Organic Chemistry Chapter 6 PDF

Organic Chemistry Chapter 6 1  Organic Reactions Reactions occur by movement of electrons Look for “reaction sites” areas where molecules can interact General types of reactions: Substitution Elimination Addition Understand how to “...

Organic Chemistry Chapter 6 1  Organic Reactions Reactions occur by movement of electrons Look for “reaction sites” areas where molecules can interact General types of reactions: Substitution Elimination Addition Understand how to “write & read” chemical reactions 2  Organic Reactions Understand how to “write & read” chemical reactions Parameters of the reaction are shown above and below the arrows; such as conditions, reactants, solvents, etc. Organic reactions may not show by-products or other products; they may specifically only show the direction of the reaction….starting reagent and product They may not be balanced (which still needs to be done in the actual experiment….) They may show a specific order …stepwise experiment… 3  Organic Reactions Generally: 1. Look for the functional groups in the reagent 2. Then look at the product and see: if there has been a change in the functional group; ex. Alcohol to Halide if the molecule has dramatically changed ex. Combustion 3. Look at the chemicals/ reagents above/below the arrows Or determine what chemicals/ reagents can cause the change 4  Writing Equations for Organic Reactions Example H2, Ni CH3CH3 + Br2 CH3CH2Br 5  Writing Equations for Sequential Reactions steps are numbered above or below the reaction arrow to show sequential steps 6 Arrows Used in Organic Reactions 7  Reactions General types of reactions: Substitution Elimination Addition 8  Substitution Reactions Substitution 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, Y replaces Z on a carbon atom. 9  Substitution Reactions one σ bond breaks & another forms at the same carbon atom. Generalization: a more electronegative element will substitute for a heteroatom on the carbon 10  Substitution Reactions one σ bond breaks & another forms at the same carbon atom Substitution reactions can also occur on aromatic rings and replace a Hydrogen…… 11  Elimination Reactions Elimination: elements of the starting material are “lost” and a π bond is formed. Two bonds are broken, and a bond is formed between adjacent atoms. 12  Elimination Reactions The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon. Notice how the reaction is written 13  Elimination Reactions The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon. Notice how the reaction is written 14  Addition Reactions Addition: elements are added to the starting material. Addition to a multiple bond to form 2 single bonds 15  Addition Reactions X and Y are added to the starting material. A π bond is broken and two σ bonds are formed. 16  Addition Reactions X and Y are added to the starting material. A π bond is broken and two σ bonds are formed In this case, water is added to the double bond (as X = H; Y = OH) 17  Relationship of Addition and Elimination Reactions Addition and elimination reactions can complement each other and can be reversible. 18  Relationship of Addition and Elimination Reactions Example: addition in one direction and elimination in the other 19  Reaction Mechanisms Mechanisms are used to study a reaction: How does it proceed… Which bonds are broken…. Which bonds are formed…. What components of the reaction are involved…. These give insight as to how the reaction proceeds and how a product is formed….. allows us to speculate on new reactions by understanding what has occurred in other reactions 20  Reaction Mechanisms Types of reaction mechanisms One-step reaction is called a concerted reaction. a starting material is converted directly to a product. A B A stepwise reaction involves more than one step. A intermediates B Intermediates can involve many steps 21  Reaction Mechanisms Example of concerted reaction One-step reaction is called a concerted reaction. a starting material is converted directly to a product. A B * * * * * * * * 22  Reaction Mechanisms Example: stepwise reaction: involving unstable reactive intermediates A stepwise reaction involves more than one step. A intermediates B Intermediates can involve many steps 23  Bonds are only broken two ways Bonds cleave: Homolysis: Homolytic Cleavage electrons are divided equally between the atoms of the bond Heterolysis: Heterolytic Cleavage electrons are divided between the two atoms unequally; the more electronegative atom gets them 24 Bond Breaking (Cleavage) 25  Bond Breaking (Cleavage) This is an intermediate known as a free radical….unstable, reactive 26  Bond Breaking (Cleavage) If the atom has a positive charge; then it is a carbocation If the atom has a negative charge; then it is a carbanion Both of these are unstable intermediates 27  Reactive Intermediates Resulting from Breaking a C-Z Bond (Z represents various functional groups) Figure 6.2 28  Intermediates Behavior Radicals and carbocations are electrophiles because they contain an electron-deficient carbon. Carbanions are nucleophiles because they contain a carbon with a lone pair. 29  Bond formation Bond formation occurs in two different ways. Two radicals can each donate one electron to form a two-electron bond. Two ions with unlike charges can come together to form the resulting two- electron bond. Bond formation always releases energy (Exothermic) Notice the arrow heads; “half” for one electron; “full” for 2 electrons 30  Bond Dissociation Energy Bond dissociation energy is the energy needed to homolytically cleave a covalent bond. H-H H.. H CH3-Cl. CH3. Cl O-O O.. O This is the C-Cl bond that is broken bond breaking requires energy: ΔHo is positive….energy “goes in”; absorbed dissociation energies with positive values; homolysis is always endothermic. bond formation releases energy; always exothermic. ΔHo is negative 31  Bond Dissociation Energy (repeat) Bond dissociation energy is the energy needed to homolytically cleave a covalent bond. Energy required to break a bond is equal to the energy required to form the bond but opposite in sign…… Enthalpy change (heat of reaction), ΔHo ΔHo = the energy that is released or absorbed in a reaction ΔHo is positive when energy is absorbed…..endothermic ΔHo is negative when energy is released…..exothermic 32  Example…Energy Associated with the 𝐇𝟐 Bond H—H bond requires +435 kJ/mol to cleave releases −435 kJ/mol when formed. 33 Notice: The cleavage of the same bond on different molecules may have a different heat of reaction CH3-H 435 kJ/mol CH3CH2-H 410 kJ/mol CH3-Cl 351 kJ/mol CH3CH2-Cl 339 kJ/mol Fortunately: these have been calculated……..so we can have an “idea” of what will happen in a reaction…… 34 Bond Dissociation Energies 35 So what does this mean: Bond dissociation energies are equivalent to comparing bond strength. More energy is needed to dissociate a strong bond Bond dissociation energies decrease down a column of the periodic table. Shorter bonds are stronger bonds as seen in the following table: Intuitively makes sense Electrons are held “tighter” if nuclei are closer in bond: C-F requires more energy than C-I; see table 6.2 36 Basically: Reactions result in bond breaking and bond forming Bond dissociation energies can be used to calculate enthapy changes in a reaction What this means is for the reaction: 1. ΔHo overall:positive; a. bonds broken in the starting material are stronger than the bonds formed in the product; b. , more energy is needed to break bonds than is released in forming bonds. 2. ΔHo overall: negative c. bonds formed in the product are stronger than the bonds broken in the starting material d. more energy is released in forming bonds than is needed to break bonds. 37 What this means is for the reaction: Always look at the bonds being broken and formed in a balanced equation Bond dissociation energies (equation); 1. are an approximation because they are calculated from gas phase reactions; 2. Do not indicate a mechanism; just the overall energay change 38 Always look at the bonds being broken and formed in a balanced equation Sample Problem 6.3 Only 1 bond formed in each Only 1 bond broken in water 39 Example: problem 6.8b Calculate ΔHo overall (multiple bonds) 2 CH3CH3 + 7O2 -------- 4CO2 + 6H2O 1. Determine ΔHo for bonds that are broken 12 C-H bonds from 2 CH3CH3 = +410 x 12 = 4920 kJ/mol 7O2 bonds = + 497 x 7 = 3479 kJ/mol 2 C-C bonds from 2 CH3CH3 = + 368 x 2 = 736 kJ/mol 2. Determine ΔHo for bonds that are formed 8 C=O from 4CO2 = -535 x 8 = -4280 kJ/mol 12 O-H from 6H2O = -498 x 12 = -5976 kJ/mol 3. Add 1 & 2; ΔHo overall = -1121 kJ/mol overall exothermic 40  Kinetics and Thermodynamics Thermodynamics describes: how the energies of reactants and products compare what the relative amounts of reactants and products are at equilibrium. Kinetics describes reaction rates (how quickly reactants are converted to products). equilibrium must favor product formation; and the reaction rate must be fast enough to form them in a reasonable time. 41  Kinetics and Thermodynamics equilibrium constant, Keq, relates the amount of starting material and product at equilibrium. 42 What does this mean: If Keq>1, then equilibrium favors the products. equilibrium lies to the right as the equation is written. If Keq1, log Keq is positive, energy is released. Equilibrium favors the products. When Keq