CHEM 3331 1nd Edition Lecture 14Outline of Last Lecture I. HaloalkanesII. UsesIII. Physical propertiesIV. Preparation V. Substitution reactionsOutline of Current Lecture I. Nucleophiles for Sn2 reactionII. Substrates and leaving groups for Sn2III. StereochemistryIV. Sn1 reactionV. RearrangementsVI. Sn1 vs. Sn2Current LectureI. Nucleophiles for Sn2 reactionsStronger nucleophiles speed up the reactionA strong nucleophile contains the following list:1. Charge- CH3O- > CH3OH. There are no positive nucleophiles. A negatively charged nucleophile is stronger than a neutral one.2. electronegativity- F- < CH3-. Less electronegativity makes a stronger nucleophile. 3. size- I->Br-> Cl- > F-. A larger size makes a better nucleophile. H2Se > H2S > H2O4. steric bulk- CH3O- > (CH3)3CO. A smaller bulk makes a better nucleophile.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.5. solvation- If we want to get F- as our nucleophile we cannot dissolve it in H20 since the hydrogen atoms neutralize the fluorine. Therefore H2O is a bad solvent. A polar aprotic solvent is needed. Usually DMF N(CH3)2OH or DMSO S(CH3)2O. II. Substrates and leaving groups for Sn2 reactionLeaving group1. it has to polarize carbon so the ucleophie has a partially positive carbon to attach to.2. it has to cleave.3. it cleaves with electron pairs4. it must be electron withdrawing5. it needs to be stable on its own.Good leaving groups are neutral (H2O, NH3, ROH, PR3) a stable anion (F-, I-, Br-, RSO3-) and weak bases.Substrates1. steric bulkCH3X> primary> secondary> tertiaryC(CH3)3CH2X even though it is primary it is still slow due to the large neighboring atoms.III. StereochemistryAs discussed in the previous lecture, the substrate undergoes an umbrella inversion. Cis-1-bromide-2-methylcyclopentane interacted with OH- causes it to become trans-1-hydroxide-2-methylcyclopentane and Br-.IV. Sn1 reactionFor an Sn1 reaction the rate=k[substrate]. 1 means unimolecular. The rate only contains substrate because it contains multiple steps. Substrate is in one ste and the nucleophile is introduced in a separate step. This means the substrate is in the rate determining step. Mechanism:1. C(CH3)3Br Br- + C(CH3)3(+)2. C(CH3)3(+) + CH3OH C(CH3)3O(+)HCH33. C(CH3)3O(+)HCH3 + CH3OH C(CH3)OCH3 + CH3O(+)H2The first mechanism is the rate determining step. Making the substrate the only molecule in the rate equation.Stability of the cation is due to conjugation. For an Sn1 reaction tertiary > secondary> primary> CH3+. For an sp2 hybridized carbon there is no Sn1 or Sn2 reaction. For solvents they need to bepolar protic solvent. Stereochemistry: Sn1 reactions result in racemization. They create two molecules with r and s configurations. V. Rearrangements1-bromo-3-methylbutane seperates into Br- and CH3C(+)HCH2(CH3)2 and CH3CH2C(+)(CH3)2This is known as a hydride shift in which the hydrogen is able to shift between the adjacent carbons shifting the positive charge for the nucleophile to join to. Hydride shift ony goes from primary to secondary to tertiary. There are also carbocation rearrangements.C(CH3)4Br Br- + C(C(+)H3)4Methyl shift- C(C(+)H3)4 C(+)(CH3)3CH3The methyl group once attached to the central carbon is moved to the end methyl group giving the central carbon a positive charge.VI. Sn1 vs. Sn2Sn1 Sn2Nucleophile doesn’t matter (can be weak) needs a strong nucleophileNeeds a strong leaving group needs a strong leaving groupPolar protic solvent polar aprotic solventRacemization inversionSubstrate should be 3>2>1> CH3+ substrate should be CH3+> 1>2>3Rate=k[substrate]
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