Organic Chemistry

John McMurry

Chapter 2

Polar Covalent Bonds; Acids and Bases - all with Video Answers

Educators

Chapter Questions

Problem 1

Which element in each of the following pairs is more electronegative?
(a) Li or H
(b) $B$ or $B r$
(c) Cl or I
(d) C or H

Alex M

Alex M

Numerade Educator

Problem 2

Use the $\delta+/ \delta$ - convention to indicate the direction of expected polarity for each of the bonds indicated.
(a) $\mathrm{H}_{3} \mathrm{C}-\mathrm{Cl}$
(b) $\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}$
(c) $\mathrm{H}_{2} \mathrm{~N}-\mathrm{H}$
(d) $\mathrm{H}_{3} \mathrm{C}-\mathrm{SH}$
(e) $\mathrm{H}_{3} \mathrm{C}-\mathrm{MgBr}$
(f) $\mathrm{H}_{3} \mathrm{C}-\mathrm{F}$

Bobby Barnes

University of North Texas

Problem 3

Use the electronegativity values shown in Figure $2-2$ to rank the following bonds from least polar to most polar: $\mathrm{H}_{3} \mathrm{C}-\mathrm{Li}, \mathrm{H}_{3} \mathrm{C}-\mathrm{K}, \mathrm{H}_{3} \mathrm{C}-\mathrm{F}, \mathrm{H}_{3} \mathrm{C}-\mathrm{MgBr}$
$\mathrm{H}_{3} \mathrm{C}-\mathrm{OH}$

Crystal Wang

Numerade Educator

Problem 4

Look at the following electrostatic potential map of methylamine, a substance responsible for the odor of rotting fish, and tell the direction of polarization of the C-N bond:

Crystal Wang

Numerade Educator

Problem 5

Ethylene glycol, $\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH},$ may look nonpolar when drawn, but an internal hydrogen bond results in an electric dipole moment. Explain.

Crystal Wang

Numerade Educator

Problem 6

Make three-dimensional drawings of the following molecules, and predict whether each has a dipole moment. If you expect a dipole moment, show its direction.
(a) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}$
(b) $\mathrm{CHCl}_{3}$
(c) $\mathrm{CH}_{2} \mathrm{Cl}_{2}$
(d) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CCl}_{2}$

Bobby Barnes

University of North Texas

Problem 7

Calculate formal charges for the nonhydrogen atoms in the following molecules:
(a) Diazomethane, $\quad \mathrm{H}_{2} \mathrm{C}=\mathrm{N}=\ddot{\mathrm{N}}:$
(b) Acetonitrile oxide, $\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}-\ddot{\mathrm{O}}:$
(c) Methyl isocyanide, $\mathrm{H}_{3} \mathrm{C}-\mathrm{N} \equiv \mathrm{C}$ :

Bobby Barnes

University of North Texas

Problem 8

Calculate formal charges for the nonhydrogen atoms in the following molecules:
(a) Diazomethane, $\quad \mathrm{H}_{2} \mathrm{C}=\mathrm{N}=\ddot{\mathrm{N}}:$
(b) Acetonitrile oxide, $\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}-\ddot{\mathrm{O}}:$
(c) Methyl isocyanide, $\mathrm{H}_{3} \mathrm{C}-\mathrm{N} \equiv \mathrm{C}$ :

Bobby Barnes

University of North Texas

Problem 9

Which of the following pairs of structures represent resonance forms, and which do not? Explain.

Alexander Cheng

Alexander Cheng

Numerade Educator

Problem 10

Draw the indicated number of resonance forms for each of the following species:
(a) The methyl phosphate anion, $\mathrm{CH}_{3} \mathrm{OPO}_{3}^{2-}(3)$
(b) The nitrate anion, $\mathrm{NO}_{3}^{-}$ (3)
(c) The allyl cation, $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}^{+}(2)$
(d) The benzoate anion (4)

Bobby Barnes

University of North Texas

Problem 11

Nitric acid (HNO $_{3}$ ) reacts with ammonia (NH $_{3}$ ) to yield ammonium nitrate. Write the reaction, and identify the acid, the base, the conjugate acid product, and the conjugate base product.

Bobby Barnes

University of North Texas

Problem 12

The amino acid phenylalanine has $\mathrm{pK}_{\mathrm{a}}=1.83$, and tryptophan has $\mathrm{p} K_{\mathrm{a}}=$ 2.83. Which is the stronger acid?

Alex M

Numerade Educator

Problem 13

Amide ion, $\mathrm{H}_{2} \mathrm{~N}^{-},$ is a much stronger base than hydroxide ion, $\mathrm{HO}^{-} .$ Which is the stronger acid, $\mathrm{NH}_{3}$ or $\mathrm{H}_{2} \mathrm{O}$ ? Explain.

Bobby Barnes

University of North Texas

Problem 14

Will either of the following reactions take place to a significant extent as written, according to the data in Table $2-3 ?$
(a) $\mathrm{HCN}+\mathrm{CH}_{3} \mathrm{CO}_{2}-\mathrm{Na}^{+} \quad \stackrel{?}{\longrightarrow} \quad \mathrm{Na}^{+}-\mathrm{CN}+\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{Na}^{+-} \mathrm{CN} \quad \stackrel{\mathrm{P}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-} \mathrm{Na}^{+}+\mathrm{HCN}$

Bobby Barnes

University of North Texas

Problem 15

Ammonia, $\mathrm{NH}_{3}$, has $\mathrm{pK}_{\mathrm{a}}=36,$ and acetone has $\mathrm{p} K_{\mathrm{a}}=19 .$ Will the following reaction take place to a significant extent?

Bobby Barnes

University of North Texas

Problem 16

What is the $K_{\mathrm{a}}$ of $\mathrm{HCN}$ if its $\mathrm{pK}_{\mathrm{a}}=9.31 ?$

Alex M

Numerade Educator

Problem 17

Using curved arrows, show how the species in part (a) can act as Lewis bases in their reactions with HCl, and show how the species in part (b) can act as Lewis acids in their reaction with $\mathrm{OH}^{-}$
(a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{HN}\left(\mathrm{CH}_{3}\right)_{2}, \mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}$
(b) $\mathrm{H}_{3} \mathrm{C}^{+}, \mathrm{B}\left(\mathrm{CH}_{3}\right)_{3}, \mathrm{MgBr}_{2}$

Bobby Barnes

University of North Texas

Problem 18

Imidazole forms part of the structure of the amino acid histidine and can act as both an acid and a base.
(a) Look at the electrostatic potential map of imidazole, and identify the most acidic hydrogen atom and the most basic nitrogen atom.
(b) Draw structures for the resonance forms of the products that result when imidazole is protonated by an acid and deprotonated by a base.

Bobby Barnes

University of North Texas

Problem 19

Of the two vitamins $A$ and $C,$ one is hydrophilic and water-soluble while the other is hydrophobic and fat-soluble. Which is which?

Bobby Barnes

University of North Texas

Problem 20

Fill in the multiple bonds in the following model of naphthalene, $\mathrm{C}_{10} \mathrm{H}_{8}$ (gray $=\mathrm{C},$ ivory $=\mathrm{H}) .$ How many resonance structures does naphthalene have? Draw them.

Bobby Barnes

University of North Texas

Problem 21

The following model is a representation of ibuprofen, a common overthe-counter pain reliever. Indicate the positions of the multiple bonds, and draw a skeletal structure (gray $=\mathrm{C},$ red $=\mathrm{O},$ ivory $=\mathrm{H}$ ).

Crystal Wang

Numerade Educator

Problem 22

cis-1,2-Dichloroethylene and trans-1,2-dichloroethylene are isomers, compounds with the same formula but different chemical structures. Look at the following electrostatic potential maps, and tell whether either compound has a dipole moment.

Alex M

Numerade Educator

Problem 23

The following molecular models are representations of (a) adenine and (b) cytosine, constituents of DNA (deoxyribonucleic acid). Indicate the positions of multiple bonds and lone pairs for both, and draw skeletal structures (gray $=\mathrm{C},$ red $=\mathrm{O},$ blue $=\mathrm{N},$ ivory $=\mathrm{H})$

Bobby Barnes

University of North Texas

Problem 24

Predict the product(s) of the acid/base reactions below, Draw curved arrows to show the formation and breaking of bonds.

Banhishikha Sinha

Banhishikha Sinha

Numerade Educator

Problem 25

Use curved arrows to draw the protonated form of each Lewis base below.

Chloe Schroeder

Chloe Schroeder

Numerade Educator

Problem 26

Use the curved-arrow formalism to show how the electrons flow in the resonance form on the left to give the one on the right.

Benjamin Angeles

Benjamin Angeles

Numerade Educator

Problem 27

Double bonds can also act like Lewis bases, sharing their electrons with Lewis acids. Use curved arrows to show how each double bond below will react with HCl and draw the resulting carbocation.

Chloe Schroeder

Chloe Schroeder

Numerade Educator

Problem 28

Identify the most electronegative element in each of the following molecules:
(a) $\mathrm{CH}_{2} \mathrm{FCl}$
(b) $\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}$
(c) $\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}$
(d) $\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Li}$

Bobby Barnes

University of North Texas

Problem 29

Use the electronegativity table given in Figure 2 - 2 to predict which bond in each of the following pairs is more polar, and indicate the direction of bond polarity for each compound.
(a) $\mathrm{H}_{3} \mathrm{C}-\mathrm{Cl}$ or $\mathrm{Cl}-\mathrm{Cl}$
(b) $\mathrm{H}_{3} \mathrm{C}-\mathrm{H}$ or $\mathrm{H}-\mathrm{Cl}$
(c) $\mathrm{HO}-\mathrm{CH}_{3}$ or $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{Si}-\mathrm{CH}_{3}$
(d) $\mathrm{H}_{3} \mathrm{C}-\mathrm{Li}$ or $\mathrm{Li}-\mathrm{OH}$

Alex M

Numerade Educator

Problem 30

Which of the following molecules has a dipole moment? Indicate the expected direction of each.

Crystal Wang

Numerade Educator

Problem 31

(a) The $\mathrm{H}-\mathrm{Cl}$ bond length is $136 \mathrm{pm} .$ What would the dipole moment of HCl be if the molecule were $100 \%$ ionic, $\mathrm{H}^{+} \mathrm{Cl}^{-} ?$
(b) The actual dipole moment of HCl is 1.08 D. What is the percent ionic character of the $\mathrm{H}-\mathrm{Cl}$ bond?

Bobby Barnes

University of North Texas

Problem 32

Phosgene, $\mathrm{Cl}_{2} \mathrm{C}=\mathrm{O},$ has a smaller dipole moment than formaldehyde, $\mathrm{H}_{2} \mathrm{C}=\mathrm{O},$ even though it contains electronegative chlorine atoms in place of hydrogen. Explain.

Bobby Barnes

University of North Texas

Problem 33

Fluoromethane $\left(\mathrm{CH}_{3} \mathrm{~F}, \mu=1.81 \mathrm{D}\right)$ has a smaller dipole moment than chloromethane $\left(\mathrm{CH}_{3} \mathrm{Cl}, \mu=1.87 \mathrm{D}\right)$ even though fluorine is more electronegative than chlorine. Explain.

Bobby Barnes

University of North Texas

Problem 34

Methanethiol, $\mathrm{CH}_{3} \mathrm{SH}$, has a substantial dipole moment $(\mu=1.52)$ even though carbon and sulfur have identical electronegativities. Explain.

Bobby Barnes

University of North Texas

Problem 35

Calculate the formal charges on the atoms shown in red.
(a) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{O} \mathrm{BF}_{3}$
(b) $\mathrm{H}_{2} \ddot{\mathrm{C}}-\mathrm{N} \equiv \mathrm{N}$ :
(c) $\mathrm{H}_{2} \mathrm{C}=\mathrm{N}=\ddot{\mathrm{N}}$
(d) $: \ddot{\mathrm{O}}=\ddot{\mathrm{O}}-\ddot{\mathrm{O}}:$
(e)

Bobby Barnes

University of North Texas

Problem 36

Assign formal charges to the atoms in each of the following molecules:
(a)
(b) $\mathrm{H}_{3} \mathrm{C}-\ddot{\mathrm{N}}-\mathrm{N} \equiv \mathrm{N}:$
(c) $\mathrm{H}_{3} \mathrm{C}-\ddot{\mathrm{N}}=\mathrm{N}=\ddot{\mathrm{N}}$

Bobby Barnes

University of North Texas

Problem 37

Which of the following pairs of structures represent resonance forms?

Ronald Prasad

Numerade Educator

Problem 38

Draw as many resonance structures as you can for the following species:

Ronald Prasad

Numerade Educator

Problem 39

1,3 -Cyclobutadiene is a rectangular molecule with two shorter double bonds and two longer single bonds. Why do the following structures not represent resonance forms?

Bobby Barnes

University of North Texas

Problem 40

Alcohols can act either as weak acids or as weak bases, just as water can. Show the reaction of methanol, $\mathrm{CH}_{3} \mathrm{OH}$, with a strong acid such as HCl and with a strong base such as $\mathrm{Na}^{+}-\mathrm{NH}_{2}$.

Bobby Barnes

University of North Texas

Problem 41

The $\mathrm{O}-\mathrm{H}$ hydrogen in acetic acid is more acidic than any of the $\mathrm{C}-\mathrm{H}$ hydrogens. Explain this result using resonance structures.

Bobby Barnes

University of North Texas

Problem 42

Draw electron-dot structures for the following molecules, indicating any unshared electron pairs. Which of the compounds are likely to act as Lewis acids and which as Lewis bases?
(a) $\mathrm{AlBr}_{3}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \quad$ (c) $\mathrm{BH}_{3}$
(d) HF
(e) $\mathrm{CH}_{3} \mathrm{SCH}_{3}$
(f) TiCl $_{4}$

Bobby Barnes

University of North Texas

Problem 43

Write the products of the following acid-base reactions:
(a) $\mathrm{CH}_{3} \mathrm{OH}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightleftharpoons ?$
(b) $\mathrm{CH}_{3} \mathrm{OH}+\mathrm{NaNH}_{2} \rightleftharpoons ?$
(c) $\mathrm{CH}_{3} \mathrm{NH}_{3}+\mathrm{Cl}^{-}+\mathrm{NaOH} \rightleftharpoons ?$

Bobby Barnes

University of North Texas

Problem 44

Rank the following substances in order of increasing acidity:

Bobby Barnes

University of North Texas

Problem 45

Which, if any, of the substances in Problem $2-44$ is a strong enough acid to react almost completely with $\mathrm{NaOH}$ (The $\mathrm{pK}_{\text {a }}$ of $\mathrm{H}_{2} \mathrm{O}$ is $15.74 .$ )

Bobby Barnes

University of North Texas

Problem 46

The ammonium ion $\left(\mathrm{NH}_{4}^{+}, \mathrm{p} K_{\mathrm{a}}=9.25\right)$ has a lower $\mathrm{p} K_{\mathrm{a}}$ than the methylammonium ion $\left(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}, \mathrm{p} K_{\mathrm{a}}=10.66\right) .$ Which is the stronger base, ammonia $\left(\mathrm{NH}_{3}\right)$ or methylamine $\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)$ ? Explain.

Alex M

Numerade Educator

Problem 47

Is tert-butoxide anion a strong enough base to react significantly with water? In other words, can a solution of potassium tert-butoxide be prepared in water? The $\mathrm{p} K_{\mathrm{a}}$ of tert-butyl alcohol is approximately 18 .

Bobby Barnes

University of North Texas

Problem 48

Predict the structure of the product formed in the reaction of the organic base pyridine with the organic acid acetic acid, and use curved arrows to indicate the direction of electron flow.

Bobby Barnes

University of North Texas

Problem 49

Calculate $K_{\mathrm{a}}$ value from the following $\mathrm{pK}_{\mathrm{a}}$ 's:
(a) Acetone, $\mathrm{pK}_{\mathrm{a}}=19.3$
(b) Formic acid, $\mathrm{p} K_{\mathrm{a}}=3.75$

Alex M

Numerade Educator

Problem 50

Calculate $\mathrm{p} K_{\mathrm{a}}$ values from the following $K_{\mathrm{a}}$ 's:
(a) Nitromethane, $K_{\mathrm{a}}=5.0 \times 10^{-11}$
(b) Acrylic acid, $K_{\mathrm{a}}=5.6 \times 10^{-5}$

Alex M

Numerade Educator

Problem 51

What is the $\mathrm{pH}$ of a $0.050 \mathrm{M}$ solution of formic acid, $\mathrm{p} K_{\mathrm{a}}=3.75 ?$

Bobby Barnes

University of North Texas

Problem 52

Sodium bicarbonate, $\mathrm{NaHCO}_{3}$, is the sodium salt of carbonic acid $\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right), \mathrm{pK}_{\mathrm{a}}=6.37 .$ Which of the substances shown in Problem $2-44$ will react significantly with sodium bicarbonate?

Alex M

Numerade Educator

Problem 53

Maleic acid has a dipole moment, but the closely related fumaric acid. a substance involved in the citric acid cycle by which food molecules are metabolized, does not. Explain.

Bobby Barnes

University of North Texas

Problem 54

Assume that you have two unlabeled bottles, one of which contains phenol $\left(p K_{a}=9.9\right)$ and one of which contains acetic acid $\left(p K_{a}=4.76\right)$ In light of your answer to Problem 2 -52, suggest a simple way to determine what is in each bottle.

Bobby Barnes

University of North Texas

Problem 55

Identify the acids and bases in the following reactions:

Susan Hallstrom

Susan Hallstrom

Numerade Educator

Problem 56

Which of the following pairs represent resonance structures?

Bobby Barnes

University of North Texas

Problem 57

Draw as many resonance structures as you can for the following species, adding appropriate formal charges to each:
(a) Nitromethane,
(b) Ozone,
(c) Diazomethane, $\mathrm{H}_{2} \mathrm{C}=\mathrm{N}=\stackrel{\pi}{\mathrm{N}}:$

Bobby Barnes

University of North Texas

Problem 58

Carbocations, which contain a trivalent, positively charged carbon atom, react with water to give alcohols:

Bobby Barnes

University of North Texas

Problem 59

We'll see in the next chapter that organic molecules can be classified according to the functional groups they contain, where a functional group is a collection of atoms with a characteristic chemical reactivity. Use the electronegativity values given in Figure $2-2$ on page 29 to predict the direction of polarization of the following functional groups.

Crystal Wang

Numerade Educator

Problem 60

The azide functional group, which occurs in azidobenzene, contains three adjacent nitrogen atoms. One resonance structure for azidobenzene is shown. Draw three additional resonance structures, and assign appropriate formal charges to the atoms in all four.

Bobby Barnes

University of North Texas

Problem 61

Phenol, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}$, is a stronger acid than methanol, $\mathrm{CH}_{3} \mathrm{OH}$, even though both contain an $\mathrm{O}-\mathrm{H}$ bond. Draw the structures of the anions resulting from loss of $\mathrm{H}^{+}$ from phenol and methanol, and use resonance structures to explain the difference in acidity.

Bobby Barnes

University of North Texas

Problem 62

Thiamin diphosphate (TPP), a derivative of vitamin B $_{1}$ required for glucose metabolism, is a weak acid that can be deprotonated by a base. Assign formal charges to the appropriate atoms in both TPP and its deprotonation product.

Bobby Barnes

University of North Texas

Problem 63

Determine if each compound or ion below has a dipole moment.
(a) Carbonate ion $\left(\mathrm{CO}_{3}^{2-}\right)$
(b) $-\ddot{\mathrm{O}}-$
(c) $\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}$

Crystal Wang

Numerade Educator

Problem 64

Use the $\mathrm{p} K_{\mathrm{a}}$ table in Appendix $\mathrm{B}$ to determine in which direction the equilibrium is favored.

Isaac Huidobro

Numerade Educator

Problem 65

Which intermolecular force is predominantly responsible for each observation below?
(a) $\mathrm{CH}_{3}\left(\mathrm{CH}_{3}\right)_{29} \mathrm{CH}_{3}$, a component found in paraffin wax, is a solid at room temperature while octane is a liquid.
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}$ has a higher boiling point than $\mathrm{CH}_{4}$.
(c) $\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H},$ which is found in vinegar, will dissolve in water but not in oil - for simplicity you may assume oil is $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{3}$

Pronoy Sinha

Numerade Educator

Problem 66

Draw the conjugate base for each compound below (the acidic hydrogen in each case is marked with an $^{*}$ ).

Crystal Wang

Numerade Educator