Why do Tertiary alcohols react much more quickly with HCl than do other types of alcohols?
Why do Tertiary alcohols react much more quickly with HCl than do other types of alcohols?
Upon protonation, tertiary alcohols form more stable carbocations than their counterparts. Since carbocation formation the RLS for SN1 & SN2, increased stability means increased rate.
If you react a tertiary alcohol with HCl, Sn1 and E1 are the two competing reactions that might occur. Sn2 is unfavorable because of steric considerations with a tertiary alcohol. Low temperature favors substitution, while higher temperatures favor elimination. Cl- is a decent nucleophile and can attack the tertiary carbocation, or it can act as a base to eliminate a hydrogen, as you have mentioned, but it is likely through an E1. How do you know it's E1 rather than E2? Because the acidic HCl protonates water, allowing it to leave and form a relatively stable tertiary carbocation.But if you are using a strong acid like HCl, what kind of nucleophile is that? The reaction can not be SN1 or SN2, can it? Are we talking an elimination reaction here? Because once the water leaves, Cl- can deprotonate another H+, causing a double bond to form. If that is the case, then how am I to know whether this will result in E1 or E2? E2 requires a strong base. So, can this be E1?
I would not so confidently say that Cl- cannot act as a base to abstract a proton. Cl- is both a decent nucleophile and base. Formation of HCl by acting as a base is not necessarily disfavored energetically, because basicity is a thermodynamic property, and the stabilization of the carbocation may result in a favorable energy change. You cannot readily disentangle Cl-'s behavior as a nucleophile vs. base, but you can consider things like temperature and solvent effects in making a guess about which pathway--substitution or elimination--will be favored.Why would it form an alkene? That won't happen because if Cl- takes off one of the H's it will form a strong acid again (HCl) and thus NOT stable. Hence, this won't be "favored"
Sn1 is favored since it leads to more "stable" product.
I would not so confidently say that Cl- cannot act as a base to abstract a proton. Cl- is both a decent nucleophile and base. Formation of HCl by acting as a base is not necessarily disfavored energetically, because basicity is a thermodynamic property, and the stabilization of the carbocation may result in a favorable energy change. You cannot readily disentangle Cl-'s behavior as a nucleophile vs. base, but you can consider things like temperature and solvent effects in making a guess about which pathway--substitution or elimination--will be favored.
Which is more stable, a carbocation and free H+ and Cl- in solution, or an alkene and HCl, which does, as you noted, dissociate into H+ and Cl- again? I don't know if you can draw a clear-cut answer here. From a thermodynamic perspective, I think abstraction of a proton might be favored, because it stabilizes the carbocation and results in a neutral molecule. Free Cl-, which is what you get after HCl dissociates, can be stabilized by solvent effects. Remember that Cl- is a fairly polarizable nucleophile that can also be stabilized by protic solvents.See I don't think Cl- would abstract a proton simply due to the fact that HCl will be formed (again) and thus it will be less stable reaction (as opposed to Sn1 reaction). I am not saying Cl- would not act as a base...It might in other reactions but not in this reaction.
Maybe QofQuimica can chime in?
Which is more stable, a carbocation and free H+ and Cl- in solution, or an alkene and HCl, which does, as you noted, dissociate into H+ and Cl- again? I don't know if you can draw a clear-cut answer here. From a thermodynamic perspective, I think abstraction of a proton might be favored, because it stabilizes the carbocation and results in a neutral molecule. Free Cl-, which is what you get after HCl dissociates, can be stabilized by solvent effects. Remember that Cl- is a fairly polarizable nucleophile that can also be stabilized by protic solvents.
This is essentially correct.If you react a tertiary alcohol with HCl, Sn1 and E1 are the two competing reactions that might occur. Sn2 is unfavorable because of steric considerations with a tertiary alcohol. Low temperature favors substitution, while higher temperatures favor elimination. Cl- is a decent nucleophile and can attack the tertiary carbocation, or it can act as a base to eliminate a hydrogen, as you have mentioned, but it is likely through an E1. How do you know it's E1 rather than E2? Because the acidic HCl protonates water, allowing it to leave and form a relatively stable tertiary carbocation.
While it is true that Cl- is not a very good base, the carbocation is a stronger (less stable) acid (Lewis acid) than the HCl is. Also, don't forget that your reaction vessel is full of water (where do you think all those protonated hydroxyl leaving groups go???) So, what you will really end up with is Cl- and H3O+. You will have to work the reaction up at the end (extract it and dry it over sodium sulfate) to remove the water.See I don't think Cl- would abstract a proton simply due to the fact that HCl will be formed (again) and thus it will be less stable reaction (as opposed to Sn1 reaction). I am not saying Cl- would not act as a base...It might in other reactions but not in this reaction.
Maybe QofQuimica can chime in?