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Chapter 4: Problem 3

Following is a structural formula for guanidine, the compound by which migratory birds excrete excess metabolic nitrogen. The hydrochloride salt of this compound is a white crystalline powder, freely soluble in water and ethanol. (a) Write a Lewis structure for guanidine showing all valence electrons. (b) Does proton transfer to guanidine occur preferentially to one of its \(-\mathrm{NH}_{2}\) groups \((\) cation A) or to its = NH group (cation B)? Explain.

Short Answer

Expert verified
Explain your answer. Answer: The nitrogen atom in the -NH2 group is more likely to gain a proton. This is because, upon gaining a proton, it forms a common and stable three-bond configuration. On the other hand, adding a proton to the nitrogen in the =NH group results in an unusual and less stable four-bond configuration.

Step by step solution

01

Draw a Lewis structure for guanidine

To draw the Lewis structure for guanidine, first identify each atom's valence electrons. Nitrogen (N) has 5 valence electrons, and Hydrogen (H) has 1 valence electron. Guanidine has two \(-\mathrm{NH}_{2}\) groups and one =NH group, so the molecule has a total of 16 valence electrons. Now, arrange these atoms to satisfy the octet rule (meaning 8 electrons surrounding each atom except hydrogen, which follows the duet rule and only needs 2 electrons). Guanidine has the following structure: H N H \| / N = N H Each line represents a bond between the two atoms, and each bond consists of 2 electrons.
02

Determine proton transfer preference in guanidine

Proton transfer to guanidine can occur in two ways: Cation A (H added to an \(-\mathrm{NH}_{2}\) group): H N H \| / N = N+ - H / \ H H Cation B (H added to the =NH group): H N H \| / N = N+ - 2H / \ H H Let's compare the two structures: In cation A, the positively charged nitrogen atom forms three bonds, which is a common configuration for nitrogen. In cation B, the positively charged nitrogen atom forms four bonds, which is an unusual and less stable configuration for nitrogen. As a result, proton transfer to guanidine occurs preferentially to one of its \(-\mathrm{NH}_{2}\) groups, forming cation A.

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Most popular questions from this chapter

Write an equation for the reaction between each Lewis acid-base pair, showing electron flow by means of curved arrows. (a) \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{3} \mathrm{~B}+\mathrm{OH}^{-} \longrightarrow\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}+\mathrm{AlCl}_{3} \longrightarrow\)

For each value of \(K_{\mathrm{a}}\), calculate the corresponding value of \(\mathrm{p} K_{\mathrm{a}}\). Which compound is the stronger acid? (a) Acetic acid, \(K_{\mathrm{a}}=1.74 \times 10^{-5}\) (b) Chloroacetic acid, \(K_{\mathrm{a}}=1.38 \times 10^{-3}\)

Answer true or false to the following statements about the mechanism of acid- base reactions. (a) The acid and base must encounter each other by a collision in order for the proton to transfer. (b) All collisions between acids and bases result in proton transfer. (c) During an acid-base reaction the lone pair on the base fills the A-H antibonding sigma orbital.

Following is a structural formula for the tert-butyl cation. (We discuss the formation, stability, and reactions of cations such as this one in Chapter 6 .) C[C+](C)C tert-Butyl cation (a carbocation) (a) Predict all \(\mathrm{C}-\mathrm{C}-\mathrm{C}\) bond angles in this cation. (b) What is the hybridization of the carbon bearing the positive charge? (c) Write a balanced equation to show its reaction as a Lewis acid with water. (d) Write a balanced equation to show its reaction as a Brønsted-Lowry acid with water.

Sodium hydride, 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 on 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.
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