Chemical properties
The features of the molecules of haloganoalkanes which make them susceptible to nucleophilic substitution:
Owing to the high electronegativity of halogens, the C—X bond is highly polar. The polarization of the C—X bond makes the electron deficient carbon atom susceptible to attack by a electron rich nucleophile.
Halogen atom can form a stable leaving ion.
The following are the fair tests:
Safety Precautions
Apparatus and Chemicals Used
Chemicals: Ethanol, C2H5OH (2 cm3)
1-chlorobutane, C4H9Cl (4 drops)
1-bomobutane, C4H9Br (4 drops)
1-iodobutane, C4H9I (4 drops)
Chlorobenzene, C6H5Cl (4 drops)
0.05 M Silver nitrate solution. AgNO3 (5 cm3)
Apparatus: Bunsen burner, tripod, gauze and bench protection sheet
Beaker, 250 cm3
Thermometer, 0-100 oC
5 test-tubes fitted with corks,
Test-tube rack,
Measuring cylinder, 10 cm3
Safety spectacles, Protective plastic gloves,
Stop-clock
Procedures
1. The apparatus shown in Fig. 1 was set up.
2. 2 cm3 of ethanol was poured into each of four test-tubes and with the letters A to D
marked.
3. 3-4 drops of 1-chlorobutane were added to A, 3-4 drops of 1-bromobutane were added to B, 3-4 drops of 1-iodobutane were added to C and 3-4 drops of chlorobenzene were added to D.
4. About 5 cm3 of silver nitrate solution was poured into the fifth test-tube.
5. All the test-tubes were put in the beaker (or water-bath) and were heated to 60oC. Remove the Bunsen burner.
6. 1 cm3 of aqueous silver nitrate was added quickly to each of the tubes A to D and the stop-clock was started. Each tube was shaked once to mix the contents, and was left in the water with the cork resting loosely on the tube to reduce evaporation.
7. The tubes were continuously watched for about ten minutes and, in a copy of Results Table 1, the time was noted when a precipitate first appeared in each tube as a definite cloudiness. The water was heat to 60oC again at intervals.
8. Continue observation at intervals for about 30 minutes more, any further changes was noted in the appearance of the precipitates.
Results Table
Results Table 1
Interpretation of the result
Comparison between haloalkane
They all shows positive results but the rate of hydrolysis is different.
Ag+ (aq) + Cl- (aq) → AgCl (s) White precipitate
Ag+ (aq) + Br- (aq) → AgBr (s) White precipitate
Ag+ (aq) + I- (aq) → AgI (s) Yellow precipitate
The reactivity of halo group on the rate of hydrolysis reaction is:
1-iodobutane> 1-bromobutane>1-chlorobutane
Bond energy: C-Cl > C-Br > C-I
Bond strength: C-Cl > C-Br > C-I
Reactivity: RCl < RBr < RI
Since we can see that bond energy required to break the C—X bond decrease down the group. So the hydrolysis becomes more readily down the group. It is mainly duo to the increase bond length of C—X bond down the group since the atomic size of halogen increase down the group. The longer the bond length, the weaker the bond strength, the more reactive the solution is.
Comparison between haloalkane and halobenzene
haloalkane shows positive result while halobenzene shows negative one.
Chlorobenzene is comparatively inert. Since the lone-pair electrons of the chlorine atom become involved in the π electron system in the aromatic ring.
The p-orbital of the chlorine atom interact with those of the ring, causing a drift of electrons towards the carbon atom to which it is attached, and thus reducing the degree of polarity of the C—Cl bond and increase the bond strength of C—Cl bond ,the C—Cl bond has partial double bond nature. That is why in the above table we can see that the bond energy of C—Cl bond in chlorobenzene is higher than those three.
Discussion on Errors
- Error: there is time delay by extracting 1cm3 of aqueous silver nitrate and adding to one tube one by one.
Improvement: use 4 stop watches separately to count for each substance for its appearance of white precipitate.
- Error: evaporation of the organic content from the test tubes.
Improvement: cover each tube with the cork resting loosely to reduce evaporation
- Error: time is noted for the saturated precipitate.
Improvement: time is noted for the instant appearance of precipitate. Since it is very hard to determine the same intensity of precipitate formed by each substance to be noted.
- Error: there is a little tap-water found in the test-tubes, which may spoil the result.
Improvement: use tissue to wrap each test tube.
- Error: the temperature fluctuates a lot. A sudden increase in temperature when the Bunsen burner is put near.
Improvement: do not open the air hole of Bunsen burner. Use luminous flame to keep the temperature constant
Questions and Answers
- Use the data listed in Table 2 to predict the likely order of reactivity of the following compounds:
1-chlorobutane, 1-bromobutane, 1-iodobutane, chlorobenzene
Ans1: chlorobenzene<1-chlorobutane<1-bromobutane<1-iodobutane
- From your experimental results, list the compounds in order of speed of hydrolysis, fastest first
Ans2: chlorobenzene<1-chlorobutane<1-bromobutane<1-iodobutane
-
Write equation for the hydrolysis reaction which takes place in this experiment.
CH2ClCH2CH2CH3 + H2O ⎯⎯⎯→ CH2OHCH2CH2CH3+ H+ + Cl
-
CH2BrCH2CH2CH3 + H2O ⎯⎯⎯→ CH2OHCH2CH2CH3+ H+ + Br-
CH2ICH2CH2CH3 + H2O ⎯⎯⎯→ CH2OHCH2CH2CH3+ H+ + I-
Conclusion:
The rate of hydrolysis of organic halogen compounds in ascending order:
Chlorobenzene <1-chlorobutane < 1-bromobutane< 1-iodobutane
The rate of hydrolysis 1-iodobutane is the fastest, followed by 1-bromobutane, 1-chlorobutane and chlorobenzene. The rate of hydrolysis of haloalkane is greater than that of halobenzene, while it increase with the decrease of bond strength between the carbon atom and the leaving group
Comment
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Mechanism- nucleophilic substitution
why in this experiment the hydrolysis undergoes Sn2 rather then Sn1?
-
Since the reactants is primary haloalkane which flavors Sn2. The primary
carbocation which is less stable is formed.
- The reactants are primary haloalkane so they are less bulky. So the steric hindrance is small, results in low energy of transition state and increase the activity in Sn2 mechanism.
-
Choice of nucleophile
strong nucleophile in high concentration flavors Sn1 while weak nucleophile in dilute solution flavors Sn2.
And this time in this experiment, H20 is weak nucleophile so it flavors Sn2.
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Bimolecular Nucleophilic Substitution , SN2
SN2 applies mainly to methyl and primary haloalkanes.
e.g.C4H9Br + OH⎯⎯⎯→C4H9OH + Br-
Mechanism:
In this mechanism, the rate determining step involves 2 molecules (H2O and RX.).Hence, we use the term ‘bimolecular’. And the rate of reaction should depend on the concentrations of both chlorobutane and hydroxide ion. Hence a second order reaction is observed.
rate = k[H2O][RX]
The energy change for this SN2 mechanism is shown below.
Energy level diagram of the Sn2 reaction
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Hydolysis for phenol( industrial process)
halobenzene only undergo nucleophilic substitution of the halogen atom at extreme condition(conc.NaOH,350 oC, 200atm to give phenol)
-
Photodecomposition of AgBr
At the end of experiment, there is some grey precipitate deposited in one of the tubes.It is mainly due to the fact that silver bromide is decomposed to give silver.
2AgBr(s) ⎯⎯⎯→2Ag(s) + Br2 (g)
Reference