Slide 1 : NUCLEOPHILIC SUBSTITUTION THE GENERAL REACTION AND TERMINOLOGY
Slide 2 : + + SUBSTITUTION REACTIONS Y takes the place of X ( Substitution ) Y “displaces” X One group takes the place of another.
Slide 3 : NUCLEOPHILIC SUBSTITUTION nucleophile substrate product leaving
group NUCLEOPHILIC DISPLACEMENT The nucleophile “displaces” the leaving group. This is a “substitution” reaction :
Nu substitutes for X (takes its place).
Slide 4 : IMPORTANT: This is a reaction at sp3 (tetrahedral) carbon atoms. 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. sp3 sp2 sp
Slide 5 : EXAMPLES
Slide 6 : EXAMPLE 1 soluble insoluble NaI is soluble in acetone, NaCl and NaBr are not iodide displaces bromide at carbon (s) * *
Slide 7 : solvolysis of t-butyl chloride in water EXAMPLE 2 The extra proton is taken by a second water molecule. * water displaces chloride at carbon E1
competes more about
this later
Slide 8 : NUCLEOPHILIC SUBSTITUTION REACTIONS ACID-BASE REACTIONS ( substitution at carbon ) ( substitution at hydrogen ) can be compared to ….. DISPLACEMENT REACTIONS
Slide 9 : base acid conjugate
acid conjugate
base nucleophile substrate product leaving
group COMPARE THESE SUBSTITUTIONS DISPLACEMENT AT CARBON DISPLACEMENT AT HYDROGEN
Slide 10 : THESE REACTIONS HAVE A
WIDE RANGE OF SUBSTRATES
Slide 11 : COMMON SUBSTRATES alkyl halides tosylates alcohols alcohols
require acid
and then
H2O leaves ( Leaving group varies ) alkyl p-toluenesulfonate ABBREVIATION see Table 10-3
Slide 12 : THERE IS ALSO A
WIDE RANGE OF NUCLEOPHILES A WIDE VARIETY OF COMPOUNDS CAN BE MADE
Slide 13 : R O R R ' C C R R ' C O O R NUCLEOPHILES A WIDE SELECTION OF NUCLEOPHILES MAKES POSSIBLE THE SYNTHESIS
OF MANY TYPES OF ORGANIC COMPOUNDS: Nucleophile Product R X alkyl halides R O H alcohols ethers C N R nitriles esters alkynes R S H thiols Class see Table 10-4 ‘ ‘ R-Y + Nu R-Nu + Y
Slide 14 : R ' N H R O R R THE NUCLEOPHILE DOES NOT HAVE TO BE CHARGED Nucleophile Product alcohols Class O R H ethers R N H 2 amines amines O H H + R B r O H H R + + B r O H R O H H + H 3 O + + B r - - Under some circumstances
water will react. ‘ ‘
Slide 15 : “LOOK-ALIKE” REACTIONS RBr + NaOH ROH + NaBr Both appear to be identical and follow
the general pattern shown above. Two reactions follow.... However, they are different !
Slide 16 : TWO “LOOK-ALIKE” REACTIONS 1) 2) ( + some alkene by E1,E2 ) 55oC 55oC RBr + NaOH ROH + NaBr high conc. NaOH low conc. NaOH
Slide 17 : Notice that substrate in reaction 1 has
no b-hydrogens ….. why not?
Slide 18 : REACTION 1
THE SN2 REACTION
Slide 19 : SN2 rate = k2 [RBr] [NaOH] bimolecular
concerted 55oC k2 = 0.022 liter/mole-sec substitution nucleophilic bimolecular -
Slide 20 : Concerted (one step) reaction Concerted (one step) reaction product starting material D H E N E R G Y Ea SN2
Slide 21 : REACTION 2
THE SN1 REACTION
Slide 22 : rate = k1 [RBr] slow fast SN1 unimolecular
two steps 55oC k1 = 0.010 liter/mole-sec substitution nucleophilic unimolecular alkene (via E1) also
Slide 23 : starting material product Ea 1 Ea 2 D H intermediate TS 2 TS 1 E N E R G Y step 1 step 2 carbocation TWO STEP REACTION SN1
Slide 24 : Ea Ea DH step 1 step 2 1 2 product E N E R G Y TWO STEP REACTION SN1