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Unformatted text preview: Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 Frontier Orbitals: The Importance of the HOMO and LUMO • It turns out that the interaction between orbitals that are close in energy is more important than the interaction between orbitals that are far apart in energy. Why is this the case? • Because of the above observation, we can understand most of the reactivity of organic molecules by examining only a small number of orbitals, known as the frontier orbitals. These are: HOMO (Highest Occupied Molecular Orbital) LUMO (Lowest Unoccupied Molecular Orbital) Show the interaction between the HOMO and the LUMO of molecules A and B. Why is this interaction the single most important orbital interaction for these species? Molecule A Molecule B unfilled orbitals filled orbitals Reading: Supplemental Handout, Section 2.4 10 32 Chemistry S-20ab Week 1 Finding the HOMO and LUMO • Given the order of the energies of molecular orbitals of organic compounds, we can make some simple generalizations that will help us locate quickly the HOMO and LUMO for a given molecule. What are those generalizations? What should we look for? — !* C–H — !* C–C — !* C–X — "* C=C — "* C=X — C nonbonding orbital (lone pair OR carbocation) — X nonbonding orbital (lone pair) — " C=C — " C=X — ! C–H — ! C–C — ! C–X • Using those guidelines, find the HOMO and LUMO for the following species: O H3C Br H 3C CH3 . HOMO: LUMO: Reading: Supplemental Handout, Section 2.4 33 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 The Shapes of Frontier Orbitals: Overlap Matters • Draw “cartoon orbitals” to represent the shapes of the specified orbitals in the following molecules: the HOMO of CH3OH the HOMO of ethylene (H2C=CH2) the LUMO of the tert-butyl carbocation ((CH3)3C+) the LUMO of methyl bromide (CH3Br) the LUMO of formaldehyde (H2C=O) Reading: Supplemental Handout, Section 2.4 12 34 Chemistry S-20ab Week 1 Molecules with Several Possible Frontier Orbitals • Most organic molecules have several possible HOMO’s and several possible LUMO’s. Consider the following molecules. Why can’t we simply specify a single HOMO or a single LUMO for these species? Identify all the possible HOMO’s and LUMO’s in these species. O CH3 O O H Reading: Supplemental Handout, Section 2.4 35 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 Arrows: Non-Bonding HOMO + Non-Bonding LUMO • For the following species: 1) 2) 3) 4) + H3C CH3 Identify the possible HOMO’s and LUMO’s Select the likely nucleophile and electrophile from these two species Show how these species will react using curved arrows Predict the immediate product of that reaction CH3 Cl Reading: Supplemental Handout, Section 2.5 14 36 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 Arrows: Non-Bonding HOMO + Antibonding LUMO • For the following species: 1) 2) 3) 4) Identify the possible HOMO’s and LUMO’s Select the likely nucleophile and electrophile from these two species Show how these species will react using curved arrows Predict the immediate product of that reaction H C H H Br HO + Reading: Supplemental Handout, Section 2.5 15 37 Chemistry S-20ab Week 1 Arrows: Bonding HOMO + Antibonding LUMO • For the following species: 1) 2) 3) 4) CH3 H3C CH3 CH3 Identify the possible HOMO’s and LUMO’s Select the likely nucleophile and electrophile from these two species Show how these species will react using curved arrows Predict the immediate product of that reaction + H Cl Reading: Supplemental Handout, Section 2.5 39 38 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 Brønsted-Lowry Acids and Bases • One of the most useful theories of acidity and basicity is the Brønsted-Lowry Theory. In this theory, acids and bases are defined as follows: A Brønsted Acid is a species that can donate a proton (H+). A Brønsted Base is a species that can accept a proton. • Every Brønsted acid, therefore, must have a conjugate base. What are some examples of Brønsted acids? For each one, identify its conjugate base. Brønsted Acid Conjugate Base • When a Brønsted acid reacts with a Brønsted base, a proton is transferred from the acid to the base. These reactions are called proton-transfer reactions. Give some examples of proton-transfer reactions using the above list of acids and bases. Reading: Section 3.4 17 39 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 Strengths of Acids and Bases: Ka and pKa • All proton-transfer reactions are reversible (at least in principle). In general, though, one direction of the reaction will predominate over the other (that is, the proton transfer will tend to be unequal). For instance, consider the reaction between the ammonium ion and the hydroxide ion. Do you have a sense of whether this reaction proceeds mainly to the right or mainly to the left? • Because so much chemistry takes place in water, we typically use water as a “standard reference” for the strengths of acids and bases. For any Brønsted acid HA, the reaction with water will reach some equilibrium: The equilibrium constant of this reaction is Ka = * Just as “pH” means “the negative log of the H+ concentration,” we use the term “pKa” to refer to “the negative log of the Ka” How can we express that fact mathematically? • Fill in the blanks: “A strong acid will have a ______________________ pKa” “A strong base will have a conjugate acid with a _____________________ pKa” Reading: Section 3.4 18 40 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 pKa Values for Common Acids • Here are some important pKa values. You should memorize these values, at least to the nearest 5 pKa units: Conjugate Acid CH4 NH3 HC H3C CH OH pKa 48 35 24 16 16 9 9 Conjugate Base H2O NH4+ HCN O H3C OH 5 H3O+ –2 HCl • • –7 For each of the above acids, fill in the conjugate base. We typically define a strong acid as an acid that is at least as strong as H3O+, and a strong base as a base that is at least as strong as OH–. Identify the strong acids and strong bases in the above list. Reading: Section 3.4 19 41 Chemistry S-20ab Chemistry E-2a: Lecture 2 Week 1 September 25, 2008 Using pKa Values to Predict Acid-Base Equilibria • Consider, again, the reaction between the ammonium ion and the hydroxide ion. What will be the predominant species present in this system at equilibrium? • Can you calculate the equilibrium constant for this reaction? How? • What does the value of the equilibrium constant tell you about the concentrations of reactants and products at equilibrium? Reading: Section 3.4 20 42 Chemistry S-20ab Week 1 Factors That Influence Acidity: The Main Atom • Examine the pKa’s for each of the following pairs of acids, and explain why one acid is stronger than the other. H OH H F +15.7 +3.2 H OH H O H2 +15.7 –1.7 H3C H2C N H H H H3C C N H +10 –10 • Compare these trends with the “low-energy trifecta” you saw earlier.... any similarities? • Now examine the four acids from the upper-right corner of the periodic table. Do the strengths of these acids follow the trend you would expect? Why or why not? H OH H F +15.7 + 3 .2 H SH H Cl +7.0 –7 43 Reading: Section 3.6 Chemistry S-20ab Week 1 Factors That Influence Acidity: Adjacent Groups • The acidity of a particular proton can be influenced by adjacent or nearby groups in the molecule. Can you explain the difference in the following pKa’s? O H3C C O H FH2C C O H O 4.76 2.66 • Whenever a molecule exhibits resonance, and the resonance allows charge to be delocalized, then the charged structure will be more stable than a comparable structure that does not have the delocalized charge. Thus, resonance can stabilize either the conjugate acid or the conjugate base, whichever is charged. Let’s look at some examples: EXAMPLE 2: EXAMPLE 1: O H3C C O H N H 4.76 5.25 CH3 H3C C O H H3C N H3C N H 16.5 9.2 Reading: Section 3.6 44 Chemistry S-20ab Week 1 Frontier Orbitals of Proton-Transfer Reactions • We can, of course, look at proton-transfer reactions in terms of the HOMO and LUMO involved in the reaction. Can you find the HOMO and LUMO of the following protontransfer reactions? H O H3C CH3 OH2 + H Cl H3C CH3 + Cl H O O + H3C CH3 H3C C C H3C CH3 + H3C C CH • What generalization can we make about the LUMO in any proton-transfer reacton? Reading: Supplemental Handout, Section 2.6 45 Chemistry S-20ab Week 1 What!s Your Role: Acid or Electrophile? • In one of the following reactions, acetone ((CH3)2C=O) plays the role of an acid; in the other it plays the role of an electrophile. Which is which, and why? Can you draw the curved arrows and identify the HOMO and LUMO of each reaction? O O OH CH3 + H3C CH3 HO H3C O H3C CH3 O + HO + H3C CH2 H 2O Reading: Supplemental Handout, Section 2.7 46 Chemistry S-20ab Week 1 What!s Your Role: Base or Nucleophile? • In one of the following reactions, acetone ((CH3)2C=O) plays the role of a base; in the other it plays the role of a nucleophile. Which is which, and why? Can you draw the curved arrows and identify the HOMO and LUMO of each reaction? O OH + H3C CH3 H 3O H3C CH3 + H 2O O CH3 O H3C CH3 + H3C CH3 H3C O CH3 CH3 + H3C O CH3 Reading: Supplemental Handout, Section 2.7 47 Chemistry S-20ab Chemistry E-2a: Lecture 3 Week 1 October 2, 2008 Pop Quiz (don’t worry, this will not be graded!) • For each of the following molecules: • Identify the HOMO • Identify the LUMO • Name the functional group O C H N O O O O O Br O 4 48 Chemistry S-20ab Chemistry E-2a: Lecture 3 Week 1 October 2, 2008 An Introduction to Alkenes • Draw the skeletal structures of all hydrocarbons with the formula C4H8. What can we note about these structures? • Some of the above structures contain a C=C double bond. What orbitals are involved in a C=C double bond? • What do we call different molecules that have the same molecular formula? Reading: Section 4.1 6 49 Chemistry S-20ab Chemistry E-2a: Lecture 3 Week 1 October 2, 2008 Naming Alkenes • Each of the following names describes an alkene. However, these names are not the correct names for these molecules. Draw the skeletal structure of the molecule and provide a correct systematic name. 4-ethyl-4-pentene 2-vinylpropane allylethane Reading: Section 4.2 7 50 Chemistry S-20ab Week 1 The Problem with Cis and Trans • How would you name the following alkenes? A: B: • We need better rules: The E/Z rules for naming alkanes require that we determine the priority of the four groups attached to the alkene, as follows: Step 1. Greater atomic number = higher priority. If atomic number is the same, heavier isotope = higher priority. If there is a tie, go on to Step 2. If the atoms are the same, consider the atoms that are attached. (For carbon, there will be 3 other atoms). List those atoms in descending order of priority, and make a pairwise comparison between the two sets. One “wins” at the first point of difference. If there is a tie, go on to Step 3. If the two sets of atoms are the same, follow the path of highest priority and move one atom away along each path. Then go back to Step 2, but with the new atoms. Continue in this fashion until one of the branches ultimately “wins” over the other. If the high-priority groups are on the same side of the double bond, then the configuration is Z (German “zusammen” = together). Otherwise, the configuration is E (German “entgegen” = opposite). Step 2. Step 3. Step 4. This is not any more difficult that figuring out NFL playoff schedules! They key concept is the application of specific rules as “tiebreakers.” Let’s look at some examples: • Determine whether the double bonds in the following alkenes has the E or Z configuration: OH Reading: Section 4.2 51 ...
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This note was uploaded on 07/21/2010 for the course CHEM S20ab taught by Professor Mccarty during the Summer '10 term at Harvard.

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