Question

Substance M is refluxed with aqueous sodium hydroxide. One of the products of this reaction reacts with alkaline aqueous iodine to give a pale yellow precipitate. Which compounds could be substance M?

`1)` `"CH"_3"CO"_2"CH"_3`
`2)` `"CH"_3"CO"_2"CH"_2"CH"_3`
`3)` `"HCO"_2"CH"("CH"_3)_2`

Answer 1

Substance M could be either 2. ethyl acetate or 3. isopropyl formate.

Explanation:

The formation of a pale yellow precipitate when iodine in sodium hydroxide reacts with an alcohol that can be oxidized to acetaldehyde or a methyl ketone is the iodoform reaction.

The equations for your saponification reactions are:

`bb((1.))color(white)(m)underbrace("CH"_3"COOCH"_3)_color(red)("methyl acetate") + "NaOH"→ underbrace("CH"_3"COONa")_color(red)("sodium acetate") + underbrace("CH"_3"OH")_color(red)("methanol")"`

`bb((2.))color(white)(m)underbrace("CH"_3"COOCH"_2"CH"_3)_color(red)("ethyl acetate") + "NaOH" → underbrace("CH"_3"COONa")_color(red)("sodium acetate") + underbrace("CH"_3"CH"_2"OH")_color(red)("ethanol")`

`bb((3.))color(white)(m)underbrace("HCOOCH"("CH"_3)_2)_color(red)("isopropyl formate") + "NaOH" → underbrace("HCOONa")_color(red)("sodium formate") + underbrace("CH"_3"CH(OH)CH"_3)_color(red)("isopropyl alcohol")`

The iodoform test

1. Methanol does not give the iodoform test.

2. Ethanol forms acetaldehyde on oxidation, so it gives the iodoform test.

`underbrace("CH"_3"CH"_2"OH")_color(red)("ethanol") stackrelcolor(blue)("I"_2"/NaOH"color(white)(mm))(→) underbrace("CH"_3"CHO")_color(red)("acetaldehyde") stackrelcolor(blue)("I"_2"/NaOH"color(white)(mm))(→)underbrace("HCOONa")_color(red)("sodium formate") + underbrace("HCI"_3)_color(red)("iodoform")`

3. Isopropyl alcohol forms acetone on oxidation, so it gives the iodoform test.

`underbrace("CH"_3"CH(OH)CH"_3)_color(red)("isopropyl alcohol") stackrelcolor(blue)("I"_2"/NaOH"color(white)(mm))(→) underbrace("CH"_3"COCH"_3)_color(red)("acetone") stackrelcolor(blue)("I"_2"/NaOH"color(white)(mm))(→) underbrace("CH"_3"COONa")_color(red)("sodium acetate") + underbrace("HCI"_3)_color(red)("iodoform")`