Q49E - Organic Chemistry (Mcmurry)

Question

How would you prepare the following substances from cyclopentanol?

More than one step may be required.

(a) Cyclopentanone 

(b) Cyclopentene

(c) 1-Methylcyclopentanol 

(d) trans-2-Methylcyclopentanol

Step-by-Step Solution

Answer

(a) 

(b)

(c)

(d)

Step 1: Conversion of alcohols into various compounds

Secondary alcohols can be converted to ketones by oxidizing with acidified ${\mathbf{CrO}}_{\mathbf{3}}$ Alcohols can be converted into alkenes by treating with ${\mathbf{POCl}}_{\mathbf{3}}$ in pyridine. A secondary alcohol can be converted to tertiary alcohol by oxidizing the alcohol to ketone followed by treatment with Grignard reagent.

Step 2: Preparation of cyclopentanol from various compounds

(a)Cyclopentanone can be prepared from cyclopentanol by reacting cyclopentanol with ${\mathrm{CrO}}_3$ and ${\mathrm{H}}_3{\mathrm{O}}^{+}$ . The reaction can be given as:

                     Reaction (a)

(b)Cyclopentene can be prepared from cyclopentanone by reacting cyclopentanol with ${\mathrm{POCl}}_3$ and pyridine. The reaction can be given as:

                                Reaction (b)

(c)1-Methylcyclopentanol can be prepared from cyclopentanol by reacting cyclopentanol in the presence of ${\mathrm{CrO}}_3$ and ${\mathrm{H}}_3{\mathrm{O}}^{+}$ to form cyclopentanone. This compound further reacts with ${\mathrm{CH}}_3\mathrm{MgBr}$ and ${\mathrm{H}}_3{\mathrm{O}}^{+}$ to form 1-Methylcyclopentanol. The reaction can be given as:    

                               Reaction (c)

(d)The compound, trans-2-Methylcyclopentanol can be formed by reacting 1-Methylcyclopentanol with ${\mathrm{H}}_3{\mathrm{O}}^{+}$ to form cyclopentene with methyl group as the substituent. This is further treated with ${\mathrm{BH}}_3$ , THF, ${\mathrm{H}}_2{\mathrm{O}}_2$ and ${}^{-}\mathrm{OH}$ to form trans-2-Methylcyclopentanol. The reaction can be given as:

                              Reaction (d)