Understanding Organic Reactions PDF

Understanding Organic Reactions Why do certain reactions occur when two compounds are mixed together whereas others do not? To answer this question we must learn how and why organic compounds react. Reactions are at the heart of organic chemistry. An understanding of chemical...

Understanding Organic Reactions Why do certain reactions occur when two compounds are mixed together whereas others do not? To answer this question we must learn how and why organic compounds react. Reactions are at the heart of organic chemistry. An understanding of chemical processes has made possible the conversion of natural substances into new compounds with different, and sometimes superior, properties. Aspirin, ibuprofen, nylon, and polyethylene are all products of chemical reactions between substances derived from petroleum. Reactions are difficult to learn when each reaction is considered a unique and isolated event. Virtually all chemical reactions are woven together by a few basic themes. In our study of organic reactions we will begin with the functional groups, looking for electron rich and electron-deficient sites, and bonds that might be broken easily. These reactive sites give us a clue as to the general type of reaction a particular class of compound undergoes. We will learn about how a reaction occurs. Does it occur in one step or in a series of steps? Understanding the details of an organic reaction allows us to determine when it might be used in preparing interesting and useful organic compounds. Writing Equations for Organic Reactions Like other reactions, equations for organic reactions are usually drawn with a single reaction arrow (→) between the starting material and product. The reagent, the chemical substance with which an organic compound reacts, is sometimes drawn on the left side of the equation with the other reactants. At other times, the reagent is drawn above the reaction arrow itself, to focus attention on the organic starting material by itself on the left side. The solvent and temperature of a reaction may be added above or below the arrow. The symbols “hv” and “∆” are used for reactions that require light or heat, respectively. When two sequential reactions are carried out without drawing any intermediate compound, the steps are usually numbered above or below the reaction arrow. This convention signifies that the first step occurs before the second, and the reagents are added in sequence, not at the same time. In this equation only the organic product is drawn on the right side of the arrow. Although the reagent CH3MgBr contains both Mg and Br, these elements do not appear in the organic product, and they are often omitted on the product side of the equation. These elements have not disappeared. They are part of an inorganic by-product (HOMgBr in this case), and are often of little interest to an organic chemist. Kinds of Organic Reactions Organic molecules undergo acid–base and oxidation–reduction reactions, substitution, elimination, and addition reactions 1. Substitution Reactions This is a reaction in which an atom or a group of atoms is replaced by another atom or group of atoms. In a general substitution reaction, Y replaces Z on a carbon atom. Substitution reactions involve σ bonds: one σ bond breaks and another forms at the same carbon atom. 2. Elimination Reactions Elimination is a reaction in which elements of the starting material are “lost” and a o bond is formed. In an elimination reaction, two groups X and Y are removed from a starting material. Two σ bonds are broken, and a π bond is formed between adjacent atoms. The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon. 3. Addition Reactions Addition is a reaction in which elements are added to a starting material. In an addition reaction, new groups X and Y are added to a starting material. A π bond is broken and two σ bonds are formed. Addition and elimination reactions are exactly opposite. A π bond is formed in elimination reactions, whereas a π bond is broken in addition reactions. Bond Breaking and Bond Making Reaction mechanism A reaction mechanism is a detailed description of how bonds are broken and formed as a starting material is converted to a product. A reaction can occur either in one step or in a series of steps  A one-step reaction is called a concerted reaction. No matter how many bonds are broken or formed, a starting material is converted directly to a product.  A stepwise reaction involves more than one step. A starting material is first converted to an unstable intermediate, called a reactive intermediate, which then goes on to form the product. A. Bond Cleavage Bonds are broken and formed in all chemical reactions. No matter how many steps there are in the reaction, however, there are only two ways to break (cleave) a bond: the electrons in the bond can be divided equally or unequally between the two atoms of the bond.  Breaking a bond by equally dividing the electrons between the two atoms in the bond is called homolysis or homolytic cleavage. Breaking a bond by unequally dividing the electrons between the two atoms in the bond is called heterolysis or heterolytic cleavage. Heterolysis of a bond between A and B can give either A or B the two electrons in the bond. When A and B have different electronegativities, the electrons normally end up on the more electronegative atom. Homolysis and heterolysis require energy. Both processes generate reactive intermediates, but the products are different in each case. Homolysis generates uncharged reactive intermediates with unpaired electrons. Heterolysis generates charged intermediates These intermediates has a very short lifetime, reacts quickly to form stable organic product. B. Reactive Intermediates Carbocations, and Carbanions Heterolysis of the C – Z bond can generate a carbocation or a carbanion. Heterolysis of the C–Z bond can generate a carbocation or a carbanion. Both carbocations and carbanions are unstable reactive intermediates in polar reactions—reactions in which a nucleophile reacts with an electrophile. : A carbocation contains a carbon atom surrounded by only six electrons. A carbanion has a negative charge on carbon.  Giving two electrons to Z and none to carbon generates a positively charged carbon intermediate called a carbocation.  Giving two electrons to C and none to Z generates a negatively charged carbon species called a carbanion. Radicals Homolysis of the C–Z bond generates two uncharged products with unpaired electrons. A reactive intermediate with a single unpaired electron is called a radical. Most radicals are highly unstable because they contain an atom that does not have an octet of electrons. Radicals typically have no charge.  Radicals and carbocations are electrophiles because they contain an electron-deficient carbon.  Carbanions are nucleophiles because they contain a carbon with a lone pair. C. Bond Formation Like bond cleavage, bond formation occurs in two different ways.  Two radicals can each donate one electron to form a two-electron bond.  Alternatively, two ions with unlike charges can come together, with the negatively charged ion donating both electrons to form the resulting two electron bond. Bond formation always releases energy. D. Kinds of Arrows A more complete summary of the arrows used in organic chemistry is given in the table. Bond Dissociation Energy The bond dissociation energy is the energy needed to homolytically cleave a covalent bond. The energy absorbed or released in any reaction, symbolized by H°, is called the enthalpy change or heat of reaction. Bond breaking requires energy, bond dissociation energies are always positive numbers, and homolysis is always endothermic. Conversely, bond formation always releases energy, so this reaction is always exothermic. The stronger the bond, the higher its bond dissociation energy. Bond dissociation energies decrease down a column of the periodic table as the valence electrons used in bonding are farther from the nucleus. Thermodynamics For a reaction to be practical, the equilibrium must favor the products, and the reaction rate must be fast enough to form them in a reasonable time. These two conditions depend on the thermodynamics and the kinetics of a reaction, respectively. Equilibrium Constant and Free Energy Changes The equilibrium constant, Keq , is a mathematical expression that relates the amount of starting material and product at equilibrium. For example, when starting materials A and B react to form products C and D, the equilibrium constant is given by the following expression.  When Keq > 1, equilibrium favors the products (C and D) and the equilibrium lies to the right as the equation is written.  When Keq < 1, equilibrium favors the starting materials (A and B) and the equilibrium lies to the left as the equation is written. For a reaction to be useful, the equilibrium must favor the products, and Keq > 1. The position of equilibrium is determined by the relative energies of the reactants and products. The free energy of a molecule, also called its Gibbs free energy, is symbolized by G°. The change in free energy between reactants and products, symbolized by G°, determines whether the starting materials or products are favored at equilibrium. Enthalpy and Entropy Entropy (S°) is a measure of the randomness in a system. The more freedom of motion or the more disorder present, the higher the entropy. Gas molecules move more freely than liquid molecules and are higher in entropy. The entropy change (S°) is the change in the amount of disorder between reactants and products. ΔS° is positive (+) when the products are more disordered than the reactants. ΔS° is negative (–) when the products are less disordered (more ordered) than the reactants. When the number of molecules of starting material differs from the number of molecules of product in the balanced chemical equation. When an acyclic molecule is cyclized to a cyclic one, or a cyclic molecule is converted to an acyclic one. Energy Diagrams An energy diagram is a schematic representation of the energy changes that take place as reactants are converted to products. An energy diagram indicates;  how readily a reaction proceeds,  how many steps are involved, and  how the energies of the reactants, products, and intermediates compare. Consider, for example, a concerted reaction between molecule A–B with anion C:– to form products A:– and B – C. If the reaction occurs in a single step, the bond between A and B is broken as the bond between B and C is formed. An energy diagram plots energy on the y axis versus the progress of reaction, often labeled the reaction coordinate, on the x axis. As the starting materials A– B and C:– approach one another, their electron clouds feel some repulsion, causing an increase in energy, until a maximum value is reached. This unstable energy maximum is called the transition state. In the transition state the bond between A and B is partially broken, and the bond between B and C is partially formed.  The energy difference between the reactants and products is H°. Because the products are at lower energy than the reactants, this reaction is exothermic and energy is released. The energy difference between the transition state and the starting material is called the energy of activation, symbolized by Ea. The energy of activation is the minimum amount of energy needed to break bonds in the reactants. It represents an energy barrier that must be overcome for a reaction to occur. The larger the Ea, the greater the amount of energy that is needed to break bonds, and the slower the reaction rate. Energy Diagram for a Two-Step Reaction Mechanism Consider following reaction. It involves heterolysis of the A–B bond to form two ions A:– and B+, followed by reaction of B+ with anion C:– to form product B– C, as outlined in the accompanying equations. Species B+ is a reactive intermediate. It is formed as a product in Step , and then goes on to react with C:– in Step. We must draw an energy diagram for each step, and then combine them in an energy diagram for the overall two-step mechanism. Step is endothermic because energy is needed to cleave the A–B bond, making ΔH° a positive value and placing the products of Step at higher energy than the starting materials. In the transition state, the A–B bond is partially broken. Step is exothermic because energy is released in forming the B–C bond, making ΔH° a negative value and placing the products of Step at lower energy than the starting materials of Step. In the transition state, the B–C bond is partially formed. In a multistep mechanism, the step with the highest energy transition state is called the rate-determining step.  Kinetics The study of reaction rates-how fast a particular reaction proceeds, is called kinetics. The rate of chemical processes affects many facets of our lives. Aspirin is an effective antiinflammatory agent because it rapidly inhibits the synthesis of prostaglandins. Butter turns rancid with time because its lipids are only slowly oxidized by oxygen in the air to undesirable by-products. All of these processes occur at different rates, resulting in benefi cial or harmful effects Factors affecting rate of reaction Energy of Activation Ea, is the energy difference between the reactants and the transition state. It is the energy barrier that must be exceeded for reactants to be converted to products. Concentration The higher the concentration, the faster the rate. Increasing concentration increases the number of collisions between reacting molecules, which in turn increases the rate. Temperature The higher the temperature, the faster the rate. Increasing temperature increases the average kinetic energy of the reacting molecules. Because the kinetic energy of colliding molecules is used for bond cleavage, increasing the average kinetic energy increases the rate. The Ea values of most organic reactions are 40–150 kJ/mol. When Ea < 80 kJ/mol, the reaction occurs readily at or below room temperature. When Ea > 80 kJ/mol, higher temperatures are needed. Rate Equations The rate of a chemical reaction is determined by measuring the decrease in the concentration of the reactants over time, or the increase in the concentration of the products over time. A rate law (or rate equation) is an equation that shows the relationship between the rate of a reaction and the concentration of the reactants. A rate law is determined experimentally, and it depends on the mechanism of the reaction. Fast reactions have large rate constants. Slow reactions have small rate constants.  A rate equation contains concentration terms for all reactants involved in a one-step mechanism.  A rate equation contains concentration terms for only the reactants involved in the rate-determining step in a multistep reaction. In the one-step reaction of A –B + C:– to form A:– + B–C, both reactants appear in the transition state of the only step of the mechanism. The concentration of both reactants affects the reaction rate and both terms appear in the rate equation. This type of reaction involving two reactants is said to be bimolecular. The order of a rate equation equals the sum of the exponents of the concentration terms in the rate equation. In the rate equation for the concerted reaction of A –B + C:– , there are two concentration terms, each with an exponent of one. Thus, the sum of the exponents is two and the rate equation is second order (the reaction follows second-order kinetics). Because the rate of the reaction depends on the concentration of both reactants, doubling the concentration of either A –B or C:– doubles the rate of the reaction. Doubling the concentration of both A –B and C:– increases the reaction rate by a factor of four. In the stepwise conversion of A –B + C:– to form A:– + B–C. The mechanism shown in Section 6.8 has two steps: a slow step (the rate-determining step) in which the A –B bond is broken, and a fast step in which the B–C bond is formed. In a multistep mechanism, a reaction can occur no faster than its rate-determining step. Only the concentrations of the reactants affecting the rate-determining step appear in the rate equation. In this example, the rate depends on the concentration of A –B only, because only A –B appears in the rate-determining step. A reaction involving only one reactant is said to be unimolecular. Because there is only one concentration term (raised to the fi rst power), the rate equation is first order (the reaction follows first-order kinetics). Because the rate of the reaction depends on the concentration of only one reactant, doubling the concentration of A –B doubles the rate of the reaction, but doubling the concentration of C:– has no effect on the reaction rate. Catalysts Some reactions do not occur in a reasonable time unless a catalyst is added. Common catalysts in organic reactions are acids and metal Enzymes The catalysts in living organisms, are usually protein molecules called enzymes. Enzymes are biochemical catalysts composed of amino acids held together in a very specific three-dimensional shape. An enzyme contains a region called its active site, which binds an organic reactant, called a substrate. When bound, this unit is called the enzyme–substrate complex. For example enzyme lactase binds lactose, the principal carbohydrate in milk. Once bound, the organic substrate undergoes a very specific reaction at an enhanced rate. In this example, lactose is converted into two simpler sugars, glucose and galactose. When individuals lack adequate amounts of lactase, they are unable to digest lactose, causing abdominal cramping and diarrhea. DRM Software Reviews