Lecture Slides on an Overview Organic Chemistry | CHEM 2010, Study notes of Organic Chemistry

Download Lecture Slides on an Overview Organic Chemistry | CHEM 2010 and more Study notes Organic Chemistry in PDF only on Docsity! 1 5. An Overview of Organic Reactions Based on McMurry’s Organic Chemistry, 6th edition, Chapter 5 2 5.1 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 An Elimination Reaction it i This one __ —— \ _ / _} ... gives these reactant... a i ° re, + H—Br two products. H H H H Bromoethane Ethylene (an alkyl halide) (an alkene) ©2004 Thomson - Brooks/Cole  _" Substitution Reaction i i These two Light ... give these reactants... H 7 H+Cl—Cl H 1 cl + H—Cl two products. H H Methane Chloromethane (an alkane) (an alkyl halide) ©2004 Thomson - Brooks/Cole  4 A Rearrangement Reaction CH,CH, H H.C H \ / Acid catalyst \ g ff \ / \ H H H CH; 1-Butene 2-Butene ©2004 Thomson - Brooks/Cole 10 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 11 Homogenic Formation of a Bond  One electron comes from each fragment  No electronic charges are involved Homogenic Formation of a a + hk 12 15 Indicating Steps in Mechanisms  Curved arrows indicate breaking and forming of bonds  Arrowheads with a “half” head (“fish-hook”) 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 a ?o Making A‘ +-B ——> A:B Homogenic bond making (radical) (one electron donated by each fragment) Heterogenic bond making (polar) +4 -D- : Av +:BT —> A: B (two electrons donated by one fragment) ©2004 Thomson - Brooks/Cole 16 17 Homolytic Breaking of Covalent Bonds  Each product gets one electron from the bond 20 Radicals  Alkyl groups are abbreviate “R” for radical  Example: Methyl iodide = CH3I, Ethyl iodide = CH3CH2I, Alkyl iodides (in general) = RI  A “free radical” is an “R” group on its own:  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 21 5.3 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 add to an alkene to give a new radical, causing an addition reaction 4 Radical Substitution Unpaired electron Unpaired electron Cm ( Rad: + aR —> Rad:A + :B Reactant Substitution Product radical product radical ©2004 Thomson - Brooks/Cole 22 25 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 HCl and CH3.  Termination – combination of two radicals to form a stable product: CH3. + CH3.  CH3CH3 26 Intitiation mae opagation “(MD (a) :Cl: + HiCH, —> H:Cl: +-CH, ee i 2 ee po (b) ‘CH, + CCl —> :Cl:CH, + :Cl- ©2004 Thomson - Brooks/Cole 27 30 Electronegativity of Some Common Elements  Higher numbers indicate greater electronegativity  Carbon bonded to a more electronegative element has a partial positive charge (+) oe Olarity Chloromethane ©2004 Thomson - Brooks/Cole H/ “H Methyllithium 31 Polarity is affected by age structure changes: SO: I Methanol—weakly electron-poor carbon ©2004 Thomson - Brooks/Cole H re —T. C H/ “HH H ‘A> H. + .H N67 £ H7 “H H 6+ Protonated methanol— strongly electron-poor carbon 32 35 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 36 Generalized Polar Reactions  An electrophile, an electron-poor species (Lewis acid), combines with a nucleophile, an electron-rich species (Lewis base)  The combination is indicated with a curved arrow from nucleophile to electrophile Polar Reactions: This curved arrow shows that electrons move from :B~ to A*. aU NS At ot S © —> A—B The electrons that moved Electrophile Nucleophile from :B~ to Atend up here (electron-poor) (electron-rich) in this new covalent bond. ©2004 Thomson - Brooks/Cole a, CH,OH AICla~ CH3* (of sane MgBrt CH 4 Water asa Water as an nucleophile electrophile ©2004 Thomson - Brooks/Cole 37 40 5.5 An Example of a Polar Reaction: Addition of HBr to Ethylene 41 5.5 An Example of a Polar Reaction: Addition of HBr to Ethylene  HBr adds to the  part of C-C double bond  The  bond is electron-rich, allowing it to function as a nucleophile  H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophileH Br  4 Addition of HBr to Ethylene O+ y—Br =< 42 The electrophile HBr is attacked by the 7 electrons of the double bond, and a new C-H ¢ 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 C—Br o bond and yielding the neutral addition product. © 2004 Thamson/Brooks Cole co -H qoc CS ?Br? } H-~nt c—C. a \ Vu Carbocation intermediate Br H \ vs -C—C.. Hd “H H H 45 46 5.6 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 47 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 Rule 2: Nu: can be ae eative or neutral Negatively charged atom Neutral _ WL, Ae ¥ \ .. = —O: + H—Br: —~> aa + ?Br: ©2004 The ~ Brooks/Col A Neutral Positively charged atom c=c + iN — +C—C—H + :Br? / \ / | H H H H ©2004 Thomson - Brooks/Cole 50 Rule 3: E can be positive or neutral Positively = atom Neutral cs ~ Ng I “436 _ | C=N: — H” ~H H/ ~C ©2004 Thomson - Brooks/Cole Stable, negatively Neutral charged ion s a + H—Bi ‘oe H + :Br? = —Br: ——=> +c) —C— : : / \ ee f oe H H ‘ar H tt ©2004 Thomson - Brooks/Cole 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 H 4 (FP Ma \ | p= So 7 e# + :Bri aH s H H ©2004 Thomson - Brooks/Cole This carbon already has eight electrons. When another electron pair moves to the carbon from CN, an electron pair in the C=O bond must leave. ©2004 Thamson - SrooksiCole 52 55 5.7 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. aA + bB cC + dD Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b 56 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 product(s)  A value of Keq less than one indicates that at equilibrium most of the material is present as the reactant(s) 4 For example: H,C=CH, + HBr => [(CH,CH,Br] Key = _{HBr][H,0=-CH,] ©2004 Thomson - Brooks/Cole = 7.5 X 10° CH,CH,Br 57 60 Numeric Relationship of Keq and Free Energy Change  The standard free energy change at 1 atm pressure and 298 K is Gº  The relationship between free energy change and an equilibrium constant is:  Gº = - RT lnKeq where  R = 1.987 cal/(K x mol) (gas constant)  T = temperature in Kelvins  ln = natural logarithm 61 Changes in Energy at Equilibrium  Free energy changes (Gº) can be divided into  a temperature-independent part called entropy (Sº) that measures the change in the amount of disorder in the system  a temperature-dependent part called enthalpy (Hº) that is associated with heat given off (exothermic) or absorbed (endothermic)  Overall relationship: Gº = Hº - TSº TABLE 5.2 Explanation of Thermodynamic Quantities: AG° = AH® — TAS®° Term Name Explanation AG® Gibbs free-energy change AH? Enthalpy change AS° Entropy change The energy difference between reactants and products. When AG* 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 difference in strength between the bonds broken in a reaction and the bonds formed, When AH”° 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° is negative, disorder decreases; when AS° is positive, disorder increases. ©2004 Thomson - Brooks/Cole 62 65 Calculation of an Energy Change from Bond Dissociation Energies 66 5.9 Describing a Reaction: Energy Diagrams and Transition States 67 5.9 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 (G‡) 70 First Step in the Addition of HBr  In the addition of HBr the transition- state structure for the first step  The  bond between carbons begins to break  The C–H bond begins to form  The H–Br bond begins to break 4 Energy Diagram for step 1 Activation energy | AG AG Energy Reactants H,C—=CHy, + HBr aoe ition state Carbocation product CH3CH, Br- Reaction progress ————> © 2004 Thomson/Brooks Cole 71 72 First Step in the Addition of HBr 75 Carbocation Intermediate Reactions with Anion  Bromide ion adds an electron pair to the carbocation  An alkyl halide produced  The carbocation is a reactive intermediate 76 Reaction Diagram for Addition of HBr to Ethylene  Two separate steps, each with a own transition state  Energy minimum between the steps belongs to the carbocation reaction intermediate. First transition state Carbocation intermediate Second transition state AG;* Energy CH,CH,Br Reaction progress ————> ©2004 Thomson - Brooks/Cole Enzymes Change a echanisms: Energy Uncatalyzed ee Enzyme catalyzed © 2004 Thomson/Brooks Cole Reaction progress —————> 80  4 Explosives: Nitroglycerine H H —— ——" ee +3HNO, —2=4> hone, + 3H,0 =i on oe : : Glycerin Nitroglycerin ©2004 Thomson - Brooks/Cole 81 IVES. Trinitrotoluene (TNT) ©2004 Thomson - Brooks/Cole CHLONO, O,NOCH, # —CH,ONO, CH,ONO, Pentaerythritol tetranitrate (PETN) 82