Number of molecules from below which cannot give iodoform reaction is :

Ethanol, Isopropyl alcohol, Bromoacetone, 2-Butanol, 2-Butanone, Butanal, 2-Pentanone, 3-Pentanone, Pentanal and 3-Pentanol.

<p>For the iodoform reaction, the compound must contain either:</p> <ul> <li><p>a methyl ketone group: </p> <p>$\mathrm{R{-}CO{-}CH_3}$</p></li> </ul> <p>or</p> <ul> <li><p>an alcohol which can be oxidised to a methyl ketone, that is: </p> <p>$\mathrm{CH_3{-}CH(OH){-}R}$</p></li> </ul> <p>Also, ethanol gives iodoform reaction because it gets oxidised to ethanal, and ethanal also gives the test.</p> <p>Now let us check each compound one by one.</p> <h3>1. Ethanol</h3> <p>Structure: </p> <p>$\mathrm{CH_3CH_2OH}$</p> <p>It gives iodoform reaction.</p> <h3>2. Isopropyl alcohol</h3> <p>Structure: </p> <p>$\mathrm{CH_3{-}CH(OH){-}CH_3}$</p> <p>This is a secondary alcohol of type </p> <p>$\mathrm{CH_3{-}CH(OH){-}R}$</p> <p>So it gives iodoform reaction.</p> <h3>3. Bromoacetone</h3> <p>Structure: </p> <p>$\mathrm{CH_3{-}CO{-}CH_2Br}$</p> <p>It contains the methyl ketone group </p> <p>$\mathrm{CH_3{-}CO{-}}$</p> <p>So it gives iodoform reaction.</p> <h3>4. 2-Butanol</h3> <p>Structure: </p> <p>$\mathrm{CH_3{-}CH(OH){-}CH_2CH_3}$</p> <p>This is of type </p> <p>$\mathrm{CH_3{-}CH(OH){-}R}$</p> <p>So it gives iodoform reaction.</p> <h3>5. 2-Butanone</h3> <p>Structure: </p> <p>$\mathrm{CH_3{-}CO{-}CH_2CH_3}$</p> <p>This is a methyl ketone.</p> <p>So it gives iodoform reaction.</p> <h3>6. Butanal</h3> <p>Structure: </p> <p>$\mathrm{CH_3CH_2CH_2CHO}$</p> <p>This is an aldehyde, but not ethanal.</p> <p>So it does <strong>not</strong> give iodoform reaction.</p> <h3>7. 2-Pentanone</h3> <p>Structure: </p> <p>$\mathrm{CH_3{-}CO{-}CH_2CH_2CH_3}$</p> <p>This is a methyl ketone.</p> <p>So it gives iodoform reaction.</p> <h3>8. 3-Pentanone</h3> <p>Structure: </p> <p>$\mathrm{CH_3CH_2{-}CO{-}CH_2CH_3}$</p> <p>There is no </p> <p>$\mathrm{CH_3{-}CO{-}}$ </p> <p>group directly attached to carbonyl carbon.</p> <p>So it does <strong>not</strong> give iodoform reaction.</p> <h3>9. Pentanal</h3> <p>Structure: </p> <p>$\mathrm{CH_3CH_2CH_2CH_2CHO}$</p> <p>This is an aldehyde, but not ethanal.</p> <p>So it does <strong>not</strong> give iodoform reaction.</p> <h3>10. 3-Pentanol</h3> <p>Structure: </p> <p>$\mathrm{CH_3CH_2{-}CH(OH){-}CH_2CH_3}$</p> <p>The carbon having $-OH$ is not attached to a $CH_3$ group in the required way. On oxidation it forms 3-pentanone, which is not a methyl ketone.</p> <p>So it does <strong>not</strong> give iodoform reaction.</p> <h3>Compounds which do <strong>not</strong> give iodoform reaction:</h3> <ul> <li><p>Butanal</p></li> <li><p>3-Pentanone</p></li> <li><p>Pentanal</p></li> <li><p>3-Pentanol</p></li> </ul> <p>Total number:</p> <p>$4$</p> <p>Therefore, the correct answer is:</p> <p>$\boxed{4}$</p> <p>So, <strong>Option A</strong> is correct.</p>