Identifying an Unknown Alcohol Lab Quiz

Lucas reagent

ZnCl2 dissolved in concentrated HCl

Role of ZnCl2 in Lucas reagent

Lewis acid that makes the solution more acidic

Lucas test reaction type

SN1 – the –OH is replaced by Cl–; the rate depends on carbocation stability

Positive Lucas test

Solution turns cloudy / a second layer forms (the alkyl chloride is nonpolar and insoluble)

Lucas test – 3° alcohol

Cloudy in less than 30 seconds

Lucas test – 2° alcohol

Cloudy in 1–3 minutes

Lucas test – 1° alcohol

No reaction within an hour

Allyl and benzyl alcohol in the Lucas test

React as fast as 3° alcohols (resonance-stabilized carbocations)

Lucas test solubility limit

The alcohol must dissolve in the reagent – about 7–8 carbons or fewer

Lucas test lab rule

If there's no change within 5 minutes, assume no reaction

TCICA

1,3,5-trichloroisocyanuric acid – pool chlorine source; releases Cl2 (an oxidizer) in acid

TCICA test solvent

Acetonitrile

TCICA positive result

Isocyanuric acid precipitate forms

TCICA – 2° alcohol

Precipitate in 15–30 seconds (fastest)

TCICA – 1° alcohol

Precipitate in 3–7 minutes (up to 20)

TCICA – 3° alcohol

No precipitate (can't be oxidized)

Why 3° alcohols resist oxidation

No H on the carbon bonded to the –OH, so no C=O can form without breaking a C–C bond

Oxidation products

1° → aldehyde/carboxylic acid, 2° → ketone, 3° → no reaction

Known positive / known negative

Controls that confirm the reagents work and the test was done correctly

Two rules of classification tests

Follow the procedure exactly (3 drops means 3) and always run known controls

Lucas controls used

1-propanol (1°), 2-propanol (2°), 2-methyl-2-propanol / tert-butyl alcohol (3°)

Derivative used

3,5-dinitrobenzoate ester (alcohol + 3,5-dinitrobenzoyl chloride)

Why make a derivative

A stable crystalline solid whose melting point can be compared to literature values

Derivative: purity vs yield

Purity matters more than yield

DMAP two functions

Acylation catalyst (forms a reactive acylpyridinium intermediate) and base (removes HCl, preventing dehydration)

Why 3° alcohols form derivatives slowly

Steric hindrance

Refractive index correction

n20 = nT + 0.00066(T − 20°C)

Why correct the refractive index

It changes with temperature, and literature values are reported at 20°C

Practice ID: Lucas 2.5 min, TCICA 20 s, mp 113–114°C, n 1.4535

Cyclopentanol (2°)