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Consider the reaction between tert-butyl alcohol and acetadelhyde in the presence of $\ce{HBr}$ in aqueous solution:

$$\ce {(CH_3)_3COH(aq) + (CH_3)CHO(aq)} \longrightarrow ?$$

Here is my assumed solution to the reaction mechanism: enter image description here

Essentially, the $\ce{H+}$ ion removes the alcohol from the tert-butyl alcohol allowing the acetadelhyde to bond with the carbocation. In this process the double bond between the oxygen and a carbon becomes a single leaving a positive region on the respective carbon. This allows the bromide ion to attach forming the molecule in the last step.

Is the product correct? Is the mechanism to get to this product correct?

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    $\begingroup$ Mostly, but it's not the product. This is acetalisation, and some bromide is intermediate, at best. $\endgroup$
    – Mithoron
    Feb 26 at 0:10
  • $\begingroup$ I guess this photo might help? Just that it's HCl and HBr also might work... $\endgroup$ Feb 26 at 1:27
  • $\begingroup$ This one is the mechanism... $\endgroup$ Feb 26 at 1:28
  • $\begingroup$ Basically from what I saw @Mithoron, the HCl gas protonates the oxygen of the carbonyl compounds and increases its electrophilicity, thus facilitating the nucleophilic attack of the alcohol. Acetals and ketals are hydrolysed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively. So HBr also would do the same thing... $\endgroup$ Feb 26 at 1:32
  • $\begingroup$ @HarikrishnanM Not HCl gas, perhaps in solution, but for something like that it would be H2SO4. $\endgroup$
    – Mithoron
    Feb 26 at 1:47

1 Answer 1

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In short, the mechanism posted by the OP is not correct.

Over here, an aldehyde is being combined with a tertiary alcohol, and the reaction is acetalisation.

We'll go through the main crux of the reaction, and the detailed mechanism.


The reaction:

Aldehydes undergo nucleophilic addition reaction in dry HCl gas (HBr also can be considered) with monohydric alcohol to form a hemiacetal. Being unstable, hemiacetals further react and form a gem-dialkoxy compound called acetal.

enter image description here

enter image description here


Mechanism:

HCl gas protonates the oxygen of the carbonyl compounds and increases its electrophilicity, thus facilitating the nucleophilic attack of the alcohol...

enter image description here

followed up by the nucleophilic attack,

enter image description here

and the deprotonation.

enter image description here

Further reaction takes place due to the instability of the acetal (thermodynamic reasons).

enter image description here

enter image description here

enter image description here

enter image description here


This would be sufficient for the OP to draw a conclusion and write the final mechanism.


Sources:

  1. Chemistry, Class 12, NCERT
  2. Acetals

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