A level chem qns

lolerchild SG

I have a few questions about organic chem,

By means of a simple chemical test, distinguish the following pairs of compounds:

1) butanol and 2-methylpropan-2-ol

2) phenylmethanol and phenol

3) pentan-3-one and pentan-3-ol

UltimaOnline SG

I have a few questions about organic chem,

By means of a simple chemical test, distinguish the following pairs of compounds:

1) butanol and 2-methylpropan-2-ol

2) phenylmethanol and phenol

3) pentan-3-one and pentan-3-ol

Pretty straightforward question. Anyway, now that the forum these days have a lot more helpful post-'A' level pple (you know who you are, feel free to help out on this forum), I'll let them guide you (you = current 'A' level students). I'll 'moderate' only when I have comments to add.

d3sT1nY SG

Eh, let's see.

1. 2-methylpropan-2-ol is a 3 degree alcohol. So it can't be oxidised.

2. Phenylmethanol has a carbon attached to the phenyl ring thus can be oxidised. Phenol can't be.

3. One is ketone and the other one alcohol. I believe there's a test to distinguish between carbonyl group and alcohol. Tollen's reagent or Fehling's solution.

Haha, that's as much as I can remember.

Eyelessz SG

1. Yep, 2-methylpropan-2-ol is 3 degree alcohol. can't be oxidised. For butanol, u can oxidise it to form carboxylic acid and test with ca(oh)2, white ppt formed

2. pretty much wad destiny said, but i was wondering if we could use 2,4 NDPH (Please confirm)

3. Use Tollen reagent, only alcohol forms the silver mirror.

UltimaOnline SG

A couple of minor issues, but overall, good attempt, d3sT1nY and Eyelessz.

2,4 NDPH is to be used for distinguishing question (3) pentan-3-one and pentan-3-ol. Only the ketone will give a positive result.

Tollen's reagent or Fehling's solution, both give positive results only with aldehydes (because they oxidize aldehydes to carboxylic acids), hence does not help to distinguish a ketone from an alcohol. Ketones can't be oxidized by Tollen's or Fehling's, or by KMnO4 or K2Cr2O7 for that matter. Primary and secondary alcohols can be oxidized, but require a stronger oxidizing agent like KMnO4 or K2Cr2O7. Note that pentan-3-ol is a secondary alcohol.

The solution to question (3), is to use 2,4 NDPH, and not Tollen's reagent or Fehling's solution.

>>> Q2) i was wondering if we could use 2,4 NDPH (Please confirm) <<<

No you can't. 2,4 NDPH only distinguishes aldehydes/ketones from non-aldehydes/ketones, and phenylmethanol and phenol (question 2) are both non-aldehydes/ketones.

basmoth SG

1) Oxidation or Lucas' Test

2) Oxidation or Neutral aq FeCl3 solution or SOCl2/PCl5 or Bromine (aq)

3) Oxidation or Brady's reagent (2,4-dinitrophenylhydrazine)

UltimaOnline SG

Very good, basmoth. Additional valid alternative methods offered.

With neutral FeCl3(aq), a purple colouration indicates presence of phenol.

With SOCl2 or PCl5, steamy fumes of HCl evolved indicates presence of (non-phenolic) alcoholic hydroxy group.

UltimaOnline SG

I can think of one more, that so far no one has suggested.

2) phenylmethanol and phenol

Notice that these 2 compounds are both, in neutral or acidic conditions, rather insoluble in water (due to the large, non-polar benzene ring).

Adding NaOH(aq), would result in deprotonation of phenol, forming the phenoxide ion, which is able to form ion-dipole interactions with water, and is thus highly soluble.

Phenylmethanol would not be deprotonated by sodium hydroxide, and thus remains insoluble.