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NUCLEOPHILIC SUBSTITUTION

NUCLEOPHILIC SUBSTITUTION. THE GENERAL REACTION AND TERMINOLOGY. R. X. R. Y. Y. X. SUBSTITUTION REACTIONS. One group takes the place of another. +. +. Y takes the place of X ( Substitution ). Y “displaces” X. NUCLEOPHILIC SUBSTITUTION. NUCLEOPHILIC DISPLACEMENT. leaving

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NUCLEOPHILIC SUBSTITUTION

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  1. NUCLEOPHILIC SUBSTITUTION THE GENERAL REACTION AND TERMINOLOGY

  2. R X R Y Y X SUBSTITUTION REACTIONS One group takes the place of another. + + Y takes the place of X ( Substitution ) Y “displaces” X

  3. NUCLEOPHILIC SUBSTITUTION NUCLEOPHILIC DISPLACEMENT leaving group substrate product nucleophile The nucleophile “displaces” the leaving group. This is a “substitution” reaction : Nu substitutes for X (takes its place).

  4. IMPORTANT: This is a reaction at sp3 (tetrahedral) carbon atoms. sp3 sp2 sp yes no Compounds that have sp2 or sp carbons generally do not give nucleophilic substitution reactions. ….. of course there are a few special cases that do react, but we won’t discuss them in this chapter.

  5. EXAMPLES

  6. EXAMPLE 1 * (s) soluble insoluble iodide displaces bromide at carbon * NaI is soluble in acetone, NaCl and NaBr are not

  7. C H 3 H C C 2 C H 3 E1 competes EXAMPLE 2 more about this later solvolysis of t-butyl chloride in water water displaces chloride at carbon * The extra proton is taken by a second water molecule.

  8. DISPLACEMENT REACTIONS NUCLEOPHILIC SUBSTITUTION REACTIONS ( substitution at carbon ) can be compared to ….. ACID-BASE REACTIONS ( substitution at hydrogen )

  9. COMPARE THESE SUBSTITUTIONS DISPLACEMENT AT CARBON nucleophile substrate product leaving group DISPLACEMENT AT HYDROGEN base acid conjugate acid conjugate base

  10. THESE REACTIONS HAVE A WIDE RANGE OF SUBSTRATES

  11. COMMON SUBSTRATES ( Leaving group varies ) see Table 10-3 alkyl halides alcohols alcohols require acid and then H2O leaves tosylates ABBREVIATION alkyl p-toluenesulfonate

  12. THERE IS ALSO A WIDE RANGE OF NUCLEOPHILES A WIDE VARIETY OF COMPOUNDS CAN BE MADE

  13. NUCLEOPHILES see Table 10-4 A WIDE SELECTION OF NUCLEOPHILES MAKES POSSIBLE THE SYNTHESIS OF MANY TYPES OF ORGANIC COMPOUNDS: R-Y + Nu R-Nu + Y Nucleophile Product Class alkyl halides R X alcohols R O H ‘ ‘ ethers R O R nitriles R C N O esters R ' C O R alkynes R ' C C R thiols S H R

  14. THE NUCLEOPHILE DOES NOT HAVE TO BE CHARGED + - H O H O R + R B r + B r H H O H Under some circumstances water will react. H - + H O R + + B r H O 3 Nucleophile Product Class alcohols R O H ‘ ‘ ethers R O R amines R N H 2 amines R ' N H R

  15. “LOOK-ALIKE” REACTIONS RBr + NaOH ROH + NaBr Two reactions follow.... Both appear to be identical and follow the general pattern shown above. However, they are different !

  16. rate = k2 [RBr] [NaOH] rate = k1 [RBr] TWO “LOOK-ALIKE” REACTIONS RBr + NaOH ROH + NaBr 1) 55oC high conc. NaOH 2) 55oC ( + some alkene by E1,E2 ) low conc. NaOH

  17. Notice that substrate in reaction 1 has no b-hydrogens ….. why not?

  18. REACTION 1 THE SN2 REACTION

  19. 55oC rate = k2 [RBr] [NaOH] k2 = 0.022 liter/mole-sec - bimolecular concerted SN2 substitution nucleophilic bimolecular

  20. transition state TS Concerted (one step) reaction Concerted (one step) reaction SN2 E N E Ea R G Y starting D H material product

  21. REACTION 2 THE SN1 REACTION

  22. 55oC rate = k1 [RBr] k1 = 0.010 liter/mole-sec unimolecular two steps slow SN1 fast substitution also nucleophilic alkene (via E1) unimolecular

  23. TWO STEP REACTION carbocation SN1 intermediate TS 1 E TS 2 N E Ea 2 R G Ea 1 Y step 1 step 2 starting D H material product

  24. carbocation intermediate TS 1 TS 2 starting material TWO STEP REACTION SN1 E N E R G Ea Y 1 Ea 2 DH step 1 step 2 product

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