Question

How will you convert? (i) propan-1-ol to propan-2-ol (ii) 3 -methylbutan-2-ol to 2 -methylbutan-2-ol

Short answer

Answer: (i) To convert Propan-1-ol to Propan-2-ol, the required reactions and reagents are: 1. Oxidation using potassium dichromate (K2Cr2O7) in acidic conditions (H2SO4), forming Propanal. 2. Reduction using lithium aluminum hydride (LiAlH4) or sodium borohydride (NaBH4), forming Propan-2-ol. (ii) To convert 3-methylbutan-2-ol to 2-methylbutan-2-ol, the required reactions and reagents are: 1. Oxidation using potassium dichromate (K2Cr2O7) in acidic conditions (H2SO4), forming 3-methylbutan-2-one. 2. Nucleophilic addition using Grignard reagent (CH3MgBr), forming 2-methylbutan-3-ol-MgBr+. 3. Acidic hydrolysis using dilute HCl, forming 2-methylbutan-2-ol.

Step-by-step solution

(i) Conversion of Propan-1-ol to Propan-2-ol

Step 1: Oxidation of Propan-1-ol to Propanal To oxidize propan-1-ol, we need to use an oxidizing agent such as potassium dichromate (K2Cr2O7) in acidic conditions (H2SO4). The resultant product will be propanal. Reaction: Propan-1-ol + [O] -> Propanal Step 2: Reduction of Propanal to Propan-2-ol Next, we need to reduce propanal to propan-2-ol. We can use a reducing agent like lithium aluminum hydride (LiAlH4) or sodium borohydride (NaBH4) for this purpose. The product will be propan-2-ol. Reaction: Propanal + [H] -> Propan-2-ol

(ii) Conversion of 3-methylbutan-2-ol to 2-methylbutan-2-ol

Step 1: Oxidation of 3-methylbutan-2-ol to 3-methylbutan-2-one In order to shift the methyl group, we first need to oxidize the secondary alcohol. We can use potassium dichromate (K2Cr2O7) in acidic conditions (H2SO4) as the oxidizing agent. The product will be 3-methylbutan-2-one. Reaction: 3-methylbutan-2-ol + [O] -> 3-methylbutan-2-one Step 2: Nucleophilic addition of Grignard reagent To shift the methyl group from the 3 position to the 2 position, we will use a Grignard reagent (CH3MgBr). The Grignard reagent will undergo a nucleophilic addition reaction with the carbonyl group of 3-methylbutan-2-one, forming an alcohol. Reaction: 3-methylbutan-2-one + CH3MgBr -> 2-methylbutan-3-ol-MgBr+ Step 3: Acidic hydrolysis Finally, we need to convert the intermediate alcohol-MgBr+ back to the desired alcohol (2-methylbutan-2-ol) through acidic hydrolysis using a dilute acid like HCl. Reaction: 2-methylbutan-3-ol-MgBr+ + HCl -> 2-methylbutan-2-ol + MgBrCl By following these steps, the conversions of the given alcohols can be achieved.

Key concepts

Oxidation and Reduction in Organic Chemistry

Understanding the process of oxidation and reduction, often abbreviated as 'redox', is critical when dealing with organic chemical reactions. Oxidation in organic chemistry typically involves the increase in the number of bonds of carbon with more electronegative elements, like oxygen, or a decrease in the number of bonds to hydrogen. Conversely, reduction is the decrease in bonds to oxygen or increase in bonds to hydrogen.
For instance, when converting propan-1-ol to propanal, an oxidizing agent such as potassium dichromate is used. This step increases the oxygen content bonded to the carbon chain, effectively removing two hydrogen atoms and creating an aldehyde, propanal. In the textbook example, further reduction of propanal to propan-2-ol is needed, which is achieved by adding hydrogen atoms using a reducing agent like lithium aluminum hydride.
It is essential for students to grasp the concept of identifying oxidation states in organic molecules and predicting the products of redox reactions. An oxidizing agent causes the oxidation, while a reducing agent leads to the reduction of the molecule in question. Recognizing the role of these agents helps to elucidate the pathways to the desired product in organic syntheses.

Grignard Reagent Reaction

Grignard reagents are a pivotal tool in synthetic organic chemistry. Named after the French chemist Victor Grignard, these compounds are magnesium halides and are represented as R-Mg-X, where R is an organic group and X is a halogen. They are highly nucleophilic and participate readily in reactions where they donate their organic group to a positively polarized atom, frequently the carbon of a carbonyl group.
An exemplary application of this is in the conversion of 3-methylbutan-2-one to 2-methylbutan-2-ol. The Grignard reagent, specifically methylmagnesium bromide in the textbook solution, attacks the carbonyl carbon of the ketone, forming a new carbon-carbon bond. This results in an intermediate alcohol, which carries the magnesium halide.

Preparation and Reactions of Grignard Reagents

Students must understand that the preparation of Grignard reagents involves the reaction of an alkyl or aryl halide with magnesium metal in a dry ether solvent. The dry conditions are crucial; any moisture can cause the reagent to decompose. The textbook's step-by-step solution underscores the role Grignard reagents play in forming carbon-carbon bonds, a fundamental process in building complex organic molecules from simpler ones.

Acidic Hydrolysis

Acidic hydrolysis is a versatile reaction extensively used in organic chemistry to cleave certain bonds under the influence of an acid. Dating back to traditional saponification reactions, acidic hydrolysis is often employed to break esters into their corresponding alcohols and acids. However, it also plays an essential role in concluding the Grignard reagent reaction.
In the conversion of 2-methylbutan-3-ol-MgBr+ to 2-methylbutan-2-ol within the solution of the exercise, the intermediate Grignard addition product undergoes acidic hydrolysis. This step involves the addition of a dilute acid, such as HCl, which helps to protonate the oxygen atom of the alcohol group, simultaneously breaking the bond with magnesium, effectively regenerating the alcohol and yielding by-products such as MgBrCl.

Understanding Mechanisms of Hydrolysis

For a comprehensive understanding, students should recognize that hydrolysis mechanisms vary depending on the functional group involved. In some cases, an acid functions as a catalyst, whereas, in others, it may participate directly in the bond cleavage process. Grasping the nuances of these reactions allows students to predict and execute syntheses that yield the desired organic products, as demonstrated in the provided textbook exercise.