Question

Using your roadnap as a guide, show hoes to comvert 4-methyl-1-pentene into S-lbethyhexanenitrile. You must use 4-1bethyl-1-pentene and sodium cyanide as the source of all carbon atoms in the target molecule. Show all reagents needed and all molecules symthesized along the way.

Short answer

Question: Outline the steps to convert 4-methyl-1-pentene into S-ethylhexanenitrile using sodium cyanide as the source of all carbon atoms. Answer: To synthesize S-ethylhexanenitrile from 4-methyl-1-pentene and sodium cyanide, follow these steps: 1. Brominate 4-methyl-1-pentene using N-bromosuccinimide (NBS) to form 1-bromo-4-methylpentane. 2. Perform an SN2 reaction with 1-bromo-4-methylpentane and sodium cyanide (NaCN) to create 1-cyano-4-methylpentane. 3. Hydrogenate 1-cyano-4-methylpentane using Palladium on carbon (Pd/C) as a catalyst and hydrogen gas (H2) to obtain S-ethylhexanenitrile.

Step-by-step solution

Bromination of 4-methyl-1-pentene

First, we need to convert the double bond in 4-methyl-1-pentene into a good leaving group. This can be done by reacting it with a bromination reagent, such as N-bromosuccinimide (NBS) in the presence of a radical initiator like benzoyl peroxide. This reaction will yield 1-bromo-4-methylpentane. 4-methyl-1-pentene + NBS → 1-bromo-4-methylpentane

Formation of nitrile

Now that we have 1-bromo-4-methylpentane, we can react it with sodium cyanide (NaCN) to form the corresponding nitrile. This is a substitution reaction (SN2), where the cyanide ion attacks the carbon atom bonded to the bromine, substituting it in the process. The resulting product is 1-cyano-4-methylpentane. 1-bromo-4-methylpentane + NaCN → 1-cyano-4-methylpentane

Hydrogenation of 1-cyano-4-methylpentane

Next, we need to convert the -CN group into a -CH2CN group to obtain S-ethylhexanenitrile. This can be done using a catalytic hydrogenation reaction. We can use a catalyst like Palladium on carbon (Pd/C) and hydrogen gas (H2) to selectively hydrogenate the nitrile group, leading to the formation of S-ethylhexanenitrile. 1-cyano-4-methylpentane + H2 (Pd/C) → S-ethylhexanenitrile The final product of these reactions is S-ethylhexanenitrile, synthesized using 4-methyl-1-pentene and sodium cyanide as the source of all carbon atoms. The steps include bromination of the alkene, substitution with cyanide, and hydrogenation of the nitrile group.

Key concepts

Bromination of alkenes

Understanding how alkenes react with brominating agents is essential in organic synthesis. Alkenes, which are hydrocarbons with a double bond, are reactive sites that can be transformed into various functional groups. Bromination is a type of halogenation—a reaction where a halogen is added across the double bond of an alkene.

The classic example involves adding bromine (Br₂) to an alkene to form a vicinal dibromide, which means the bromine atoms are attached to adjacent carbon atoms. This reaction is highly selective and occurs via a mechanism involving the formation of a bromonium ion intermediate.

However, for the synthesis of nitriles, we're interested in introducing a single bromine atom to the alkene to obtain a good leaving group for further reactions. We generally carry out this reaction using N-bromosuccinimide (NBS) in the presence of light or a radical initiator like benzoyl peroxide. This results in the bromination at the allylic position, adjacent to the double bond, yielding 1-bromo-4-methylpentane when applied to 4-methyl-1-pentene.

SN2 reaction

The SN2 reaction, or bimolecular nucleophilic substitution reaction, is a fundamental concept in organic chemistry. In an SN2 reaction, a nucleophile with a lone pair of electrons aggressively attacks an electrophilic carbon center and displaces a leaving group in a single concerted step.

This reaction has a characteristic 'backside attack' because the nucleophile approaches from the opposite side of the leaving group, leading to an inversion of stereochemistry at the carbon center. The rate of the reaction depends on both the concentration of the nucleophile and the substrate.

In the synthesis of nitriles, the SN2 mechanism is critical. The nucleophile in this case is the cyanide ion (CN^-) from sodium cyanide (NaCN). Cyanide substitutes for a bromine atom on 1-bromo-4-methylpentane, a good leaving group, to yield 1-cyano-4-methylpentane efficiently. The effectiveness of the SN2 reaction for this purpose makes it an indispensable tool in constructing carbon-nitrogen bonds, such as those found in nitriles.

Catalytic hydrogenation

Catalytic hydrogenation is a reduction process where molecular hydrogen (H₂) is added to a molecule in the presence of a metal catalyst. This reaction is used extensively to reduce multiple bonds to single bonds—alkynes to alkenes or alkenes to alkanes. However, it is also useful for a variety of other functional groups.

For our educational exercise, we focus on the hydrogenation of nitriles to form amines. Specifically, the -CN group of 1-cyano-4-methylpentane is hydrogenated to form a -CH₂CN group, using a catalyst such as palladium on carbon (Pd/C). The catalyst provides a surface for the reaction to occur, significantly increasing the rate of hydrogen addition.

The addition of hydrogen to nitriles requires careful control because over-hydrogenation can turn nitriles into primary amines. In the synthesis of S-ethylhexanenitrile, selective hydrogenation is applied to achieve the desired functionality without over-reduction. The result is the addition of hydrogen atoms specifically to the -CN group, giving us the target molecule without affecting the rest of the hydrocarbon chain.