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Chapter 4: Problem 46

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.

Short Answer

Expert verified
Question: Predict the C-C-C bond angles, determine the hybridization of the carbon with the positive charge, write the balanced equation of the tert-butyl cation's reaction with water as a Lewis acid, and explain whether the tert-butyl cation can react with water as a Brønsted-Lowry acid. Answer: The C-C-C bond angles in the tert-butyl cation are 120 degrees. The central carbon with the positive charge has sp2 hybridization. The balanced equation for the reaction with water as a Lewis acid is C4H9+ + H2O → C4H9OH2+. The tert-butyl cation cannot act as a Brønsted-Lowry acid, as it lacks a proton to donate.

Step by step solution

01

1. Analyze the tert-butyl cation structure

We have the following tert-butyl cation structure: C[C+](C)C. The central carbon (the one with the positive charge) is bonded to three other carbons.
02

2. Predict C-C-C bond angles

Since the central carbon is surrounded by three atoms and has no lone pairs, the molecule adopts a trigonal planar geometry. In a trigonal planar molecule, the bond angles are 120 degrees. Therefore, the \(\mathrm{C}-\mathrm{C}-\mathrm{C}\) bond angles are 120 degrees.
03

3. Identify the hybridization of the central carbon

As the central carbon is surrounded by three atoms and has no lone pairs, its hybridization matches the required number of atomic orbitals to form three sigma bonds. It needs three orbitals, which comes from mixing one s and two p orbitals, resulting in an sp2 hybridization.
04

4. Write a balanced equation for the reaction as a Lewis acid with water

A Lewis acid is a species that can accept a lone pair of electrons. In this case, the tert-butyl cation (as a Lewis acid) can accept a lone pair from the water molecule (as a Lewis base). The balanced equation is: \(\mathrm{C_4H_9^+} + \mathrm{H_2O} \rightarrow \mathrm{C_4H_9OH_2^+}\)
05

5. Write a balanced equation for the reaction as a Brønsted-Lowry acid with water

A Brønsted-Lowry acid is a species that can donate a proton (H+). The tert-butyl cation doesn't have an available proton to donate; hence, it cannot act as a Brønsted-Lowry acid. There is no balanced equation for this case since there's no reaction.

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Most popular questions from this chapter

Offer an explanation for the following observations. (a) \(\mathrm{H}_{3} \mathrm{O}^{+}\)is a stronger acid than \(\mathrm{NH}_{4}^{+}\). (b) Nitric acid, \(\mathrm{HNO}_{3}\), is a stronger acid than nitrous acid, \(\mathrm{HNO}_{2}\). (c) Ethanol and water have approximately the same acidity. (d) Trifluoroacetic acid, \(\mathrm{CF}_{3} \mathrm{COOH}\), is a stronger acid than trichloroacetic acid, \(\mathrm{CCl}_{g} \mathrm{COOH}\).

Unless under pressure, carbonic acid \(\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)\) in aqueous solution breaks down into carbon dioxide and water, and carbon dioxide is evolved as bubbles of gas. Write an equation for the conversion of carbonic acid to carbon dioxide and water.

Answer True or False to the following statements about energy diagrams and reactions. (a) A reaction coordinate diagram is used to visualize the change in the internal energy of chemical structures that occurs during chemical reactions. (b) Thermodynamics is the study of the energies of structures that are represented by wells on reaction coordinate diagrams. (c) Kinetics is the study of the rates of chemical reactions. (d) One part of a reaction mechanism would be the understanding of which bonds break and form during a reaction. (e) Thermal reactions occur via collisions between molecules, and the more energy in those collisions the greater the rate of the reactions. (f) The enthalpy of a reaction is the sole determinant of whether it will or will not occur. (g) An exergonic reaction will always occur during the life span of the standard human being.

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.

As we shall see in Chapter 19 , hydrogens on a carbon adjacent to a carbonyl group are far more acidic than those not adjacent to a carbonyl group. The anion derived from acetone, for example, is more stable than is the anion derived from ethane. Account for the greater stability of the anion from acetone. $$ \begin{array}{cc} \stackrel{O}{\|} & \\ \mathrm{CH}_{3} \mathrm{CCH}_{2}-\mathrm{H} & \mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{H} \\ \text { Acetone } & \text { Ethane } \\ \mathrm{p} K_{\mathrm{a}} 22 & \mathrm{p} K_{\mathrm{a}} 51 \end{array} $$
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