Transcript 幻灯片 1

第七章
卤代烃Halohydrocarbon
教材:徐寿昌 主编 高等教育出版社
Teaching contents
• Classification of Halogen-Substituted Hydrocarbons (TO)
• Nomenclature of Halogen-Substituted Hydrocarbons (TO)
• Preparation of Halogen-Substituted Hydrocarbons (TO)
• Structure of Halogen-Substituted Hydrocarbons (TO)
• Chemical Properties of Halogen-Substituted Hydrocarbons (TO)
Classification of Halogen-Substituted Hydrocarbons
• The compound that one (or more) hydrogen atom in a hydrocarbon is substituted by halogen atom is called halogensubstituted hydrocarbon.
• According to the hydrocarbon, halogen-substituted hydrocarbon
can be classified into:
• Alkyl halogen, alkenyl halogen, alkynyl halogen, aryl halogen.
• According to the degree of carbon, halogen-substituted hydrocarbon can be classified into:
H
C C X
H
H
C C X
C
C
C C X
C
primary
secondary
tertiary
halide
halide
halide
Nomenclature of Halogen-Substituted Hydrocarbons(1)
• Alkyl halides are named in the same way as alkanes, by treating
the halogen as a substituent on a parent alkane chain. There are
three rules:
• Rule 1 Find the longest carbon chain and name it as the parent.
If a double or triple bond is present, the parent chain must
contain it.
• Rule 2 Number the carbon atoms of the parent chain, beginning
at the end nearer the first substituent, regardless of whether it is
alkyl or halo. Assign each substituent a number to its position on
the chain.
• Rule 3 If the parent chain can be properly numbered from either
end by rule 2, begin at the end nearer the substituent (either
alkyl or halo) that has alphabetical precedence.
Preparation of Halogen-Substituted Hydrocarbons
• For example:
CH3
Br
CH3CHCH2CHCHCH2CH3
CH3
CH3
Br
CH3CHCH2CHCHCH2CH3
Br
5-Bromo-2,4-dimethylheptane
4,5-Dibromo-2-methylheptane
CH3
Cl
CH3CHCH2CH2CHCH3
2-Chloro-5-methylhexane
•
Cl
1-Chlorocyclohexene
Br
Br
ortho-Dibromobenzene
2－甲基－5－氯己烷
BACK
Preparation of Halogen-Substituted Hydrocarbons (1)
Cl
• Free radical substitution of
alkane
Cl2
+
hv
HCl
Br
• Additions of small cycloalkanes
+
HBr
Br
• Addition of Halogens to Alkenes
+
Br2
CCl4
Br
Br
• Hydrohalogenation of Alkenes
+ HBr
Ether
+ HBr
ROOR
Heat
Br
Preparation of Halogen-Substituted Hydrocarbons (2)
• Hydrohalogenation of Alkynes
CH3CH2C
Br
Br
CH HBr/HOAc
CH3CH2C
CH2
HBr
Br
• Free Radical Substitution of Alkenes
Br
+ NBS
CCl4
Heat
• Aromatic Halogenation
Br
+ Br2
FeBr3
+ HBr
• Bromination of Alkylbenzene Side Chains
Br
CH2CH3
+ NBS
CCl 4
CH3CH2CCH3
Preparation of Halogen-Substituted Hydrocarbons (3)
• Preparing Alkyl Halides from Alcohols
• The most general method for preparing alkyl halides is to make them
from alcohols. The simplest method for converting an alcohol to an
alkyl halide involves treating the alcohol with HCl, HBr, or HI.
ROH + HX
R X + H2O
(X=Cl, Br, I)
• Primary and secondary alcohols are best converted into alkly halides
by treatment with such reagents as thionyl chloride (SOCl2) or
phosphorus tribromide (PBr3).
N
N
N
N
CH2 OH
•
+ SOCl2
N
+ SO2 + HCl
N
CH2Cl
BACK
Structure of Halogen-Substituted Hydrocarbons (1)
• The structure of alkyl halides
• The carbon-halogen bond in an alkyl halide results from
the overlap of a carbon sp3 orbital with a halogen orbital.
Thus, alkyl halides carbon atoms have an approximately
tetrahedral geometry. Since halogens are more electronegative than carbon. The C-X bond is therefore polar,
with the carbon atom bearing a slight positive charge (δ+)
and the halogen a slight negative charge (δ-).
Xδ
_
+
δ
C
•
Structure of Halogen-Substituted Hydrocarbons (2)
• The structure of vinylic halides and aryl halides
• p, π-conjugated system in vinylic halides and aryl halides
0C. 0C. ..
0
00 X
0
. . ..
.
..
.
BACK
Chemical Properties of Halogen-Substituted Hydrocarbons (1)
• Nucleophilic Substitutions of Alkyl Halides (TO)
• The SN2 Reaction and SN1 Reaction (TO)
• Elimination Reactions of Alkyl Halides (TO)
• Reactions of Halides: Grignard Reagents (TO)
• Organometallic Coupling Reactions (TO)
• Nucleophilic Aromatic Substitutions (TO)
• Benzyne---Elemination/Addition mechanism (TO)
• Reactions of Allyl Halides and Benzyl Halides (TO)
Chemical Properties of Halogen-Substituted Hydrocarbons (2)
• Nucleophilic Substitutions of Alkyl Halides
• Alkyl halides can undergo substitution of the X group by
the nucleophile (Nu):
Nucleophilic
Substitution
•
Nu- +
C
X
C
Nu-=OH-, RO-, CN-, NH3, X-, NO3-
Nu + X-
Chemical Properties of Halogen-Substituted Hydrocarbons (3)
For example:
CH3CH2CH2OH
CH3CH2CH2 C
CH
NaC CH
CH3CH2CH2 SCH3
NaSCH3
CH3CH2CH2OCH3
NaOH/H2O/
CH3CH2CH2Br
AgNO3/C2H5OH
NaI/CH3COCH3
CH3CH2CH2ONO2
CH3CH2CH2 I
NaOCH3/HOCH3/
NaCN/C2H5OH/H2O
CH3CH2CH2 CN
NH3/HOC2H5/
CH3CH2CH2 NH2
NEXT
Chemical Properties of Halogen-Substituted Hydrocarbons (4)
• The SN2 Reaction
• In every chemical reaction, there is a direct relationship between
reaction rate and reactant concentrations. When we measure this
relationship, we measure the kinetics (动力学）of the reaction. For
example, let’s look at the kinetics of a simple nucleophilic substitution
of CH3Br with OH- to yield CH3OH plus Br -.
HO-
H
+ H C Br
H
-
CH3OH + Br
Reaction Rate = k [ HO- ] [ CH3 Br ]
• This equation says that the rate of disappearance of reactant is equal to
a constant of k times the alkyl halide concentration times the hydroxide
ion concentration. So the rate of this reaction is dependent on the
concentrations of two species, and the reaction is second-order reaction.
Chemical Properties of Halogen-Substituted Hydrocarbons (5)
• The SN2 Reaction
• The essential feature of the SN2 mechanism is that the reaction takes
place in a single step without intermediates. For example:
HO-
H3C
H
C Br
CH2CH3
(S)-2-Bromobutane
[
δ
HO
H CH3
C
δ
Br
]
CH2CH3
Transition state
HO C
CH3
H
+ Br-
CH2CH3
(R)-2-Bromobutane
• This reaction occurs through a transition state in which the new HO-C
bond is partially forming at the same time that the old C-Br is partially
breaking. The transition state for this inversion has the remaining three
bonds to carbon in a planar arrangement. The stereochemistry at
carbon is inverted. ( Walden Inversion, 沃尔顿翻转)
Chemical Properties of Halogen-Substituted Hydrocarbons (6)
• The SN1 Reaction
• We might expect that the reaction of a tertiary substrate (hindered)
with water to be the slowest of substitution reactions. Remarkably,
however, the opposite is true.
RBr + H2O
H
H C
H
Relative
reactivity
<1
Br
H
H3C C
H
1
ROH +
Br
CH3
H C Br
CH3
12
HBr
CH3
H3C C Br
CH3
1,200,000
• What happened? Clearly, these reactions can’t be taking place by the
SN2 mechanism we have been discussing. This alternative mechanism is
called the SN1 reaction.
Chemical Properties of Halogen-Substituted Hydrocarbons (7)
• The reaction of (CH3)3CBr with H2O looks analogous to the reaction of
CH3Br with OH-, and we might therefore expect to observe secondorder kinetics. In fact, we do not.
CH3
CH3
C Br
CH3
+
H2O
CH3
CH3
C OH
CH3
+
HBr
• We find instead that the reaction rate is dependent only on the alkyl
halide concentration and is independent of the H2O concentration. In
other words, the reaction is a first-order process.
Reaction rate
= k [(CH3)3CBr]
Chemical Properties of Halogen-Substituted Hydrocarbons (8)
The mechanism of the reaction of 2-bromo-2-methylpropane with H2O:
CH3
CH3
C
Br
Rate-limiting
step
CH3
[
CH3
CH3
C+
]
+
Br-
CH3
Carbocation
CH3
[
CH3
CH3
] + H2O
C+
Fast step
[ CH3
CH3
C
+
OH2
CH3
+
OH2
]
CH3
CH3
[ CH3
C
] + H2O
CH3
CH3
C OH
CH3
+
+ H3O
Chemical Properties of Halogen-Substituted Hydrocarbons (9)
• Stereochemistry of the SN1 Reaction:
• Since an SN1 reaction occurs through a carbocation intermediate, its
stereochemical outcome should be different from that for an SN2
reaction. Since carbocations are planar and are achiral. The carbocation
intermediate can be attacked by a nucleophile equally well from either
side, leading to a 50:50 mixture of enantiomers – a racemic mixture. For
example:
Cl
CH3
CH2CH3
C
CH3
CH2CH3
H2O/Ethanol
HO C
CH2CH2CH2CH(CH3)2
(R)-6-Chloro-2,6-dimethyloctane
CH3CH2
+
CH2CH2CH2CH(CH3)2
CH3
C
OH
CH2CH2CH2CH(CH3)2
40% R
60% S
(retention)
(inversion)
Chemical Properties of Halogen-Substituted Hydrocarbons (10)
• The factors that effect the SN1 and SN2 reactions
SN1
SN2
Alkyl
Tertiary>Secondary>Primary
Primary
>Secondary>Tertiary
Leaving Group
RI>RBr>RCl>RF
RI>RBr>RCl>RF
Nucleophile
Nucleophile cannot affect the
reaction rate
Reaction rate is related to
nucleophilicity of
nucleophiles
Solvent
Polar solvents stabilizing
the carbocation
Polar aprotic best,
protic worst
NEXT
Chemical Properties of Halogen-Substituted Hydrocarbons (11)
• Elimination Reactions of Alkyl Halides
• When a nucleophile/Lewis base reacts with an alkyl halide,
the nucleophile can attack at a neighboring hydrogen and
cause elimination of HX to form an alkene.
Substitution
H
C C
+
OH
Br
H
Elimination
-
C C
+ OH
Br
OH
H
C C
C= C
+ Br-
+ H2O + Br-
Chemical Properties of Halogen-Substituted Hydrocarbons (12)
• Regiochemistry of Elimination of Alkyl Halides- Zaitsev’s Rule
• What products result from loss of HX from an unsymmetrical halide?
According to a rule of formulated in 1875 by the Russian chemist
Alexander Zaitsev, base-induced elimination reactions generally give the
more highly substituted (more stable) alkene product. For example:
Br
CH3CH2ONa
CH3CH2CHCH3
CH3CH2OH
2-Bromobutane
CH3CH=CHCH3 + CH3CH2CH=CH3
2-Butene
1-Butene
• The reaction gives mixtures of butene products because elimination
reactions can take place through two different mechanistic pathways:
the E1and E2 reactions.
Chemical Properties of Halogen-Substituted Hydrocarbons (13)
• The E2 Reaction
• The E2 reaction (for elimination, bimolecular) occurs when an alkyl
halide is treated with a strong base, such as hydroxide ion or alkoxide
ion (RO-). It is the most commonly occurring pathway for elimination
and can be formulated as shown below:
:
B
R
H
R
R
C
C
δ+
R
X
B
[
...H..
.C...C. RR
.
R
.Xδ
R
_
]
R
R
C
C
R
R
+
+B
_
H + :X
Transition state
• Like the SN2 reaction, the E2 reaction takes place in one step without
intermediate. As the attacking base begins to abstract H+ from a carbon
next the leaving group, the C-H bond begins to break, a C=C bond
begins to form, and the leaving group begins to depart, taking with it
the electron pair from the C-X bond.
Chemical Properties of Halogen-Substituted Hydrocarbons (14)
• The stereochemistry of E2 reaction
• As shown by a large number of experiments, E2 reactions always occur
with a periplanar geometry, meaning that all four reacting atoms-the
hydrogen, the two carbons, and the leaving group- lie in the same plane.
Two such geometries are possible: syn periplanar geometry, in which the
H and X are on the same side of the molecule, and anti periplanar
geometry, in which the H and the X are on opposite sides of the
molecule. Of the two choices, anti periplanar geometry is energetically
preferred because it allows the substituents on the two carbons to adopt
a staggered relationship, whereas syn geometry requires that the
substituents on carbon be eclipsed. For example:
Ph
C
C
Ph
Br
H
Br2
H
Ph
Br
H
Ph
H
Br
Br
=
Ph
Ph
H
H
Br
Ph
C
C
KOH
Ethanol
Ph
H
Chemical Properties of Halogen-Substituted Hydrocarbons (15)
• The E1 Reaction
• Just as the E2 Reaction is analogous to the SN2 reaction,
there is a close analog to the SN1 reaction called the E1
reaction (for elimination, unimolecular). The E1 reaction
can be formulated as shown below:
Base
Cl
CH3 C
CH3
CH3
Rate-limiting
CH3
[ CH
3
+
C
H
C H
H
]
Fast
CH3
C
CH3
CH2
Chemical Properties of Halogen-Substituted Hydrocarbons (16)
The factors that effect the E1 and the E2 reactions
E1
E2
Alkyl
Tertiary>Secondary>Primary
Tertiary>Secondary>Primary
Leaving Group
RI>RBr>RCl>RF
RI>RBr>RCl>RF
Nucleophile
Nucleophile cannot affect the
reaction rate
Reaction rate is related to
nucleophilicity of nucleophiles
Solvent
Polar solvents stabilizing
the carbocation
Have some effects
NEXT
Chemical Properties of Halogen-Substituted Hydrocarbons (17)
• Reactions of Halides: Grignard Reagents
• Organohalides, RX, react with magnesium metal in dry ether or
tetrahydrofuran (THF) solvent to yield organo-magnesium halides,
RMgX. The products, called Grignard reagents after their discoverer,
Victor Grignard (1912 Nobel Prize winner), are examples of organometallic compounds because they contain a carbon-metal bond.
R X + Mg
o o
Ether
or THF
o
R Mg
X
where R=1 , 2 , 3 , alkyl, aryl, or alkenyl
X=Cl, Br, or I
Chemical Properties of Halogen-Substituted Hydrocarbons (18)
• The reactions of Ethylmagnesium bromide:
CH3CH3 + HOMgBr
CH3CH2 CO2H
H3O+
H2O
CH3CH2 CO2MgBr CO2
CH3CH2 MgBr
O2
CH3CH2 OMgBr
CH3C CH
CH3CH3 + CH3C C MgBr
H2O
CH3CH2 OH
Chemical Properties of Halogen-Substituted Hydrocarbons (19)
• Organometallic Coupling Reactions
• Many other kinds of organometallic compounds can be prepared in a
manner similar to that of Grignard reagents. For example, alkyllithium
reagents, RLi, can be prepared by the reaction of an alkyl halides with
lithium metal. For example:
CH3CH2CH2CH2 Br
2 Li
Pentane
CH3CH2CH2CH2 Li
+ Li Br
• One of the most valuable reactions of alkyllithiums is their use in
making lithium diorganocopper compounds, R2CuLi, called Gilman
reagents. For example:
2 CH3 Li + CuI
(CH3)2Cu Li
Et2O
(CH3)2Cu Li
+ CH3CH2CH2CH2 Br
+
Li I
Et2O
0 oC
CH3CH2CH2CH2 CH3 + LiBr + CH3 Cu
• Gilman reagents are useful because they undergo organometallic
coupling reactions with alkyl halides to yield hydrocarbon product.
Chemical Properties of Halogen-Substituted Hydrocarbons (20)
• Nucleophilic Aromatic Substitutions
• Aromatic substitution reactions usually occur by an electrophilic
mechanism. Aryl halides that have electron-withdrawing substituents,
however, can also undergo nucleophilic aromatic substitution. For
example:
Cl
O2N
OH
_
NO2
1) OH
O2N
NO2
2) H3O+
NO2
+
_
Cl
NO2
2,4,6-Trinitrophenol (100%)
• Nucleophilic substitutions on an aromatic ring proceed by the addition
/elimination mechanism shown below:
_
OH
O 2N
NO2
NO2
OH
Cl OH
Cl
O 2N
NO2
O 2N
NO2
+
]
[
NO2
NO2
_
Cl
Chemical Properties of Halogen-Substituted Hydrocarbons (21)
• Benzyne---Elimination/Addition mechanism
• Halobenzenes without electron-withdrawing substituents do not react
with nucleophiles under most conditions. At high temperature and
pressure, however, even chlorobenzene can be forced to react.
Cl
OH
1) NaOH/H2O
P
2) H2O
• Mechanism of reaction:
_
OH
Cl
H
OH
_
HCl
[
]
H2O
Chemical Properties of Halogen-Substituted Hydrocarbons (22)
• Reactions of Allyl Halides and Benzyl Halides
CH2=CHCH2OH
NaOH/H2O
CH2=CHCH2OCH3
NaOCH3/HOCH3
CH2=CHCH2Cl
Mg/Ether
CH2=CHCH2MgCl
CuCN/Nitrobenzene
CH2=CHCH2CN
PhCH2OH
Na2CO3/H2O
PhCH2OCH3
NaOCH3/HOCH3
PhCH2Cl
CuCN
PhCH2CN
Mg/Ether
PhCH2MgCl