Class Note for CH 231 at UA-Elem Organic Chem I (2)
Class Note for CH 231 at UA-Elem Organic Chem I (2)
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This 5 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Alabama - Tuscaloosa taught by a professor in Fall. Since its upload, it has received 15 views.
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Date Created: 02/06/15
Chapter 5 Outline I 51 Kinds of Organic Reactions 0 Addition reactions Two reactants add together to make a single product 0 Elimination reactions A single reactant gives two products Essentially the reverse of an addition 0 Substitution reactions Reaction where two reactants exchange parts 0 Rearrangement reactions Conversion of a single reactant to a di erent constitutional isomer I 52 How Organic Reactions OccuriMechanisms 0 Reaction Mechanism describes the step by step process of how a reaction occurs Usually each change in bonding is shown as a separate step Arrows are used to show the movement of electrons 0 Types of bond breaking and forming reactions Bond breaking I Homolytic Bond breaks to give two radical species I Heterolytic Bond breaks so that one atom gets both electrons A cation and an anion are formed Bond forming I Homogenic Reaction of two radical species where each provides an electron to form the bond I Heterogenic Reaction of an electron pair donor with an electron pair acceptor Homolytic and homogenic reactions are known as radical reactions as single electron species are often referred to as free radicals Heterolytic and heterogenic reactions are known as polar reactions as they usually involve ionic species as reactants or products I 53 Radical Reactions 0 Radicals are highly reactive species and will often react with nonradical reactants Substitution The radical species can abstract remove an element from a molecule by a homolytic bond cleavage A new radical species is formed Addition Radicals often add to Tcbonds This addition results in the formation of a new radical at the other atom of the Tcbond o Chlorination of alkanes An example of a radical substitution reaction is the conversion of alkanes to chlorinated alkanes by reaction with chlorine and light Initiation In the presence of light the weak ClCl bond is homolytically broken to give two Cl species radicals I Propagation There are two propagation steps They form a continuous loop that will keep going until the reaction runs out of radical species see termination Note that each step below produces a new radical that is used in the next step I The C1 radical abstracts an H from the alkane to give HCl and a carbon radical I The carbon radical reacts with C12 to form the CCl bond and a new Cl radical that can repeat the rst propagation step I Termination Any reaction that results in the consumption of reactions will terminate the chain process I Reaction of two Cl radicals to give C12 I Reaction of a Cl radical with a carbon radical I Reaction of two carbon radicals I 54 Polar reactions 0 Polar reactions are the most common in organic chemistry Most reactions in organic chemistry involve polar bonds where one part is partially positive and the other part partially negative Nonpolar bonds such as CH and CC rarely are reactive Polar reactions are Lewis acidLewis base reactions I An electronpair donor Lewis base donates its electrons to the electron pair acceptor Lewis acid I In these reactions the Lewis base is often called the nucleophile nucleus lover because it is seeking an atom nucleus with which to share electrons Typical nucleophiles include atoms with lone pairs N O halides as well as Tcbonds I The Lewis acid is called the electrophile electron lover because it is seeking a pair of electrons to share Typical electrophiles include acids H30 HCl etc and carbon attached to more electronegative elements I 55 Example of a Polar Reaction Addition of HBr to Ethylene O O Ethylene reacts with HBr to give bromoethane How does this reaction occur When thinking about polar reactions we always want to think about which species would be the electrophile and which would be the nucleophile I HBr is a strong acid so we would expect it to act as the electrophile Therefore ethylene must be the nucleophile initially I The Tcelectrons of ethylene can react with the partially positive H in HBr I The H is added to one of the alkene carbons The other carbon now has a positive charge carbocation The other product is bromide I In the second step the carbocation is now our electrophile and bromide is the nucleophile A lone pair on bromide is used to make a bond with the carbocation to give bromoethane This reaction can occur with any alkene We39ll talk more about this in the next chapter 56 Drawing Curved Arrows in Polar Reaction Mechanisms O The arrow is drawn from the electron source nucleophile to the electrophile 39 The nucleophile must have a pair of electrons available These can be a lone pair or a Tcelectron pair 39 The electrophile must be able to accept electrons It could have a incomplete octet carbocation or boron or be the positive end of a polar bond In this case the bond must be broken The nucleophile can be negatively charged or neutral The charge will increase by 1 in the reaction The electrophile can be positive or partially positive The overall charge on the electrophile species will decrease by l The octet rule must be followed In making a new bond the normal valence cannot be exceeded If the electrophile already has a full valence then it must break a bond as the new bond is being formed If the electrophile is below its valence then no bond breaking is required 57 Reaction Equilibria Rates and Energy Changes 0 All reactions are equilibria in principle meaning that they can proceed forward and back 39 The equilibrium is defined by the equilibrium constant K which is the ratio of the product concentrations over the reactants 39 For the reaction of ethylene with HBr K 71 X 107 Thus the reaction proceeds to 9999999 completion For all practical purposes the reaction goes to completion and there is no ethylene or HBr left Reactions for which the products are favored have K values gt1 while those that are disfavored have K values lt l The value of K is determined by whether the reaction is energetically favorable or not 39 Gibbs free energy change AG is used to determine the energy ow of a reaction 39 Energetically favorable reactions have negative AG values and are called exergonic 39 Energetically unfavorable reactions have positive AG values and are called endergonic 39 AG RTlnK so the free energy change is directly related to K AG is made up of two energy terms AG AH 7 TAS 39 AH enthalpy This is the heat change in a reaction 39 AS entropy This is the change in disorder A positive AS is favored which is why AS is subtracted from AH If a reaction is endergonic it means that it is favorable for it to occur That does not mean the reaction will occur in a reasonable time I The rate of the reaction depends on the activation energy This is the change in energy going from the starting point to the highest energy point along the reaction This point is known as the transition state The higher the transition state energy the slower the reaction I 58 Describing ReactionsiBond Dissociation Energies o The bond dissociation energy is the amount of energy required to homolytically cleave a bond Bond cleavage is always endothermic so the BDE is a positive value 0 The overall AH of a reaction can be determined by the change in bond energies I AH energy of bonds broken 7 energy of bonds formed I If the formed bonds are stronger than the broken bonds the reaction will be exothermic I 59 Describing ReactionsiEnergy Diagrams o The energetic path that a reaction takes is represented using a reaction coordinate diagram I The vertical aXis represents energy I The horizontal aXis the reaction progress ie how far the reaction has proceeded I Thus the energy is plotted as a function of reaction progress I In an endergonic reaction the product will be at higher energy than the starting material I In an exergonic reaction the product will be at lower energy than the starting material I The reaction energy will increase initially even for an exergonic reaction reach a peak and then decrease to the product I The structure at the peak of the curve is called the transition state This species is the least stable between the starting material and product It has no lifetime I The difference in energy between the starting point and the transition state is the activation energy AG1 I The rate of the reaction is directly proportional to the activation energy A large activation energy leads to a slow reaction while a small activation energy gives a fast reaction I Very fast reactions have activation energies lt 80 ldmol while reactions that are slow at room temperature hours to days have activation energies of 100 ldmole or higher I 510 Describing aReactions Intermediates o For reactions with more than one step in the mechanism there will intermediate species that are formed I Intermediates are different from transition states because they exist with a measurable lifetime 39 On a reaction coordinate a reaction intermediate will be a valley between the transition states of each step 39 Each step of the mechanism will have a transition state 39 There will be one fewer intermediate than the number of steps in the reaction 0 For a multistep reaction the slowest step will be the one with the highest transition state energy
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