Chemical Properties and Reactions

The functional group of a compound determines the kinds of reactions that the compound undergoes. • All compounds with a certain functional group react in the same way. • The family or class to which an organic compound belongs is also determined by its functional group. • All compounds within a certain family or class react in the same way. Recall Chemical Properties and Reactions • The functional group of alkenes is the carbon– carbon double bond. • The π bond of a double bond is weak. • Therefore, the π bond is easily broken when an alkene undergoes a reaction. Chemical Properties Chemical Properties and Reactions • Alkenes undergo addition reactions. • Specifically, electrophilic addition reactions. • Atoms or groups of atoms are added to the two sp2 carbons (for each to form four bonds after the π bond breaks). • An alkene is an electron-rich molecule — it is a nucleophile. • Recall. The π bond of an alkene consists of a cloud of electrons above and below the σ bond. Reactions Chemical Properties and Reactions Chemical Properties and Reactions Reactions • Electrophilic addition reactions are the characteristic reactions of alkenes. Chemical Properties and Reactions Learning Objectives • Why alkenes react. • The kinds of reagents with which they react. • The mechanisms by which the reactions occur. • The products that are formed. • Synthesis of various compounds. Chemical Properties and Reactions Reactions and Synthesis • You learn not only about the reactivity of compound A but also about one way that compound B can be synthesized. • Many different reagents can add to alkenes and various compounds are synthesized as a result. • All compounds with a carbon–carbon double bond react in the same way. Functional Group and Reactions Chemical Properties and Reactions • Organic chemistry is all about the interaction between electron-rich species and electrondeficient species. • Electron-deficient species are attracted to electron-rich species. • Electron-deficient species = Electrophile • Electron-rich species = Nucleophile Functional Group and Reactions Chemical Properties and Reactions • An electrophile looks for a pair of electrons. Electrophiles and Nucleophiles Chemical Properties and Reactions • A nucleophile has a pair of electrons it can share. Nucleophiles react with Electrophiles. • The mechanism of a reaction describes the stepby-step process by which reactants are changed to products. Reaction Mechanism Chemical Properties and Reactions • Curved arrows are drawn to show how the electrons move as new covalent bonds are formed and existing covalent bonds are broken (i.e., as reactants are converted to products). • Each arrow represents the simultaneous movement of two electrons (an electron pair) from a nucleophile (electron donor, at the tail of the arrow) toward an electrophile (electron acceptor, at the point of the arrow). • A curved arrow indicates where the electrons start from and where they end up. • An arrowhead with two barbs signifies the movement of two electrons. Curved Arrows Chemical Properties and Reactions • The tail of the arrow is positioned where the electrons are in the reactant. • The tail always starts at a lone pair or at a bond. • The head of the arrow points to where these same electrons end up in the product. • The arrow always points at an atom or a bond. • The curved arrows allow to follow the electrons and see what bonds are broken and what bonds are formed. Curved Arrows (cont’d) Chemical Properties and Reactions • You need to learn how to draw curved arrows. • See the Tutorial in the required textbook. • Step 1. Reaction Steps Chemical Properties and Reactions • Step 2. Reaction Steps Chemical Properties and Reactions • The overall reaction involves the addition of 1 mole of HBr to 1 mole of the alkene (2-butene) to form 2-bromobutane. • Addition reaction. • The first step of the reaction is the addition of an electrophile (H+) to the alkene. • Electrophilic addition reaction. Reaction Mechanism - Summary Chemical Properties and Reactions Chemical Properties and Reactions Energy Changes • In each step of the two-step process, the reactants pass through a transition state as they are converted into products. Transition states represent the highest-energy structures that are involved in the reaction. Chemical Properties and Reactions Reaction Coordinate Diagram • The reaction coordinate diagram describes the reaction pathway, i.e., the energy changes that occur when reactants are converted to products. ΔG° > 0 ΔG° < 0 Chemical Properties and Reactions Reaction Coordinate Diagram • Transition states • Partially formed bonds • Can never be isolated • Intermediates • Fully formed bonds • Can be isolated (depending on stability) Exergonic Chemical Properties and Reactions Rate-Determining Step • If a reaction has two or more steps, the step that has its transition state at the highest point on the reaction coordinate diagram is the rate-determining step or rate-limiting step. • The rate-determining step controls the overall rate of the reaction. Chemical Properties and Reactions Rate-Determining Step • In the example, the free energy of activation (energy barrier) for the first step of the reaction is greater than that for the second step. • In other words, the rate constant for the first step is smaller than that for the second step. • The rate-determining step for the reaction of 2- butene with HBr is the first step. • I.e., the addition of the proton (the electrophile) to the alkene (the nucleophile) to form the carbocation.