Chem 227 1st Edition Lecture 9 Outline of Last Lecture I. 5.10 Chirality at Nitrogen, Phosphorus, and SulfurII. 5.11 ProchiralityIII. 5.12 Chirality in Nature and other Chiral environmentsIV. 6.1 Kinds of Organic ReactionsV. 6.2 How organic reactions occur: mechanisms and stepsOutline of Current Lecture I. 6.3 Radical ReactionsII. 6.4 Polar Reactionsa. Polarizabilty and Polar reactionsb. Electrophiles and nucleophilesIII. 6.5 An example of a Polar reaction: Addition of HBr to EthyleneIV. 6.6 Using curved arrows in polar reaction mechanismsV. 6.7 Describing a Reaction: Equilibria, Rates, and Energy changesa. Numeric Relationship of Keq and Gb. Thermodynamic quantitiesVI. 6.8 Bond Dissociation EnergiesVII. 6.9 Describing a Reaction: Energy Diagrams, # of steps, Transition states and intermediatesVIII. 6.10 IntermediatesIX. 6.11 A comparison between biological Reactions and Laboratory ReactionsCurrent Lecture6.3 Radical Reactions- Not as common as polar reactions- Radicals react to complete electron octet of valence shell- A radical can break a bond in another molecule, giving substitution- A radical can add to an alkene to give a new radical- Initiation – hemolytic formation of two reactive species with unpaired electronsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Propagation – reaction with another molecule to generate a new radical- Termination – combination of two radicals to form a non radical product: CH3 + CH3  CH3CH3 (making a nonreactive product)6.4 Polar reactions- Molecules can contain unsymmetrical electron distributions. This causes ‘delta +’ and ‘delta – ‘ zones- Helpful to know the difference in the electronegativity of different functional groups in order to know how this is going to worko What can we expect from reaction of a Grignard reagent with an aldehyde? The oxygen in the aldehyde would have ‘delta –‘ and the carbon a ‘delta +’ The Mg in the Grignard reagent ( R’-MgX )would have ‘delta +’ and the carbon would have ‘delta –‘  Then react with H3O+ for protonation – you get an alcohol- We can have an educated guess on what will happen when you react one functional group with anotherPolarizability and Polar reactions- Polarization is a change in electron distribution as a response to change in electronicnature of the surroundings- Polarizability is the tendency to undergo polarization - An electrophile, an electron-poor species, combine with a nucleophile, an electron-rich species- An electrophile is a lewis acid- A nucleophile is a lewis base- Very important concept in O chemSome nucleophiles and electrophiles (Red= electron rich, Blue= electron poor)6.5 An example of a Polar reaction: Addition of HBr to Ethylene- HBr adds to the pi part of a C-C double bond- The pi bond is electron-rich, allowing it to function as a nucleophileo The electrons used in the formation of the bond will be provided by ethylene- H-Br is electron deficient at the H since Br is more electronegative, making HBr an electrophileMechanism of Addition of HBr to Ethylene- Good exercise to understand mechanism:o Describe with words what is happening in the diagram on the lefo Or Produce a picture consistent with the description on the righto Or not having the arrows and being able to put them in the right place- PRACTICE!!6.6 Using curved arrows in polar reaction mechanisms- Curved arrows are a way to keep track of changes in bonding in a reaction- The arrows track “electron movement”- Review handout on “use of curved arrow” !!!6.7 Describing a Reaction: Equilibria, Rates, and Energy changes- Reactions may go either forward or backward to reach equilibrium- If Keq is 10, most of the material is present as products- If Keq is .1, then most of the material is present as the reactantFree Energy and EquilibriumThe ratio of products to reactants is controlled by their relative Gibbs free energy (G)- This energy is released on the favored side of equilibrium- If Keq >1, energy is released (exergonic)o If the products are favored- If Keq <1, energy is absorbed (endergonic)o If the reactants are favoredNumeric Relationship of Keq and G- 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: o Gº = - RT ln Keq where o R = 1.987 cal/(K x mol)o T = temperature in Kelvino ln Keq = natural logarithm of Keq o OWL calculations: determination of spontaneity of a reaction based on  H and S data G = H - TSo MAKE SURE everything is in the same KJ/mol unitThermodynamic quantities6.8 Bond Dissociation Energies- One of the magnitudes we will need for the calculations from the previous section (6.7)- Bond Dissociation energy (D): amound of energy required to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25ºC- The energy is mostly determined by the type of bond, independent of the moleculeo The C-H bond in methane requires a net energy input of 106 kcal/mol to be broken at 25 ºC. o Table 6.3 lists energies for many bond types- Changes in bonds can be used to calculate net changes in heat6.9 Describing a Reaction: Energy Diagrams, # of Steps, Transition States, and Intermediates- 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‡)- A diagram to show all the characteristics/information that can be found on an energy diagram: (this is not related to previous reaction shown)- First step in Additiono Transition state structure for the first step of HBr addition to an alkeneo The  bond between carbons begins to break The C–H bond begins to formo The H–Br bond begins to break6.10 Intermediates- If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product (“intermediates”)- Each step has its own free energy of activation6.11 A comparison between biological Reactions and Laboratory Reactions- Laboratory reactions are usually carried out in organic solvent- Biological reactions in aqueous medium inside cells- They are promoted by catalysts that lower the activation barrier- The catalysts are usually proteins, called enzymes- Enzymes provide an alternative mechanism that is compatible with the