8-1 Alkyl Halides & Radical Rx’s Structure Nomenclature Physical Properties Halogenation of Alkanes Mechanism of Halogenation Allylic Halogenation Examples Radical addition Note the Chapter Summary and Key Rx’s Cl Cl 2 Δ chapter 8 ss18 12
Dec 21, 2015
8-1
Alkyl Halides & Radical Rx’sStructureNomenclaturePhysical PropertiesHalogenation of AlkanesMechanism of HalogenationAllylic HalogenationExamplesRadical additionNote the Chapter Summary and Key Rx’s
ClCl2
Δ
chapter 8
ss18 12
8-2
Chapter 8
H3C
CH3
H
H OH
CH
H3C F
HO
CH
H3CF
O
CO
H3C
H
OH
O
CO
H3C
OH
H
+
+
XH
H2 / M
Cl
IC
H
H3C D
NC
CH
H3CD
(-):CN:
CH2
OH O
HCH3
CH3
H
Br Mg CH3
δ+ δ−
LiCu
2
free radical substitution mechanism
allylicbenzylicHammondradical addition
Chapter 7
H3C
CH3
H
H OH
CH
H3C F
HO
CH
H3CF
O
CO
H3C
H
OH
O
CO
H3C
OH
H
+
+
XH
H2 / M
Cl
IC
H
H3C D
NC
CH
H3CD
(-):CN:
CH2
OH O
HCH3
CH3
H
Br Mg CH3
δ+ δ−
LiCu
2
8-48.1 Structure of ‘R-X’
haloalkane
sp3
(alkyl halide)
R-XH3C-Cl
haloarene
sp2
(aryl halide)
haloalkene
sp2
(a vinyl halide)
C
C
H H
CH3 X
X
8-58.1 Structure of ‘R-X’
Haloalkane (alkyl halide) sp3
R-X
H3C-Clmethyl chloride
R C
H
X
H1o halide
R C
H
X
R'2o halide
X
R C
R"
R'3o halide
8-6
IUPAC - halides (X) are substituentsSubstituent names: halo
fluoro, chloro, bromo, iodo
8.2 Nomenclature
#-haloalkane
#-halocycloalkane
(R)-4-bromo-1-chloro-4-fluoro-1-cyclopentenestructure ?
Br
F
Cl
8-88.3 Physical Properties
“polar covalent bond” - dipole- mismatch of electronegativity -size
H3C
C BrH3C
Hδ δ
8-9
8.4 Halogenation of Alkanes
X2 = Cl2, Br2 seldom F2 (too reactive - exothermic)or I2 (endothermic, unreactive)
hv = ultraviolet light, Δ = heat
substitution of X for H
H3C
CH3C
H3CH
+X X Δ
orhv
H3C
CH3CH3C X
H X+δ+
δ−
8-10
Substitution, products and by-products+ other R-X’s
H
CHH
H
+ Cl ClΔorhv
HC
HH Cl
H X+δ+ δ−
Cl
CHH Cl
hvCl2
Cl
C
ClH
Cl ClC
ClCl
Cl
hvCl2
hv Cl2
8-11Generally halogenation not useful - mixtures (separate)
A few rx’s are useful, e.g.:
+ Br2
ΔBr + HBr + other Br's
Others - allylic & benzylic
8-12Substitution, products and by-products
H3C CH3 + Br2hv
H3C CH2Br + HBr + diBr + etc
monobromination
bromination favors 3o > 2o > 1o
+ HBr + diBr +...
(92) (8)
CH3
CH2 + Br2
CH3
CH3
C
CH3
Br
CH3
CCH2Br
+Δ
H H H
CH3C
BrCH3
CH3C
HH
+ HBr + diBr +...
CH3
C
HCH3
CH3C
HH
+ Br2Δ
8-13Br+Br Br Brinitiation
H3CC
H3C HBr Br+
H3CC
H3C H
Br+ Br
H3CC
H3CH
HBr+
H3CC
H3C HH Br+propagation
terminationsH3C
CH3C H
Br H3CC
H3C H
Br
Br + BrBrBr
H3C
CH3C
H CH3
C CH3
H H3C
CH3C
H
CH3
C CH3
H
and/or
and/or
8-14
(92) (8)
Regioselective for 3o > 2o > 1o C-H
(57) (43)
>
>
ΔC
CH3
CH3
HHBr2
C
CH3
CH3
BrH +C
CH2Br
CH3
HH + HBr + other-Brs
ΔC
CH3
CH3
HHCl2
C
CH3
CH3
ClH +C
CH2Cl
CH3
HH + HCl + other-Cls
8-16Order of stability of R(+) / R.
R CR
R
R CH
R
R CH
H
H CH
H
>
>
>
E
R CR
R
R CH
R
R CH
H
H CH
H
>
>
>
E
same order
8-18
Selectivity 3o > 2o > 1o, but Cl and Br are different
Br2 1600 80 1Cl2 5 4 1
3o 2o 1o
major mono-X product
ΔCH3
H2C
CH3C CH3
H
+ X2
CH3H2C
CH3C CH3
X
+ HX + other R-X
Cl. more reactive less selective than Br.
8-19
Hammond’s Postulate Cl vs Br
Hammond’s Postulate: the structure of the transition state: – for an exothermic reaction looks more like the
reactants of that step
- for an endothermic reaction looks more like the products of that step
8-20
Hammond’s PostulateIn halogenation of an alkane, the rate-limiting step
is hydrogen abstraction– this step is endothermic for brominationand exothermic for chlorination.
ΔH°(kcalmol)
CH3 CH2-H + Br + H-BrCH3 CH2
+98 -88 +10
CH3 CH2-H + Cl + H-ClCH3 CH2
+98 -103 -5
8-21
Hammond’s Postulate
• transition state resembles the alkane and chlorine atom• little radical character on carbon in t.s.• regioselectivity only slightly influenced by radical
stability
For chlorination (hydrogen abstraction is exothermic):
8-22Hammond
R
C H
HR
Br
late t.s.
PE
SM prog rx Prod
stability of R. important SM prog rx Prod
PE
early t.s. - like SM
R
C H
HR
Cl
site of collision important
8-23
Halogenation (free radical substitution)
H H
+ Br2
Δ(heat)
H Br
+ H-Br
cool
H H
Br
Br
8-24Allyl Radical - resonance
CC
H2C
H
CC
H2C
H
HH
H
Br
H
H
Br2
Δ+ H-Br
Br
CH2C
H2C
H
H2CC
CH2
H
CH2C
H2C
H
8-25H
H
H
H
NBSH
Br
H
H
NBS for Br2
N-Br
O
O
N-H
O
O
HBr(trace) + Br2with progress rx
N
O
O
N
O
O
+hv
Br Brinitially
H-Br
8-26NBS for Br2
CH
H
+ HBrBr
CH3
Br2Δ
CH
H
Br
Br Br Br Br+
NBSΔ
+ H-BrN
O
O
H+
N
O
O
Br + N
O
O
CH
H
H
8-27Free Radical Stability
C
H
HC
H
HC
H
HC
H
H
CH
H
8-28
allylic
H2CC
CH2
H
benzylic
CH
H
Free Radical Stability
R CR
R
R CH
R
R CH
H
H CH
H
>
>
>
E
8-33
NBSΔ
H H
H
H
HH
H
H
H
H
H
H
H
Br
H H Cl2
hv
H
H
H
H
H
H
H
H
Br
+
HCl
+ H-Cl
H
H
H
H
8-34
Radical ADDITION - note rx conditions!!! Add H-X in the presence of peroxides
reverses normal addition“anti-Markovnikov”
H-XROOR H
X
H-XX
HMarkovnikov“normal”
H :Br
8-35
RO OR +RO OR
Radical ADDITION - note rx conditions!!! Add H-X in the presence of peroxides
reverses normal addition“anti-Markovnikov”X = Cl, Br, I
H-XROOR H
X
end
RO H X RO H + X
X X
XH X
HX
+ X
XX X X