1.13 Acid-Base Reactions: Lone-Pair Donors & Acceptors

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1 1.13 Acid-Base Reactions: Lone-Pair Donors & Acceptors I, Cl, N 3, 3 P 4 pka 10 to 5 Super strong acids 3 + pka 1.7 RC 2 pka ~ 5 acids Ph pka ~ 10 get 2, R pka ~ 16 weaker RCC (alkynes) pka ~ 26 RN 2 pka ~ 36 Extremely weak acid RC 3 pka ~ 60 Not acidic at all Lone pair donor + Lone pair acceptor = Acid Base reaction Rate determining step is Bimolecular (only one step involved) Acid Base reactions are generally very fast (proton,, is accessible) Acid Base reactions appear as components of other mechanisms

2 4.8 Reaction of a Tertiary Alcohol with -Cl/-/-I = S N 1 3 C Cl 3 C 3 C C 3 C 3 C C 3 C Cl (+ 2 ) Cl Cl 3 C 3 C C 3 C Cl (- 2 ) C 3 C 3 C C 3 3 alcohol + X alkyl halide = S N 1 reaction Rate determining step is Unimolecular = CARBCATIN FRMED Carbocation stabilized by yperconjugation (3 > 2 > 1 > C 3 3) Stepwise process, reactive intermediate (carbocation) involved

3 4.12 Primary Alcohols (& C 3 ) with -Cl/-/-I S N 2 via - X R + C alcohol (or C 3 ) + X alkyl halide = S N 2 reaction Rate determining step is Bimolecular (no carbocation formed) Reaction slowed by Steric Crowding (C 3 > 1 > 2 > 3 ) Concerted process, no reactive intermediate involved

4 4.15 Cl/ Substitution on Alkanes Free Radicals heat Cl Cl 2 Cl Initiation Cl C 3 Cl C 3 Propagation Cl Cl C 3 Cl C 3 Cl Propagation 3 C C 3 3 C C 3 Termination Cl C 3 Cl C 3 Termination Alkane + Cl 2 / 2 and heat/light = Radical halogenation reaction Non polar mechanism with homolytic bond breaking and forming Selectivity observed with abstraction of (3 > 2 > 1 C bond) omination more selective than chlorination ( radical selective)

5 5.9 Reaction of 3 /2 Alcohol with 2 S 4 / 3 P 4 E1 3 C 3 C C 3 C 2 S 4 or 3 P 4 heat 3 C C (+ 2 ) 3 C C regenerated 3 C 3 C C 3 C (- 2 ) 3 C C 3 C C 2 3 or 2 alcohol + 2S 4 or 3P 4 alkene = E1 reaction Rate determining step is Unimolecular = CARBCATIN FRMED Carbocation stabilized by yperconjugation (3 > 2 > 1 > C 3 ) Product distribution based on alkene stability (substitution pattern)

6 5.14 Reaction of 3 /2 /1 Alkyl alide with Base E2 3, 2, or 1 alkyl halide + base alkene = E2 reaction Rate determining step is Bimolecular = no intermediate formed Zaitsev outcome based on alkene stability (substitution pattern) Useful, predictable process since no intermediates are formed

7 6.4 Electrophilic Addition of -X to Alkenes R R -X X C C R X=Cl,,I R alkene alkyl halide 3 C + 3 C C 3 C 3 3 C C 3 Alkene + X (X = Cl,, I) gives alkyl halide addition product(s) Two step process: slower step is carbocation formation utcome based on carbocation stability (Markovnikoff rule) l) Carbocation formed so rearrangements are a possibility

8 6.9 Acid-catalyzed ydration of Alkenes 3 C + 3 C + 2 C 3 C C C C C 3 Alkene + dilute 2 S 4 ( 3 + ) gives alcohol laddition product(s) Three step process, the first being slow formation of carbocation Majorproductformed via more stabilized carbocation (Markovnikoff) Carbocation(s) generated so rearrangements will be possible

9 6.11 Addition-xidation ydration of Alkenes C 3 B syn addition C 3 C 3 Na/ 2 2 B B C 3 C 3 C 3 B Alkene + BR 2 gives addition based on sterics and electronics First step is concerted syn addition; no intermediate(s) formed Second d( (oxidation) step retains ti original ii lc BR 2 stereochemistry t verall outcome is opposite to that obtained using dilute 2 S 4

10 6.14 Addition of alogens to Alkenes 2 in CCl 3 (cold) alkene vicinal dibromide Attack from opposite side preferred nly anti product tindicates that t this is not a syn addition pathway ne product only suggests a modified carbocation intermediate Formation ofthebromonium ion explains stereochemicaloutcome outcome Similar reaction with 2 / 2 gives only anti addition of

11 6.18 Radical Addition of - to Alkenes, 2 2 major + minor Alkene + and peroxide gives alkyl halide addition product(s) Radical process with usual steps (initiation, propagation, termination) utcome is based on relative radical stability (Markovnikoff process) No rearrangements observed with free radical intermediates here

12 6.20 zonolysis of Alkenes Zn, 2 Remarkable process that breaks both the and bonds in the alkene First step is concerted syn addition; no intermediate(s) formed Subsequent steps involve breaking of weak bonds and the C C C bond Malozonide product is then reduced to give the carbonyl products

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