Topic on " SN1 nucleophilic substitution reactions"
ABSTRACT
My research question is “how do various solvents affect the rate of SN1 nucleophilic substitution reactions, (if at all) with tertiary butyl bromide, stating which solvent is the most effective.”
Tertiary butyl bromide is a tertiary alkyl halide which can react with various nucleophiles in reactions known as nucleophilic substitution reactions. This process is unimolecular, which means the reaction rate (kinetic rate) only depends upon the nature/concentration of the tertiary alkyl halide. The nucleophile or the solvent (in this case ethanol, ammonia and sodium hydroxide) has no impact on the reaction rate at all. This process can also be termed as Solvolysis, where the nucleophile is a solvent molecule.
The aim of all the experiments is to ascertain whether the nucleophile really has an effect on the kinetic rate of the nucleophilic substitution reaction. This is carried out by the process of titrating the tertiary butyl bromide against the various other solvents along with different quantities of those solvents. Initial and final concentrations are noted down along with the timings, graphs are plotted and the reaction rates are calculated.
In the end, the concentration-time graphs are plotted and prove that the nucleophilic substitution reactions depend only on the tertiary butyl bromide and not the nucleophiles (solvents). The reaction rate obtained for tertiary butyl bromide is almost the same with all the other solvents. For further proof, even the reaction constant is somewhat similar. All the graphs obtained are those of the first order in which the concentration is inversely proportional to the time. Hence there actually is no more “effective” solvent.
1. INTRODUCTION
Tertiary butyl bromide is a type of organic compound, known as an alkyl halide. Tertiary butyl bromide is a clear, colourless to slightly yellow liquid. Halogenoalkanes or alkyl halides are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms, fluorine, chlorine, bromine or iodine, in result, this functional group is polarized so that the carbon is electrophilic and the halogen is nucleophilic[i].
It is used as during organic synthesis[ii] as intermediates and refrigerants, solvents, blowing agents, aerosol propellants, fire extinguishing media, and in semiconductor device fabrication[iii]. Having studied topics such as organic chemistry and chemical kinetics, and observing the number of reactions and mechanisms, it was evident that I was very interested in it. The reason I chose Tertiary butyl bromide as my main substance was because it is extremely versatile in application and is very reactive[iv]. My investigation is centred on the reaction rates (speed of the reaction) of the nucleophilic substitution reactions between Tertiary butyl bromide and various solvents. (Solvolysis)
I often found myself asking how these commercially important chemical reactions could be made more efficient/quicker and the above statement was my answer. I will use the simple method of titrating Tertiary butyl bromide against different concentrations of different solvents, noting down observations, record the time required for each change to occur and simultaneously identify the products formed. Hence I will be able to determine whether the solvent has any effect on the nucleophilic substitution reaction or not.
2. RESEARCH QUESTION:
How do various solvents affect the rate of SN1 nucleophilic substitution reactions, (if at all) with tertiary butyl bromide, stating which solvent is the most effective.
3. BACKGROUND INFORMATION:
A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else.
Nucleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia. [v]
In other words, it can also be said that a nucleophiles participate in chemical reactions by donating electrons to species called electrophiles in order to form chemical bonds.
In Latin, the term ‘nucleophile’ means ‘nucleus-loving’. Nucleophilicity is often used to compare an atom's relative affinity to another's[vi].
Where in this case, the relative affinity between the tertiary alkyl halide and the nucleophiles(solvents) is crucial.
3.1. Nucleophilic Substitution Reaction is a type of chemical reaction in which one nucleophile electron donor replaces another as a covalent substitute of some atom[vii].
There is a great similarity between the mechanisms of this type of reaction and those of acid-base reactions.
In addition, strong bases are very similar to strong nucleophiles in substitution reactions. An important fact to be taken into consideration is that in an acid-base reaction, a proton is transferred from the weak base to the strong base. In a similar fashion, nucleophilic substitution reactions involve the transfer of a carbon group from the leaving group (weak base) to the nucleophile[viii].
There are two different types of mechanisms. The SN1 mechanism and the SN2 mechanism.
3.2. SN1 Reaction:
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary alkyl halides, the alternative SN2 reaction occurs. Among inorganic chemists, the SN1 reaction is often known as the dissociative mechanism[ix]. Such reactions occur when conditions exist that favour the ionization of the organic reactant.
There are various conditions that need to be fulfilled in order to carry out the for SN1 reactions
1) The Leaving Group: The leaving group must be a weak base.
2) The Carbon Group: The rate-determining step involves production of a carbocation, and this step will occur faster for those compounds that yield the more stable carbocations. For example, tertiary compounds react faster than secondary compounds. Primary compounds react extremely slowly. Carbocation intermediates are also stabilized by dispersal of the positive charge through delocalization of electrons. SN1 reactions that produce such resonance-stabilized carbocation intermediates are also quite fast.
3) The Solvent: Polar solvents ionise substances, hence the rate of an SN1 reaction is directly dependent on this property. The reaction occurs faster in more polar solvents[x].
3.3. Tertiary butyl bromide:
Molecular formula : (CH3) 3CBr
It is also known as 2-bromo-2-methyl-propane ( 2-bromo-2-methylpropane).
Melting point -20 ° C, boiling point of 72 - 74 ° C, refractive index – 1.4279, and relative density of 1.189. Do not dissolve in water, soluble in alcohol, ether and other organic solvents. In the presence of alkali easily lost generation of hydrogen bromide isobutene; Easy hydrolysis into alcohol; with magnesium, lithium, etc. [xi]
Tertiary butyl bromide is prepared by reacting acetone with Grignard reagent (methyl magnesium bromide), followed by hydrolysis which gives 2-methyl propan-2-ol (t-butanol) , which on reacting with HBr results in the formation of tertiary butyl bromide.[xii]
CH3(CO)CH3 +CH3MgBr---------------->CH3C(CH3)2OMgBr +H2O----> (CH3)3COH +HBr---------->(CH3)3CBr
3.4. EFFECTS OF THE SOLVENT:
Basically, the actual terminology of the process being used in the experiments is called solvolysis, where the effect of different solvents on the reaction rate is observed. Solvolysis is a special type of nucleophilic substitution where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction. For water, the term is hydrolysis; for alcohols, it is alcoholysis; for ammonia, it is ammonolysis[xiii].
Since the SN1 reaction involves formation of an unstable carbocation intermediate in the rate-determining step, anything that can facilitate this will speed up the reaction. The normal solvents of choice are both polar (to stabilize ionic intermediates in general) and protic (to solvate the leaving group in particular). Typical polar protic solvents include water and alcohols, which will also act as nucleophiles.
Begin by considering what makes a good nucleophile. The nature of the nucleophile is such that it may affect the rate of the reaction rate. A strong nucleophile will be much more preferred than a weak one when attacking the electrophile (carbon atom).
The slow step of the SN1 reaction involves formation of two ions. Solvation of these ions is crucial to stabilizing them and lowering the activation energy for their formation. Very polar ionizing solvents such as water and alcohols are needed for the SN1[xiv].
3.4.1. Ethanol:
Ethanol (ethyl alcohol, grain alcohol) is a clear, colourless liquid with a characteristic, agreeable odour. In dilute aqueous solution, it has a somewhat sweet flavour, but in more concentrated solutions it has a burning taste. Ethanol, C2H5OH, is an alcohol, a group of chemical compounds whose molecules contain a hydroxyl group, –OH, bonded to a carbon atom. Ethanol's hydroxyl group is able to participate in hydrogen bonding. At the molecular level, liquid ethanol consists of hydrogen-bonded pairs of ethanol molecules, the ethanol molecule has a hydrophilic -OH group that helps it dissolve polar molecules and ionic substances. [xv]
The nucleophile ethanol is a species which is strongly attracted to the positively charged region in something else. Oxygen is much more electronegative than hydrogen and carbon and so drags bonding electrons towards it. That produces a significant amount of negative charge on the oxygen atom. The oxygen also has two active lone pairs of electrons[xvi]. One of these attacks the tertiary butyl bromide.
3.4.5. Hypothesis:
It is presumed knowledge that in nucleophilic substitution reactions, the nature or the concentration of the nucleophile has no effect on the reaction rate. The rate soly depends on the concentration of the tertiary alkyl halide, in this case tertiary butyl bromide.
My assumption is that maybe by trying different solvents and different concentrations of those solvents, we can alter the reaction rate of the nucleophilic substitution reactions by using solvents such as sodium hydroxide, ethanol and ammonia.
3. Apparatus:
The main experimental process being used here is titration, the general equipment required included:
1.) Burettes
2.) Pipettes
3.) Funnels
4.) Erlenmeyer flasks.
5.) White tiles to be placed under the flask so that any colour change can be easily noticed.
6.) Stopwatch
7.) Graduated cylinder (100 ml ± 1ml)
8.) Droppers
9.) Conical flask
Chemicals:
1.) Tertiary Butyl Bromide
2.) Sodium Hydroxide
3.) Ethanol
4.) Distilled water
5.) Indicator Phenolphthalein
4. Method:
1.) Safety equipment is to be worn, i.e. goggles as the fumes from the chemicals may cause inhalatory problems and also agitates the eyes. (For example, ammonia’s pungent odour makes it difficult o deal with).
2.) The burette is washed with distilled water to remove any impurities that might tend to interfere with the reaction.
3.) 2 ml of tertiary butyl bromide is added to a flask using the graduated cylinder.
4.) Solutions are made for each solvent of concentrations 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%.
5.) The solutions of each concentration of each solvent is added into the burette one by one using the graduated cylinder.
6.) 2-3 drops of phenolphthalein indicator is added to the conical flask containing 2ml of tertiary butyl bromide [1M].
7.) The solvent is then allowed to slowly trickle down into the flask from the burette and the reaction takes place and the timing is recorded when a significant colour change is observed.
8.) A definite time period of 0.3 seconds is taken when emptying the solvent into the flask containing the tertiary butyl bromide, where the flow of solvent is stopped at the end of each successive time period.
9.) It is possible to take more trials, repeating the experiment several times so as to obtain more credible sets of readings.
5. Results:
Table No:1
Sr.No. | Initial Conc. of Tert.Butyl Bromide. (mol ml-1) x 10-3 [1 M] | Volume of Tert.Butyl Bromide. (ml) | Conc. of Sodium Hydroxide (NaOH) in % | Volume of Sodium Hydroxide. (ml) | Final Conc. of Tert.Butyl Bromide after titration. (mol ml-1) x 10-3 in [M] | Time Taken. (in seconds) |
1 | 1 | 2 | 100 | 1 | 1 | 0 |
2 | 1 | 2 | 90 | 2 | .90 | .3 |
3 | 1 | 2 | 80 | 2 | .80 | .6 |
4 | 1 | 2 | 70 | 2.1 | .73 | 0.9 |
5 | 1 | 2 | 60 | 2.2 | .63 | 1.2 |
6 | 1 | 2 | 50 | 2.2 | .48 | 1.5 |
7 | 1 | 2 | 40 | 2.3 | .41 | 1.8 |
8 | 1 | 2 | 30 | 2.6 | .39 | 2.1 |
9 | 1 | 2 | 20 | 3.7 | .37 | 2.4 |
6.1. RATE OF REACTION:
From the graph above, which is a concentration vs. time graph; it can be inferred that the concentration is inversely proportional to the time which proves that the graph is of a first order reaction.
As proved from the experimental results that the reaction follows the first order; the following equations can be used to find out the rate constant:
r = -k(CTBB)
k dCTBB/dt=KcTBB
dCTBB/CTBB=- kdt
ʃ dCTBB/CTBB=- ʃ kdt
as, ʃ dx/x=ln (x)
ln[C(TBB)final /C(TBB)initial ]=-kt
ln C(TBB)final – ln C(TBB)initial =-kt
ln C(TBB)final = ln C(TBB)initial -kt
As the above equation represents y= mx + c(which is straight line equaton) :
Where y-axis = ln C(TBB)final and x-axis=t
Intercept (C) = ln C(TBB)initial
Slope (m) = -k
Table No.2
Sr.No. | Initial Conc. of Tert.Butyl Bromide. (mol ml-1) x 10-3 | Volume of Tert.Butyl Bromide. (ml) | Conc. of Ammonia (NH3) in % | Volume of Ammonia. (ml) | Final Conc. of Tert.Butyl Bromide after titration. (mol ml-1) x 10-3 | Time Taken. (in seconds) |
1 | 1 | 2 | 100 | 1 | 1 | 0 |
2 | 1 | 2 | 90 | 1.7 | .792 | 0.3 |
3 | 1 | 2 | 80 | 1.7 | .709 | 0.6 |
4 | 1 | 2 | 70 | 1.8 | .6424 | 0.9 |
5 | 1 | 2 | 60 | 1.9 | .5632 | 1.2 |
6 | 1 | 2 | 50 | 1.9 | .4244 | 1.5 |
7 | 1 | 2 | 40 | 2.0 | .3608 | 1.8 |
8 | 1 | 2 | 30 | 2.2 | .3432 | 2.1 |
9 | 1 | 2 | 20 | 3.2 | .3256 | 2.4 |
Table No.3
Sr.No. | Initial Conc. of Tert.Butyl Bromide. (mol ml-1) x 10-3 | Volume of Tert.Butyl Bromide. (ml) | Conc. of Ethanol (C2H5OH) in % | Volume of Ethanol. (ml) | Final Conc. of Tert.Butyl Bromide after titration. (mol ml-1) x 10-3 | Time Taken. (in seconds) |
1 | 1 | 2 | 100 | 1 | 1 | 0 |
2 | 1 | 2 | 90 | 1.3 | .6089 | 0.3 |
3 | 1 | 2 | 80 | 1.3 | .5428 | 0.6 |
4 | 1 | 2 | 70 | 1.4 | .4944 | 0.9 |
5 | 1 | 2 | 60 | 1.4 | .4335 | 1.2 |
6 | 1 | 2 | 50 | 1.3 | .3242 | 1.5 |
7 | 1 | 2 | 40 | 1.3 | .2771 | 1.8 |
8 | 1 | 2 | 30 | 1.7 | .2676 | 2.1 |
9 | 1 | 2 | 20 | 2.5 | .2508 | 2.4 |
Sample Calculations :
1.Rate of Reaction
For NaOH
Theoretical Value:
r= d C(TBB)/dt
(Final Conc. TBB - Intial Conc. of TBB) / (Final Time-Initial time) ( using value of Sr.No.4)
= 1-.73/.3
= 0.90 mole ml-3 sec-1
2. Graphical Value:
From Fig. No.1
= 0.98 mole ml-3 sec-1
3. Rate Constant:
Now calculation to show the value of rate constant i.e ‘k’
put the value in equation:
ln C(TBB)final = ln C(TBB)initial -kt
Taking the values of Sr.no.3 (see table no.1)
Value of ln C(TBB)final=ln(.73)
=-.314
Value of ln C(TBB)initial=ln(1)
=0
t=.3
-.314=0-(k x (.3))
k=1.0466 sec-1
4 .Percentage Uncertainty
% Error = (Graphical value - Theoretical value)/ (Theoretical Value) x100
= ((.98-.90)/.90)x100
= 8.88%
*TBB=Tertiary Butyl Bromide.
Similar calculations have been carried out in order to find the final concentration of tertiary butyl bromide with NH3 and Ethanol respectively to obtain the following results:
For NH3 :
1. Theoretical Value: ( Using the value of Sr.No.3)
.988 mole ml-3 sec-1
2. Graphical Value:
.970 mole ml-3 sec-1 (See Fig. No.2)
3. Rate Constant:
1.14 sec-1
4. Percentage of Uncertainty:
1.821%.
For C2H5OH :
1. Theoretical Value: ( Using the value of Sr.No.3)
1.306 mole ml-3 sec-1
2. Graphical Value:
.89 mole ml-3 sec-1 (see Fig. No.2)
3. Rate Constant:
1.65 sec-1
4. Percentage of Uncertainty:
31.85 %.
Table NO.4
Solvent Used | Theoretical value(For Rate Of Reaction(r)mole ml-3 sec-1 | Graphical value(For Rate Of Reaction(r) mole ml-3sec-1) | Rate Constant in sec-1(k) | Percentage Of Uncertainty (in %) |
NaOH | .90 | .98 | 1.04 | 1.8 |
NH3 | .988 | .970 | 1.14 | 1.8 |
C2H5OH | 1.306 | .89 | 1.65 | 31.8 |
6. CONCLUSION:
As previously mentioned, the research question is:
How do various solvents affect the rate of SN1 nucleophilic substitution reactions, (if at all) with tertiary butyl bromide, stating which solvent is the most effective.
At the end of all my experiments, I realized that my initial hypothesis that the solvents (nucleophiles) might affect the rate of the nucleophilic substitution reactions between tertiary butyl bromide and the solvents
(Sodium hydroxide, ethanol and ammonia) was wrong. It was eventually proved that SN1 nucleophilic substitution reactions are not dependent in any way on the nature/concentration of the nucleophile.
On trying different types of nucleophiles in the reactions and plotting their concentration vs. time graphs, I came to the conclusion that the reaction rate when using different solvents was almost the exact same.
(The gradient of the graph is the reaction rate.)
The type of graph obtained between tertiary butyl bromide and each solvent was that of the first order, proving that the kinetic rate did not depend upon the solvent.
To answer the second part of my question, there was obviously no “effective” solvent as all the nucleophiles had no impact at all on the reaction rates of the nucleophilic substitution reactions
I also calculated the rate constants for all the reactions and as seen from the experimental results, the values (1.04, 1.14, 1.65) are pretty uniform, proving once again that there was no effect of the nucleophiles on the kinetic rates of all the reactions.
For more information: contact me at kapil.bvp@gmail.com
