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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
Alkyl Halides
401
2P + 3Br2 2PBr
3ROH + PBr3 3RBr + H3PO
3
3
Alcohol Alkyl Phosphorous
bromide acid
3CH3 CH2 CH2OH + 3Pl3 3CH3 CH2 CH2l + H3PO
n-Propyl alcohol
or P/l
2
n
-Propyl iodide
3
(1-Propanol) (1-Iodopropane)
This method is useful for the preparation of lower alkyl bromides or iodides. Moreover, this method gives
good yield of primary alkyl halides, while secondary and tertiary alkyl halides are obtained in low yields.
c. By the action of thionyl chloride (SOCl
)
2
Alkyl chlorides
of pyridine (base).
Pyridine
ROH + SOCl2 RCl + SO2 + HCl
alcohol Thionyl Alkyl
chloride chloride
2. From Alkenes: Alkyl halides can be prepared by the addition of halogen acids to alkene.
H
Ethylene Ethyl halide
C CH2 + HX CH3 CH2 X
2
The reaction takes place through electrophilic addition.
R CH CH2 + HX R CHX CH
3
Alkene Alkyl halide
(Unsymmetrical)
From Alkanes: Alkyl halides can be prepared by direct halogenation of alkanes in the presence of light,
CH4 + Cl2 CH3Cl + HCl
Methane Methyl Chloride
(Chloromethane)
C2H6 + Br2 C2H5Br + HBr
Ethane Ethyl bromide
(Bromoethane)
Light or
250–400°C
Light or
250–400°C
402
The reaction with the iodine being reversible can take place in the presence of an oxidizing agent like
iodic acid (HIO) or mercuric oxide (HgO).
R H + l2 R l + Hl
HlO
or HNO
3
3
4. From Silver Salt of Fatty Acid (Hunsdiecker’s or Borodine–Hunsdiecker’s Reaction):
carboxylic acid in carbon tetrachloride solution can be decomposed by chlorine or bromine to form alkyl
halides with one carbon atom less than in acid.
C6H5COOAg + Br2 C6H5Br + AgBr + CO
Silver benzoate Phenyl bromide
C
COOAg + Br2 C2H5Br + AgBr + CO
2H5
Silver propionate Ethyl bromide
CCl
Reflux
CCl
Reflux
2
2
5. By Halogenation Exchange
a. Finkelstein reaction: An alkyl iodide may be prepared by treating an alkyl chloride or bromide with
sodium iodide in methanol or acetone solution.
C
Ethyl bromide Ethyl iodide
Br + Nal C2H5l + NaBr
2H5
The reaction is possible because sodium iodide is soluble in methanol and acetone whereas sodium
chloride and sodium bromide is insoluble.
b. Reaction of Grignard’s reagent with I
RMgX + l2 R l + MgXl
Where X = Cl or Br
2
6. From Alkyl Hydrogen Sulphate: Alkyl iodides can be prepared by treating alkyl hydrogen sulphates
with an aqueous solution of potassium iodide.
C
Ethyl hydrogen Ethyl iodide
Sulphate
HSO4 + Kl C2H5l + KHSO
2H5
4
11.5 POLAR NATURE OF ALKYL HALIDES
The alkyl halides are polar in nature due to difference in the electronegativities of carbon and halogen atoms
The polarity decreases as we move down the halogen group because of decrease in electronegativity of
C–F > C–Cl > C–Br > C–I
Alkyl Halides
properties. Only because of the polar nature of alkyl halides, these are more reactive than its parent family
(i.e. hydrocarbons).
11.6 PHYSICAL PROPERTIES
1. Colour
while methyl iodide and some higher members are sweet smelling liquids; still higher members are
colourless solids.
2. Polarity
Solubility
Haloalkane CH3I CH3Br CH3CI CH3F
b.p. (K) 276.6 194.6
403
Haloalkane CH
b.p. (K) 276.6
Haloalkane CH
(Isomeric) (n
b.p. (K)
Note
3CH2CH2CH2CH4
Br CH3CH2Br CH3CH2CH2Br CH3(CH2)3Br
3
Br (CH3)2CHCH2Br (CH3)3 CBr
utyl bromide) (Isobutyl bromide) (tert
This is because of their inability to form hydrogen bonding with water and even failure to break the
hydrogen bonds already existing in water molecules. They are, however, soluble in typical organic solvents
67
).
4. Density: Alkyl chlorides are generally lighter than water; however, alkyl bromides and iodides are
RI > RBr > RCI
Among the alkyl halides, methyl iodide has the highest density. However, as the size of the alkyl
group increases, the densities of alkyl halides go on decreasing. All aryl halides are, however, heavier
than water.
5. The boiling points of alkyl halides are much higher than alkanes of comparable molecular masses.
This is because alkyl halides, being polar in nature, exhibit stronger van der Waals forces than alkanes,
404
2RI
7. Alkyl iodides become violet or brown on standing because they undergo decomposition in the presence
of light.
11.7 CHEMICAL PROPERTIES
reduction, etc. It is because of this reason that alkyl halides are considered to be the most useful synthetic
reagents in organic chemistry.
1. Replacement of halogen by hydroxyl group (formation of alcohol): When treated with aqueous
hydrolysis to form alcohols.
d+ d– + –
a.
R X + KOH
Alkyl halide (Aqueous) Alcohol
R OH + KX
d+ d– + –
b.
R X + AgOH
Alkyl halide (Moist silver oxide) Alcohol
+ –
C2H5 –Br + KOH C2H5 OH + KBr
Ethyl bromide (Aqueous) Ethyl alcohol
R OH + AgX
2. Replacement of halogen by alkoxy group (formation of ether): When treated with sodium alkoxide
a.
C2H5 Br + NaO CH3 C
Ethyl Sodium Ethyl methyl ether
bromide methoxide (Methoxyethane)
b.
C2H5 Br + NaOC2H5 C
Ethyl bromide Sodium Diethyl ether
(Bromo ethane) ethoxide (Ethoxyethane)
R X + RONa R O R + NaX
SN
SN
2
2
O CH
2H5
O C2H
2H5
+ NaBr
3
5
+ NaBr
Alkyl Halides
CH3 C N + H2OCH3CONH
2
Conc. H2SO4 or
Alkaline H
2O2
Methyl cyanide Acetamide
(Ethanamide)
This reaction is known as Williamson’s synthesis.
d+ d–
c.
2R X + Ag
Alkyl Dry silver Ether
halide oxide
O ROR + 2AgX
2
Replacement of halogen by carboxylate group (formation of esters): When treated with alcoholic
+ – + –
— Br + AgOOCCH3 CH3COOC2H5 + AgBr
C
2H5
Ethyl bromide Silver acetate Ethyl acetate
(Bromoethane) (Ethyl ethanoate)
4. Replacement of halogen by cyanide group (formation of alkyl cyanides): When treated with alcoholic
potassium cyanide, alkyl halides form alkyl cyanides (alkane nitriles).
Alkali metal cyanides are predominantly ionic and the cyanide ion is available as the nucleophile.
↔ + – and the resonance hybrid structure
– ion is an ambident nucleophile.
occurs through carbon because carbon atom with a lone pair is a better donor than the nitrogen atom
405
Alkyl cyanides are the important synthetic intermediates as they can be used to prepare many other
compounds such as amides, carboxylic acids and amines.
a. Formation of amides: On partial hydrolysis with concentrated hydrochloric acid or alkaline
hydrogen peroxide, alkyl cyanides form amides.
406
.
Ag – C � N + R–X R – N � C + AgX
Alkyl carbylamines
. .
–
R æ N+ C– + 4(H) RNHCH
3
Na/Alcohol
Alkyl methylamine
b. Formation of carboxylic acids: On complete hydrolysis with dilute mineral acid or alkali, alkyl
cyanides form monocarboxylic acid. Thus,
c. Formation of primary amines: On reduction with nascent hydrogen obtained by the action of
sodium and ethanol, alkyl cyanides from primary amines. This reaction is known as Mendius
reaction. Thus,
Reduction can also be achieved with H24).
5. Replacement of halogen by isocyanide (formation of carbylamines): When treated with silver cyanide
pair) available for bond formation. As a result, alkyl isocyanides are the chief products.
These on reduction give secondary amines, for example,
6. Replacement of halogen by nitrite group (formation of alkyl nitrite): When treated with potassium
nitrite, alkyl halides form alkyl nitrites.
O) like cyanide ion is an ambident nucleophile since it has two donor sites
(oxygen and nitrogen) through which it can attack an alkyl halide. Alkali metal nitrites are ionic
22
electron while nitrogen has no such extra electron and moreover anion has lower electronegativity
than neutral atom) to form predominantly alkyl nitrites.
Na+ O— N O + R X R O N O + Na+ X
Alkyl nirite
+d d–
—
Alkyl Halides
22
Na2S
a
7. Replacement of halogen by nitro group (formation of nitroalkanes): When treated with silver nitrite
2
tive atom, i.e. only nitrogen, are available for bond formation. As a result, silver nitrite predominantly
gives nitro compounds.
Replacements of halogen by disulphide group (formation of thioalcohol): When treated with sodium
C2H5 Br + N+aS–H C2H5 SH + NaBr
Ethyl bromide Sodium Ethyl thioalcohol
(Bromo ethane) hydrogen or
Sulphide Ethyl mercaptan
(Ethanethiol)
407
9. Replacement of halogen by mercaptide group (formation of thioethers)
10. Replacements of halogen by alkynide group (formation of higher alkynes): When treated with sodium
alkynide (R
11. Reaction with ammonia (formation of 1°, 2° and 3° amines and quaternary ammonium salt—ammonolysis
of halides):
408
b. A serious disadvantage of this reaction is the formation of more than one class of amines.
ammonolysis of alkyl halides.
NOTEWORTHY POINTS
Hunsdiecker reactions proceed through free-radical mechanism.1.
The yield of alkyl halide is: primary > secondary > tertiary.2.
Hunsdiecker reaction is used to reduce the length of carbon chain (descent of series).3.
Alkyl iodides cannot be prepared by this reaction because these form esters with silver salts.4.
2RCO2Ag + I
This reaction is called Birnbarun Simonini reaction.
RCO2R + CO2 + 2AgI
2
11.8 NUCLEOPHILIC SUBSTITUTION REACTIONS
Alkyl halides are highly reactive in nature. This is due to the fact that halogen atom is held rather loosely by
the carbon atom and hence can be easily removed.
CH3 X > C2H5 X > C3H7 X
Explanation for reactivity of alkyl halides:
d+ d–
R – X
Alkyl Halides
��
tert-Butyl bromide
tert-Butyl alcohol
11.8.1 Types of Nucleophilic Substitution Reactions
2 (nucleophilic substitution, bimolecular) reactions
1 (nucleophilic substitution, unimolecular) reactions
Kinetics of nucleophilic substitution reactions
It seems probable that the two mechanisms involved in alkyl halides with hydroxyl ions are the same with
1.
2.
409
These two reactions appear to be similar and are expected to follow similar mechanistic path. However,
both the reactions follow different kinetic order as illustrated below.
Rate = k [CH3Br] [KOH]
It means that the reaction results from the collision between methyl bromide and hydroxyl ion. This is
found to be so.
In second reaction, the rate is dependent on the concentration of alkyl halide only and is independent of
base concentration, i.e.
Rate = k [C4H9Br]
account for this difference, two different mechanisms are proposed for these reactions.
1
mechanism.
2
2 reactions are given below.
11.8.2 Mechanisms of SN2 (Substitution Nucleophilic Bimolecular)
–
–
410
HO + H – C – Br HO C Br HO – C – H + Br
–
HH
H
+δ –δ
δ –
δ –
HHHH
–
sp2-hybridized
carbon
p-Orbital of
sp
2
-carbon
H
Br
Carbon and
three H-atom
lie in a plane
Transition state for a SN
2
reaction
H
HO
–δ
H
C
2) proceeds through a transition state involving two
Transition state of a SN2 reaction
In the transition state, the central carbon atom is sp2
p-
bond is partly broken.
Factors which influence SN2 reactions
1. Reactivity of alkyl halides: The basic requirement of mechanism is the backside attack of the incoming
nucleophile on the carbon atom carrying the halogen atom. This type of attack is quite easy in the case of
primary halides as the small size of hydrogen atom does not hinder the attack. However, in case of tertiary
alkyl halides, there are three bulky groups attached to carbon atom carrying a halogen atom. These alkyl
groups cause overcrowding and hinder the incoming nucleophile. This type of hindrance is called steric
hindrance
2 reaction decreases as the bulk of the substituents increases.
CH3X > Primary alkyl halide > Secondary alkyl halide > Tertiary alkyl halides > Neopentyl halides
2. Nature of the halogen atom: 2 reactions, the negative charge is normally
distributed over the nucleophile and the leaving group. The better the leaving group, more stable is the
transition state due to greater stabilization of the negative charge by the leaving group and hence more rapid
is the reaction.