[Doctor.Fate]

HCl is the catalyst in this reaction.

This is what I have so far, but I'm fairly certain that it is incorrect:


Another way I thought about doing this is to have some protonation action with the OH group and the HCl, so H₂O would be formed, since it is a good leaving group. But I am lost from this point on.

Any help would be greatly appreciated, and thank you!

EDIT: I updated the image with a more correct representation of the problem.

[orgopete]

I suggest you steer away from alkoxide anions in your mechanism with HCl present.

Hint, if HCl protonates water, can't it protonate any of the oxygens in this reaction?

[Doctor.Fate]

I suggest you steer away from alkoxide anions in your mechanism with HCl present.

Hint, if HCl protonates water, can't it protonate any of the oxygens in this reaction?

So I protonate the OH first, and that gives me H₂O. The water then leaves since water is a good leaving group. Now I have my main product, H₂O, and Cl^-. Hopefully I did not misunderstand your hint, but isn't it a rule that OH needs to be protonated first?

[TwistedConf]

You have all sorts of wrong here.

You need to go and study acid-catalyzed formation and hydrolysis of acetals. Until you understand that basic chemistry, this will make no sense.

[orgopete]

I suggest you steer away from alkoxide anions in your mechanism with HCl present.

Hint, if HCl protonates water, can't it protonate any of the oxygens in this reaction?

So I protonate the OH first, and that gives me H₂O. The water then leaves since water is a good leaving group. Now I have my main product, H₂O, and Cl^-. Hopefully I did not misunderstand your hint, but isn't it a rule that OH needs to be protonated first?

There are three oxygens, try another.

[khemophore]

The intermediate after the first step of the reaction is wildly unstable and not at all likely to form. An alkoxide is not a good leaving group, particularly in acidic solution and when there is no strong entropic driving force (unlike in, e.g., the final elimination step of an aldol condensation).

There's also no particular reason to suppose the reaction would follow an SN1 mechanism (you're producing a secondary carbocation only weakly stabilized by resonance with the neighboring oxygen, the substrate is not severely sterically hindered, and the reaction is intramolecular). In order for SN1 to even be possible, it is doubly important to produce a stable carbocation and expel a good leaving group (since SN1 is kinetically unaided by the nucleophile, unlike SN2), and your mechanism has neither. Furthermore, even if SN1 did occur, it would not yield the desired product, since carbocation rearrangement via hydride shift from the adjacent tertiary carbon is likely to occur. Ultimately, an SN2 mechanism is more correct.

Finally, remember what the object of acid catalysis in these sorts of reactions is: to improve the quality of the leaving group by protonating it, thereby activating the substrate to nucleophilic attack. That's where your mechanism should start. Hopefully the rest is fairly obvious.

[Dan]

There's also no particular reason to suppose the reaction would follow an SN1 mechanism (you're producing a secondary carbocation only weakly stabilized by resonance with the neighboring oxygen, the substrate is not severely sterically hindered, and the reaction is intramolecular).

Lots of problems here.

It is an acyloxonium ion - it is highly stabilised. The stability of this oxonium is essentially the basis of acetal/ketal reactivity. Check any organic chemistry textbook.

R₂⁺C-O-R  R₂C=O⁺-R

Consider that 2,2,-dimethoxypropane hydrolyses easily in dilute aqueous acid, whereas methyl isopropyl ether is very stable (if this was an S_N2 reaction, the opposite would be expected).

Ketal hydrolysis (and formation) is very likely to occur by S_N1:

1. The carbocation is highly stabilised (favours S_N1)
2. The nucleophile (alcohol) is weak (disfavours S_N2)

the substrate is not severely sterically hindered,

3. The corresponding S_N2 pathway would require S_N2 substitution at a quaternary centre. Quaternary centre = severely sterically hindered.

Every organic chemistry text will show the S_N1 pathway for ketals (and acetals). While acetals can undergo S_N2 (or S_N2-like) reactions under highly specialised conditions (e.g. in the active site of some glycosidases), it is certainly not their usual reaction pathway. The reactivity difference between ethers and acetals is due to relatively low energy S_N1 pathways compared to S_N2.

[khemophore]

I stand corrected. I was definitely dead wrong on this, thanks for the lesson.