Question

Show how you would use a simple chemical test to distinguish between the following pairs of compounds. Tell what you would observe with each compound.

(a)isopropyl alcohol andtert-butyl alcohol

(b)isopropyl alcohol and butan-2-one, CH3COCH2CH3

(c)hexan-1-ol and cyclohexanol

(d)allyl alcohol and propan-1-ol

(e)butan-2-one andtert-butyl alcohol

Short answer

(a)

(b)

(c)

(d) Jhnbn,

(e)

Step-by-step solution

Lucas Chemical Test

Lucas chemical test is the best method to distinguish between alcohols ( 1⁰,2⁰and 3⁰) with the help of reagent anhydrous ZnCl₂ and concentrated HCl.

Lucas chemical test follows S_N1 mechanism; thus, carbocation stability is seen during the reaction. This test is given easily by tertiary alcohol, then by secondary alcohol, and primary alcohol does not give this chemical test at room temperature.

Step 2: Distinguishing between the given pair of compounds

1. Isopropyl alcohol is the secondary alcohol, and tert-butyl alcohol is the tertiary alcohol, so they both will be distinguished by Lucas reagent ( ZnCl₂/ HCl). Isopropyl alcohol will react in about 1 to 5 minutes with Lucas reagent as secondary carbocation form is less stable than tertiary carbocation. tert-butyl alcohol reacts with Lucas reagent instantly as they form a very stable tertiary carbocation.

Lucas reaction distinguishing Isopropyl alcohol and tert-butyl alcohol

b. Isopropyl alcohol is the secondary alcohol, and butane- 2-one is the secondary ketone; when both are treated with Lucas reagent ( / HCl), Isopropyl alcohol shows a reaction in 1 to 5 minutes, while with butan-2-one, there is no reaction with Lucas reagent.

Lucas reaction distinguishing Isopropyl alcohol and butane-2-one

c. Hexan-1-ol is a primary alcohol, and cyclohexanol is the secondary alcohol, so they both will be distinguished by Lucas reagent ( / HCl). Hexan-1-ol reacts very slowly with Lucas reagent, or there is no observable reaction at room temperature, while cyclohexanol reacts in about 1 to 5 minutes with Lucas reagent.

Lucas reaction distinguishing Hexan-1-ol and cyclohexanol

d. Allyl alcohol and propan-1-ol will be distinguished with the help of Lucas reagent ( / HCl). When treated with Lucas reagent, allyl alcohol forms tertiary carbocation; thus, it will instantly react with Lucas reagent while propan-1-ol is primary alcohol reacts very slowly with Lucas reagent, or there is no observable reaction at room temperature.

Lucas reaction distinguishing Allyl alcohol and propan-1-ol

e. Butan-2-one is a secondary ketone, and tert-butyl-alcohol is a tertiary alcohol. When both are treated with Lucas reagent ( / HCl), tert-butyl-alcohol alcohol shows instantly, while with butan-2-one, there is no reaction with Lucas reagent.

Lucas reaction distinguishing Butan-2-one and tert-butyl-alcohol