Question

The following projected synthesis for $\mathrm{n}$ -butane is not very efficient. Why? ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}+{\mathrm{CH}}_3\mathrm{Br}+2\mathrm{Na}\to {\mathrm{CH}}_3{\left({\mathrm{CH}}_2\right)}_2{\mathrm{CH}}_3+2\mathrm{NaBr}$

Short answer

The inefficiencies of this projected synthesis for n-butane mainly stem from the formation of various undesirable side products due to competing reactions involving nucleophilic species, leading to reduced yield and challenging purification. Additionally, the use of sodium as a highly reactive and potentially dangerous reagent increases risks and operational costs. These factors contribute to the low efficiency and increased complexity of the synthesis process.

Step-by-step solution

1. Identify the reactants and the products

The reactants are 1-bromopropane ($C{H}_{3}C{H}_{2}C{H}_{2}Br$), methyl bromide ($C{H}_{3}Br$), and sodium ($Na$). The products are n-butane ($C{H}_3(C{H}_2{)}_{2}C{H}_3$) and sodium bromide ($NaBr$).

2. Understand the mechanism of the reaction

The reaction involves a two-step mechanism, the given reactants undergo a nucleophilic substitution (SN2) reaction, which involves an exchange of halogen (bromine) by a hydride (H) provided by the sodium. First, sodium (a strong reducing agent) will donate electrons to the halogen atoms, leading to the formation of ionic species: $C{H}_{3}C{H}_{2}C{H}_{2}Br+Na\to C{H}_{3}C{H}_{2}C{H}_2^{-}+N{a}^{+}+B{r}^{-}$ $C{H}_{3}Br+Na\to C{H}_3^{-}+N{a}^{+}+B{r}^{-}$ Subsequently, a nucleophilic attack occurs between the negatively charged carbon and a carbon of another alkyl halide, resulting in the formation of n-butane and sodium bromide: $C{H}_{3}C{H}_{2}C{H}_2^{-}+C{H}_3^{-}+2B{r}^{-}\to C{H}_3(C{H}_2{)}_{2}C{H}_3+2NaBr$

3. Identify the inefficiencies in the reaction path

One major inefficiency in this synthesis is the formation of various undesirable side products due to competing reactions. Since the reaction involves nucleophilic species, they can potentially attack any carbon bound to a halogen. These side reactions will lead to the formation of various isomers and products, which will reduce the overall efficiency and yield of n-butane. The mixture of products makes the separation and purification of n-butane more challenging and energy-consuming. Another inefficiency is the use of sodium, a highly reactive and potentially dangerous reagent that requires specific handling and storage conditions. Using such a hazardous substance increases the risks and operational costs associated with the synthesis. In conclusion, the inefficiencies of this projected synthesis of n-butane mainly stem from the undesirable side reactions leading to a mixture of products and the use of a hazardous reagent (sodium). These factors contribute to the low efficiency and increased complexity of the synthesis process.

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of reaction in organic chemistry where an electron-rich 'nucleophile' seeks out an electron-poor 'electrophile' or positively polarized atom, typically carbon. The nucleophile generously donates a pair of electrons to form a new bond, while simultaneously displacing a leaving group, often a halide ion.

For example, in the synthesis for n-butane from the exercise, the nucleophiles are the carbanions formed from bromopropane and methyl bromide, while the electrophile is the carbon atom the carbanions attack. The leaving group is the bromide ion, which is displaced when the nucleophile forms a bond with the alkyl halide.

Key Factors in Nucleophilic Substitution:

• Strength of the nucleophile: Stronger nucleophiles are more effective at displacing leaving groups.
• Leaving group ability: Better leaving groups (like bromide) will dissociate more easily, facilitating the reaction.
• Substrate reactivity: The type of alkyl halide affects the rate and outcome of the reaction.

Understanding these concepts will help in predicting and controlling the course of a reaction, which is crucial for efficient organic synthesis.

SN2 Reaction Mechanism

The SN2 reaction mechanism (Substitution Nucleophilic Bimolecular) describes a one-step process where the nucleophile and substrate collide and react in a single concerted reaction. The '2' in SN2 signifies that the rate of the reaction depends on the concentration of two reacting species: the nucleophile and the electrophile.

In the exercise, sodium acts as the electron donor, creating negatively charged carbanions that serve as nucleophiles. These nucleophiles attack the electrophilic carbons of opposite charged alkyl halides simultaneously, with the halide leaving groups being pushed out.

Characteristics of SN2 Reactions:

• Bimolecularity: Both the nucleophile and the electrophile determine the rate of the reaction.
• Steric hindrance: SN2 reactions are sensitive to steric effects; bulky groups near the reaction site can inhibit the process.
• Inversion of configuration: SN2 reactions result in an inversion of configuration at the reactive carbon, akin to an umbrella turning inside out.

The SN2 mechanism is essential in understanding how the reaction occurs in a controlled manner and is directly relevant to the problems of inefficiency observed in the n-butane synthesis discussed.

Alkyl Halide Reactivity

Alkyl halides, also known as haloalkanes, are central to many nucleophilic substitution reactions. Their reactivity is influenced by both the halogen atom attached and the nature of the carbon backbone.

Primary alkyl halides, like the 1-bromopropane used in the exercise, generally react faster in SN2 reactions due to less steric hindrance compared to secondary and tertiary alkyl halides. Reactivity in nucleophilic substitutions decreases as we move from primary to tertiary halides.

Impact on Reactivity:

• Alkyl group structure: More complex structures hinder the approach of nucleophiles.
• Halogens: The bond strength and size of the halogen affect how easily it can be displaced.

These reactivity principles must be considered when designing syntheses like that of n-butane because using the wrong type of alkyl halide can drastically lower yields and create unwanted side products.

Organic Synthesis Inefficiencies

The exercise mentions inefficiencies in the synthesis of n-butane. In organic synthesis, inefficiencies can arise from numerous sources, impacting both yield and the cost-effectiveness of the process.

In the case at hand, the side reactions and production of multiple isomers are a prime cause of inefficiency. Since the reactive intermediates can potentially react with several species in the mixture, the selectivity of the reaction is reduced, and the purification of the desired product becomes more complex and expensive.

Contributors to Inefficiency:

• Side reactions: Competing reactions can lead to unnecessary by-products.
• Use of hazardous reagents: Reagents like sodium can pose safety risks and additional costs.
• Purification difficulties: A complex mixture of products complicates extraction and purification, reducing overall yield.

Understanding these inefficiencies is critical for improving reaction conditions and choosing more efficient synthetic routes in future organic chemistry endeavors.