# Why doesn't a halide anion react with primary or secondary alcohols using SN2?

In the tenth edition of Organic Chemistry by Solomons and Fryhle, section 6.18A of the "Ionic Reactions" chapter, in the last paragraph, they have mentioned that:

Use of a weakly basic anion such as a chloride ion ($\ce{Cl^-}$ ), or an acetate ion ($\ce{CH3CO2^-}$) or a weakly basic and a highly polarizable one such as $\ce{Br^-}$, $\ce{I^-}$ or $\ce{RS^-}$ increases the likelihood of substitution (SN2). (emphasis mine)

But when we react alcohols with concentrated hydrohaloic acids (is that the right term?) with alcohols, we see that the rate of reaction decreases from a tertiary to a primary alcohol, indicating that the reaction is SN1. For primary alcohols, it is shown in the textbook to proceed through (SN2) pathway. [There is no such reaction mentioned with hydrobromic or hydroiodic acid (atleast I haven't come across such a reaction)].

Elsewhere, I have read about the Lucas test, which involves reacting the alcohol with hydrochloric acid and zinc chloride, where nearly no reaction occurs with primary alcohols.

My question is, why does this reaction not occur using an SN2 pathway for secondary and primary alcohols in the Lucas test? Why is it biased towards an SN1 reaction?

• I would have thought that especially the primary alcohols react via an $\mathrm{S_N2}$ mechanism, but you’ve gotten me curious. – Jan Aug 24 '16 at 11:09

The issue is that when carrying out the Lucas test you have a polar protic solution (due to the alcohol which you are testing), in which case Cl$^-$ is well solvated and thus not a very good nucleophile. As you are not using a strong nucleophile, you disfavor the S$_\text{N}$2 mechanism, but since you have a primary alcohol, you are not really favoring S$_\text{N}$1 either. Hence, why no reaction happens and the Lucas test works to identify primary alcohols. I think your question of Why is it biased towards an S$_\text{N}$1 reaction? might be a bit misleading because the reaction is neither biased towards S$_\text{N}$1 nor S$_\text{N}$2 for primary alcohols - it simply does not happen at all (at room temperature). In the case of secondary and tertiary alcohols, however, I would definitely say the reaction is biased towards S$_\text{N}$1 since you have polar protic conditions, a weak nucleophile, and a more stable carbocation intermediate.

Upon raising the temperature, I suspect that the reaction would then proceed via an S$_\text{N}$2 mechanism for a primary alcohol (as, despite not having a strong nucleophile, the barrier would be lower than via S$_\text{N}$1), but remain S$_\text{N}$1 for secondary and tertiary alcohols. Hope this helps!

• Thank you for your answer! That makes a lot of sense. I might add that for chlorination using $\ce{PCl3}$, the chloride ion attackes the alkyl phosphite intermediate thru SN2 mechanism, and it is able to do so because the solvent is polar aprotic (I assume so!). This was a part of the reason why I had asked for the difference! – FreezingFire Aug 24 '16 at 17:10

Hydroxyl group is not good leaving group that's why the reaction in the Lucas test with primary alcohol don't occur via SN2 reaction. In the Lucas test Hydroxyl group first, form hydronium ion in the presence of strong acid and ZnCl2. Hydronium is better leaving group thus form carbonium ion immediately. As, you know tertiary carbonium ion is much more stable than secondary and primary carbonium ion. Lucas test also occur with primary and secondary alcohol at elevated temperature but with tertiary alcohol the reaction with HCl occur even at room temperature.

• Once the hydroxyl group has been protonated, why doesn't the halide ion displace it by SN2? That is my question. The reaction could switch to an SN2 mechanism for primary alcohols, but that doesn't happen! – FreezingFire Aug 24 '16 at 15:18
• It is not always the case, there are some other important criteria for SN2 reaction as well that you have to considered. Under these condition that you have mentioned the prefer pathway is the formation of carbonium ion irrespective of primary, secondary and tertiary carboinium ion. – Khan Aug 24 '16 at 15:30
• What important criteria for SN2 are you referring to? Also, AFAIK, carbocations (carbonium ions) don't form in case of primary alcohols unless the conditions are pretty extreme. My question is for room temperature conditions. – FreezingFire Aug 24 '16 at 17:13
• SN2 reaction normally occur in polar aprotic solvent like DMSO, DMF, whereas SN1 reaction usually occur polar protic solvent like Alcohol. – Khan Aug 25 '16 at 2:38
• Oh, okay! We use water as solvent for the Lucas test (isn't it?). So that promotes SN1. You can add this in your answer, I think. Thanks for hearing me out! – FreezingFire Aug 25 '16 at 5:12