Alkenes:Formation by elimination reactions McM 11'12, 11'13

1
Alkenes Formation by elimination reactions
(McM 11.12, 11.13) Substitution
reactions Cleavage of ethers (McM
18.5) Organometallic coupling react. (McM
10.9, lab ex 10) Dienes MO description,
conjugated diene (McM 14.2) Aromatic compounds
Aromaticity (repetition McM 15.3-15.8)
Electrophilic aromatic subst and substituent
effects (McM 16.5-16.7, 16.12) Nucleophilic
aromatic subst (McM16.8-16.) Other react on
aromatic compounds (McM 16.10-16.11)
2
Dienes (McM 14.2)
3
Molecular orbitals of H2 (cf fig 1.20) - s-bond
Molecular orbitals of p-bond (cf fig. 1.21)
MO 1,3-butadiene cf fig 14.2
Energy absorb UV spectra
LUMO
hn 1 e-
HOMO
4 p electron in 2 bonding orbitals (ground state)
4
MO orbitals benzene - aromaticity (McM 15.4 -
15.5)
1,3,5-hexatriene only bonding MO shown
Benzene
5

• Criteria for Aromaticity (Hückel)
• (Monocyclic) ring
• Planar
• No of p-electrons in conjugation 4n2 (n 0, 1,
  2,....)

Benzene The 3 bonding MOs are filled Filled
shell of MOs (cf. filled shell of atomic
orbitals nobel gasses) Less than 6 p-electrons
Half-filled orbital(s) - radical character More
than 6 Electron in antibonding
orbitals Unstable, high-enegy species
6
Ex. of an anti-aromatic compound - Cyclobutadiene
7
Electrophilic Aromatic Substitution and
Substituent Effects (McM. 16.5 - 16.6)

• Halogenation (bromination)
• Nitration
• Sulfonation
• Alkylation (Friedel Craft)
• Acylation (Friedel Craft)

• R influence
• Reactivity
• Regiochemistry

8
R Electron donating
By resonance ex. RO-, HO-, O-, R2N-, RHN,
H2N-. RCOHN-
The ring is activated for E-fil ar subst More
reactive than benzene
9
Reaction in o or p pos.
Activating, o/p directing
10
R Electron donating
The ring is activated for E-fil ar subst More
reactive than benzene
By inductive effect ex. alkyls
Activating, o/p directing
11
R Electron withdrawing
By resonance effect -CN, -COR, COH, CO2H,
CO2R, -NO2, -SO2R etc
Especially electron poor in o/p pos
The ring is deactivated for E-fil ar subst Less
reactive than benzene
12
Deactivating, m directing
13
R Electron withdrawing
By inductive effect Alternative A -NR3
Deactivating, m directing
Alternative B -Halogens
Deactivating, but o/p directing!!
Electron donor by resonance (lone pairs) weak
effect
Inductively strong withdrawing effect
14
Regiochemistry in E-fil aromatic subst of
disubst. benzene derivs. (McM 16.7)
Synth of trisubst. benzene deriv. - Planning of a
good reaction sequence (McM 16.12)
15
Nucleophilic Aromatic Substitution (McM 16.8 -
16.9)
16
Mechanisms

• SNAr
• SN1 Via diazonium salts (McM 24.8)
• Benzyne
• (SRN1 Involves radicals)
• (VNS Vicarious nucl. Subst.)

17

• SNAr

• Only on electron defficient arenes (EWG o/p to X,
  Anion stabilizing effect)
• (Aromatic heterocycles, McM 28.6)
• 1st step rate limiting (Aromaticity broken)
• XFgtClgtBrgtI

18

• Benzyne

Triple bond between sp2 C p-p overlap sp2-sp2
overlap - weak bond Benzynes unstable / reactive
intermed.

• Reactivity of benzynes
• Adds nucleophiles
• Dienophile in Diels Alder react.

19

• Reactivity of aromatic side chains
• Ox. of alkyl side chains
• Halogenation benzylic position (c.f. Lab ex. 1)
• No ox. cleavage of double bonds in benzene
• Reduction of benzene (hydrogenation) difficult