Reactions. Reactions. Elimination. 2. Elimination Often competes with nucleophilic substitution. 2. Elimination Alkyl halide is treated with a base

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1 eactions 1 eactions 2 2. limination Alkyl halide is treated with a base B: 2. limination ften competes with nucleophilic substitution LIMINATIN Nu: SUBSTITUTIN Nu Bimolecular B: limination B * * 3 Kinetics 2 ate determining step involves both reactants rate = k [base] [-] Second order kinetics 4 B + 2 = limination, 2 nd order 1

2 Zaitsev s ule In some cases a number of elimination products are possible: Stereochemistry ccurs in anti-periplanar geometry Anti Periplanar B trisubstituted trisubstituted disubstituted the most substituted products dominate nly one isomer is formed t B: 3 3 t t t S N 2 and 2 7 S N 2 in synthesis 8 S N 2 favoured with Low temp 1º substrates good n phileeg. nucleophile ' ' product alcohol ether 2 favoured with igh temp 3º substrates Strong/bulky base tert-butoxide ' N ' N alkyne nitrile ether 2

3 Nucleophilic Substitution 9 S N 1 10 Alternative alled S N 1 poor nucleophile, 3 o substrate different kinetics and stereochemistry are observed Slow 3 fast rate = k [-] fast 3 3 S N 1 11 limination 12 Stereochemistry carbocation intermediate Alternative alled 1 poor base, 3 o substrate First order kinetics mechanism again involves carbocation rate = k [-] Mixture of enantiomers formed 3

4 slow 3 fast 3 :B Stereochemistry no requirement for anti-periplanar geometry Substrate can lose a proton from any neighbouring position Zaitsev s ule most substituted alkene will dominate SN1 and 1 Difficult to differentiate Both involve carbocation intermediate 15 rganic hemistry 16 In general: Temp = sub Temp = elim Alkanes Alkenes Alkynes Benzene S N 1 and 1 much less useful than S N 2 and 2 4

5 Structure 17 Stability 18 The Kekule Proposal Stability of Benzene possible constitutional isomers for benzene ( 6 6 ) unusually high stability compare with alkenes N ATIN N ATIN Ladenburg Dewar Kekule l N ATIN WY? - esonance and Aromaticity Structure 19 esonance Theory 20 esonance theory of benzene All bonds are equivalent! π electrons are delocalised around the ring 1.esonance forms are imaginary benzene has a single hybrid structure which combines the characteristics of both resonance forms esonance forms ybrid structure 5

6 esonance Theory 21 Aromaticity esonance forms only differ in the position of π electrons 3. neither the position or hybridisation of the atoms change 4. The more resonance forms there are, the more stable the molecule. special characteristic of certain resonance stabilised systems equirements cyclic planar conjugated overlapping p orbitals between all atoms (4n + 2) π electrons We call these molecules resonance stabilised Aromaticity 23 Aromaticity 24 xample 1: Benzene ther xamples? cyclic planar conjugated 6 π electrons cyclic planar conjugated 6π electrons cyclic planar conjugated 6π electrons 6

7 eactions 25 eactions 26 lectrophilic Aromatic Substitution Benzene undergoes substitution NT addition 2 lectrophilic Aromatic Substitution Step 1 2 never observed slow 2 requires catalyst alogenation 27 eactions 28 The Intermediate ation stabilised by resonance lectrophilic Aromatic Substitution Step 2 - fast nly variation is the lectrophile 7

8 eactions 29 alogenation alogenation omination benzene is treated with bromine and a catalyst (usually Fe 3 ) omination The catalyst activates the electrophile ( 2 ) 2 + Fe 3 + Fe 4 2 catalyst + alogenation + - Fe 3 31 alogenation ther alogenations hlorine l 2 Fel 3 l 32 Iodine - + I 2 ul 2 I 8

9 eactions 33 eactions Nitration the electrophile is generated by reacting nitric acid with sulfuric acid mechanism N 2 N 2 N 2 - N S S 4 + N 2 lectrophilic Aromatic substitution 35 Substituted Benzenes 36 Substituent ffect on eactivity N 2 - Nitration resonance stabilised ANIUM IN N S S 4 + N 2 Nitronium ion phenol is 1000 x more reactive than benzene nitrobenzene is 20,000,000 x less reactive than benzene 9

10 Substituent ffects 37 Substituent ffects ing Activating Substituents donate electrons to the ring Best activators have lone pairs. eg Phenol Stabilises the arenium ion it forms more readily Faster reaction = N 2 lone pair N 2 lone pair lone pair lone pair inductive 2. ing Deactivating withdraw electrons from the ring destabilise the arenium ion Slower reaction N N carbonyl containing nitrile nitro Substituent ffects Directing N 2 N 2 39 Substituent ffects ing Activators Direct to ortho and para positions Lone pairs allow extra resonance structure 40 ortho 50% meta N 2 N 2 para 50% = N 2 lone pair N 2 lone pair lone pair lone pair inductive + 10

11 Substituent ffects 41 Substituent ffects 42 ing Deactivators Direct to meta position ortho and para are destabilised N nitrile N nitro carbonyl containing alogens special case electron withdrawing electronegative electron donating lone pairs Deactivating rtho para directing + Substitent ffects 43 Phenol 44 Summary Strongest activators N 2 3 Ar Strongest deactivators N 2 eactions 1. Acid + N pk a = 18 very poor acid Directing: Act/Deact: T-/PAA- ATIVATS T-/PAA- DATIVATS MTA- DATIVATS + pk a = 10 much better acid. Why? 11

12 Phenol 45 Phenol 46 Acid onjugate base is SNAN stabilised eactions 2. Nucleophile + S N 2 Analgesics Phenol 47 N PAATAML ASPIIN 12