Question

What product(s) will be obtained when 1 -bromobutane is treated with (a) Aqueous \(\mathrm{KOH}\) (b) Alcoholic KOH (c) Na/ether (d) Mg/ether (e) NaI/acetone (f) \(\mathrm{NaCN}\) (g) \(\mathrm{AgCN}\) (h) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa}\)

Short answer

Products: (a) Butan-1-ol; (b) 1-Butene; (c) n-Octane; (d) Butylmagnesium bromide; (e) 1-Iodobutane; (f) Butanenitrile; (g) 1-Butyl isocyanide; (h) Ethyl butyl ether.

Step-by-step solution

Aqueous KOH Reaction

When 1-bromobutane is treated with aqueous \( \mathrm{KOH} \), a nucleophilic substitution reaction occurs. The hydroxide ion \( \mathrm{OH}^- \) from \( \mathrm{KOH} \) attacks the carbon attached to bromine, replacing \( \mathrm{Br}^- \) and forming butan-1-ol. The product is butan-1-ol (\( \mathrm{C_4H_9OH} \)).

Alcoholic KOH Reaction

1-bromobutane with alcoholic \( \mathrm{KOH} \) undergoes an elimination reaction, known as dehydrohalogenation. The hydroxide ion acts as a base, removing a hydrogen from the \( \beta \)-carbon, resulting in the formation of 1-butene as the major product.

Sodium in Ether (Wurtz Reaction)

When 1-bromobutane reacts with sodium in ether, a Wurtz reaction occurs. Two molecules of 1-bromobutane react to form n-octane (\( \mathrm{C_8H_{18}} \)) after eliminating two bromine atoms, which bond with sodium to form sodium bromide (\( \mathrm{NaBr} \)).

Grignard Reagent Formation

Treating 1-bromobutane with magnesium in ether forms a Grignard reagent. The reaction creates butylmagnesium bromide (\( \mathrm{C_4H_9MgBr} \)). This is an important organometallic compound used for further chemical synthesis.

Sodium Iodide in Acetone

In this reaction, the \( \mathrm{SN2} \) mechanism leads to the exchange of bromine for iodine. The product is 1-iodobutane (\( \mathrm{C_4H_9I} \)), with sodium bromide precipitating as it is insoluble in acetone.

Reaction with Sodium Cyanide

1-bromobutane reacts with \( \mathrm{NaCN} \) via an \( \mathrm{SN2} \) mechanism, where the cyanide ion replaces bromine, forming butanenitrile (\( \mathrm{C_4H_9CN} \)). This reaction extends the carbon chain by introducing a nitrile group.

Silver Cyanide Reaction

When 1-bromobutane is treated with \( \mathrm{AgCN} \), an isocyanide is formed instead of a normal cyanide compound. This is due to the different bonding nature of \( \mathrm{AgCN} \) allowing the formation of 1-butyl isocyanide (\( \mathrm{C_4H_9NC} \)).

Ethoxide Ion Reaction

1-bromobutane undergoes a reaction with \( \mathrm{C_2H_5ONa} \) through an \( \mathrm{SN2} \) substitution, where the ethoxide ion replaces bromine, yielding ethyl butyl ether (\( \mathrm{C_4H_9OC_2H_5} \)).

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental process in organic chemistry, pivotal for transforming molecular structures by replacing an atom or group with a nucleophile. In this reaction, a nucleophile, rich with electrons, targets an electron-deficient carbon atom, often attached to a leaving group such as a halide. For 1-bromobutane, this process involves the attack of the hydroxide ion (\(\mathrm{OH}^-\) from aqueous \(\mathrm{KOH}\)) on the carbon bonded to bromine. The result is the replacement of the bromine atom with a hydroxyl group, producing butan-1-ol (\(\mathrm{C_4H_9OH}\)). This type of reaction commonly proceeds through an \(\mathrm{SN1}\) or \(\mathrm{SN2}\) mechanism, with SN2 being more prevalent in primary halides like 1-bromobutane, ensuring a swift and clean substitution.

Elimination Reaction

Elimination reactions are key to creating alkenes from alkyl halides through the loss of elements (such as a hydrogen and a halogen) from adjacent carbon atoms. When 1-bromobutane interacts with alcoholic \(\mathrm{KOH}\), it undergoes dehydrohalogenation. Here, the hydroxide ion acts as a base rather than a nucleophile, removing a hydrogen from the \(\beta\)-carbon. This process results in the formation of a double bond, converting 1-bromobutane into 1-butene (\(\mathrm{C_4H_8}\)). Known as an \(\mathrm{E2}\) reaction (bimolecular elimination), this mechanism is favored by strong bases and occurs in a single concerted step without intermediates.

Wurtz Reaction

The Wurtz reaction is a classic method used in organic chemistry to couple two alkyl halides, extending carbon chains. When 1-bromobutane is treated with sodium metal in ether (a non-protic solvent that helps stabilize reactive intermediates), a Wurtz reaction ensues. Two molecules of 1-bromobutane combine to form n-octane (\(\mathrm{C_8H_{18}}\)), alongside the formation of sodium bromide (\(\mathrm{NaBr}\)) as a byproduct. This reaction is an example of homolytic cleavage and radical coupling, highlighting it as a vital synthesis route for producing higher alkanes from simpler precursors.

Grignard Reagents

Grignard reagents are invaluable organometallic compounds in synthetic organic chemistry, often employed to form carbon-carbon bonds. Formed by reacting an alkyl halide with magnesium metal in an ether solvent, these reagents are characterized by their highly reactive carbon-magnesium bond. For instance, treating 1-bromobutane with magnesium in ether yields butylmagnesium bromide (\(\mathrm{C_4H_9MgBr}\)). This compound serves as a strong nucleophile, able to attack electrophiles like carbonyl compounds to create alcohols or contribute to even more complex organic syntheses. The formation of Grignard reagents is sensitive to moisture; hence, reactions are typically performed under anhydrous conditions to prevent unwanted side reactions with water.