Hazard warnings
As all organic compounds have harmful vapors and con be toxic by absorption through the skin. Some are flammable. We must:
- keep the stoppers in the bottles as much as possible
- keep the bottles away from flames
- wear safety spectacles and gloves
- carry out experiment in good ventilation
Procedure
- Apparatus were set up as shown in Fig. 1. All test tubes were made sure not containing any tap-water.
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2cm3 of ethanol was pour into each of four test-tubes and they were marked with letters A to D
- 3-4 drops of 1-chlorobutane was added to A, 3-4 drops if 1-bromobutane was added to B, 3-4 drops of 1-iodobutane to C and 3-4 drops chlorobenzene was added to D.
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5cm3 of silver nitrate solution was pour into the fifth test-tube.
- All the test-tubes were standed in the beaker and heated to 60℃. Then, the Bunsen burner was removed.
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1cm3 of aqueous silver nitrate was quickly added to each of the tubes A to D and the stopclock was started. Each tube was shake once to mix the contents, and was leaved in the water with the cork resting loosely on the tube to reduce evaporation.
- The tubes were being watched continuously for about ten minutes and the time when a precipitate first appears in each tube as a definite cloudiness was noted. The water was heated to 60 again at intervals.
- Tubes were being observed at intervals for about 30 minutes more, noted for any further changes in the appearance of the precipitates.
Results Table
Triad 1 Test 2
Melting Point of 2,4-dinitrophenylhydrazone/℃ 123-128 127-128
Structural formula of the Unknown carbonyl compound
Chemical name of the unknown carbonyl compound propanone
Interpretation of results
From the experimental results, compounds in order of speed of hydrolysis, from fastest rate first are:
1-iodobutane > 1-bromobutane > 1-chlorobutane > chlorobenzene
We can compare the rate of hydrolysis of halogen-compounds in 2 categories. The first one is among the haloalkanes (1-chlorobutane, 1-bromobutane and 1-iodobutane), and the second one is between haloalkane and halobenzene.
- Among the haloalkanes (1-chlorobutane, 1-bromobutane and 1-iodobutane)
All three haloalkanes give positive result, i.e. precipitate formed, in the experiment. The equations are:
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 rates of hydrolysis among 3 haloalkanes, fastest rate first are:
1-iodobutane > 1-bromobutane > 1-chlorobutane
Consider the bond energy of haloalkanes:
Since the rate determining step involves the bond breaking of the between the C－X bond in the haloalkane, where X = Cl, Br or I. The greater the bond energy, the greater the bond strength and the lower the reactivity, therefore:
Bond energy: C－Cl > C－Br > C－I
Bond strength: C－Cl > C－Br > C－I
Reactivity: RCl < RBr < RI
As least energy is required to break the C－I bond, it is the weakest bond and most reactive, therefore the rate of hydrolysis of 1-iodobutnae is the greatest, vice versa.
- Between haloallane and halobenzene
All the haloalkanes give positive result but the halobenzene (chlorobenzene) give a negative result, i.e. no observable change in the experiment.
The reactivity of halobenzene is much lower than that of haloalkanes. In chlorobenzene, the lone-pair electrons of chlorine become involved in the π-electron system of the benzene ring.
The p-orbital of the Cl atom interact with those of the ring, causing a drift of electrons towards the degree of polarity of C－Cl bond, the C－Cl bond has partial double bond nature. This also account for the high C－Cl bond energy in chlorobenzene, 365 kJ mol-1, compare with those of halogalkanes.
Therefore, chlorobenzne (halobenzenes) is comparatively inert and only undergo nucleophilic substitution of the Cl atom with extreme difficulty.
Discussion on Errors
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There was time lag on adding AgNO3 solution into different halogen compounds. For example, the stopclock was started when adding the AgNO3 solution into the first test tube. Time lag existed when adding AgNO3 solution into following tubes. Thus, time measured for the first appearance of precipitate is longer than the actual one.
We can use separate stopclock for each set of data, i.e. one stopclock is responsible for measuring the time for first appearance of precipitate of one chemical only. Thus, the time lag in measuring can be reduced. Or we can have more technicians involved in the experiment to pour the AgNO3 solution into the test tubes at the same time.
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There were impurities left of the test tubes. Once the AgNO3 solution was added, the solution gave precipitate immediately. The time measured was not because of the release of halogen during the substitution.
We can make sure the test tubes do not contain any water. Clear test tubes should be used.
- All the halogeno-compounds used in the experiment are volatile. The chemicals may evaporate during the experiment, leading to lost of chemicals.
Cover the tubes with stoppers loosely to minimize the effect of evaporation of chemicals.
- The temperature of the water bath was not constant, which affected the rate of reaction. Thus, the time for first appearance of precipitate was affected.
Use an electrical water bath to keep the temperature of the chemicals constant.
Conclusion
The rate of hydrolysis of 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 increases with the decrease bond strength between the carbon atom and the leaving group.
Questions and Answers
- chlorobenzene < 1-chlorobutane < 1-bromobutane < 1-iodobutane
- 1-iodobutane > 1-bromobutane > 1-chlorobutane > chlorobenzene
- 1-chlorobutane:
1-bromobutane:
1-iodobutane:
Comment
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SN2 mechanism
The hydrolysis of organic halogen compounds is a kind of nucleophilic substitution. As all the halogen compounds used in this experiment is primary halogen compounds, the primary carbocation is unstable and the steric hindrance is small, which flavored the SN2 mechanism.
Therefore, halogen compounds in this experiment undergo SN2 mechanism more readily, here is the general equation for the SN2 mechanism is:
The term SN2 stands for Substitution reaction, Nucleophilic, 2nd order. According to the SN2 mechanism, there is a single transition state because bond-breaking and bond-making occur simultaneously, which is a one-step reaction mechanism. For this to occur, the nucleophile approach from the backside of the carbon-leaving group bond (so-called backside attack). There is not intermediate in an SN2 reaction, just a transition state. A transition state has no real lifetime, it is the highest energy point on the reaction coordinate as starting materials transition into products.
In this reaction, a new bond is formed between the nucleophile, H2O, and the carbon atom, while the carbon-halogen bond is broken. The departing halogen is referred to as the leaving group. The species being attacked by the nucleophile is referred to as the electrophile.
SN2 is a second order kinetics, rate of reaction is totally depended on both [OH-] and [RX]
Rate = k [RX] [OH-]
The energy level diagram of the SN2 reaction:
- Photodecomposition of AgBr (s)
At the end of the experiment, it was found that the solution containing AgBr(s) turns from pale yellow to grey. This is due to the decompostition of AgBr(s) in the presence of sunlight.
2AgBr (s) → 2Ag(s) + Br2 (g)
- Nucleophilic substitution of halobenzenes
As the lone-pair electrons of the halogen become involved in the π-electron system in the aromatic ring, the halogbenzenes are comparatively inert and only undergo nucleophilic substitution with extreme difficulty
Under 300 ℃, 200 atm, halobenzenes undergo mucleophilic substitution with presence of conc. NaOH to give phenol, C6H5OH.
Reference
- Study Guide Chapter 30
P. 7, 12-13
2. Haloalkanes
3. AUS-e-TUTE
