Procedures
- Comparison of the rates of hydrolysis of chloro-, bromo- and iodobutane
- A 60C water bath was prepared.
- 2cm3 of ethanol was poured respectively into 3 clean and dry test tubes.
- 1cm3 of 0.1M silver nitrate solution was poured into each of the 3 test tubes.
- The 3 test tubes are put in the water bath for several minutes until the 3 test tubes are in the same temperature of about 60.
- 5 drops of 1-chlorobutane was added to the first test tube. Then 5 drops of 1-bromobutane to the 2nd one and 5 drops of 1-iodobutane to the 3rd one as quickly as possible.
- The 3 test tubes were shaken.
- The speeds at which precipitates appear were recorded.
- The colors and the densities of precipitates were recorded.
- Comparison of the rates of hydrolysis of primary, secondary and tertiary haloalkanes
- PartA is repeated at room temperature using 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane.
- The speeds at which precipitates appear were recorded.
- The colors and the densities of precipitates were recorded.
- Comparison of the rates of hydrolysis of aliphatic and aromatic haloalkanes
- PartA is repeated at room temperature using 1-bromobutane and bromobenzene.
- The speeds at which precipitates appear were recorded.
- The colors and the densities of precipitates were recorded.
Results
- Comparison of the rates of hydrolysis of chloro-, bromo- and iodobutane
- Comparison of the rates of hydrolysis of primary, secondary and tertiary haloalkanes
- Comparison of the rates of hydrolysis of aliphatic and aromatic haloalkanes
Conclusion
- Iodo- is more reactive than bromo- than chlorobutane respectively.
- Tertiary is more reactive than secondary than primary haloalkane respectively.
- Aliphatic is more reactive than aromatic haloalkanes.
Discussion
The halide ion departs from the haloalkane as a leaving group with the breakage of the C-X bond. For the halide ions in the above experiment part A, iodobutane is the most reactive, followed by bromobutane and chlorobutane respectively. It’s because the size of I- is largest among three, the electrons are the most further away, this makes the electrons cloud easier to be polarized. I- is said to be more polarizable. As the electrons are far away from the nucleus, the C-I bond is much weaker than C-Br and C-Cl bond and hence, I- is the best leaving group, followed by Br- and Cl- respectively. As a result, iodobutane is most reactive.
Kinetic studies show that the nucleophilic substitution reactions proceed by two different reaction mechanisms. They are bimolecular nucleophilic substitution (Sn2) and unimolecular nucleophilic substitution. (Sn1)
For Sn2, it just involved 1 step. There are no intermediates. The reaction proceeds through the formation of an unstable arrangement of atoms called the transition state. The rate of reaction depends on both concentration of haloalkane and hydroxide ion. Two species are involved in the rate determining step of the reaction.
Rate = k [haloalkane] [OH-]
For Sn1, it involved 2 steps and one intermediate is formed. The first step is the rate determining step and a carbocation is formed. The rate of reaction is independent of the concentration of hydroxide ion.
Rate = k [haloalkane]
In part B of the experiment, the experimental condition favours Sn1 reactions because the substrate bromobutane is rather bulky. Due to the effect of steric hindrance, bulky substituents on or near that carbon atom have a dramatic inhibiting effect. This causes the energy of the transition state increases, and a higher activation energy of the reaction is resulted.
The result shows that tertiary haloalkane react in the fastest rate as it gives the tertiary carbocation intermediate in the reaction. The central positive carbon atom of tertiary carbocation is surrounded by 3 electrons releasing methyl groups. The electrons releasing groups help reduce the positive charge on central carbon atom by exerting positive inductive effects. Thus 3° butyl carbocation is the most stable among the three carbocations. The greater the number of alkyl groups attached to the central carbon atom, the more dispersion of the charge and the more stable is the carbocation. Therefore 3° butyl carbocation is the easiest to form followed by 2° and 1°. Thus tertiary haloalkanes is the most reactive followed by 2° and 1°.
In part C of the experiment, it shows that 1-bromobutane is more reactive than bromobenzene, i.e. aliphatic haloalkanes is more reactive than the aromatic ones. It is because of the delocalization of electrons throughout the benzene ring and the halogen atom caused the C-Br bond of a halobenzene being stronger than haloalkane. The C-Br bond of a halobenzene has some partial double bond character due to it partial bond character, which make the breaking the C-X bond requires a larger amount of energy, substitution reactions of halobenzenes become more difficult to occur, just like the case in the above experiment, when the halobenzene is attacked by nucleophile H2O. Besides, the delocalization of electrons makes the polarity of the C-X bond decreased, and thus the electropositive carbon centre is much less susceptible to nucleophilic attack. Also, the delocalized electrons in the benzene ring tend to repel any approaching nucleophiles. These explain halobenzenes are unreactive towards Sn2 reaction.
Further, benzene cations are highly unstable because the loss of acromaticity. This explains the unreactivity of halobenzenes towards Sn1 reaction.
In the experiment, the nucleophile is the water in aqueous silver nitrate. H2O is acted as the nucleophile but not ethanol because ethanol had a low Ka which is not easily ionized. Although H2O itself is not a very strong nucleophile, it is the nucleophile in this experiment by providing OH- to attack the electropositive carbon centre. The relative strength of nucleophiles is correlated with 2 features:
- Negative charged nucleophile is stronger than neutral one.
- If the nucleophilic atom is the same, the nucleophilicity follows the order of basicity. (the power to donate e-)
In this experiment, ethanol is acted as a solvent to dissolve water and silver nitrate which is polar with halogeno-compound.
Silver nitrate can be used to test for halide ions because the halide ions displace out will form silver halide with the Ag+ ions. As AgCl, AgBr, AgI are solid and are not soluble in water, they shows as precipitates.
Ag+ (aq) + Cl- (aq) → AgCl (s)
Ag+ (aq) + Br- (aq) → AgBr (s)
Ag+ (aq) + I- (aq) → AgI (s)
Sources of error and Way of Improvement
The water bath is set up by a Bunsen flame and the temperature is measured with thermometer. However, the temperatures always fluctuate. The temperature can influence the reaction rate greatly. A thermostatic bath can improve the accuracy of the experiment by keeping the temperature at 60C constantly. In the experiment, though it is difficult to keep the temperature at 60C constantly, the temperature in the three test tubes should keep the same as a fair test.
A dropper is used when adding the halogeno-compound to the test tubes. However, dropper is not an accurate apparatus; the size of a drop can be very different. The amount of halogeno-compound added will directly affect the reaction rate thus the amount of precipitate. To improve, a pipette can replace the dropper. In using a pipette, more time will be need, it is quite impossible to compare the 3 test tubes at the same time. Some changes will be needed:
1. Use a timer to record the time for precipitates to appear. Or
2. Prepare the measured halogeno-compound before adding to ethanol and silver nitrate.
