Download Rates of Hydrolysis of Some Halogeno-compounds

Title : Rates of Hydrolysis of Some Halogeno-compounds
Theory :
In this experiments we are going to study the effect of the structure of
the halogeno-compounds on the rate of hydrolysis.
1-chlorobutane, 1-bromobutane and 1iodobutane can be classified as
'Haloalkanes' while bromobenzen can be classified as 'Halobenzenes'.
Haloalkanes (鹵烷) (also known as alkyl halides) and halobenzenes
(鹵苯) (also known as aryl halides) are organic compounds containing a
halogen atom as the functional group. Haloalkanes have the general
formula CnH2n+1 X or simply R - X where R is any simple alkyl or
substituted alkyl group and X is a halogen atom. Halobenzenes have
the general formula Ar - X, where Ar is a phenyl or substituted phenyl
group. A halogen atom is attached directly to an aromatic ring in the
halobenzene. Haloalkanes undergo a number of reactions and are
useful in making a large number of organic compounds. In contrast,
halobenzenes are chemically unreactive.
Haloalkanes can be classified into primary, secondary and tertiary
according to the nature of the carbon atom that the halogen atom is
attached to.
The three classes of haloalkanes show different
reactivities.
H
R
C
H
X
R
C
R"
X
R
C
H
R'
R'
Primary
Secondary
Tertiary
haloalkane
haloalkane
haloalkane
X
In halobenzene, the halogen atom is bonded directly to the benzene
ring.
For example,
Chlorobenzene
1,2-Dichlorobenzene
(Chloromethyl) benzene is not a halobenzene because the halogen
atom is not directly attached to the benzene ring but is located in an
alkyl side chain. (Chloromethyl) benzene is thus a phenyl-substituted
haloalkane and not halobenzene.
The typical reaction of a haloalkane is nucleophilic substitution. The
carbon-halogen bond in the haloalkane is polarized with a small positive
charge on the carbon atom and so in the reaction, the haloalkane acts
as an electrophile.
C +
X-
In nucleophilic substitution, a nucleophile attacks the C+ and
displaces a halide ion from the haloalkane. A large variety of
compounds can be formed depending on the nucleophile used in the
reaction.
The nucleophilic substitution of haloalkanes is a very
important type of synthetic reaction. This reaction can be applied in
the preparation of alcohols, ethers, esters, nitrides and amines when
substitution occurs with by —OH, —OR, —OOCCH3, —CN and —NH2
groups respectively.
Besides substitution, haloalkanes may undergo elimination of the
hydrogen halide to form an alkene. An alkyne can be formed from the
elimination reaction of an alkene. Both elimination and substitution are
brought about by basic, electron-rich reagents. Hence there is always
competition between the two types of reactions.
Halobenzenes are comparatively unreactive in nucleophilic
substitution reactions. The low reactivity is related to the structure of
the compound. The p-orbitals on the carbon atoms of the benzene ring
and that on the halogen atom overlap sideways to form a delocalized
-bond system (resonance structure).
However nucleophilic
substitution of the halogen atom may be promoted by the presence of
electron-withdrawing groups, e.g. —NO2 and —CN in the ring.
The consequences of the delocalization of -electrons throughout
the benzene ring and the halogen atom are:
1. the carbon-halogen bond is strengthened by its partial -bond
character. The breakage of the bond requires a larger amount of
energy and so the substitution reaction becomes more difficult, and
2. the polarity of carbon-halogen bond is decreased, making the C
atom much less susceptible to nucleophilic attack.
The halogen atom exhibits both negative inductive effect and positive
resonance effect on the benzene ring. As the overall effect is electron
withdrawing, the reactivity of halobenzenes is lower than that of
benzene. However, the back-donation of electrons enhances electron
density at 2- and 4- positions, thus halobenzenes form 2, 4-directing
derivatives. The aromatic ring to which the halogen atom is attached
can undergo typical substitution reactions of benzene.
Chemicals
Ethanol, 0.1 M AgNO3, 1-chlorobutane, 1-bromobutane, 1-iodobutane,
bromobenzen
Apparatus
Test tube, -10-110C thermometer, 10cm3 measuring cylinder, teat
pipette, 250 cm3 beaker
Procedure
[Hazard Warning:
Ethanol and 1-chlorobutane are flammable,
1-bromobutane and 1-iodobutane are harmful; and
bromobenzene is irritant.]
A. To compare the rates of hydrolysis of chloro-, bromo-, and
iodo-alkane
1. 2cm3 of ethanol was added to three separate test tubes and
was placed in a water bath at about 60C.
2. 1cm3 of 0.1M silver nitrate (V) solution was added to each
test-tube.
3. 5 drops of 1-chlorobutane was added to the first test-tube; 5
drops of 1-bromobutane was added to the second test-tube; 5
drops of 1-iodobutane was added to the third test-tube.
4. The test tubes were shaken and the order in which the
precipitates appear was observed.
The colour of the
precipitates formed in each case was noted.
B. To compare the rate of hydrolysis of aliphatic and aromatic
halogen-compound.
Part A was repeated first at room temperature and then in hot
water (at 60C) with 1-bromobutane and bromobenzene.
(Take care that there is enough ethanol present to dissolve the
aroma halogeno-compound. A slight turbidity on mixing may
be due to an emulsion the organic compound with water. If
this happens, add a few drops of ethanol and shake until the
solution is clear.)
Result:
Part A
Reaction
Time needed for the
precipitate to appear
Observations
A.1-chlorobutane
B.1-bromobutane
C.1-iodobutane
Part B
Reaction at 60C
Time needed for the
precipitate to appear
Observations
Time needed for the
precipitate to appear
Observations
1-bromobutane
Bromobenzene
Reaction at room
temperature
1-bromobutane
Bromobenzene
(HALOGENO-COMPOUNDS.DOC)