Question

Write equations for the substitution reaction of \(n\) -bromopentane, a typical primary haloalkane with the following reagents: (a) \(\mathrm{NaN}_{3};\) (b) \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3};\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CNa};\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SNa}\).

Short answer

The reactions of \(n\) -bromopentane with varied reagents produce pentyl azide, pentyltrimethylammonium bromide, 4-pentyne, and pentyl ethyl sulfide, respectively.

Step-by-step solution

Reaction with \(\mathrm{NaN}_{3}\)

When \(n\) -bromopentane reacts with sodium azide \(\mathrm{NaN}_{3}\), the azide ion \(\mathrm{N}_3^-\) behaves as a nucleophile and substitutes the bromine atom to form pentyl azide. The reaction equation can be written as: \( \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{Br} + \mathrm{NaN}_{3} \rightarrow \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{N}_{3} + \mathrm{NaBr}\)

Reaction with \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\)

Similarly, when \(n\) -bromopentane reacts with trimethylamine \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\), the nitrogen atom in the amine substitutes the bromine atom in the haloalkane to form pentyltrimethylammonium bromide. The reaction can be expressed as: \( \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{Br} + \mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \rightarrow \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{Br}\)

Reaction with \(\mathrm{CH}_{3}\mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CNa}\)

In a reaction with sodium acetylide \(\mathrm{CH}_{3}\mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CNa}\), the acetylide ion \(\mathrm{C} \equiv \mathrm{C}^-\) replaces the bromine atom in the haloalkane to form 4-pentyne. This can be expressed in the following equation: \( \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{Br} + \mathrm{CH}_{3}\mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CNa} \rightarrow \mathrm{C}_{5}\mathrm{H}_{9}\mathrm{C}\equiv\mathrm{CH} + \mathrm{NaBr}\)

Reaction with \(\mathrm{CH}_{3}\mathrm{CH}_{2} \mathrm{SNa}\)

When \(n\) -bromopentane reacts with sodium ethyl sulfide \(\mathrm{CH}_{3}\mathrm{CH}_{2} \mathrm{SNa}\), the sulfide ion \(\mathrm{S}^-\) substitutes the bromine atom, resulting in pentyl ethyl sulfide. The reaction can be written as: \( \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{Br} + \mathrm{CH}_{3}\mathrm{CH}_{2} \mathrm{SNa} \rightarrow \mathrm{C}_{5}\mathrm{H}_{11}\mathrm{S}\mathrm{CH}_{2}\mathrm{CH}_{3} + \mathrm{NaBr}\)

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of chemical reaction where an electron-rich species, the nucleophile, replaces a leaving group attached to a carbon atom. In the context of haloalkane reactions, this process involves the exchange of a halogen atom, such as bromine, with a nucleophile, which could be anything from azide to alkynes.

In the case of n-bromopentane, a variety of nucleophiles are used, resulting in the formation of different organic products. The nucleophile aggressively attacks the carbon atom that carries the halogen, forming a new bond, while the bromide ion departs, often with a counter ion such as sodium. This mechanism allows the synthesis of a range of organic compounds from a simple haloalkane.

• When NaN3 is used, the azide ion (\textbf{N}\(_{3}^{-}\)) is the nucleophile.
• With N(CH3)3, the nucleophilic center is the nitrogen atom in trimethylamine.
• CH3CH2C≡CNa introduces a carbon nucleophile in the form of an acetylide ion.
• CH3CH2SNa involves a sulfide ion as the nucleophile.

Haloalkane Reactions

Haloalkanes, also known as alkyl halides, are composed of an alkane with one or more halogen atoms attached. These compounds are highly reactive in nucleophilic substitution reactions due to the polar nature of the carbon-halogen bond. In n-bromopentane, the carbon-bromine bond is polar, making the carbon a good target for attack by nucleophiles.

The reactions discussed showcase the versatility of haloalkanes in organic synthesis. For instance, reacting n-bromopentane with various nucleophiles produces different organic structures that are valuable in further chemical processing or as end products themselves. The nature of the nucleophile and the conditions of the reaction can significantly impact the reaction course and the final product formed.

• The reaction with sodium azide forms pentyl azide, a compound that can be utilized in the synthesis of other nitrogen-containing molecules.
• Trimethylamine leads to the formation of a quaternary ammonium salt, which has applications in phase transfer catalysis.
• Reacting with sodium acetylide results in the creation of an alkyne, a useful building block in organic chemistry.
• Sodium ethyl sulfide forms a thioether, a compound type that features in pharmaceuticals and organosulfur chemistry.

Chemical Reaction Mechanisms

Understanding the chemical reaction mechanisms involved in nucleophilic substitution reactions is pivotal for predicting the outcome of these reactions. In primary haloalkanes like n-bromopentane, the most common mechanism is the SN2 reaction. This mechanism is characterized by a 'backside attack' by the nucleophile and a simultaneous displacement of the leaving group, resulting in an inversion of configuration at the carbon atom if it's chiral.

The reaction conditions, such as solvent and temperature, play crucial roles in determining the reaction path and the speed at which the reaction proceeds. For instance:

• In a polar aprotic solvent, the SN2 mechanism is usually favored due to better solvation of the nucleophile and the leaving group.
• Temperature can affect the nucleophile's reactivity and the stability of the transition state.

Considering the exercise solutions, each step represents a classic SN2 reaction where the nucleophiles have both the kinetic ability and thermodynamic drive to displace the bromide ion efficiently. This concept of the reaction mechanism is central to visualizing how molecular interactions dictate the formation of new substances in organic chemistry.

Organic Synthesis

Organic synthesis is the construction of organic compounds via chemical processes, and nucleophilic substitution reactions like those of n-bromopentane form an integral part of it. Such reactions are vital for creating a diverse range of organic molecules with varying complexity for use in pharmaceuticals, materials science, and other industries.

The exercise examples illustrate how to manipulate simple starting materials to synthesize more complex molecules. Each product has specific properties and potential applications, demonstrating the creativity involved in organic synthesis. Organic synthesis is often about choosing the correct reagents and reaction conditions to build desired molecular structures with precision and efficiency.

• The production of pentyl azide could be a step towards generating nitrogen-based compounds.
• The formation of pentyltrimethylammonium bromide could be used to synthesize phase transfer catalysts.
• Creating 4-pentyne opens pathways to making polymers or pharmaceuticals.
• Pentyl ethyl sulfide illustrates the introduction of sulfur into organic molecules for various applications.

Through understanding and applying concepts of nucleophilic substitution, haloalkane reactions, chemical reaction mechanisms, and organic synthesis, chemists can design and execute pathways to create an array of valuable organic compounds from relatively simple precursors.