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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5852_Библиотеки_им_академика_М_И_Перельмана.pdf
Alkyl Halides
HS :: CN >≅ : I > CH CH O : > : OH > CHO : > : CI > (CH )N : >CH COO : > HO : > : F
32 65 33 32
ΘΘΘΘΘΘ ΘΘΘ
The stability of an anion is inversely proportional to its basicity. Therefore, best leaving groups are weak
est bases.
:
:
IQ < BrQ < CIQ < F
:
IQ < BrQ < CIQ < F
:
Q
:
:
:
:
Q
–I) kJ mol–1
Hence the order or reactivity of alkyl halides having same alkyl group but different halogens is as
R I > R Br > R CI > R F
Nucleophilicity or strength of the nucleophile: 2 reaction is formed by
the attack of the nucleophilic reagent on alkyl halide, stronger the nucleophilic reagent, more rapid would be
2
as its ability to donate a pair of electrons to the carbon atom.
Thus, greater is the nucleophilicity of a nucleophile, the more
2 reaction. As the nucleophiles have
complete octet and a lone pair of electrons, therefore all the
nucleophiles should be basic in nature. However, their
nucleophilicity does not always parallel their base strength.
For instance, the hydroxide ion is a strong base and is also a
Stronger the base, stronger will be nucleophile, but
poor leaving group and vice-versa, e.g. OH
strong base; it is a good nucleophile and poor leaving group while water is a weak base, therefore, it
is a poor nucleophile and stronger leaving group.
REMEMBER
–
ion is a
good nucleophile but iodide ion is a very good nucleophile
though it is very weak base. In a series of nucleophiles belonging
to the same family, the nucleophilicity increases with the increase in the size of the atom bearing the unshared
:
:
:
:
IQ > BrQ > CIQ > F
Q
411
The electrons in the larger atoms are less tightly bound by the nucleus because of lower electronegativity
2 displacements. In other
words, because of their bigger size, the atoms become more polarizable and hence better nucleophiles.
2 reaction. Thus the relative nucleophilicities of some important nucleophiles
2
4. Effect of solvent on SN2 reaction: 2
2
2 reaction.
412
(–)–2-Bromooctane
SN
2
–
(+)–Octan–2-ol
tert-Butyl bromide Carbonium ion
Slow
CH
3
CH
3
CH
3
+δ –δ
CH
3
CH3 æ C æ Br
H
3
C æ C+ + Br
—
(CH3)3C+ + O–H (CH3)3C æ OH
Fast
11.8.3 Stereochemistry of SN2 Reaction
2 conditions (high
Thus if the alkyl halide is optically active and laevorotatory, the substitution product is also optically
Walden inversion.
11.8.4 SN1 Reaction
A nucleophilic substitution reaction in which the rate of kinetics depends upon the concentration of only one
1 reaction.
Mechanism of SN1 (substitution nucleophilic unimolecular) reaction: The mechanism suggested for the
reaction between tert
1.
2.
concentration of tertk)
Factors which inuence SN
1. Effect of solvent:1 reaction is polar, therefore polar solvents will stabilize
Rate = k [RX]
reactions
1
1 reaction.
Alkyl Halides
2. Nature of nucleophiles:
1 reaction, the strength of the nucleophile has no effect
on the rate o reaction.
Effect on structure of alkyl halides: We have discussed above that the carbocations are
1 reactions. Therefore, the more stable the carbocation formed,
1 reaction. As the order of the stability of various carbocations follows
Benzyl > Allyl > Tertiary > Secondary > Primary > Methyl carbocation
1
Benzyl > Allyl > Tertiary > Secondary > Primary > > Methyl halides
However, it may be noted here that this order of reactivity of alkyl halides is opposite to that of the
2 reactions.
4. Effect of nature of the leaving group E
transition state leading to the formation of the carbocation and hence more should be the reactivity of
1 reactions. Thus the order of reactivity of the different leaving groups follows the
:
:
:
:
IQ > BrQ > CIQ > F
Q
for the
act
413
Stereochemistry of SN1:
hindered because of the presence of halides ion, so attack from this side is less. Thus the major attack from
this side which is just opposite to the halide ion. Hence partial racemization takes place in which inverted
product is major and retention product is minor.
NOTEWORTHY POINTS
Reactivity of halides towards SN1.
Reactivity of halides towards SN2.
In reaction SN3.
In reaction SN4.
inversion.
Strong nucleophile and its high conc. favour SN5.
attack of nucleophile may take place from either side.
1
attack of nucleophile may take place from backside only and always lead to Walden’s
2
mechanism is 3º > 2º > 1º.
1
mechanism is 1º > 2º > 3º.
2
mechanism.
2
414
11.9 ELIMINATION REACTIONS
Reaction with alcoholic potash—Dehydrohalogenation
(formation of alkenes): When heated with concentrated
alcoholic potassium hydroxide, alkyl halides eliminate
halogen acid to form alkenes. This reaction known as
dehydrohalogenation involves the loss of halogen and the
hydrogen atom from a carbon adjacent to the one losing
the halogen (b
H X
b a
R C C H + KOH R CH CH2 + KX + H2O
(Alc.)
H H
A concentrated alc. KOH contains hydroxide ions which
being more basic than OH ions preferentially withdraw a
proton from alkyl halide to form an alkene, while aqueous
KOH contains only OH ions which being a better nucleophile preferentially bring about hydrolysis of alkyl halides
to the corresponding alcohols.
Saytzeff’s rule: If the structure of an alkyl halide is such that it can
undergo dehydrohalogenation in two different ways, the preferred
product is the alkene that contains least number of hydrogen atoms
on doubly bonded carbon atoms. The highly substituted alkene is the
REMEMBER
REMEMBER
In but-2-ene, trans-isomer is the major
product while cis-isomer is minor.
major product.
H Br H H
b a b
H C C C C H + KOH CH2 CH CH2 CH3 + CH3
(Alc.) 1-Butene 2-Butene
H H H H (Minor product 20%) (Major product 80%)
(Less substituted)
CH CH CH
3
Order of dehydrohalogenation of alkyl halides is 3º > 2º > 1º.
b
and hence higher the probability of attack.
2. The more branched alkyl halide leads to more branched alkenes and, therefore, more stable alkenes.
Alkyl Halides
11.9.1 Mechanism
E
(Elimination bimolecular)
2
The dehydrohalogenation of a majority of primary alkyl halides occurs by E2 mechanism. Kinetic studies of
here is proportional to the concentration of both the alkyl halide and the base. Thus,
Rate ∝ [Alkyl halide] [Base]
415
This means that E
hydroxide ion (hydrogen attached to carbon atom which is next to the carbon atom carrying the halogen
atom), the carbon hydrogen bond begins to break. The carbon–carbon p bond starts forming and the leaving
group (i.e. the halogen atom) starts leaving the a
carbon halogen bond. This is explained below with the help of dehydrohalogenation of bromoethane.
As elimination of a proton by a base and simultaneous expulsion of halide ion takes place in one step,
there is no opportunity of rearrangement. It is clear from the reaction that the transition state has acquired
H Cl
CH3 CH2 CH CH2 + Alc. KOH CH3CH2CH CH2 + H2O + KCl
In case of an alkyl halide having two different types of hydrogens, two alkenes can be obtained. It is thus
clear that more substituted the alkene the more stable the transition state and hence more of this alkene
E
(Elimination unimolecular)
1
The rate of reaction in these reactions depends only on one reactant molecule.
1 reactions are shown by tertiary alkyl halides and in solution of low base concentration.
416
C2H5Br CH2 � CH2 + HBr
573 K
R � X + 2Na + X � R R � R + 2NaX
Alkyl Higher
halide alkane
Dry ether
The complete mechanism may be depicted as shown below.
Step I.
E
1
X
+ . .
C C C C +X …(Slow)
H H
(Carbonium)
Step II.
+
C C + B– > C C < + H : B … (Fast)
H
Action of heat:
alkenes.
11.10 MISCELLANEOUS REACTIONS
11.10.1 Wurtz Reaction (Formation of Higher Alkanes)
When treated with sodium metal in dry ether, alkyl halides form higher alkanes. Thus,
Limitations
′), i.e. alkanes containing odd number of carbon atoms cannot be
prepared by this method.
Alkyl Halides
Butane
SN
2
2. Methane cannot be prepared by this method.
Ionic mechanism
It operates when the reactions are carried out in solution. It has been suggested that an atom of sodium
reacts with an alkyl halide, form an alkyl sodium compound having a carbanion character. The alkyl sodium
thus formed provides the nucleophile alkyl carbanion which the halide ion forms from another molecule of
2
417
This mechanism also explains the formation of side products. For example, ethyl iodide on treatment
with sodium metal in the presence of ether gives n
the minor products.
11.10.2 Corey–House Reaction
even and odd number of carbon atoms. Higher alkanes including branched chain alkanes can also be prepared
by this method. This method was developed in 1970. It consists of a reaction between an alkyl halide and a
lithium dialkyl copper.
418
′
′
2
2
Dry ether
Alkyl magnesium halide
(Grignard reagent)
R – X + Mg R – Mg + X
(R and R′ may be same or different.)
For good yields, the alkyl halide (R′–X) is generally primary one but alkyl group (R) in the organometal
11.10.3 Reaction with Magnesium (Formation of Grignard Reagent)
reagent).
chloride.
2H5H7X
Tertiary alkyl halides particularly iodides form alkenes.
11.10.4 Formation of TEL
Alkyl Halides
MethaneMethyl bromide
(Bromomethane)
Bromoethane
Ethane
Reduction
,
n-Butylbromide
11.10.5 Reduction (Formation of Alkanes)
1. H2
4).
alcohol.
Note:4 42H52H5
(diethylether), respectively.
4. HI on heating in presence of red P.
419
11.10.6 Rearrangement on Heating (Isomerization)
When heated at 570 K in the presence of aluminium chloride as a catalyst, alkyl halides undergo rearrangement,
for example,
420
11.11 USES OF ALKYL HALIDES
tors.
4 is an antihelminthic medicine against hookworm.
NOTE WORTHY POINTS
1. The function of zinc chloride is to act as a dehydrating agent and favour the forward reaction.
2. 2º and 3º alcohols on reaction with KBr/H
alkenes as major products.
3. PBr
4. Phosphorous halides are generally used to prepare lower alkyl bromides and iodides in the laboratory.
and PI5 are highly unstable compounds therefore only chlorides are obtained with PCl5.
5
preferably undergo elimination reaction to produce
2SO4
REVISION QUESTIONS
2. How is ethyl bromide prepared? How does it react with the following reactions?
(a) Aq. KOH
(c) Alc. KOH
2
(a) n-Propyl bromide is treated with alcoholic KOH