Question

Write equations for the substitution reaction of \(n\) -bromobutane, a typical primary haloalkane, with the following reagents: (a) \(\mathrm{NaOH} ;\) (b) \(\mathrm{NH}_{3} ;\) (c) \(\mathrm{NaCN}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{ONa}\).

Short answer

The substitution reactions of n-bromobutane with various reagents form: (a) butan-1-ol; (b) butylamine; (c) butanenitrile; (d) butyl ethyl ether.

Step-by-step solution

Understand the reagent

Firstly, identify and understand the structure of the reagent, n-bromobutane (or 1-bromobutane). It is a primary haloalkane with the bromine atom attached to an end carbon of the four-carbon chain. The formula is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\).

Reaction with NaOH

Reacting n-bromobutane with \(\mathrm{NaOH}\) will replace the bromine atom with a hydroxyl group through a nucleophilic substitution, forming butan-1-ol. The equation is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{NaOH}\) \(\rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} + \mathrm{NaBr}\).

Reaction with NH3

When n-bromobutane reacts with \(\mathrm{NH}_{3}\), the bromine atom is replaced by an amine functional group, forming butylamine. The equation is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{NH}_{3} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} + \mathrm{HBr}\).

Reaction with NaCN

In the reaction with \(\mathrm{NaCN}\), the bromine atom is replaced by a cyanide group, forming butanenitrile. The equation is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{NaCN}\) \(\rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CN} + \mathrm{NaBr}\).

Reaction with CH3CH2ONa

Finally, reaction with \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{ONa}\) causes the bromine atom to be replaced by an ethoxide group, forming butyl ethyl ether. The equation is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{ONa} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3} + \mathrm{NaBr}\).

Key concepts

Primary Haloalkane

Primary haloalkanes are organic compounds characterized by a halogen atom, such as bromine, chlorine, or iodine, attached to the first carbon atom of an alkyl chain. This carbon atom is connected to at least two hydrogen atoms, distinguishing it as a primary position.

Take n-bromobutane, for example. This molecule is comprised of a four-carbon butane chain with a bromine atom attached to the first carbon, making it a primary bromoalkane. Its structural formula, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{Br}\), showcases the halogen atom bonded to a carbon that's only linked to one other carbon.

Due to their structure, primary haloalkanes are more reactive in certain types of chemical reactions, such as nucleophilic substitution, which can be used to create a wide range of other compounds, showcasing their importance in organic synthesis.

Organic Synthesis

Organic synthesis is the process through which complex organic molecules are constructed from simpler ones through chemical reactions. It is a critical aspect of creating pharmaceuticals, plastics, and other essential materials.

In the case of haloalkanes, organic synthesis can involve various nucleophilic substitution reactions. A nucleophile, which is a species rich in electrons, attacks the carbon atom bonded to the halogen, resulting in the substitution of the halogen with the nucleophile.

For instance, when n-bromobutane is treated with sodium hydroxide \(\mathrm{NaOH}\), ammonia \(\mathrm{NH}_{3}\), sodium cyanide \(\mathrm{NaCN}\), or sodium ethoxide \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{ONa}\), the bromine atom is replaced by different functional groups, thus creating alcohols, amines, nitriles, and ethers respectively. This versatility is what makes primary haloalkanes a valuable starting point in synthetic organic chemistry.

Chemical Reaction Mechanisms

A chemical reaction mechanism is a step-by-step description of how a chemical reaction occurs, including the breaking and formation of bonds, the intermediates, and the changes in energy throughout the process.

In nucleophilic substitution reactions, two main mechanisms are observed: the S_N1 and S_N2 mechanisms. S_N1 involves a two-step process where the bond between the carbon and the halogen breaks first, creating a positively charged intermediate called a carbocation. The nucleophile then quickly attacks this carbocation, forming a new product.

On the other hand, S_N2 reactions are single-step and involve a simultaneous attack by the nucleophile and departure of the leaving group. This results in an inversion of stereochemistry at the carbon center, often compared to an 'umbrella flipping inside out'.

Primary haloalkanes usually undergo the S_N2 mechanism because the steric hindrance around the carbon-halogen bond is minimal, allowing the nucleophile to approach and react directly with the carbon atom.