C(CH3)3Cl + OH- → C(CH3)3OH + Cl-
The substitution reaction between hydroxide and tert-butyl chloride is carried out above in a mixture of water and acetone and universal indicator. Initially there is sodium hydroxide present in universal indicator. Thus we begin with a violet solution. The tert-butyl chloride/acetone is added and the reaction begins. As the hydroxide is consumed the pH lowers and thus we can track the reaction progress as the solution progresses from violet to blue to green. Curiously the solution continues past a neutral pH as all of the hydroxide is consumed and the solution continues through yellow and orange and eventually red. The pKa of the tert-butyl alcohol formed is 16.5 and thus unlikely to cause a shift down below a pH of 4 even at high concentrations. The culprit behind the continuing decrease in pH is the excess tert-butyl chloride. This undergoes a reaction with the solvent, water, to form tert-butyl alcohol and hydrochloric acid.
C(CH3)3Cl + H2O → C(CH3)3OH + H+ + Cl-
Substitution reactions can occur via two different mechanisms. The first possible mechanism is called SN1 and involves the bond between the leaving group (here chloride) and the carbon to break via a collision with solvent. This leads to a carbocation intermediate forming and the chloride anion. After this step is complete, the nucleophile (hydroxide) forms a bond with the carbocation intermediate. This mechanism is unimolecular in that the first step (which is the rate-determining step) only depends on one molecule.
Figure 1: SN1 mechanism for tert-butyl chloride reacting with hydroxide
The second possible mechanism involves the bond between the leaving group and carbon to break as a result of a collision with the nucleophile. Thus the new bond forms as the old bond breaks. This results in an activated complex where one bond is breaking as the other forms which can be seen in the mechanism below.
Figure 2: SN2 mechanism for tert-butyl chloride reacting with hydroxide
Whether a substitution reaction will be SN1 or SN2 depends on several features of the chemicals involved. The strength of the nucleophile, the leaving group and the solvent used all impact which will occur. But the most important factor is the carbon that the leaving group is attached to. If this carbon is tertiary (three other carbons attached) the mechanism will likely be SN1. If the carbon is primary (one other carbon attached) the mechanism will likely be SN2. If the carbon is secondary (two other carbons attached), then the other factors will likely determine the outcome. In this particular reaction the SN1 mechanism is the primary course of reaction because the tert-butyl chloride has the leaving group (chloride) attached to a tertiary carbon. A tertiary carbon is able to have a more stable carbocation intermediate because the carbocation can be stabilized slightly by the neighboring carbon atoms. A primary carbon is not able to do this sufficiently to make this pathway favorable because of the limited electron density of the hydrogen atoms. Additionally, the carbon where the reaction takes place is quite bulky due to the three carbons being attached. This is called steric hindrance, where the rest of the molecule prevents a collision from happening with the carbon attached to the leaving group. For a primary carbon, steric hindrance is less of an issue. Once the carbocation intermediate is formed, the carbon site is planar and more susceptible to collision especially given the large amount of positive charge.
If the carbon were not tertiary of primary then other factors would determine the reaction mechanism. The solvent used will affect whether the mechanism is SN1 or SN2. If the solvent is capable of hydrogen bonding (water, ethanol, etc.) the solvent is referred to as polar protic and will increase the likelihood of SN1 occurring. A polar protic solvent will do two things to make SN1 more likely. The first is that a polar protic solvent interacts strongly with the nucleophile causing it to not react as successfully because the negative charge of the nucleophile is drawn to the positive end of the polar protic solvent. The second is that the polar protic solvent is capable of stabilizing the carbocation intermediate with its negatively charged end of the molecule. Polar aprotic solvents (DMSO, acetone, etc.) are polar but not capable of hydrogen bonding. These will increase the likelihood of SN2 mechanism happening. A nonpolar solvent (toluene, hexane, etc.) is unlikely to adequately facilitate some of the substitution reactions, but if it will then it will again increase the chances of the mechanism being SN2. The leaving group and nucleophile also will both influence the mechanism. A leaving group that is weakly bonded to the carbon will facilitate SN1 and a strong nucleophile will increase the likelihood of SN2 happening.
