General formula- Rhal, (r is alkyl group, and Hal is a halogen atom)
In this experiment I will investigate the effects of the nature of the halogen atom (Hal) has on the reactivity of the halogenoalkane.
The rates of hydrolysis of different halogenoalkanes can be investigated by:
* Alkaline hydrolysis with NaOH(aq)/H2O, reflux:
R-X + OH–>R-OH + X
* Acidification with dilute nitric acid. This removes excess NaOH, which would otherwise form a precipitate with the silver nitrate in the next stage. Preventing detection of silver halides.
* Addition of AgNO3(aq) shows the presence of aqueous halide ions
Ag(aq) + X (aq)–>AgX(s)
The intensity of any precipitate indicates the extent and rate of hydrolysis.
Two factors that effect the rate of hydrolysis
Polarity predicts that the more polar C-Cl bond would attract water most easily and would give the faster reaction.
Bond enthalpy predicts the opposite that the C-I bond would be broken the most easily, therefore giving the fastest reaction.
For this reaction it is the bond empathy that is more important than polarity
So as the C-X bond weakens the rate of hydrolysis increases.
From research I have done on the inter net and from preliminary experiments in the classroom, I predict that the 1-iodobutane with produce a precipitate first, next the 1-bromobutane, and finally the 1-chlorobutane. This is because C-I bond in 1-iodobutane would be broken most easily and that C-Cl bond would be broken least easily.
Apparatus and chemicals needed
* Test tubes
* Test tube rack
* Stop clock
* 250cm beaker
* Bunsen burner
* Heat proof mat
* Silver nitrate solutions, 0.01mol dm
To make sure my results are accurate and reliable I will:
* When measuring the liquid and aqueous chemicals in this experiment I will measure to the bottom of the meniscus to make sure my measurements are as accurate as possible.
* Use a thermometer in both the water and inside the tubes themselves to make sure the hydrolysis takes place at the same temperature.
* In the repeated experiments I will use chemicals from the same bottles as chemicals mixed in different batches may vary slightly in their concentrations and/or properties.
* I will repeat the experiments twice and take an average of the two to increase the reliability of my results.
* To avoid contamination of the chemicals I will either use a different pipette or thoroughly wash the other pipette out before measuring a different chemical.
Safety is exceptionally important and it is essential to follow safety procedures. It is important to wear eye protection when working with any chemicals that are irritant. Also long hair should be tied back. The halogenoalkanes are harmful when inhaled so every effort should be made not to breathe them in. Also as the chemicals used are highly flammable (1-iodobutane is flammable) it is important to take special care while working with and around them especially when near naked flame. Stoppers should be replaced straight away after use and kept well away from naked flames. Also to cuts and broken skin should be covered, and hands should be washed after handling the chemicals. If contact with skin does occur it should be washed with cold water immediately.
* Water was heated in the beaker to just over 50oC(measured with thermometer in water) then the heat was removed and beaker left to rest.
* Next six test tubes were placed in a rack.
* Into three 1cm of ethanol was added (to each), these tubes were then labelled chloro, bromo and iodo then 2 drops of the corresponding halogenoalkane was added.
* Into the other three beakers 1cmof 0.01moldm was added to each.
* Next all six beakers were placed in the water in the beaker now with the temperature of 50oC.
* After ten minutes the temperature inside the beakers had risen to 50oC.
* one tube of warm silver nitrate solution was then add to each of the test tubes containing the halogenoalkanes and ethanol.
* These tubes were then shaken and observed for a period of two minutes and times were noted as to when and if a precipitate was formed and if a precipitate was formed within the two minutes the time was recorded.
* The experiment was then repeated using the same procedures.
I conclude that 1-iodobutane is reactive than, both 1-chlorobutane and 1-bromobutane, and that 1-bromobutane is more reactive than 1-chlorobutane. I can also predict that 1-astobutane would be even more reactive, and 1-flurobutane less reactive.
I feel that my experiment went very well and my results appear to be both accurate and reliable. I did not include my trial results as they were done using only rough measurements and if they had be included in the average they would have made my results less reliable.
I also made sure I carried out the experiment safely and accurately following the instructions as in my plan.
I feel that the first thing that made my experiment less reliable was the fact that when keeping the experiment at 50oC I did not use a mechanical water bath but one made my self. The water bath that I used was probably not as effective as an electrically powered water bath at keeping the temperature constant but I feel that my results were not greatly effected by this mirror difference.
The second point where I can see a small fault in accuracy is the use of a standard pipette for measuring the chemical, a more accurate method of measuring would have been to use a graduated pipette but again I do not believe this has greatly effected my results.
The thirdly that I have identified as causing my results to be less accurate is the fact that the finishing point of the experiment was only judged with the human eye and time recorded using human reflexes. So it was perhaps wrong to have recorded the times to 1/100 of a second, as it would take many hundredths of a second for the stopwatch to be pressed.
Finally that the finishing point of the experiment was not clearly defined so it was hard to tell when the exact same amount of precipitate had been formed.
If I were to do the experiment again I may have investigated the effect of the positioning of the halide atom in the molecule using for example both 1-bromobutane and 2-bromobutane.