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We were asked the reaction between 1 mole acetone and 1 mole $\ce{Br2}$ in basic medium. Since only a limited amount of bromine was given I got the final answer as 1-bromopropan-2-one (mono bromo acetone). However, in the given solution the entire haloform reaction was shown with $\ce{CHBr3}$ and acetate ion as the final product.

I am not sure how a single mole of $\ce{Br2}$ is sufficient for this(at least 3 moles are needed, I believe).

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Let's take a look at the first product you propose: 1-bromopropan2-one.

The mechanism includes deprotonation by hydroxide to form an enolate. The resulting enolate is better stabilized with more electronegative atoms (Br in this case) and will form more readily than another molecule of acetone turning into an enolate ion. Thus, as the reaction proceeds, the product distribution will be determined by the most stable enolate anion. This will be the tribromoacetyl intermediate enolate ion and will drive the reaction forward. So a whole mol of bromine will be converted completely to the tribromoacetyl intermediate with a 1/3 mol of acetone. Notice how only a 1/3 will react to give the monosubtituted intermediate, than 1/3 mol of bromine will react to give disubstituted product, and lastly the last 1/3 mol of bromine will react to form the trisubstitued product. The last step is displacement by hydroxide via the nucleophilic acyl substitution mechanism (not SN2!) to form acetate and release bromoform. This reaction isn't really great if bromine isn't in excess. enter image description here

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  • $\begingroup$ I understand why the carboanion formed is more stable (inductive effect of bromine) however since the question specified 1 mole bromine how is the attack by the enolate ion taking place the second and third time around , these products should be formed in ''excess Br2'' not '' 1 mole Br2'' $\endgroup$ – Arn_kap Jun 20 '16 at 18:37
  • $\begingroup$ @Arn_kap One-third of the acetone will react fully to give acetic acid. The other two-thirds will not react. $\endgroup$ – orthocresol Jun 20 '16 at 19:07
  • $\begingroup$ @orthocresol So am I right in interpreting it in terms of molecules that one molecule of monobromo acetate formed will go ahead and form di and tri substituted as opposed to considering in moles 1:1 ratio used (acetone and bromine) wherein actually it was 1/3:1 $\endgroup$ – Arn_kap Jun 20 '16 at 19:45
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  1. $3$ $\ce{H}$ atoms out of total $6$ in a molecule of acetone is brominated.

  2. So $\frac{3}{2}$ moles $\ce{Br}$ molecules is needed to react with 1 mol acetone.

  3. Hence $1$ mol $\ce{Br}$ molecule will react with $2/3$ mol acetone. Thus acetone is the limiting reagent and the reaction is completed.

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