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CHEM 2124 – Quick Review

CHEM 2124 – Quick Review. Common inorganic reagents: CaCl 2 , Na 2 SO 4 , HCl, H 2 SO 4 , HNO 3 , HC 2 H 3 O 2 = CH 3 COOH, NaOH, KOH Acetate salts or groups often abbreviated OAc. NaOAc, EtOAc Et stands for ethyl, Me stands for methyl – EtOAc, EtOH, MeOH. CHEM 2124 – Quick Review.

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CHEM 2124 – Quick Review

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  1. CHEM 2124 – Quick Review • Common inorganic reagents: CaCl2, Na2SO4, HCl, H2SO4, HNO3, HC2H3O2 = CH3COOH, NaOH, KOH • Acetate salts or groups often abbreviated OAc. NaOAc, EtOAc • Et stands for ethyl, Me stands for methyl – EtOAc, EtOH, MeOH

  2. CHEM 2124 – Quick Review • Functional groups Alkenes, alkynes, aromatic rings, alcohols, haloalkanes (alkyl halides), esters, ethers, thiols, aldehydes, ketones, carboxylic acids, amines, amides, cyano (nitrile) • Polar vs. nonpolar molecules • Intermolecular forces – hydrogen bonding, dipole-dipole, London dispersion forces

  3. CHEM 2124 – Quick Review • Relative strengths of acids and bases • Acids: HCl > HF > HCOOH > CH3COOH > HCN > NH4+ > CH3OH > H2O > CH3CH2OH Lower pKa = stronger acid • Bases: Ethoxide ion > hydroxide ion > methoxide ion > ammonia > cyanide ion > acetate ion > formate ion > fluoride ion > chloride ion Lower pKb = stronger base Conjugate bases of weaker acids are stronger bases.

  4. CHEM 2124 – Quick Review • Nucleophiles - “nucleus loving” - electron rich (lone pair, often negative charge) • Strong bases can act as strong nucleophiles • Conjugate acids of strong bases are weak nucleophiles • Ex: EtOH, H2O, NH3 • Some weak bases are excellent nucleophiles • Ex: CN • Ions with highly polarizable shells are excellent nucleophiles • Iodide > bromide > chloride >> fluoride • SH > OH

  5. CHEM 2124 – Quick Review • SN1 vs. SN2 • SN2 reactions • Promoted by strong nucleophiles • Promoted by aprotic solvents (DMF, THF, acetone) • 2nd order kinetics (rate = k[nuc][substrate]) • Backside attack gives inversion of configuration (RS, SR) • Methyl substrate > 1 > 2 not with 3

  6. CHEM 2124 – Quick Review • SN1 reactions • Favored with weak nucleophiles • Promoted by protic solvents (H2O, alcohol) • Promoted by AgNO3 if halide substrate • 1st order kinetics (rate = k[substrate]) • Carbocation intermediate • Gives racemization (R  50/50 R + S) • 3 > 2 • Always accompanied by E1 to some extent

  7. CHEM 2124 – Quick Review • E1 and E2 • E2 • Favored with strong bases - especially bulky bases (Ex: tBu-O) • Solvent not a big factor • 3 > 2 > 1 • 2nd order kinetics • Coplanar transition state required - preferably anti-coplanar • No rearrangements

  8. CHEM 2124 – Quick Review E1 • Bases can be strong or weak • Promoted by protic solvents • Promoted by AgNO3 if halide substrate • 3 > 2 > 1 • 1st order kinetics • No special geometry required • Rearrangements common - e.g. hydride and methyl shifts - to get more stable carbocation intermediate

  9. CHEM 2124 – Quick Review • Exothermic reactions • Less stable reactants  more stable products Ea

  10. CHEM 2124 – Quick Review • Endothermic reactions • More stable reactants  less stable products Ea

  11. CHEM 2124 – Quick Review • Multi-step reactions • Reactants  reactive intermediates  products

  12. CHEM 2124 – Quick Review • Reactive Intermediates • Carbocations • 3 > 2 > 1 > methyl • Radicals • Have an unpaired electron, 3 > 2 > 1 > methyl • Carbenes • sp2 hybridized C with lone pair

  13. CHEM 2124 – Quick Review • Reactive intermediates continued… • Carbanions – negative charge on C H3C:  > 1 > 2 > 3 Relative stabilities of reactive intermediates determined by inductive effects. Reactive intermediates are generally stabilized by resonance effects.

  14. CHEM 2124 – Quick Review • Hybridization • sp3 – bond angles ~109.5 • sp2 – bond angles ~120 • sp – bond angles 180 • Geometries • Tetrahedral (sp3) • Trigonal planar (sp2) • Linear (sp)

  15. CHEM 2124 – Quick Review • Formal Charges Formal charge = group #  1/2 bonding e  e’s in lone pairs Quick method: Group # - # of marks

  16. CHEM 2124 – Quick Review • Conformation vs. Configuration • Conformation or shape of a large molecule changes as a result of rotations about single (sigma) bonds. • Conformational change = change in shape • Configuration – fixed arrangement of atoms in space. (cis/trans, R/S, E/Z) • Stereoisomers have a fixed configuration. • Configurations cannot change without breaking bonds. • Steric hindrance can affect stabilities of different conformations or configurations.

  17. CHEM 2124 – Quick Review

  18. CHEM 2124 – Quick Review • R and S designations • Cahn-Ingold-Prelog system • Based on assigning priorities to groups or atoms attached to chiral centers (usually C). • Priority based on atomic mass • Priority based on first atom of difference • Assignment of R or S designation is made based on following priority 1,2,3 groups around clockwise (R) or counterclockwise (S) when group 4 (lowest priority – commonly H) is in a back position. • R and S designations are absolute configurations.

  19. CHEM 2124 – Quick Review • Relative configurations are assigned experimentally based on rotation of plane-polarized light (+/- or d/l). • Racemic mixture – a 50/50 mixture of +/- (or R/S) isomers. • Racemization – conversion of a formally optically active reactant to a racemic mixture.

  20. CHEM 2124 – Quick Review • E and Z designations • Similar to Cahn-Ingold-Prelog system for R,S • Based on assigning priorities to groups or atoms attached to C of C=C. • Priority based on atomic mass • Priority based on first atom of difference • Assignment of E or Z designation is made based on whether the highest priority groups on each C of C=C are opposite sides of C=C (E for Entgegen) or on the same side of C=C (Z for Zusammen). • E for Epposite - Z for Zame side

  21. CHEM 2124 – Quick Review • Mechanisms – Example 1: Hydrohalogenation of an alkene CH3-CH=CH2 + H-Br  Issues to consider: 1) Which electrons attack which atoms? 2) Regioselectivity – atoms added have a prefered orientation. (Markovnikov vs. anti-Markovnikov) 3) Stereochemistry?

  22. CHEM 2124 – Quick Review • Mechanisms continued – Example 2: Dehydrohalogenation CH3CHBrCH2CH3 + OH  Zaitsev (Saytseff) product vs. Hofmann product

  23. CHEM 2124 – Quick Review • Stabilities of alkenes Tetrasubstituted > tri > di > mono > un Trans > geminal > cis

  24. CHEM 2124 – Quick Review • Stoichiometry is still important!! • Determining limiting reactants • Calculating maximum or theoretical yields • Calculating percentage yields Limiting reactant – the reactant that determines the maximum yield possible. Used up first in the reaction and therefore limits the amount of product that can be made.

  25. CHEM 2124 – Quick Review • Limiting reactant + maximum yield Ex: If you mix 0.100 g of acetone with 0.330 g of benzaldehyde in the presence of base in the following reaction, what is the maximum yield of dibenzalacetone.

  26. CHEM 2124 – Quick Review • Percentage yield Amount recovered Theoretical yield Ex: If you recover 37 mg of product from a reaction with a theoretical yield of 0.106 g what is the percentage yield? If the lab manual says you should expect to recover 50 mg of product in this reaction, what does this mean? X 100

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