Starting from Grignard's reagent, how would you prepare i) propan ol ii) propan 2 ol. Mention the chemical test to distinguish them. Write down the structural formula of neo pentyl alcohol and its…

Preparation of Propan-1-ol and Propan-2-ol from Grignard Reagent

i) Preparation of Propan-1-ol (1-Propanol)

Grignard reagents are organomagnesium compounds of the form R-Mg-X, where R is an alkyl or aryl group and X is a halogen (usually Cl, Br, or I). To prepare propan-1-ol, we start with ethylmagnesium bromide (C₂H₅MgBr) and react it with formaldehyde (H₂C=O).

bromoethanebromoethane+Mg+dry etherheat (gentle)MgBr(OH)

Reaction Steps:

1. Formation of Grignard Reagent: Ethyl bromide (C₂H₅Br) reacts with magnesium (Mg) in dry ether to form ethylmagnesium bromide. $$\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{Br}+\mathrm{Mg}\stackrel{\mathrm{dry}\mathrm{ether}}{\to }\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$$

formaldehydeformaldehydeH₂O (workup)propan-1-olpropan-1-ol+Mg(OH)Br

2. Reaction with Formaldehyde: The Grignard reagent (C₂H₅MgBr) is then reacted with formaldehyde (H₂C=O), which acts as a carbonyl compound with no alkyl group attached to the carbonyl carbon. $$\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}+\mathrm{H}{}_2\mathrm{C}=\mathrm{O}\stackrel{\mathrm{ether}}{\to }\mathrm{C}{}_2\mathrm{H}{}_5-\mathrm{CH}{}_2-\mathrm{O}-\mathrm{MgBr}$$ The intermediate formed is an alkoxide, which upon hydrolysis (addition of water) yields propan-1-ol. $$\mathrm{C}{}_2\mathrm{H}{}_5-\mathrm{CH}{}_2-\mathrm{O}-\mathrm{MgBr}+\mathrm{H}{}_2\mathrm{O}\to \mathrm{C}{}_2\mathrm{H}{}_5-\mathrm{CH}{}_2-\mathrm{OH}+\mathrm{Mg}(\mathrm{OH})\mathrm{Br}$$

Final Product: $$\mathrm{CH}{}_3-\mathrm{CH}{}_2-\mathrm{CH}{}_2-\mathrm{OH}\text{(Propan-1-ol)}$$

---

ii) Preparation of Propan-2-ol (Isopropyl Alcohol)

To prepare propan-2-ol, we start with methylmagnesium bromide (CH₃MgBr) and react it with acetaldehyde (CH₃CHO).

acetoneacetoneH₂O (workup)propan-2-olpropan-2-ol+Mg(OH)Br

bromoethanebromoethane+Mg+dry etherheat (gentle)MgBr(OH)

Reaction Steps:

1. Formation of Grignard Reagent: Methyl bromide (CH₃Br) reacts with magnesium (Mg) in dry ether to form methylmagnesium bromide. $$\mathrm{CH}{}_3\mathrm{Br}+\mathrm{Mg}\stackrel{\mathrm{dry}\mathrm{ether}}{\to }\mathrm{CH}{}_3\mathrm{MgBr}$$

2. Reaction with Acetaldehyde: The Grignard reagent (CH₃MgBr) is then reacted with acetaldehyde (CH₃CHO), which has a methyl group attached to the carbonyl carbon. $$\mathrm{CH}{}_3\mathrm{MgBr}+\mathrm{CH}{}_3\mathrm{CHO}\stackrel{\mathrm{ether}}{\to }(\mathrm{CH}{}_3){}_2\mathrm{C}-\mathrm{O}-\mathrm{MgBr}$$ The intermediate formed is an alkoxide, which upon hydrolysis yields propan-2-ol. $$(\mathrm{CH}{}_3){}_2\mathrm{C}-\mathrm{O}-\mathrm{MgBr}+\mathrm{H}{}_2\mathrm{O}\to (\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{OH}+\mathrm{Mg}(\mathrm{OH})\mathrm{Br}$$

Final Product: $$\mathrm{CH}{}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}{}_3\text{(Propan-2-ol)}$$

---

Chemical Test to Distinguish Propan-1-ol and Propan-2-ol

Propan-1-ol and propan-2-ol can be distinguished using the Lucas test or oxidation test.

Propan-1-ol (Primary)Propan-1-ol (Primary)Propan-2-ol (Secondary)Propan-2-ol (Secondary)

1. Lucas Test (for Secondary vs. Primary Alcohols)

• Lucas reagent is a mixture of ZnCl₂ and concentrated HCl.
• Propan-2-ol (secondary alcohol) reacts immediately with Lucas reagent to form a cloudy solution due to the formation of an alkyl chloride, which is insoluble in the reagent. $$(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{OH}+\mathrm{HCl}\stackrel{\mathrm{ZnCl}{}_2}{\to }(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{Cl}+\mathrm{H}{}_2\mathrm{O}$$
• Propan-1-ol (primary alcohol) does not react appreciably at room temperature and remains clear.

2. Oxidation Test (Using Acidified Potassium Dichromate)

• Propan-2-ol is easily oxidized to acetone (propanone), which has a characteristic fruity odor. $$(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{OH}+[\mathrm{O}]\stackrel{\mathrm{K}{}_2\mathrm{Cr}{}_2\mathrm{O}{}_7/\mathrm{H}{}_2\mathrm{SO}{}_4}{\to }(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{O}+\mathrm{H}{}_2\mathrm{O}$$
• Propan-1-ol is oxidized first to propanal (which has a pungent odor) and then further to propanoic acid (which has a sour smell).

---

Structural Formula and IUPAC Name of Neo-Pentyl Alcohol

Neo-pentyl alcohol is a tertiary alcohol with the structure: $$\mathrm{CH}{}_3-\mathrm{C}(\mathrm{CH}{}_3){}_2-\mathrm{CH}{}_2-\mathrm{OH}$$ Its IUPAC name is 2,2-dimethyl-1-propanol.