Lecture_24

Lecture_24 - Lecture 24 Chapter 8: Subs3tu3on and...

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Unformatted text preview: Lecture 24 Chapter 8: Subs3tu3on and Elimina3on Inversion of Configura3on Which of the following alkyl halides gives the slowest SN2 reac3on?  A) CH3CH2Cl  B)  C)  D)  E) Which of the following alkyl halides gives the slowest SN2 reac3on?  A) CH3CH2Cl  B)  C)  D)  E) Which of the following compounds will undergo an SN2 reac3on most readily?  A) (CH3)3CCH2I  B) (CH3)3CCl  C) (CH3)2CHI  D) (CH3)2CHCH2CH2CH2Cl  E) (CH3)2CHCH2CH2CH2I Which of the following compounds will undergo an SN2 reac3on most readily?  A) (CH3)3CCH2I  B) (CH3)3CCl  C) (CH3)2CHI  D) (CH3)2CHCH2CH2CH2Cl  E) (CH3)2CHCH2CH2CH2I Factors that Effect SN2 Reac3ons  The Leaving Group –  The weaker the basicity, the beDer it leaves –  I>Br>Cl>F •  Fluorides are such strong bases, they don’t do SN2 –  Polarizability Factors that Effect SN2 Reac3ons  The Nucleophile –  Basicity: how well it shares lone pair with a proton –  Nucleophilicity: how readily it will aDack δ+ C –  Stronger bases are beDer nucleophiles •  OH- > H2O NH2- > NH3 Factors that Effect SN2 Reac3ons  The Nucleophile –  Size: bigger atoms- electrons not held Sghtly –  But this does not make up for I’s low basicity •  Iodine is sSll a poor nucleophile (in an aproSc solvent) Factors that Effect SN2  Solvent –  In a proSc solvent, smallest atom is poorest nucleophile, even though it’s a stronger base –  Iodide is weak base, so weak solvent interacSons Factors that Effect SN2  Solvent –  Fluoride would be beDer in nonpolar solvent –  QuesSon is: what aproSc solvent is polar enough? •  Ionic compounds don’t dissolve in nonpolar solvents Factors that Effect SN2  Steric Effects Reversibility of Reac3on  Depends upon the basicity/nucleophilicity of leaving group compared to that of the nucleophile.  If difference is small, reacSon is reversible Reversibility or Not  Reversible because differences in energies not large Reversibility or Not  Not reversible. HCl is a stronger acid than H2O –  Cl- is a weaker base than OH- SN1  SubsStuSon Nucleophilic Unimolecular –  First Order •  Rate only depends upon [RX] •  As methyl groups removed from R, rate decreases •  Product is racemic –  Mechanism SN1  ReacSon coordinate diagram Factors that Affect SN1  Rate –  Leaving group: ease of leaving –  CarbocaSon: stability of  Order of reacSvity of alkyl halides:  Nucleophile: no effect on rate –  Solvolysis (when solvent is nucleophile) Which halide reacts most rapidly via an SN2 mechanism?  A  B  C  D  E Which halide reacts most rapidly via an SN2 mechanism?  A  B  C  D  E Which of the following is the strongest nucleophile in an aqueous solu3on?  A) HO-  B) F-  C) Cl-  D) Br-  E) I- Which of the following is the strongest nucleophile in an aqueous solu3on?  A) HO-  B) F-  C) Cl-  D) Br-  E) I- What product results from the SN2 reac3on between (R)- 2- chloropentane and hydroxide?  A) (R)- 2- pentanol  B) (S)- 2- pentanol  C) racemic pentanol  D) 1- pentanol  E) 3- pentanol What product results from the SN2 reac3on between (R)- 2- chloropentane and hydroxide?  A) (R)- 2- pentanol  B) (S)- 2- pentanol  C) racemic pentanol  D) 1- pentanol  E) 3- pentanol Factors that Affect SN1  Sn1 or Sn2? That is the quesSon. –  We have some limited control (8.9, 8.10) Stereochem of SN Reac3ons  SN2- To predict stereoisomers, need to know: –  ConfiguraSon of reactant –  The mechanism  SN1- racemic product –  ParSal racemizaSon: more of one that other –  Complete racemizaSon: equal amounts Stereochem of SN Reac3ons  Why parSal racemizaSon in SN1? –  InSmate Ion Pair: front fact parSally blocked •  Forces more “backside aDack” The specific rota3on of op3cally pure (R)- sec- butyl alcohol is - 13.52°. An op3cally pure sample of (R)- sec- butyl bromide was converted into the corresponding sec- butyl alcohol via an SN2 reac3on. What is the specific rota3on of the product, assuming 100% yield?  A) - 13.52°  B) between 0° and - 13.52°  C) between 0° and +13.52°  D) +13.52°  E) zero The specific rota3on of op3cally pure (R)- sec- butyl alcohol is - 13.52°. An op3cally pure sample of (R)- sec- butyl bromide was converted into the corresponding sec- butyl alcohol via an SN2 reac3on. What is the specific rota3on of the product, assuming 100% yield?  A) - 13.52°  B) between 0° and - 13.52°  C) between 0° and +13.52°  D) +13.52°  E) zero ...
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