Organic Chemistry I
Organic Chemistry I CHEM 2010
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Date Created: 10/11/15
5 An Overview of Organic i Reactions Based on McMurry s OrganC ChemSL734 6th edition Chapter 5 51 Kinds of Organic Reactions In general we look at what occurs and try to learn how it happens What includes reactivity patterns and types of reaction How refers to reaction mechanisms 51 Kinds of Organic Reactions Addition reactions two molecules combine Elimination reactions one molecule splits into two Substitution parts from two molecules exchange Rearrangement reactions a molecule undergoes changes in the way its atoms are connected An Addition Reaction H H III 131139 These two add to give reactants i C C H Br 5 H I ll H this product H H H H Ethylene Bromoethane an alkene an alkyl halide 2004 Thomson v BrooksICole An Elimination Reaction H Br H H This one Base gives these reactant II ClJ ll H 5 C C H Br two products H H H H Bromoethane Ethylene an alkyl halide an alkene 2004 Thomson BrooksCole g A Substitution Reaction H i These two Light give these reactants H C H 01 01 H j Cl H Cl two products H H Methane Chloromethane an alkane an alkyl halide 2004 Thomson BrooksICole A Rearrangement Reaction CH30H2 H H3C H Acid catalyst C C gt C C H H H CH3 lButene 2Butene mson BrooksCole 2004 The 52 How Organic Reactions Occur Mechanisms In a clock the hands move but the mechanism behind the face is what causes the movement In an organic reaction by isolating and identifying the products we see the transformation that has occurred The mechanism describes the steps behind the changes that we can observe Reactions occur in defined steps that lead from reactant to product Steps in Mechanisms A step usually involves either the formation or breaking of a covalent bond Steps can occur individually or in combination with other steps When several steps occur at the same time they are said to be concen ea Types of Steps in Reaction Mechanisms Formation of a covalent bond Homogenic or heterogenic Breaking of a covalent bond Homolytic or heterolytic Oxidation of a functional group Reduction of a functional group 10 Homogenic Formation of a Bond One electron comes from each fragment No electronic charges are involved A 4 A19 Homogenic Formation of a pm A 4 gtr 12 Heterogenic Formation of a Bond One fragment supplies two electrons Lewis Base One fragment supplies no electrons Lewis AL Combination can involve electronic charges Common in organic chemistry e 9 o gt 13 Heterogenic Formation of a 5 Bond X39 e gtr 14 Indicating Steps in Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads with a half head fishhookquot indicate homolytic and homogenic steps called radical processes the motion of one electron Arrowheads with a complete head indicate heterolytic and heterogenic steps called polar processes the motion of an electron pair 5 15 Bond Making AAAB A B Homogenic bond making radical one electron donated by each fragment AG B gt A B Heterogenic bond making polar two electrons donated by one fragment 2004 Thomson BrooksCole 16 Homolytic Breaking of Covalent Bonds Each product gets one electron from the bond M w r Heterolytic Breaking of Covalent Bonds Both electrons from the bond that is broken become associated with one resulting fragment A common pattern in reaction mechanisms As 6 is Bond Breaking AA 13 gt A B Homolytic bond breaking radical one electron stays with each fragment Heterolytic bond breaking polar two electrons stay with one fragment A AB AB39 2004 Thomson BrooksCole 19 Radicals Alkyl groups are abbreviate R for radical Example Methyl iodide CH3I Ethyl iodide CH3CHZI Alkyl iodides in general RI A free radical is an R group on its OWl lI CH3 is a free radical or simply radical Has a single unpaired electron shown as CH3 Its valence shell is one electron short of being complete 20 53 Radical Reactions and How They Occur Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron giving substitution in the original molecule A radical can addto an alkene to give a new radical causing an addition reaction 21 Radical Substitution Unpaired electron Unpaired electron A Rad A33 gt RadiA B Reactant Substitution Product radical product radical 2004 Thomson BrooksCole 22 V Radical Addition Unpaired electron Unpaired electron Rad Km Rad C C 39 Reactant Alkene Addition product radical radical 2004 Thomson BrooksCole 23 Chlorination of methane a radical substitution reaction H H I Light H ll H 01 01 gt H f Cl H Cl H H Methane Chlorine Chloromethane 2034 Thomson BrooksCole 24 Steps in Radical Substitution details in Chapter 10 Three types of steps Initiation homolytic formation of two reactive species with unpaired electrons Example formation of Cl atoms form Cl2 and light Propagation reaction with molecule to generate radical Example reaction of chlorine atom with methane to give HCI and CH3 Termination combination of two radicals to form a stable product CH3 CH3 9 CH3CH325 Light 231 26 Propagation a gt H3l CH3 b 613 2161 gt qnoH3 91 2004 Thomson BrooksCole 27 Termination 1 1 a 321 1 Nil CH3 gt 33I3CH3 Possible termination steps H3O CH3 H3O CH3 2004 Thomson BrooksCole 28 54 Polar Reactions and How They Occur Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom The more electronegative atom has the greater electron density 29 Electronegativity of Some Common Elements Higher numbers indicate greater electronegativity Carbon bonded to a more electronegative element has a partial positive charge 5 30 Polarity 5 5 0 H H H Chloromethane 2004 Thomson BrooksCole 5 UT 8 H H H Methyllithium 31 Polarity is affected by Methanol weakly electronpoor carbon 2004 Thomson BrooksiCole V structure changes A H H O 39M H H H Protonated methanol strongly electronpoor carbon 32 4 TAILE 51 Polarity Patterns in Some Common Functional GrOUps Functional Functinnal Campound gmup Compound group type structure type structure M KM Alcohol F i DH Carbonyl Cl l X if xx r 7 Alkene C 0 Myquot 39W 1 Cm39imxyllc 361d C Symmetrical imnpulm 39H l 54 1 Alkyl halide 39 y airf 39 xx Carimxyhc and chlaride l H 5 Anune IF NHQ H Aid 1 1 quotFquot 9 ny 9 u n H 7 Ether i I Lr IE 39 33 And a few more N 9 393 M5 Nlt le L39 E N Ester H c M Eff C Gngnanl Mggr reagent t lHquotf KELHHE j vquot 5f a Alkyllnhlum l n C I 2004 Thnmson Brnnkstole 34 Polarizability Polarization is a change in electron distribution as a response to change in electronic nature of the surroundings Polarizability is the tendency to undergo polarization Polar reactions occur between regions of high electron density and regions of low electron density 5 391 Because of iodine s high polarizability 5 the carbon iodine bond behaves as if C it were polar 2004 Thomson BrooksCole 35 Generalized Polar Reactions An electrophile an electronpoor species Lewis acid combines with a nucleophile an electronrich species Lewis base The combination is indicated with a curved arrow from nucleophile to electrophile 36 Polar Reactions This curved arrow shows that electrons move from B to A A A 1B A B The electrons that moved Electrophile Nucleophile from B to Aend up here electronpoor electronrich in this new covalent bond 2004 Thomson BrooksCole ngoH Alch CH H okH gtCH339 MgBr CH4 Water as a Water as an nucleophile electrophile 2004 Thomson BrooksCole 37 Electrophiles amp Nuclephiles randnth H3N H25 T T T T 61 Some nuCl39EOPI39LlleS elect 5 39039 Same electmphiles Hg0 EH3 C 5 91 Baron poor 1 r W 2004me Cole 38 Problem 55 BF3 electrophile g or nucleophile BF3 39 55 An Example of a Polar Reaction Addition of HBr to Ethylene H H H Br i CC H Br H H H H Ethylene Hydrogen bromide Bromoethane nucleophile electmphile 2004 Thomson BrooksCole 40 55 An Example of a Polar Reaction Addition of HBr to Ethylene HBr adds to the 7 part of CC double bond The 7 bond is electronrich allowing it to function as a nucleophile HBr is electron deficient at the H since Br is much more electronegative making HBr an electrophile 5H Br lt 41 Addition of HBr to Ethylene iT Br 42 g HBonds as Nucleophiles Hbonding electron pairs can function as Lewis bases Carbon carbon a head Calb0 ncm bon 7r bond stronger less accessible weaker more accessible electrons bonding electrons 2004 TIwmnleole Cole 43 Mechanism of Addition of HBr to Ethylene HBr electrophile is attacked by 7 electrons of ethylene nucleophile to form a carbocation intermediate and bromide ion Bromide adds to the positive center of the carbocation which is an electrophile forming a C Br 6 bond The result is that ethylene and HBr combine to form bromoethane All polar reactions occur by combination of an electronrich site of a nucleophile and an electrondeficient site of an electrophile The electl ophile HBI is attacked by the 7139 electrons of the double bond and a new C H 0r bond is Formed This leaves the other carbon atom with a charge and a vacant p orbital Br donates an electron pair to the positively charged carbon atom forming a CBr Ubond and yielding the neutral addition product a 2004 Thumsuru mols Cole Carbmation intermediate l 45 56 Using Curved Arrows in Polar Reaction Mechanisms Curved arrows are a way to keep track of changes in bonding in polar reaction The arrows track electron movement Electrons always move in pairs in polar reactions Charges change during the reaction One curved arrow corresponds to one step in a reaction mechanism 46 Rules for Using Curved Arrows The arrow goes from the nucleophilic reaction site to the electrophilic reaction site The nucleophilic site can be neutral or negatively charged The electrophilic site can be neutral or positively charged 47 Using Curved Arrows Rule 1 electrons move from gNu oE Electrons usually flow from one of these nucleophiles 2004 Thomson BrooksCole Electrons usually ow to one of these electrophiles 2004 Thomson Brookleole O N E C E CC I Nu Nu Nu Ifquot O quot mid af To f g I 5 C Halogen l KR 39 gr 39 49 Rule 2 Nu can be negative or V neutral Negatively charged atom Neutral m CHS QF H g gt OHS 395 13 H Neutral Positively charged atom H1 m Hl ITI 020 gt C C H 13 H H H H 2004 Thomson BrooksCole 50 Rule 3 E can be positive or neutral Positively charged atom Neutral CW 1 C 6 E N C H N 2004 Thomson BrooksCale Stable negatively Neutral charged ion 0 0 H 339 C I H Bquot Z r gt 39 z I39 H H J H I 2004 Thomson BrooksCole 51 Rule 4 Octet rule This hydrogen already has two electrons When another electron pair moves to the hydrogen from the double bond the electron pair in the H Br bond must leave H H I A C C H1 r 00 Hilir gt H H H H 2004 Thomson BrooksCole H H C l C 6 E N 5 C H H H Ck H N This carbon already has eight electrons When another electron pair moves to the carbon from CN an electron pair in the 00 bond must leave 2004 Thomson BrooksCole 52 Practice Prob 52 Add curved arrows 0 H H W l C 39 H C Br 9 C Brquot quot H30 CH2 l H3O CH2 CH3 H 2004 Thomson Brookleole 53 a Solution f H H30 CH2 2004 Thomson BrooksCole H H30 0 II C CH2 CH3 Br 54 57 Describing a Reaction Equilibria Rates and Energy Changes Reactions can go in either direction to reach equilibrium The multiplied concentrations of the products divided by the multiplied concentrations of the reactant is the equilibrium constant Keq Each concentration is raised to the power of its coefficient in the balanced equation aAbB cCdD Keq ProductslReactants Cc D I I AaB39 55 Magnitudes of Equilibrium Constants If the value of Keq is greater than 1 this indicates that at equilibrium most of the material is present as products A value of Keq less than one indicates that at equilibrium most of the material is present as the reactants 56 For example H20 CH2 HBI CH3CH2BI39 9 1 Thomson Brooks HBr HZCCH2 CH3CH2BI39 75 x 107 57 ree Energy and Equilibrium if The ratio of products to reactants is controlled by their relative Gibbs free energy This energy is released on the favored side of an equilibrium reaction The change in Gibbs free energy between products and reacts is written as AG If Keq gt 1 energy is released to the surrounding exergonic reaction If Keq lt 1 energy is absorbed from the surroundings endergonic reaction 58 Free Energy and Equilibrium Keq gt 1 energy out AGquot negative Keq lt 1 energy in AGquot positive 2004 Thomson BrooksCole 59 Numeric Relationship of Keq and Free Energy Change The standard free energy change at 1 atm pressure and 298 K is AGO The relationship between free energy change and an equilibrium constant is AGO RT aneq where R 1987 calK X mol gas constant T temperature in Kelvins ln natural logarithm 60 Changes in Energy at g Equilibrium Free energy changes AG0 can be divided into a temperatureindependent part called entropy ASO that measures the change in the amount of disorder in the system a temperaturedependent part called enthalpy AHO that is associated with heat given off exothermic or absorbed endothermic Overall relationship AGO AHO TASC 61 TABLE 52 Explanation ofThermodynam ic Quantities AG AH TASquot Term Name Explanation AGC Gibbs freeenergy change AH Enthulpy change ASquot Entropy change The energy difference between reactants and products When AGquot is negative the reaction is exergonic has a favorable equilibrium constant and can occur spon taneously When AG is positive the reac tion is endergonic has an unfavorable equilibrium constant and cannot occur spontaneously The heat of reaction or di Terence in strength between the bonds broken in a reaction and the bonds formed When AHquot is negative the reaction releases heat and is exothermic When AH is positive the reaction absorbs heat and is endothermic The change in molecular disorder during a reaction When AS 1s negatlve disorder a decreases when AS is pOSItlve dlsorder increases 2004 Thomson BrooksCole 62 Ethylene HBr AG 3 448 kJmol AHO 841kJm01 AS Z 0132kJK mol T 298 K HBI 2 3 CH3CH2BI 2004 Thomson BrooksCole AGO AHO TASO 63 58 Describing a Reaction Bond Dissociation Energies Bond dissociation energy D Heat change that occurs when a bond is broken by homolysis The energy is mostly determined by the type of bond independent of the molecule The CH bond in methane requires a net heat input of 105 kcalmol to be broken at 25 OC Table 53 lists energies for many bond types Changes in bonds can be used to calculate net changes in heat 64 Calculation of an Energy Change from g Bond Dissociation Energies H H l H lj H 01 01 gt H 3 C1 H Cl H H Product bonds formed Reactant bonds broken C Cl D 351 kJmol C H D 438 kJmol H Cl D 432 kJmol 01 01 D 243 kJmol Total D 783 kJmol Total D 681 kJmol AH 681 kJmol 783 kJmol 102 kJInol 2004 Thomson BrooksCole 65 59 Describing a Reaction Energy V Diagrams and Transition States Carbocation 2004 Thomson BrooksCole 66 59 Describing a Reaction Energy Diagrams and Transition States The highest energy point in a reaction step is called the transition state The energy needed to go from reactant to transition state is the activation energy AG1 Transition state Carbocation product CHSCHZ Br Activation energy I AG Reaction progress gt AGquot Reactants HZC CH2 HBr 2110 mmmmmmmmmmm Is 67 i Energy Diagram for step 1 Transition state Carbfcation product CchHz Br Activation energy I AG AGquot Reactants H20CH2 HBr Reaction progress gt 2004 Th mmmmmmmm le 68 Eiergy Energy c Reaction progress Reaction progress 2004 Thomson BrooksCole E Reaction progress Reaction progress 69 First Step in the Addition of HBr In the addition of HBr the transition state structure for the first step The 7 bond between carbons begins to break The C H bond begins to form The H Br bond begins to break 70