Results Recorded
#Water is present in all reactions from the AgNO3(aq) which acts as an reactant.
Calculations
No calculation is needed, visual compare only. Hurray!
Discussions
Introduction
The reactions involved are all nucleophilic substitution which involves SN1 and SN2 mechanisms. The reaction rate of the two reaction path ways determines by several factors which includes the structure of the substrate, the strength of the nucleophile and the stability of the leaving group.
In our experiment, all reactions involve water acting as the nucleophile; the lone pair electrons of the oxygen atom seek an electron deficient site to attack. Therefore the strength of the nucleophile is the same in all reactions.
The nature of all the reactions is to substitute a hydroxyl group to a halide group, forming an alcohol product from the haloalkane. The leaving group is the halide ion which varies from chlorides, bromides and iodides depending on the substrates. Therefore the bonding energy / bonding length also varies depending on the halide group.
Another important factor affecting the reaction rate is the structure of the hydrocarbon skeleton. Alkyl groups are electron releasing while phenyl group may form resonance structure in the reaction intermediate state. Such differences affect the bonding strength of the C-X bond and as well as the stability of the reaction intermediate, which in turn influences the reaction rates. Examples of the hydrocarbon skeleton affecting reaction rates will be discussed later on when explaining the reaction of chlorobenzene.
1-chlorobutane, 1-bromobutane, 1-iodobutane
The reaction involved in 1-chlorobutane, 1-bromobutane and 1-iodobutane are very similar, they are all primary haloalkanes and they possess the same hydrocarbon skeleton – butane. Their reaction pathways are also similar. The substrate is favour to SN2 mechanism as it is a primary haloalkane, the carbon atom where the halogen atom is attached to, has two hydrogen attached to it. The molecule is favoured to the backside attack of the SN2 pathway. Steric hindrance is not significant.
The reaction rate of the three similar substrates are contrasted by the carbon halogen C-X bonding. The shorter the bond length, the stronger the bond; the stronger the bond the more difficult it is for the successful attack to occur. The bond length depends on (1) the atomic size of the halogen atom and (2) the polarity of the bond.
Halogen atoms has an increasing atomic size according to: Cl > Br > I . The covalent bonding is the interaction of the outermost shell of electron between the halogen atom and the carbon atom. The smaller the atomic radius, the halogen atom nucleus will exert a stronger the nuclear attraction to the bonding electrons. Therefore Cl atoms have the strongest covalent bonding when compared with Br and I. The polarity of the bond depend on the difference in electronegativity between the bonding atoms, as Cl is more electronegative than Br than I, the polarity of the bond also follows the order: Cl > Br > I .
Concluding the above two effect, the order of increasing reaction rate of hydrolysis should be: 1-chlorobutane < 1-bromobutane < 1-iodobutane.
1-chlorobutane, 2- chlorobutane, 2-chloro-2-methylpropane
The difference between 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane is the level of substitution of the halogen attached atom. 1-chlorobutane has only one alkyl group attached to the ‘Halogen attached carbon atom’, therefore it is a primary haloalkane. 2-chlorobutane is a secondary haloalkane and 2-chloro-2-methylpropane is a tertiary haloalkane.
The relative rates of SN1 reactions of haloalkanes: (From Textbook)
The experimental condition favours SN1 reaction (don’t bother the reason behind), and thus the reaction rate determines mainly on the stability of the carbocation intermediate formed in the reaction. We know that tertiary haloalkane forms tertiary carbocation which is more stable than secondary and primary carbocation, thus the rate of reaction follows the pattern:
1-chlorobutane (primary) < 2-chlorobutane (secondary) < 2-chloro-2-methylpropane (tertiary)
Chlorobenzene
In chlorobenzene, the p-orbital on the carbon atom of the benzene ring side-way overlaps with the p-orbital of chlorine atom and form a delocalized π bonding system. The carbon-chlorine bonding gains extra stability by the (1) partial π bonding character. The delocalizing of electrons through the π bonding system reduces the polarity of the bond. The chlorine atom is less negative causing the group harder to leave as an anion, carbon atom is less positive and becomes much less susceptible to nucleophilic attack.
The delocalized π bonding system in the benzene ring also cause a high density electron cloud to form at the back-side of the carbon atom. Nucleophile which is either negatively charged or slightly negative is repelled from the back side of the molecule, thus un-favoring the SN2 reaction.
The forming of a benzene cation in SN1 reaction is also a very unstable intermediate because it requires the breaking the benzene resonance structure in its formation. This requires a very large activation energy thus its reaction is not favored. Combining the number of disadvantages in the process of hydrolysis of chlorobenzene to phenol, the reaction will only proceed under severe condition with sodium hydroxide as the nucleophile.
Notes on Procedure Precautions
The test tubes used for this experiment should be completely free of halide ions in order to obtain accurate result while the silver ions form precipitations.
The use of alcohol is to act as a common solvent to increase the contact chance of reactants: H2O and the haloalkanes. Its importance is to increase the rate of reaction and obtain reliable results.
Results
In the experimental results, we see a strange result for the reaction involving the secondary haloalkane, the tube produce no observable result which was expected. This might be due to insufficient temperature for the SN1 reaction to overcome the EA in the rate determining step.
The experiment should include a warming step which provides some kinetic energy to the molecules. The manual suggest a water bath at 60°, however, due to lab limitation, this step is skipped, thus the reaction may occur at a very slow rate. The absence of precipitation of 1-chlorobutane might also be related to the low reaction rate.
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