Transcript Organic Halides (Haloalkanes) (Alkyl Halides)

Functional Group #1
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(Haloalkanes)
(Alkyl Halides)
4/11/2016
Organic Halides
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Structure
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Cl
|
CH=C-CH2-CH3
|
Cl
1,2-dichloro-1-butene
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• R-X
where R = alkyl group
and X = halide (F, Cl, Br, I)
bromoethane
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Chlorine
Chloro-
Bromine
Bromo-
Iodine
Iodo-
Prefixes for the first four
members of the halogen family
Prefixes for halides are inserted
into the name of the structure in
the exact same fashion as any
alkyl group. Remember to write
them in alphabetical order
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Fluoro-
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Fluorine
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Nomenclature
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• The presence of a halide makes the molecule more
polar.
• Since water is also polar and “like dissolves like”, alkyl
halides are soluble in water. The more halides
connected to the parent chain, the more polar the
molecule.
• The polar nature of the molecule means that boiling
and melting points of alkyl halides are much higher
than alkanes with a similar number of carbons
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Properties
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• CFC’s (Chlorofluorocarbons) are used in
refrigerators and air conditioners (ex. Freon)
(their use is now restricted due to the fact that
some of them damage the ozone layer)
• polymers such as Teflon (non-stick surfaces as in
cookware) and PVC (polyvinylchloride) used
plumbing
• DDT (dichlorodiphenyltrichloroethane) – now
banned pesticide
• PCB’s (polychlorinated biphenyls) – electrical
transformers
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Uses
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Reactions – Preparing Alkyl Halides
• Substitution reactions with Halogens
Ex. benzene + Br → bromobenzene + HBr
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• Halogenation (by addition)
Ex. C2H4 + HCl → C2H5Cl
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C=C and C≡ C are weak bonds that are easily broken
making molecules that contain them more reactive
than alkanes
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• Alkenes/Alkynes readily react with halides of acids to
form alkyl halides.
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• When Alkene or Alkyne reacts with a hydrogen halide or water
the H bonds to the Carbon with the most H already attached.
The Rich get Richer! See the
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Markovnikov’s Rule
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Markovnikov’s Rule
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Two more examples:
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Reactions - Reactions with aromatics
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C6H6 + Cl2 ===========> C6H5Cl + HCl
Chlorobenzene
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FeCl3 catalyst
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Substitution
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Rapid. 3° C+ is stabilized by
protic solvent (H2O)
3º
..
(Lucas Test)
(CH3)3C OH +
SN1
..
H Cl
+
(CH3)3C OH
.. 2
- H2O
(CH3)3C +
..
(CH3)3C Cl : + H2O
..
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• Alkyl halides can be prepared from alcohols
by reaction with HX, i.e., the substitution of
a halide on a protonated alcohol.
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Preparation of Alkyl Halides from Alcohols
.. :Cl :
..
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Very slow. Protic solvent inhibits the
nucleophile
1º
SN2
CH3CH2
..
OH
..
.
+
H
Cl
D
CH3CH2
+
OH2
.. : Cl :
..
CH3CH2Cl
+
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• OH- is a poor leaving group, i.e., is not
displaced directly by nucleophiles.
Reaction in acid media protonates the OH
group producing a better leaving group
(H2O). 2 and 3 alcohols react by SN1 but
Me° and 1 alcohols react by SN2.
H2O
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Br
:P
1º
SN2
..
CH3(CH2)4CH2 OH
..
Br
Br
ether
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•Alternative to using hydrohalic acids (HCl, HBr,
HI), alcohols can be converted to alkyl halides by
reaction with PBr3 which transforms OH- into a
better leaving group allowing substitution (SN2) to
occur without rearrangement.
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Preparation of Alkyl Halides from Alcohols
+ PBr2
CH3(CH2)4CH2 O
..
H
CH3(CH2)4CH2Br
Br
-
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Preparation of Alkenes from Alkyl Halides
KOH in ethanol
+
Br
KBr
+
KOH

cyclohexene (80 % yield)
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Bromocyclohexane +
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-HBr
H2O
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2° and 3° alkyl halides can be dehydrohalogenated with a strong
base such as OH- producing an alkene.
Clearly, this is an E2 reaction.
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Summary of SN /Elimination Reactions
• Alkyl Halide Substrate Reactivity:
H
C
H
Br
CH3
C
H
unhindered substrates favor S N2
do not form a stable C +
do not react by S N1 or E1
3º
CH3
Br
CH3
C
H
Br
CH3
CH3
C
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H
2º
Br
CH3
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1º
hindered substrates. S N2 increasingly unfavorable, E2 is OK
form increasingly stable C +
favors SN1 and E1. E2 is OK
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H
methyl
E2 reactions possible with strong bases
E2 reactions possible with strong bulky bases (t-butoxide)
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Reactivity of Nucleophiles
HS-
CN-
I-
CH3O-
HO-
NH3
Cl-
H2 O
125,000
125,000
100,000
25,000
16,000
1000
700
1
good nucleophiles which are
also strong bases favor elimination
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good nucleophiles which are
weak bases favor SN reactions
pKb = 23
pKb = 22
pKb = 21
pKb = 11
pKb = -1.7
pKb = -2
pKb = -21
I-
Br -
Cl-
F-
HO-
RO-
H2N-
30,000
10,000
200
1
0
0
0
good leaving groups favor both
substitution and elimination reactions
poor leaving groups make both
substitution and elimination
reactions unfavorable
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Leaving Group Activity
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Poor nucleophiles that are also weak
bases (H2O, ROH, CH3COOH, etc.) do
not undergo any reaction unless a C+ is
formed first
If a C+ can form (as with a 2º, 3º, any
benzylic, or any allylic halides), then E1
and SN1 generally occur together.
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Note
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