Organic Chemistry CHM 314 Dr. Laurie S. Starkey, Cal Poly Pomona Structure & Shape of Organic Molecules - Chapter 1 (Wade)

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1 rganic hemistry M 314 Dr. Laurie S. Starkey, al Poly Pomona Structure & Shape of rganic Molecules - hapter 1 (Wade) rganic (living things, chemistry of carbon) Examples of rganic ompounds: Inorganic (rocks, minerals, metals, glass) 1-1 Products medicine pesticides dye/paint/ink gasoline/fuels cosmetics Materials paper cotton tires/rubber nylon/polyester plastic/vinyl in Nature hormones/steroids DNA protein/fats/sugars flavors/fragrances molecular bio. = organic rxns Review Some hemistry Basics (Wade 1-1 to 1-8) electronic configurations and the Periodic Table electrons (e-) are held in atomic orbitals around the nucleus (s, p, d, f), and s orbitals are more stable (lower Energy) than p orbitals fluorine is the most electronegative element (pulls electron density toward itself) Which is more electronegative, or N? oxygen is the second-most electronegative element and (no significant difference) all elements want to "look like" the Noble gases (have same electronic configuration) atom is stable if it has a filled valence shell (octet rule) Energy 2p 2s 1s Periodic trends: Ex: Na + a 2+ Br 8 e = s 2 p 6 (or 1s 2 for e) F ionic bonds are formed between atoms if they have a large difference in electronegativities transfer e - Na l Na l a salt (network structure) unstable, reactive stable, filled octets covalent bonds are formed between atoms if they have similar electronegativities share e - = unstable, reactive stable, filled octets methane molecule represents 2 shared e - (covalent bond) 1-2

2 polar covalent bonds arise if there is a difference in electronegativities between atoms 1-2 polar bond ( is more elcgtronegative than ) nonpolar bond (there is no significant difference in electronegativities) polarity of molecule depends on geometry (more in Wade hapter 2) nonpolar linear molecule: equal and opposite polar bonds, so no net dipole moment 3 3 polar bent molecule: has a net dipole moment nonpolar no polar bonds: no net dipole possible (regardless of geometry) Na (Na) ionic ionic: full charges are the ultimate in polarity hydroxide ( - ) is called a covalent ion Na has both ionic and covalent bonds!line drawings (Wade 1-10) are a short-hand way to draw carbon structures end points and intersections represent atoms omit s attached to s ctane (gasoline component) Benzene 3 3 Isopropyl Alcohol (rubbing alcohol) Acetic acid (vinegar) ( 3 ) 3 l 2 l l

3 Drawing Lewis Structures (Wade 1-4 to 1-8) example l 2 N Drawing Lewis Structures 1) draw skeleton - connectivity 1-3 2) count total # of valence electrons (valence e = group no.) 3) subtract charge (if any) 4) fill in missing electrons (fill octets) 5) determine formal charges (if any) example Formal harges calculate for each atom determine "electron count" = all nonbonded + 1/2 bonded/shared compare "electron count" with valence missing an electron > + charge extra electron > charge Typical, stable bonding (know by inspection) Atom example # bonds # lone pairs "e - count" N X X = halogen (F, l, Br, I)

4 practice: Draw the Lewis structure of nitric acid, N Understanding Resonance (Wade 1-9) 1. Any compound for which more than one Lewis structure may be written is accurately described by no single structure. The actual structure is a resonance hybrid of them all (NT "flipping back and forth" between resonance forms). The various structures are called contributing structures or resonance forms. 2. The stability of a resonance hybrid is greater than that which would be expected of any of the contributing structures. The hybrid is therefore said to be stabilized by resonance or resonance stabilized (by an amount of energy called the resonance energy). 3. The greater the stability (the lower the energy) of a contributing structure, the greater will be its contribution to the total structure of the hybrid. 4. Resonance is the interaction of electrons in p orbitals. nly pi and nonbonded electron density is reorganized in resonance (no σ bonds break). No atoms move (no change in bond length, angles). Since electrons are not being added or removed, there is no change in overall charge. Rules for Estimating Stability of Resonance Structures 1. The greater the number of filled octets, the greater the stability. You might notice that a structure with more covalent bonds is more stable (since more atoms will have complete octets) less important more important 2. The structure with fewer formal charges is more stable. If the species already has a net charge, creation of new charges is not favorable. For such charged compounds, the goal in drawing resonance forms is to delocalize the charge relocate it to as many different positions as possible. N 2 N 2 N 2 more important (best Lewis structure) less important least important 3. ther things being equal, a structure with a negative charge on the more electronegative element will be more stable. Similarly, a positive charge on a less electronegative element is more stable. important resonance not important resonance (unlikely)

5 important resonance not important resonance (unlikely) 4. Resonance forms that are equivalent have no difference in stability and contribute equally "allyl" carbocation both forms are equal in energy Looking for Resonance Delocalization of Electrons (3 main types/patterns*) 1. lone pair next to a pi bond (allylic lone pair) vacancy (missing octet) next to a pi bond (allylic carbocation) pi bond between two different elements (carbonyl-like resonance) **electrons move toward the more electronegative element** *Note: we will encounter a fourth important type of resonance later, called aromatic resonance, that is found in molecules like benzene benzene practice: draw and rank all possible resonance forms (explain rankings) N 2

6 Topic utline 1) Definitions (1-12 to 1-14) 2) Periodic Trends 3) Inductive Effects 4) Resonance Effects 5) ommon Acids and Bases Acid-Base Reactions: Proton Transfers (Wade 1-13) See also (at the end of the books): Wade Appendices 4 and 5 for pk a Tables and Solutions Manual Appendix 2, "Summary of Acidity and Basicity" 1-6 1) Definitions: acids and bases can be defined by Lewis or Bronsted-Lowry theories Lewis Acid: electron-pair acceptor (also called an Electrophile, E + ) * has a vacancy * common Lewis acids: All 3 BF 3 Lewis Base: electron-pair donor (also called an Nucleophile, Nu:) * has a lone pair or a pi bond examples: N 3 base All 3 "acid" 3 3 "Acid-Base" reaction usually means Bronsted-Lowry type Acid: (proton) donor Base: (proton) acceptor (Bronsted-Lowry definitions) A general "proton-transfer" reaction B A base acid Two acids are in competition - forward and reverse reactions are in equilibrium. **Equilibrium lies in the direction of the acid/base pair ** Which is the stronger acid? Use pk a table (Wade Appendix 4 and 5) or predict

7 Predict the products, label them (conj. acid, conj. base) and predict direction of equilibrium: 1-7 N 3 acid base 2) Periodic Trend (across row) compare these acids: 4 N 3 2 pk a draw the conj. bases: why such a large difference in pk a? Look at conjugate bases! onclusion: the stronger acid is the one with the most stable (less reactive, weaker) conjugate base! Energy 4 N 3 2 parent acids are ~same energy (neutral, stable compounds)

8 2) Periodic Trend (down column/family) 1-8 compare these acids: F l Br I pk a draw the conj. bases: why such a large difference in pk a? Look at conjugate bases! 3) Inductive Effects Which is the stronger acid? 3 vs. F 3 draw the conj. bases: F is an electron-withdrawing group (EWG) ther EWG: N N X

9 Inductive effects decrease with distance (more bonds to travel through) l 2 2 l 2 F 2 pk a ) Resonance Effects 3 2 vs. 3 I, pk a 16 II, pk a 5 II is 100,000,000,000 (100 BILLIN) times more acidic than I!! Why?! ompare conj. bases! B-I B-II Example: Which is most acidic? Least acidic?

10 Example: Which is most acidic? Least acidic? 2 II I 3 2 III 1-10 Example: Which is more basic (stronger base)? 3 I F 3 II

11 Example: Which is most basic (strongest base)? 1-11 N 2 N N 2 I II III 5) ommon Acids and Bases (Table 1.5) see pk a Table (Wade Appendices 4 and 5) strong acids weak acids very weak acids extremely weak acids (not acids!) pk a < 0 0 < pk a < 16 pk a > 16 pk a > 40 Using a pk a table to predict direction of equilibrium N 4 2 N 3 3 pk a

12 alifornia State Polytechnic University, Pomona rganic hemistry, M 314, Dr. Laurie S. Starkey 1-12 hapter 1 Summary (Wade textbook) I. Review of General hemistry concepts ( ) a. atomic structure; energy of atomic orbitals (s, p) b. electronegativity - ability of an atom to attract electron density i. fluorine is most electronegative element (oxygen is second-most!) ii. periodic trends iii., N=l electronegativity c. a filled valence shell (full octet) imparts stability d. covalent vs. ionic bonds e. bond and molecule polarity (δ+ and δ ) II. Line Drawings (1-10) III. Lewis Dot Structures ( ) a. structures show sigma (σ), pi (π) and nonbonded electrons b. formal charges (1-7) c. recognize "typical" configurations for common atoms (,, N,, X) IV. Resonance (1-9 + handouts, homework) a. moves π and nonbonded electrons in p orbitals to stabilize a molecule b. three general types (things to look for): i. lone pair next to a π bond (allylic lone pair) ii. vacancy next to a π bond (allylic empty p orbital) iii. π bond between two different atoms (carbonyl-like) c. curved arrows show the redistribution of electron density d. resonance forms can be ranked (find most important contributor, etc.) V. Acid/Base Reactions (1-13, Appendix 2 in Solutions Manual, homework assignment) a. Definitions ( ) i. Lewis acid/base (e- pair acceptor/donor) ii. Bronsted-Lowry acid/base (proton, +, donor/acceptor) iii. curved arrows to show reaction mechanisms b. Periodic trends in acid strength i. R > R3 and I > l. Why? ompare conjugate bases... ii. The stronger acid has the more stable (weaker) conjugate base! c. Inductive effects on acid strength i. electron-withdrawing groups (EWG) stabilize negative charges ii. inductive effects decrease with distance d. Effect of resonance i. acid strength: resonance can stabilize a conjugate base ii. base strength: resonance can tie up and stabilize a lone pair e. ommon acids (Table 1-5 and pka Table in Appendix 5, pp ) i. use pka table to identify strong/weak/very weak acids ii. determine direction of equilibrium, given pka table Suggested Textbook Problems: see syllabus

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