Q17-72E - Organic Chemistry (Mcmurry)

Question

Identify the reagents a–f in the following scheme:

Step-by-Step Solution

Answer

(a) ${\mathrm{NaBH}}_4$ , ${\mathrm{H}}_3{\mathrm{O}}^{+}$

(b) ${\mathrm{PBr}}_3$  

(c) Mg/ether, ${\mathrm{CH}}_2\mathrm{O}$ 

(d) Dess- Martin periodinane, ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ 

(e) ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2\mathrm{MgBr}$ , ${\mathrm{H}}_3{\mathrm{O}}^{+}$

(f) ${\mathrm{POCl}}_3$ , pyridine

Step 1: Reagent to convert ketone to alcohol

Sodium borohydride is used as a reagent to transform aldehydes and ketones to alcohol in acidic medium.${\mathrm{NaBH}}_4$ converts ketone to secondary alcohol

Step 2: Reagent to convert alcohol to alkylhalide

When alcohol reacts with phosphorus tri bromide it changes into alkyl halide to be specific alkyl bromide.

Step 3: Reagent to convert alkylhalide to alkylated alcohol

Alkyl halide can be converted to Grignard reagent and then addition of formaldehyde results in addition of single carbon primary alcohol

Step 4: Reagent to convert alcohol to aldehyde

The Dess-Martin Periodinane (DMP) converts alcohol to aldehydes or ketones, this reaction is performed in dichloromethane or chloroform

Step 5: Reagent to convert aldehyde to alkylated alcohol

The reaction of a Grignard reagent with an aldehyde results in formation of a secondary alcohol in acidic medium.  

Step 6: Reagent to convert alcohol to alkene

Phosphorus oxychloride is used with pyridine as dehydrating agent. They make hydroxyl a good leaving group and pyridine removes a β-proton resulting in alkene formation.

Step 7: Reagent a

Reagent a is ${\mathrm{NaBH}}_4$ with ${\mathrm{H}}_3{\mathrm{O}}^{+}$ as it will convert cyclohexanone to cyclohexanol, a secondary alcohol. 

Step 8: Reagent b

Reagent b is ${\mathrm{PBr}}_3$ which will convert cyclohexanol to cyclobromide as it adds halide to alcohol.

Step 9: Reagent c

Here two steps occur for this concersion:

• In first step Mg/ether are used to convert alkyl halide to grignard reagent.
• In second step the formed Grignard reagent will react with ${\mathrm{CH}}_2\mathrm{O}$ to give primary alcohol.

Step 10: Reagent d

Here Dess-Martin periodinane is used in medium having ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ so as to convert  alcohol to aldehyde. 

Step 11: Reagent e

For adding an alkyl group to aldehyde Grignard reagent is used. Here to form the desired product ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2\mathrm{MgBr}$ is used as Grignard reagent. This occur in acidic medium, ${\mathrm{H}}_3{\mathrm{O}}^{+}$ . 

Step 12: Reagent f

For converting alcohol to alkene ${\mathrm{POCl}}_3$ with pyridine can be used as it acts as a dehydrating agent