Question

Sodium hydride, \(\mathrm{NaH}\), is available commercially as a gray-white powder. It melts at \(800^{\circ} \mathrm{C}\) with decomposition. It reacts explosively with water and ignites spontaneously upon standing in moist air. (a) Write a Lewis structure for the hydride ion and for sodium hydride. Is your Lewis structure consistent with the fact that this compound is a high- melting solid? Explain. (b) When sodium hydride is added very slowly to water, it dissolves with the evolution of a gas. The resulting solution is basic to litmus. What is the gas evolved? Why has the solution become basic? (c) Write an equation for the reaction between sodium hydride and 1-butyne, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\). Use curved arrows to show the flow of electrons in this reaction.

Short answer

Question: Draw the Lewis structures of the hydride ion and sodium hydride, discuss if they are consistent with the given properties of the compound, identify the gas evolved when sodium hydride is added to water and explain why the solution becomes basic, write an equation for the reaction between sodium hydride and 1-butyne and show the flow of electrons using curved arrows. Answer: The Lewis structures of the hydride ion (H-) and sodium hydride (Na+H-) show an ionic bond between sodium and hydrogen, which is consistent with sodium hydride being a high-melting solid. When sodium hydride is added to water, hydrogen gas (H2) is evolved, and the solution becomes basic due to the formation of sodium hydroxide (NaOH). The reaction between sodium hydride and 1-butyne produces an alkynide ion (CH3CH2C≡C-) and a sodium cation (Na+), with the flow of electrons indicated by curved arrows in the reaction mechanism.

Step-by-step solution

(a) Draw Lewis structures and discuss if it is consistent with the given properties

To draw the Lewis structure for the hydride ion (\(\mathrm{H}^{-}\)) and for sodium hydride (\(\mathrm{NaH}\)), first, determine the valence electrons in each atom. Hydrogen has 1 valence electron while sodium has 1 as well. In the hydride ion, hydrogen gains an electron to achieve a stable electronic configuration, with its one orbital completely filled. For sodium hydride, sodium donates its one valence electron to the hydrogen atom, forming an ionic bond. Hydride ion (\(\mathrm{H}^{-}\)) Lewis structure: ``` H(-) ``` Sodium hydride (\(\mathrm{Na}^{+}\mathrm{H}^{-}\)) Lewis structure: ``` [Na+] [H(-)] ``` Since sodium hydride has an ionic bond between sodium and hydrogen, it forms a crystal lattice structure in the solid state. This structure requires a significant amount of energy to break, leading to a high melting point. So, the Lewis structure of sodium hydride is consistent with the fact that it is a high-melting solid.

(b) Identify the gas evolved and explain why the solution is basic

When sodium hydride (\(\mathrm{NaH}\)) is added to water, it reacts to produce sodium hydroxide (\(\mathrm{NaOH}\)) and hydrogen gas (\(\mathrm{H}_{2}\)). The balanced chemical equation for the reaction is: \(\mathrm{NaH} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{NaOH} + \mathrm{H}_{2}\) The gas evolved in this reaction is hydrogen gas (\(\mathrm{H}_{2}\)). The solution becomes basic because sodium hydroxide (\(\mathrm{NaOH}\)) is a strong base, which dissociates completely in water to produce hydroxide ions (\(\mathrm{OH}^{-}\)), making the solution alkaline: \(\mathrm{NaOH} \rightarrow \mathrm{Na}^{+} + \mathrm{OH}^{-}\)

(c) Write an equation for the reaction between sodium hydride and 1-butyne and show the flow of electrons

The reaction between sodium hydride (\(\mathrm{NaH}\)) and 1-butyne (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{C} \equiv \mathrm{CH}\)) is an acid-base reaction, where the hydride ion from sodium hydride acts as a base, and the acidic hydrogen from 1-butyne acts as an acid. The reaction generates an alkynide ion (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{C}\equiv\mathrm{C}^{-}\)) and sodium cation (\(\mathrm{Na}^{+}\)). The equation for the reaction is given by: \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{C} \equiv \mathrm{CH} + \mathrm{NaH} \rightarrow \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{C} \equiv \mathrm{C}^{-} \mathrm{Na}^{+}\) To show the flow of electrons with curved arrows, first, identify the acidic hydrogen atom, which is the one connected to the triple-bonded carbon. The curved arrow starts from the hydride ion's lone pair of electrons and points towards the acidic hydrogen atom, indicating the formation of a new bond. The next curved arrow starts from the bond between the acidic hydrogen and the triple-bonded carbon and points towards the latter, showing the bond's breaking. The resulting flow of electrons is as follows: ``` H+ H- C≡C + Na -> C≡C- + Na+ ```

Key concepts

Lewis Structures

Lewis structures are diagrams that visualize the bonding between atoms of a molecule and the lone pairs of electrons that may exist. These structures are pivotal in organic chemistry because they help us understand the arrangement of electrons in molecules.

In the case of sodium hydride (NaH), the Lewis structure for the hydride ion (H^-) shows hydrogen with an extra electron, represented by two dots surrounding the symbol for hydrogen (H). This indicates that hydrogen usually having one valence electron now has a complete shell, giving it a stable electronic arrangement. For NaH, the sodium ion (Na^) transfers its one valence electron to hydrogen, which can be represented by placing H next to Na+, separated by a dot to indicate the ionic bond between them.

This depiction is crucial in understanding why NaH is solid and has a high melting point, as the ionic bonding creates a crystalline lattice structure that requires substantial energy to disrupt.

Ionic Bonding

Ionic bonding is a type of chemical bond that occurs when an atom transfers one or more electrons to another atom, resulting in the formation of positively and negatively charged ions that attract each other. This electrostatic attraction results in the formation of an ionic compound.

Sodium hydride contains an ionic bond between sodium and hydrogen. Sodium, which has one valence electron, donates this electron to the hydrogen atom, obtaining a positive charge (Na^), while hydrogen gains this electron, acquiring a negative charge (H^-). This transfer creates a strong and stable bond that leads to the formation of a lattice structure, contributing to the high melting point observed in such compounds.

Acid-Base Reaction

An acid-base reaction is a type of chemical reaction where an acid and a base interact with each other. In organic chemistry, this often results in the transfer of a proton (H+) from the acid to the base, creating new products as a result.

When sodium hydride reacts with water, we observe an acid-base reaction. NaH serves as a base, and the water acts as the acid. NaH reacts with water to produce sodium hydroxide (NaOH) and hydrogen gas (H2). Here, sodium hydroxide dissociates in water to form OH^- ions, which increases the pH of the solution, making it basic. Additionally, the evolution of hydrogen gas is indicative of this type of reaction where water (the acid) donates a proton to the base (H^- from NaH) to form H2 gas.

Electron Flow in Chemical Reactions

Understanding electron flow in chemical reactions is essential for predicting how these reactions will proceed. It involves tracking the movement of electrons as bonds are formed and broken during a reaction.

In the reaction between sodium hydride and 1-butyne, the electron flow can be mapped with curved arrows. These arrows show the action of the base (the hydride ion from NaH) as it donates an electron pair to the acidic hydrogen atom on 1-butyne. Subsequently, the pair of electrons bonding the acidic hydrogen to the carbon atom in 1-butyne moves towards the carbon atom, forming a negatively charged alkynide ion. This flow of electrons illustrates the creation of a new bond and highlights the acid-base nature of this reaction, where electron donation is key to forming the reaction's products.