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ALKYL HALIDES. by Parinya Theramongkol Department of Chemistry Khon Kaen University. Structure : The Functional Group. R -X. Alkyl group. Halogen atom. เป็นตัวกำหนดลักษณะเชิงโครงสร้างและควบคุมคุณสมบัติของสาร. The Functional Group. Classification & nomenclature. - PowerPoint PPT Presentation
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ALKYL HALIDES
by
Parinya Theramongkol
Department of Chemistry
Khon Kaen University
Structure : The Functional Group
R-X
Alkyl group Halogen atom
เป็�นตั�วกำหนดลั�กำษณะเชิ�งโครงสร�งแลัะควบค�มค�ณสมบ�ตั�ของสร
The Functional Group
Classification & nomenclature
แบ่�งตามชนิดของอะตอมคาร์�บ่อนิที่��แฮโลเจนิเกาะอยู่��
R C
H
H
X R C
R
H
X R C
R
R
X
Primary
(1o)
Secondary
(2o)
Tertiary
(3o)
Common vs IUPAC names
CH3CH2CH2CH2Br
CH3CHCH3
Cl
CH3CHCH2ClCH3
common IUPAC
n-Butyl bromide 1-Bromobutane
Isopropyl chloride 2-Chloropropane
Isobutyl chloride 1-Chloro-2-methylpropane
(1o)
(1o)
(2o)
CH3CH2CHCHCH3
Cl
CH3
CH3CH2CCH2CHCH3
Br
CH3 CH3
CH3CHCHCH2CHCH3
CH2CH3CH3
I
3-Chloro-2-methylpentane
4-Bromo-2,4-dimethylhexane
(2o)
(3o)
?
PREPARATION
1. From alcohols
R-OH R-XHX or PX3
OH Brconc.HBr
n-Propyl alcohol n-Propyl bromide
OHPBr3
Br
1-Phenylethanol 1-Bromo-1-phenylethane
2. Halogenation of certain hydrocarbons
R-HX2
R-X + HX
H3C CH3
CH3H3C
H3C CH3
CH2ClH3C
Cl2 , heat or light
Neopentane Neopentyl chloride
CH3 CH2Br
Toluene Benzyl bromide
Br2 , reflux, light
3. Addition of hydrogen halide to alkenes
C CHX
C CH X
4. Addition of halogens to alkenes and alkynes
C CX2
C CX X
C C2X2
C CX X
X X
5. Halide exchange
R-X I R-I + X+
Notes on preparation :
•The most general and practical way to make RX is to prepare from alcohols.
•RXs are almost never prepared by direct halogenation of alkanes.
•RI is often prepared from the corresponding bromide or chloride by treatment with a solution of NaI in acetone.
REACTIONS
1.Nucleophilic aliphatic substitution
R-W R-Z W+ :Z-
+Substrate
NucleophileLeaving group
R C C X
Nucleophilic siteElectrophilic site
Nucleophilic substitution
R-X
H2O
:OR'
C CR'
:I
:CN
R-I
R-CN
R-OH
R-OH
R-OR'
C CR'R
X+-
+- Alcohol
+
-
Alcohol
+- Ether
+ Alkyne
+- Alkyl iodide
+- Nitrile
:OH
See more examples on text p.174
2. Dehydrohalogenation : elimination
C CH X
C Cbase
3. Preparation of Grignard reagent
R-X Mg RMgX+dry ether
4. Reduction
R-X M H RH M X+-
++
+ + +
Br
Br Na, CH3OH
7,7-dibromonorcarene Norcarene
M = Li , Na, K
ClMg
MgClD2O
D
t-Butyl chloride 2-Deutero-2-methylpropane
The SN2 Reaction:
Kinetics :
the concentrations of both reactantsthe reaction rate
CH3Br CH3OH Br+ +- -
OH
-rate = k[CH3Br][OH ]
Second - order kinetics
substitution nucleophilic bimolecular
Mechanism & stereochemistry of SN2 reaction
- Br-C Br
HO C BrHO - -
CHO
transition statepentavalence carbon atom !
Nucleophile attacks on the back-side of the C-X bond
Bond-making and bond-breaking occur simultaneously
Product has a complete inversion of configuration
C BrH
C6H13
CH3
NaOHC HHO
C6H13
CH3
SN2
(-)-2-Bromooctane[] = - 39.6 o
(+)-2-octanol[] = + 10.3 o
SN2 Reactivity. Steric hindrance
R-Br Cl DMF R-Cl Br+ +- -
H CH
HBr H3C C
H
HBr H3C C
CH3
HBr H3C C
CH3
CH3
Br
Methyl Ethyl Isopropyl tert-Butyl
> > >
37 1.0 0.02 0.0008
relative rate(SN2)
Reactivity : CH3W > 1o > 2o > 3o
The SN1 Reaction: substitution nucleophilic unimolecular
Br+ +- -OHBr
H3C
H3CH3C
OHH3C
H3CH3C
Kinetics :
the concentration of alkyl halidethe reaction rate
rate = k[RBr]
First - order kinetics
Mechanism & stereochemistry of SN1 reaction
Br
H3C
H3CH3C
slowBr
H3C
H3CCH3 - (1)
reactive intermediatecarbocation
H3C
H3CCH3
+ OH-
OH
H3C
H3CH3C
(2)fast
Step 1 : ionization = rate determining step
Step 2 : combination
A reactive intermediate which is a group of atoms that contains a carbon atom bearingonly 6 electrons.
CARBOCATION
Br
H3C
H3CH3C
BrH3C
H3C CH3 -ionization
tetrahedral trigonal
H3CCH3
CH3
120o
sp2
empty p-orbital
Structure of carbocation
Mechanism & stereochemistry of SN1 reaction
Ionization of the C-X bond to generate a carbocationis the rate determining step.
Reaction proceeds with racemization.
R C
C2H5
CH3
W CH3OH R C
C2H5
CH3
OCH3 W H++ +-+
Optically active Opposite configuration ;Lower optical purity
SN1 : racemization plus inversion
X
H3C
HR
CH3
HR
+ -X
H2O
H2O
ba
OH
H3C
HR
(b) retension
HO
CH3
HR
(a) Inversion(predominates)
ionization
carbocation
SN1 Reactivity. Ease of formation of carbocation
R-W CF3COOH R-OCCF3
OH-W+ +
H3C C
CH3
CH3
W H3C C
H
CH3
W H3C C
H
H
W H C
H
H
W
t-Butyl Isopropyl Ethyl Methyl
> > >
106> 1.0 10-4 10-5< <Relative
Rate(SN1)
Reactivity in SN1 : 3o > 2o > 1o > CH3W
Rate of formation of C+ : 3o > 2o > 1o > CH3+
Stabilization of C+ : Polar effects
Polar effects : ผลัที่!"เกำ�ดข#$นตัรงจุ�ดเกำ�ดป็ฏิ�กำร�ย อ�นเน)"องมจุกำกำรให�หร)อร�บอ�เลั+คตัรอนของกำลั�,มข�งเค!ยง
stabilityCharge dispersion
CG CG
G = e- donating group G = e- withdrawing group
Disperses charge
Stabilizes cation
intensifies charge
Destabilizes cation
Rearrangement of carbocations
A less stable C+ can rearrange itself in order tobecome a more stable C+.
Br C2H5OHOC2H5
SN1
Br
C2H5O
C2H5OHOC2H5
OC2H5-
SN1
SN2
rearrangement
No rearrangement
rearrangement
+
CCH
CC
H
C CH
+
CCR
CC
R
C CR
1,2-shifts
Two common migrations are a hydride shift and an alkyl shift
Migratory mode :
A hydride shift
An alkyl shift
SN1SN2 vs.
(a) second-order kinetics(b) complete stereochemical inversion(c) absence of rearrangement(d) the reactivity sequence CH3W > 1o > 2o > 3o
(a) first-order kinetics(b) racemization(c) rearrangement(d) the reactivity sequence 3o > 2o > 1o > CH3W
SN2
SN1
