Question

Using your roadmap as a guide, show how to convert 4 -methyl-1-pentene into 5 -methylhexanenitrile. You must use 4 -methyl-1-pentene and sodium cyanide as the source of all carbon atoms in the target molecule. Show all reagents needed and all molecules synthesized along the way.

Short answer

Question: Convert 4-methyl-1-pentene to 5-methylhexanenitrile using sodium cyanide as the source of all carbon atoms. List the reaction steps and products for each step. Answer: Step 1: Conversion of 4-methyl-1-pentene to 4-methyl-1-pentanol using hydroboration-oxidation reaction (BH3·THF, H2O2, NaOH). Product: 4-methyl-1-pentanol. Step 2: Conversion of 4-methyl-1-pentanol to 4-methyl-1-pentyl chloride using halogenation reaction (SOCl2). Product: 4-methyl-1-pentyl chloride. Step 3: Conversion of 4-methyl-1-pentyl chloride to 5-methylhexanenitrile using nucleophilic substitution reaction (NaCN). Product: 5-methylhexanenitrile.

Step-by-step solution

Conversion of 4-methyl-1-pentene to 4-methyl-1-pentanol

First, we need to convert the double bond present in 4-methyl-1-pentene into an alcohol group. To do this, we use hydroboration-oxidation reaction. Add BH3·THF (Borane-tetrahydrofuran complex) followed by H2O2 (hydrogen peroxide) and NaOH (sodium hydroxide). This will give 4-methyl-1-pentanol as the product. The reaction mechanism proceeds through syn addition, generating an alkylborane intermediate which is then oxidized to form the alcohol. The reaction can be represented as: 4-methyl-1-pentene + BH3·THF → (Intermediate) → 4-methyl-1-pentanol + H2O2 + NaOH

Conversion of 4-methyl-1-pentanol to 4-methyl-1-pentyl chloride

Next, we need to convert the alcohol (OH) group in 4-methyl-1-pentanol into a chloride (Cl) group. To do this, we use a halogenation reaction by reacting the alcohol with SOCl2 (Thionyl chloride). The reaction produces 4-methyl-1-pentyl chloride as the product. The reaction can be represented as: 4-methyl-1-pentanol + SOCl2 → 4-methyl-1-pentyl chloride + SO2 + HCl

Conversion of 4-methyl-1-pentyl chloride to 5-methylhexanenitrile

Finally, we need to expand the carbon chain and introduce a nitrile group. This can be accomplished through a nucleophilic substitution reaction (SN2) with NaCN (sodium cyanide). Sodium cyanide will act as a nucleophile and replace the chloride group in 4-methyl-1-pentyl chloride, generating the target molecule 5-methylhexanenitrile. The reaction can be represented as: 4-methyl-1-pentyl chloride + NaCN → 5-methylhexanenitrile + NaCl This completes the conversion of 4-methyl-1-pentene into 5-methylhexanenitrile using sodium cyanide as the source of all carbon atoms.

Key concepts

Hydroboration-Oxidation Reaction

The hydroboration-oxidation reaction is a two-step process widely used in organic chemistry to convert alkenes to alcohols. The beauty of this reaction lies in its stereospecificity and anti-Markovnikov selectivity. In the first step, a borane, like BH3·THF, adds across the carbon-carbon double bond of 4-methyl-1-pentene. This occurs through a syn addition mechanism, meaning that the hydrogen and boron atoms add to the same side of the double bond.
Following this, hydrogen peroxide (H2O2) in the presence of a base such as NaOH is used to oxidize the intermediate alkylborane to create the alcohol group on the more substituted carbon. The result is a molecule of 4-methyl-1-pentanol, where the former alkene is now a friendly hydroxyl (-OH) functional group, setting the stage for further transformations.

Alkyl Halide Formation

Part of mastering organic synthesis involves converting functional groups to facilitate further reactions. To transform 4-methyl-1-pentanol into an alkyl halide, we employ thionyl chloride (SOCl2) to replace the -OH group with a chloride ion.
This conversion is crucial because the resulting alkyl halide, 4-methyl-1-pentyl chloride, is a reactive species that can further undergo nucleophilic substitution. The use of SOCl2 is particularly advantageous as it converts alcohols to chlorides without water, which ensures that the product is not contaminated by hydrolysis. In addition, the byproducts SO2 and HCl are gaseous, which simplifies purification. This step lays the groundwork for introducing more complex functional groups into the molecule like the nitrile group in subsequent steps.

Nucleophilic Substitution Reaction

Nucleophilic substitution reactions are the cornerstone of functional group interconversions. Specifically, the SN2 mechanism plays a pivotal role in crafting organic molecules. This bimolecular process happens when a nucleophile, such as cyanide ion (CN-) provided by NaCN, swiftly displaces a leaving group— in our case, chloride ion (Cl-) from the 4-methyl-1-pentyl chloride.

Why SN2?

This pathway is chosen for its backside attack, which is favored in primary and secondary halides, creating an inversion of stereochemistry. The result of this crucial step is the nitrile-containing molecule, 5-methylhexanenitrile. The -CN group is essential for the final structure and showcases the versatility of nucleophilic substitution in extending carbon chains and introducing new functionalities.

Organic Molecule Conversion

Conversions of organic molecules involve a series of transformations to proceed from a simple starting material to a product with a desired functional group. The process showcased here transforms 4-methyl-1-pentene into 5-methylhexanenitrile using a strategic sequence of reactions tailored to maintain the integrity of the carbon skeleton while altering the functional groups.

Choosing the Right Path:

It's not just about knowing the reactions, but also about understanding their order and the way each step affects the next. From the addition of an alcohol group to the formation of an alkyl halide, and finally to the introduction of a nitrile group, each step must be correctly executed to ensure the successful synthesis of the target molecule. These conversions underscore the importance of having a deep understanding of reaction mechanisms and the ability to design a coherent synthesis roadmap.