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CHEM 2331H
T.Andrew Taton

*exam2 problem4 *alkyl/hydride shifts *finalexam2 problem 7 mechanism: can enlarge ring in an E1 rxn (1,2 alkyl shift), relives ring strain Stereoisomers (differences between each types and how to distinguish them, all the processes) • ISOMERS: different compounds (not identical) w/ same molecular formula o Constitutional/structural isomers: differ in order in which atoms = bonded together; different connections among atoms; different ways atoms = connected; (just changing 2 double bonds to 1 triple bond doesn’t make it constitutional isomer (technically resonance + shifts)…different atoms need to be connected to different things)  If need to draw these, 1 try drawing single long chain…then for 2 one, try branches/rings o Stereoisomers: isomers that differ only in orientation of atoms in space; same connectivity, different spatial arrangement  ENANTIOMERS: mirror-image isomers; stereochemistry ofALL chiral centers = inverted (= mirror image) ((R)  (S) and (S)  (R)) • = chiral: handed; can’t be superimposed on its mirror image (always works), doesn’t have mirror plane of symmetry (sometimes, not always true, but achiral for sure if does have mirror plane of symmetry) (rotation about single bonds = allowed) o *Hint: all flat (planar) molecules = achiral b/c mirror plane of symmetry = plane of molecule itself….thin half slice looks like other half slice o Molecule can have chiral conformations, but if can find at least 1 achiral conformer, molecule = achiral o usually b/c of Chiral center: tetrahedral atom w/ 4 different groups attached  Stereocenter: atom for which exchange of 2 groups leads to stereoisomer; asymmetric carbons = chirality centers  If molecule has no chiral centers, almost always achiral  Odd # chiral centers  always chiral  Even # chiral centers  may or may not be chiral  Cahn-Ingold-Prelog Notation: (R) vs (S) Configuration of Chiral Centers, Steps: • 1)Assign priority #s to each group attached to chiral center o Highest atomic # = priority 1 o If tie, look @ next atoms along chain, see which groups attached to them, whichever has highest atomic # element = higher priority o If both have same high atomic # atom, one w/ more of the higher one = high priority (multiple bonds = multiple copies of that atom) o #4 usually = H • 2) “Look” at molecule from perspective so that lowest priority (#4) pointed behind o If 123 = clockwise, then (R) o If counterclockwise, then (S) o *Don’t always have to rotate and redraw, if H = pointed out in front, see direction of 123 and then take opposite of that • Properties o most physical properties = identical (bp, mp, density, refractive index)   harder to separate  Resolution: convert to diastereomers  separate back, convert to desired enantiomer product • can do this to racemic mixture • more common nowadays: form diastereomeric salts, separate, add strong acid, get desired product o rotates plane-polarized light in opposite directions by same amt (specific rotation)  + = clockwise, - = counterclockwise • Racemate/Racemic Mixtures: perfect 1:1 mixture of enantiomers; don’t rotate polarized light o chem processes usually produce racemates o Note: achiral starting materials can’t react w/ achiral reagents to create chiral products • But beware of MESO COMPOUNDS: molecules that contain chiral centers, but = achiral o ex: if have compound w/ 2 chiral centers (opposite assignments, 1 (R) and 1 (S)) and each chiral center has same exact groups attached to it, then it = achiral (meso) o If molecule contains same # (R) and (S) stereocenters and those stereocenters have identical groups attached, then it = achiral and meso o General: Can invert stereochemistry (turn R into S and S into R) and can find same stereochemistry w/ same groups somewhere in original, then meso/achiral  DIASTEREOMERS: not mirror-image isomers; stereochemistry of SOME chiral centers = inverted (needs to have at least 2 chiral centers to be a diastereomer b/c needs to have something that doesn’t change while the other changes) • Cis-trans/geometric isomers: differ in cis-trans arrangement on ring/double bond • Other diastereomers (2+ chirality centers) • Properties o physical properties = different o  can be separated • *Note: 2 compounds can be diastereomers and one of them can be achiral; chirality doesn’t matter here, only matters for enantiomers o To see if an atom in a molecule = enantiotopic/diastereotopic (if there’s 2 of the same element attached to something), replace each w/ magic element X…if 2 new atoms = enantiomers molecule = enantiotopic; if diastereomers diastereotopic; if same neither Characteristics of all: *stabilized by hyperconjugation: neighboring C-H bonds stabilize empty p orbital in a carbocation, C-C stabilizes as well • nucleophile + electrophile  product (nucleophile attached to electrophile) + leaving group - - o Nuc: + R-X  Nuc-R + X • The Hammond Postulate: For similar rxns (only 1 variable changed), differences in starting materials/product stabilities will be reflected in transition state (to lesser degree) o Position of activation energy’s closer to starting state than products for exothermic rxns, same concept for endothermic rxns o Whichever option has smaller energy difference to activation energy = more favorable, not necessarily the one that starts at a lower energy o Product energy doesn’t directly affect rxn, but does influence transition state, which makes path more favorable  more exothermic rxn runs faster, though can’t just assume that • Attacking a certain area is hard, esp. if it’s a tertiary carbon (has 3 bulky groups blocking the way)…needs to physically have a route to come in • Multistep rxn mechanisms: use electron pushing, focus on getting from starting materials to products, makes sure each step = balanced (atoms, electrons, and charge = conserved), don’t draw multiple steps as 1 (each step, including acid-base exchange, creates new, discrete species), make sure intermediates = compatible w/ rxn conditions o Remember to show formal charges on atoms (draw them on the atom) • For energy diagrams: highest hill determines rate o all = ultimately exothermic • For alkenes, esp. in Elimination rxns, how to name them: o Look @ 2 ends of double bond separately o Assign priority #s like we do for R and S, but here, just 2 options o If same #s on same side of bond (like cis), then (Z) o If different #s on same side of bond (like trans), then (E) • How to choose which one? o Does the electrophile have 2 C’s?  Yes: Elimination possible  No  Substitution All Substitution Elimination • Good Leaving group needed • rxn mechanisms that substitute 1 • Lose leaving group and proton • Substrate must be sp hybridized functional group for another (same Generates alkene (most • Exothermic overall; not equilibrium, outcome) stable alkene product) nd • Require good leaving group (OH- = • Proton must be 1 C away from especially 2 order? Leaving group: group that leaves once bad leaving group under acidic (α to) leaving group (so steric nucleophile attacks conditions b/c not stable) hindrance around leaving • Good leaving groups (according to • Occur at sp hybridized C’s only group doesn’t matter as much) Hammond Postulate) • Both pathways compete with each …if no such H available… will do respective order o 1) Wants to take electrons other (choose whichever one has substitution (electronegative) lower transition state/rate- • New double bond stretches  higher determining step energy) btwn 2 C’s that used to have • Prefer better nucleophiles than bases H and leaving group electronegativity stabilizes transition (larger atoms) • Occurs in base, not state • Resonance-stabilized species = nucleophile, but has o 2) Polarizable (stabilizes better nucleophiles than bases similarities to it transition state) • Both pathways compete with  *some groups can be each other good nucleophiles and • Prefer better bases than good leaving groups nucleophiles (HO, RO, (esp. o 3) Products are stable; rxn tBuO), RC(triplebond)C , - shouldn’t be able to work R 3, H N3 ethoxide) (H O,2 better in reverse ROH), methanol (1 or 2?) • Smaller atoms = better bases than nucleophiles • Preferred over respective order substitution when product alkene = highly substituted • Zaitsev’s Rule: Elimination Rxns produce most substituted alkenes (b/c more stable, usually on an inner C, not terminal one); exceptions if
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