8-1 Chpt.8 Alkyl Halides & Radical Rx’s Structure Nomenclature Physical Properties Halogenation of...
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Transcript of 8-1 Chpt.8 Alkyl Halides & Radical Rx’s Structure Nomenclature Physical Properties Halogenation of...
8-1Chpt.8 Alkyl Halides & Radical Rx’s
StructureNomenclaturePhysical PropertiesHalogenation of AlkanesMechanism of HalogenationAllylic Halogenation
Note the Chapter Summary and Key Rx’s
ClCl2
8-28.1 Structure of ‘R-X’
Haloalkene
sp2
(vinyl halide)
C
CH X
H H
Haloalkane
sp3
(alkyl halide)
R-XH3C-Cl
Haloarene
sp2
(aryl halide)
X
8-3
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
8-4Common Names: (alkyl halide)
or (special names)
(isopropyl bromide) vs 2-bromopropane
(chloroform) vs trichloromethane
H-CCl3
H3C C
Br
CH3
H
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
8-68.3 Physical Properties
“polar covalent bond” - dipole- mismatch of electronegativity -size
H3C
C BrH3C
H
8-7
8.4 Halogenation of Alkanes
X2 = Cl2, Br2 seldom F2 (too reactive - exothermic)or I2 (endothermic, unreactive)
hv = ultraviolet light, = heat
Substitution of X for H
H3C
CH3C
H3CH
+X X
orhv
H3C
CH3C
H3CX
H X+
8-8
Substitution, products and by-products+ other
R-X’s
H
CHH
H
+ Cl Clorhv
HC
HH Cl
H X+
Cl
C
ClH
Cl
Cl
CHH Cl
ClC
ClCl
Cl
hvCl2
hvCl2
hv Cl2
8-9Generally halogenation not useful - mixtures (separate)
A few rx’s are useful, e.g.:
+ Br2
Br + HBr + other Br's
Others - allylic & benzylic
8-10
Substitution, 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
+
H H H
CH3C
BrCH3
CH3C
HH
+ HBr + diBr +...
CH3
C
HCH3
CH3C
HH
+ Br2
8-11Br+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 CH3H
and/or
and/or
8-12
CH3
CH2 + Br2
CH3
CH3
CH
CH3
Br
CH3
CH2
CH2Br
+ + HBr + diBrs +
(92) (8)
Regioselective for 3o > 2o > 1o C-H
(57) (43)
CH3
CH2 + Cl2CH3
CH3
CH
CH3
Cl
CH3
CH2
CH2Cl
+ + HCl + diCls +
>
>
8-13radical stability like carbocation - electron deficient
R CR
RR C
H
R>
R. is electron deficient (not charged)
> etc.
R CR
RR C
H
R> > etc.
8-14Order of stability of 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’ for R.
write condensed
8-15
EE >R3C
+ R2CH+ RCH2
+ H3C+
>>
Order of stability of R(+)
E
>R3C
. R2CH. RCH2
. H3C.
>>
‘Same’ for R.
8-16
Selectivity 3o > 2o > 1o, but Cl and Br are different
Br2 1600 80 1Cl2 5 4 1
3o 2o 1o
CH3
CH2 + X2
CH
H3C CH3
CH3
CH2
CH3C CH3
X
+ HX + other RX
major mono-X product
8-17
Hammond’s Postulate
8-18
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
8-19
Hammond’s Postulate• In halogenation of an alkane, the rate-
limiting step is hydrogen abstraction
– this step is endothermic for bromination
H°(kcal/mol)
-88 +10.0+98++CH3CH3 •Br CH3CH2• HBr
-5.0-103+98+ +CH3CH3 •Cl CH3CH2• HCl
and exothermic for chlorination.
8-20
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):
8-21Hammond
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 SMR
C H
HR
Cl
site of collision important
8-22
Halogenation (free radical substitution)
H H
+ Br2
(heat)
H Br
+ H-Br
cool
H H
Br
Br
8-23Allyl Radical - resonance
CC
C
H
H
H H
H CC
C
H
H
HH
H
CC
C
H
H
HH
H
8-24NBS
Br
NBS for Br2
N
O
O
N
O
O
Br X+
hv orROOR HBr
N-Br
O
O
N-H
O
O
HBr+ Br2
X+XXX R. + HX
8-25NBS for Br2
CH
H
+ HBrBr
CH3Br2
CH
H
Br
Br Br Br Br+
NBS
+ H-BrN
O
O
H+
N
O
O
Br + N
O
O
CH
H
H
8-26Free Radical Stability
C
H
H
C
H
HC
H
HC
H
HC
H
H
8-27
CC
C
H
H
HH
H
allylic
Free Radical Stability
C
H
H
benzylic
>>
>
R CR
R
R CH
R
R CH
H
H CH
H
8-28
H3C
H3C
H H
H
HH H
Br2
H3C
H3C
Br HH
HH H
H3C
H3C
H
H
BrH H
H
+H-Br
H3C
H3C
H HH
HH Br
H3C
H3C
H H
Br
H
H
H
+ + +
Reaction
8-29
H3C
H3C
H H
H
HH H
Br2
Br-Br
H3C
H3C
H HH
HH H
Br
H3C
H3C
Br HH
HH H
Br
H3C
H3C
H
H
BrH H
H
Br
+H-Br
H3C
H3C
H HH
HH Br
H3C
H3C
H H
Br
H
H
H
+ + +
+
Reaction
initiation
propagation
step 1rate determining
8-30
H HH
HH H
H3C
H3C
H3C
H3C
H HH
HH H
Br
BrH-Br
H-Br
H
H
HH H
H3C
H3C
H3C
H3C
H
H
HH H
H
H
HH H
H3CH3C
H3C
H3C
H
H
HH H
Br Br
Br Br
Br Br
Br Br
H3C
H
H
BrH H
H
H3C
H3C
Br HH
HH H
H3C
H3C
H3C
H
H
BrH H
H
and
H3C
H3C
Br HH
HH Hand
Br
Br
+
+
+
+
propagation
step 1rate determining
step 1rate determining
step 2
step 2
two different allylic sites
8-31
H
H
HH H
Br BrH3C
H
H
BrH H
H
H3C
Br HH
HH H
and
H3CH3C
H3C
H3C
Br+
Br
H
H
HH H
H3CH3C
H
H
HH H
H3CH3C Br
step 2
termination - one of many
8-32Radical Addition of HBr to Alkenes• Addition of HBr to alkenes gives either
Markovnikov addition or non-Markovnikov addition depending on reaction conditions– Markovnikov addition occurs when radicals are absent– non-Markovnikov addition occurs when peroxides or
other sources of radicals are present
+ HBr
no peroxides
Br
2-Methyl-propene
2-Bromo-2-methylpropane
Markovnikovaddition
HBrBrperoxides
+
2-Methyl-propene
1-Bromo-2-methylpropane
Non-Markovnikovaddition
8-33Radical Addition of HBr to Alkenes– addition of HCl and HI gives only
Markovnikov products– to account for the the non-Markovnikov
addition of HBr in the presence of peroxides, chemists proposed a radical chain mechanism
• Chain initiation
R O
R-O O-R
H Br
R O RO
HR O Br
Two alkoxy radicalsA dialkylperoxide
Bromineradical
Step 1: +
+Step 2: +
8-34
Radical Addition of HBr to Alkenes
• Chain propagation
Br
BrBr H
Br
BrH Br+
Step 3:
Step 4:
+
+
A 3° radical
1-Bromo-2-methylpropane
8-35Radical Addition of HBr to Alkenes
• Chain termination
• This pair of addition reactions illustrates how the products of a reaction can often be changed by a change in experimental conditions– polar addition of HBr is regioselective, with Br adding to the more
substituted carbon– radical addition of HBr is also regioselective, with Br adding to the
less substituted carbon
Br BrBr Br
BrBr Br
BrStep 6:
Step 5: +
+