these messages this article relies excessively on references to primary sources please improve this article by adding secondary or tertiary sources find sources physical organic chemistry news newspapers books scholar jstor june 2015 learn how and when to remove this message this article cites its sources but does not provide page references please help improve it by providing page numbers for existing citations june 2015 learn how and when to remove this message learn how and when to remove this message physical organic chemistry a term coined by louis hammett in 1940 refers to a discipline of organic chemistry that focuses on the relationship between chemical structures and reactivity in particular applying experimental tools of physical chemistry to the study of organic molecules specific focal points of study include the rates of organic reactions the relative chemical stabilities of the starting materials reactive intermediates transition states and products of chemical reactions and non covalent aspects of solvation and molecular interactions that influence chemical reactivity such studies provide theoretical and practical frameworks to understand how changes in structure in solution or solid state contexts impact reaction mechanism and rate for each organic reaction of interest application edit physical organic chemists use theoretical and experimental approaches work to understand these foundational problems in organic chemistry including classical and statistical thermodynamic calculations quantum mechanical theory and computational chemistry as well as experimental spectroscopy e g nmr spectrometry e g ms and crystallography approaches the field therefore has applications to a wide variety of more specialized fields including electro and photochemistry polymer and supramolecular chemistry and bioorganic chemistry enzymology and chemical biology as well as to commercial enterprises involving process chemistry chemical engineering materials science and nanotechnology and pharmacology in drug discovery by design scope edit physical organic chemistry is the study of the relationship between structure and reactivity of organic molecules more specifically physical organic chemistry applies the experimental tools of physical chemistry to the study of the structure of organic molecules and provides a theoretical framework that interprets how structure influences both mechanisms and rates of organic reactions it can be thought of as a subfield that bridges organic chemistry with physical chemistry physical organic chemists use both experimental and theoretical disciplines such as spectroscopy spectrometry crystallography computational chemistry and quantum theory to study both the rates of organic reactions and the relative chemical stability of the starting materials transition states and products 1 page needed chemists in this field work to understand the physical underpinnings of modern organic chemistry and therefore physical organic chemistry has applications in specialized areas including polymer chemistry supramolecular chemistry electrochemistry and photochemistry 1 page needed history edit the term physical organic chemistry was itself coined by louis hammett in 1940 when he used the phrase as a title for his textbook 2 chemical structure and thermodynamics edit thermochemistry edit main articles thermochemistry and chemical thermodynamics organic chemists use the tools of thermodynamics to study the bonding stability and energetics of chemical systems this includes experiments to measure or determine the enthalpy δ h entropy δ s and gibbs free energy δ g of a reaction transformation or isomerization chemists may use various chemical and mathematical analyses such as a van t hoff plot to calculate these values empirical constants such as bond dissociation energy standard heat of formation δ f h and heat of combustion δ c h are used to predict the stability of molecules and the change in enthalpy δ h through the course of the reactions for complex molecules a δ f h value may not be available but can be estimated using molecular fragments with known heats of formation this type of analysis is often referred to as benson group increment theory after chemist sidney benson who spent a career developing the concept 1 page needed 3 4 the thermochemistry of reactive intermediates carbocations carbanions and radicals is also of interest to physical organic chemists group increment data are available for radical systems 1 page needed carbocation and carbanion stabilities can be assessed using hydride ion affinities and pk a values respectively 1 page needed conformational analysis edit one of the primary methods for evaluating chemical stability and energetics is conformational analysis physical organic chemists use conformational analysis to evaluate the various types of strain present in a molecule to predict reaction products 5 page needed strain can be found in both acyclic and cyclic molecules manifesting itself in diverse systems as torsional strain allylic strain ring strain and syn pentane strain 1 page needed a values provide a quantitative basis for predicting the conformation of a substituted cyclohexane an important class of cyclic organic compounds whose reactivity is strongly guided by conformational effects the a value is the difference in the gibbs free energy between the axial and equatorial forms of substituted cyclohexane and by adding together the a values of various substituents it is possible to quantitatively predict the preferred conformation of a cyclohexane derivative in addition to molecular stability conformational analysis is used to predict reaction products one commonly cited example of the use of conformational analysis is a bi molecular elimination reaction e2 this reaction proceeds most readily when the nucleophile attacks the species that is antiperiplanar to the leaving group a molecular orbital analysis of this phenomenon suggest that this conformation provides the best overlap between the electrons in the r h σ bonding orbital that is undergoing nucleophilic attack and the empty σ antibonding orbital of the r x bond that is being broken 6 page needed by exploiting this effect conformational analysis can be used to design molecules that possess enhanced reactivity the physical processes which give rise to bond rotation barriers are complex and these barriers have been extensively studied through experimental and theoretical methods 7 8 9 a number of recent articles have investigated the predominance of the steric electrostatic and hyperconjugative contributions to rotational barriers in ethane butane and more substituted molecules 10 non covalent interactions edit main article noncovalent bonding cryptand with a metal cation demonstrating host guest chemistry cryptands are tricyclic compounds that tightly encapsulate the guest cation via electrostatic interactions ion dipole interaction chemists use the study of intramolecular and intermolecular non covalent bonding interactions in molecules to evaluate reactivity such interactions include but are not limited to hydrogen bonding electrostatic interactions between charged molecules dipole dipole interactions polar π and cation π interactions π stacking donor acceptor chemistry and halogen bonding in addition the hydrophobic effect the association of organic compounds in water is an electrostatic non covalent interaction of interest to chemists the precise physical origin of the hydrophobic effect originates from many complex interactions but it is believed to be the most important component of biomolecular recognition in water 1 page needed for example researchers elucidated the structural basis for folic acid recognition by folate acid receptor proteins 11 the strong interaction between folic acid and folate receptor was attributed to both hydrogen bonds and hydrophobic interactions the study of non covalent interactions is also used to study binding and cooperativity in supramolecular assemblies and macrocyclic compounds such as crown ethers and cryptands which can act as hosts to guest molecules acid base chemistry edit main article acid base reaction the principles of induction and resonance can be used to explain the different acid dissociation constant or pk a values for phenol a and p nitrophenol b for b the electronegative nitro group stabilizes the conjugate base phenoxide anion via induction and through resonance by delocalizing the negative charge the properties of acids and bases are relevant to physical organic chemistry organic chemists are primarily concerned with brønsted lowry acids bases as proton donors acceptors and lewis acids bases as electron acceptors donors in organic reactions chemists use a series of factors developed from physical chemistry electronegativity induction bond strengths resonance hybridization aromaticity and solvation to predict relative acidities and basicities the hard soft acid base principle is utilized to predict molecular interactions and reaction direction in general interactions between molecules of the same type are preferred that is hard acids will associate with hard bases and soft acids with soft bases the concept of hard acids and bases is often exploited in the synthesis of inorganic coordination complexes kinetics edit main article chemical kinetics physical organic chemists use the mathematical foundation of chemical kinetics to study the rates of reactions and reaction mechanisms unlike thermodynamics which is concerned with the relative stabilities of the products and reactants δ g and their equilibrium concentrations the study of kinetics focuses on the free energy of activation δ g the difference in free energy between the reactant structure and the transition state structure of a reaction and therefore allows a chemist to study the process of equilibration 1 page needed mathematically derived formalisms such as the hammond postulate the curtin hammett principle and the theory of microscopic reversibility are often applied to organic chemistry chemists have also used the principle of thermodynamic versus kinetic control to influence reaction products rate laws edit main article rate equation the study of chemical kinetics is used to determine the rate law for a reaction the rate law provides a quantitative relationship between the rate of a chemical reaction and the concentrations or pressures of the chemical species present 12 page needed rate laws must be determined by experimental measurement and generally cannot be elucidated from the chemical equation the experimentally determined rate law refers to the stoichiometry of the transition state structure relative to the ground state structure determination of the rate law was historically accomplished by monitoring the concentration of a reactant during a reaction through gravimetric analysis but today it is almost exclusively done through fast and unambiguous spectroscopic techniques in most cases the determination of rate equations is simplified by adding a large excess flooding all but one of the reactants catalysis edit main article catalysis reaction coordinate energy diagram for uncatalysed and catalysed reactions the latter without and with change in mechanism citation needed trace a presents the uncatalysed reaction pathway orange with its free energy of activation trace b presents a catalysed reaction pathway blue with its lowered relative free energy of activation trace c presents a complex altered reaction pathway brown for the same reaction proceeding by a different mechanism involving two kinetically discrete intermediates the study of catalysis and catalytic reactions is very important to the field of physical organic chemistry a catalyst participates in the chemical reaction but is not consumed in the process 12 page needed a catalyst lowers the activation energy barrier δ g increasing the rate of a reaction by either stabilizing the transition state structure or destabilizing a key reaction intermediate and as only a small amount of catalyst is required it can provide economic access to otherwise expensive or difficult to synthesize organic molecules catalysts may also influence a reaction rate by changing the mechanism of the reaction 1 page needed kinetic isotope effect edit main article kinetic isotope effect although a rate law provides the stoichiometry of the transition state structure it does not provide any information about breaking or forming bonds 1 page needed the substitution of an isotope near a reactive position often leads to a change in the rate of a reaction isotopic substitution changes the potential energy of reaction intermediates and transition states because heavier isotopes form stronger bonds with other atoms atomic mass affects the zero point vibrational state of the associated molecules shorter and stronger bonds in molecules with heavier isotopes and longer weaker bonds in molecules with light isotopes 6 page needed because vibrational motions will often change during a course of a reaction due to the making and breaking of bonds the frequencies will be affected and the substitution of an isotope can provide insight into the reaction mechanism and rate law substituent effects edit the study of how substituents affect the reactivity of a molecule or the rate of reactions is of significant interest to chemists substituents can exert an effect through both steric and electronic interactions the latter of which include resonance and inductive effects the polarizability of molecule can also be affected most substituent effects are analyzed through linear free energy relationships lfers the most common of these is the hammett plot analysis 1 page needed this analysis compares the effect of various substituents on the ionization of benzoic acid with their impact on diverse chemical systems the parameters of the hammett plots are sigma σ and rho ρ the value of σ indicates the acidity of substituted benzoic acid relative to the unsubstituted form a positive σ value indicates the compound is more acidic while a negative value indicates that the substituted version is less acidic the ρ value is a measure of the sensitivity of the reaction to the change in substituent but only measures inductive effects therefore two new scales were produced that evaluate the stabilization of localized charge through resonance one is σ which concerns substituents that stabilize positive ...
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