Question

Using your old and new roadmaps as a guide, show how to convert 1 -bromo- 3 -methylbutane into 2,7-dimethyloctane. You must use 1-bromo-3-methylbutane as the source of all carbon atoms in the target molecule. Show all reagents and all molecules synthesized along the way.

Short answer

Question: Describe the process of converting 1-bromo-3-methylbutane into 2,7-dimethyloctane using a series of organic reactions. Answer: The conversion of 1-bromo-3-methylbutane to 2,7-dimethyloctane can be achieved through the following steps: 1. Prepare a Grignard reagent from 1-bromo-3-methylbutane by reacting it with magnesium metal in anhydrous ether. 2. Extend the carbon chain by reacting the Grignard reagent with ethyl bromide. 3. Brominate the six-carbon chain using N-bromosuccinimide (NBS) in the presence of a radical initiator, such as benzoyl peroxide. 4. Prepare another Grignard reagent from the brominated six-carbon chain and react it with methyl bromide to further extend the carbon chain by two carbons. 5. Hydrogenate the molecule using a catalyst like palladium on carbon (Pd/C) and hydrogen gas (H₂) to remove the C=C double bond and obtain the desired 2,7-dimethyloctane.

Step-by-step solution

Preparation of Grignard Reagent

First, we need to prepare a Grignard reagent from 1-bromo-3-methylbutane. The Grignard reagent is an alkyl or aryl magnesium halide, which can help to extend the carbon chain. To form the Grignard reagent, treat 1-bromo-3-methylbutane with magnesium metal in anhydrous ether: CH_3CH_2C(CH_3)CH_2Br + Mg -> CH_3CH_2C(CH_3)CH_2MgBr

Carbon Chain Extension

Now, to extend the carbon chain, we can react the Grignard reagent with ethyl bromide. This reaction will form a six-carbon chain with a new alkyl group. CH_3CH_2C(CH_3)CH_2MgBr + BrCH_2CH_3 -> CH_3CH_2C(CH_3)CH_2CH_2CH_2CH_3 + MgBr_2

Bromination of the six-carbon chain

Next, we need to convert the six-carbon chain into a compound with a reactive site for further carbon chain extension. We can achieve this by treating the compound with N-bromosuccinimide (NBS) in the presence of a radical initiator, such as benzoyl peroxide. This will generate an allylic bromination reaction: CH_3CH_2C(CH_3)CH_2CH_2CH_2CH_3 + NBS -> CH_3CH_2C(CH_3)CH_2CHBrCH_2CH_3 + succinimide

Preparation of Grignard Reagent and Carbon Chain Extension

As in steps 1 and 2, we can prepare a Grignard reagent from the brominated six-carbon chain and react it with methyl bromide to extend the carbon chain by two more carbons: CH_3CH_2C(CH_3)CH_2CHBrCH_2CH_3 + Mg -> CH_3CH_2C(CH_3)CH_2CHMgBrCH_2CH_3 CH_3CH_2C(CH_3)CH_2CHMgBrCH_2CH_3 + CH_3Br -> CH_3CH_2C(CH_3)CH_2CH(CH_3)CH_2CH_3 + MgBr_2

Hydrogenation

At this point, we have successfully synthesized the desired 2,7-dimethyloctane molecule. To complete the reaction, we just need to remove the C=C double bond and convert it to a single bond. This can be achieved by hydrogenation of the molecule using a catalyst like palladium on carbon (Pd/C) and hydrogen gas (H_2). Here's the final step: CH_3CH_2C(CH_3)CH_2CH(CH_3)CH_2CH_3 + H_2/Pd/C -> CH_3CH_2CH(CH_3)CH_2CH(CH_3)CH_2CH_3 After following these five steps, we have succeeded in converting 1-bromo-3-methylbutane into the target molecule, 2,7-dimethyloctane.

Key concepts

Grignard Reagent Preparation

The Grignard reagent is a cornerstone in organic synthesis, acting as a doorway to crafting complex molecules from simpler components. To fabricate a Grignard reagent, you begin with an alkyl or aryl halide, like 1-bromo-3-methylbutane. This halide undergoes a reaction with magnesium turnings in a dry ether solvent, resulting in an organomagnesium compound.

It is critical that the reaction is anhydrous (devoid of water), since the presence of moisture would lead to the destruction of the reactive Grignard reagent. This sensitivity towards water highlights the importance of using ether as a solvent because ethers can form a protective layer around the Grignard reagent, effectively sheltering it from moisture and other potential reactants.

Carbon Chain Extension

The objective of carbon chain extension is to lengthen the carbon skeleton of an organic molecule, which is pivotal in constructing larger, more complex structures from smaller ones. The Grignard reagent, due to its nucleophilic nature, is excellent for this purpose. It can open doors to new carbon-carbon bonds when reacted with electrophiles like alkyl halides.

Upon treating the Grignard reagent with an alkyl halide, such as ethyl bromide, a nucleophilic substitution occurs, and a new carbon-carbon bond is formed. This vital tactic of carbon chain extension enriches synthetic reach, enabling chemists to diligently assemble intricate molecules one piece at a time.

Allylic Bromination

The concept of allylic bromination revolves around selectively inserting a bromine atom at an allylic position, which is the carbon next to a carbon-carbon double bond. This kind of bromination is incredibly useful because it creates a highly reactive site for further chemical modifications.One prevalent method for accomplishing this is the use of N-bromosuccinimide (NBS) in the presence of a radical initiator like benzoyl peroxide. Under the right conditions, NBS can replace a hydrogen atom located at the allylic position with bromine. The resulting allylic bromide serves as an excellent starting point for more complex transformations, including further Grignard reactions.

Hydrogenation

The final step in our synthetic journey is hydrogenation - the process of adding hydrogen to an organic compound. Specifically, this involves turning double bonds into single bonds by treating them with hydrogen gas in the presence of a catalyst, such as palladium on carbon (Pd/C).

The conversion of a double bond to a single bond during hydrogenation is a form of reduction; this process adds stability to the molecular structure and is of high significance in preparing saturated compounds like 2,7-dimethyloctane from their unsaturated precursors. Hydrogenation thus serves as a crucial tool to finalize the desired structural properties of organic products.