Problem 29 - Basic Principles of Organic Chemistry

Question

Propenenitrile (acrylonitrile, \(\mathrm{CH}_{2}=\mathrm{CHCN}\) ) will polymerize readily at \(-50^{\circ}\) in a polar solvent [e.g., dimethylmethanamide, \(\left.\mathrm{HCON}\left(\mathrm{CH}_{3}\right)_{2}\right]\) under the influence of sodium cyanide, \(\mathrm{NaCN}\). Show the initiation and propagation steps of this reaction, and predict the structure of the polymer. Why is a polar solvent necessary? Why does this polymerization proceed but not that of propene under the same conditions?

Step-by-Step Solution

Answer

A polar solvent stabilizes carbanion during polymerization, leading to successful acrylonitrile polymerization but not propene.

Step 1: Initiation Step

In the initiation step, the sodium cyanide acts as a nucleophile, attacking the carbon-carbon double bond of the acrylonitrile molecule. This results in the opening of the double bond and formation of a carbanion. The sodium ion does not participate directly in this reaction. The presence of the polar solvent helps to stabilize the resulting carbanion, making this step favorable.

Step 2: Propagation Step

During propagation, the carbanion formed in the initiation step attacks another acrylonitrile monomer, adding it to the growing chain. This step results in the transfer of the negative charge to the newly added acrylonitrile unit, which continues the chain reaction by continuously attacking and adding more acrylonitrile monomers. This process repeats, leading to the formation of long polymer chains.

Step 3: Prediction of Polymer Structure

The structure of the resulting polymer is composed of repeated acrylonitrile units linked together. The polymer can be represented as: \(\mathrm{-[CH_2-CH(CN)]}_n-\), where \(n\) represents the repeating units of the polymer chain.

Step 4: Role of Polar Solvent

A polar solvent is necessary because it stabilizes intermediate ionic states such as carbanions or transition states in the reaction. In this case, it helps stabilize the carbanion formed during the initiation step, which is crucial for the polymerization to proceed.

Step 5: Comparison with Propene Polymerization

Unlike acrylonitrile, propene lacks an electron-withdrawing group like the cyanide group in acrylonitrile that stabilizes the carbanion formed during the initiation step. Therefore, without stabilization, propene is less reactive under these conditions, leading to unsuccessful polymerization.

Key Concepts

Initiation Step

In the initiation step of acrylonitrile polymerization, sodium cyanide (\(\text{NaCN} \)) plays a crucial role as a nucleophile. It attacks the carbon-carbon double bond in the acrylonitrile \(\left(\mathrm{CH}_{2}= ext{CHCN}\right)\), causing it to open up.
This reaction produces a carbanion, a molecule containing a negatively charged carbon atom.
Although sodium ions are present, they do not directly involve themselves in this process.A significant factor in this step is the assistance a polar solvent provides. It helps stabilize the carbanion by surrounding and interacting with it, making the formation of this ion more favorable.
The stabilization offered by the solvent is essential for the reaction to proceed smoothly and efficiently.

Propagation Step

Once the carbanion is formed during initiation, it enters the propagation step where it continually attacks additional acrylonitrile monomers. With each attack, the monomer becomes a part of the growing polymer chain.
This process transfers the negative charge to the now-attached acrylonitrile unit, allowing the cycle of attack and attachment to continue repeatedly. The repetition results in long polymer chains.

• The cycle involves the carbanion as a continuous chain-starter.
• The new unit can effectively hold the charge, enabling further chain growth.
• This step signifies the main chain-building stage in polymerization.

Ultimately, the polymer structure is a sequence of acrylonitrile units, represented as \(\mathrm{-[CH_2-CH(CN)]}_n-\), where \(n\) is the number of repeating units.

Role of Polar Solvent

Polar solvents like dimethylmethanamide perform a vital role in the polymerization process of acrylonitrile. They provide the necessary stability to the ionic intermediates that form, specifically the carbanions. By stabilizing these charged species, the solvent lowers the energy barrier for the reactions.

• This stabilization allows the initiation step to occur more readily.
• Polar solvents can interact via dipole moments, favoring the environment for such reactions.
• Without the solvent, the intermediate carbanions would be too unstable, slowing or stopping the polymerization process.

In essence, the polar solvent's function is to maintain a conducive medium that allows the charged species to propagate effectively and efficiently.

Comparison with Propene Polymerization

Propene (\(\text{CH}_2=\text{CHCH}_3\)) does not polymerize under the same conditions as acrylonitrile due to key chemical differences. Primarily, propene lacks an electron-withdrawing group like acrylonitrile's cyanide group, and this absence impacts polymerization.

• The cyanide group in acrylonitrile helps stabilize the carbanion.
• Without such a group, propene does not form stable carbanions as easily.
• This lack of stability makes propene less reactive and thus unable to undergo polymerization as readily.

To conclude, the presence of electron-withdrawing groups is crucial for stabilizing intermediate species and enabling successful polymerization in certain conditions.