chapter 2 hw (lecture 3) 3 2 acids and bases€¦ · · 2017-10-07chapter 2 hw (lecture 3) 3 ......

CHAPTER 2 HW (Lecture 3)

68 CHAPTER 2 | Polar Covalent Bonds; Acids and Bases

▲ Problems linked to Key Ideas in this chapter

2.32 Assign formal charges to the atoms in each of the following molecules:

(a) (b) (c)N N N

CH3

CH3

NH3C

H3C H3C

O

N N N

Resonance2.33 Which of the following pairs of structures represent resonance forms?

and

and

(a) (b)

and and

(c) (d)

–O

–

O

–O

–O

–

O

–

O

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

(b) (c)

(e)

CH2–

–+

+

(a)

H3C

H

H

H

CH2 CH3(d) H3C

NH2

CH2N NH2

CH CH CH CHH2C

C

O

S+

2.35 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?

Acids and Bases2.36 Alcohols can act either as weak acids or as weak bases, just as water can. Show

the reaction of methanol, CH3OH, with a strong acid such as HCl and with a strong base such as Na! "NH2.

2.37 ▲ The O ! H hydrogen in acetic acid is more acidic than any of the C ] H hydro-gens. Explain this result using resonance structures.

Acetic acidHOC

C

H H

H

O

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Exercises 69


2.38 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) AlBr3 (b) CH3CH2NH2 (c) BH3(d) HF (e) CH3SCH3 (f) TiCl4

2.39 Write the products of the following acid–base reactions:(a) CH3OH ! H2SO4 ^ ?(b) CH3OH ! NaNH2 ^ ?(c) CH3NH3

! Cl" ! NaOH ^ ?

2.40 Rank the following substances in order of increasing acidity:

CH3COHCH3CCH3

Acetic acid(pKa = 4.76)

Acetone(pKa = 19.3)

2,4-Pentanedione(pKa = 9)

CH3CCH2CCH3OH

Phenol(pKa = 9.9)

O O O O

2.41 Which, if any, of the substances in Problem 2.40 is a strong enough acid to react almost completely with NaOH? (The pKa of H2O is 15.74.)

2.42 The ammonium ion (NH4!, pKa " 9.25) has a lower pKa than the methyl-

ammonium ion (CH3NH3!, pKa " 10.66). Which is the stronger base, ammo-

nia (NH3) or methylamine (CH3NH2)? Explain.

2.43 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 pKa of tert-butyl alcohol is approximately 18.

CH3

CH3

CH3CK+ –O Potassium tert-butoxide

2.44 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.

+

Acetic acidPyridine

N

?OHH3C

C

O

2.45 Calculate Ka values from the following pKa’s:(a) Acetone, pKa " 19.3 (b) Formic acid, pKa " 3.75

2.46 Calculate pKa values from the following Ka’s:(a) Nitromethane, Ka " 5.0 # 10"11 (b) Acrylic acid, Ka " 5.6 # 10"5


Exercises 69


2.38 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) AlBr3 (b) CH3CH2NH2 (c) BH3(d) HF (e) CH3SCH3 (f) TiCl4

2.39 Write the products of the following acid–base reactions:(a) CH3OH ! H2SO4 ^ ?(b) CH3OH ! NaNH2 ^ ?(c) CH3NH3

! Cl" ! NaOH ^ ?

2.40 Rank the following substances in order of increasing acidity:

CH3COHCH3CCH3

Acetic acid(pKa = 4.76)

Acetone(pKa = 19.3)

2,4-Pentanedione(pKa = 9)

CH3CCH2CCH3OH

Phenol(pKa = 9.9)

O O O O

2.41 Which, if any, of the substances in Problem 2.40 is a strong enough acid to react almost completely with NaOH? (The pKa of H2O is 15.74.)

2.42 The ammonium ion (NH4!, pKa " 9.25) has a lower pKa than the methyl-

ammonium ion (CH3NH3!, pKa " 10.66). Which is the stronger base, ammo-

nia (NH3) or methylamine (CH3NH2)? Explain.

2.43 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 pKa of tert-butyl alcohol is approximately 18.

CH3

CH3

CH3CK+ –O Potassium tert-butoxide

2.44 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.

+

Acetic acidPyridine

N

?OHH3C

C

O

2.45 Calculate Ka values from the following pKa’s:(a) Acetone, pKa " 19.3 (b) Formic acid, pKa " 3.75

2.46 Calculate pKa values from the following Ka’s:(a) Nitromethane, Ka " 5.0 # 10"11 (b) Acrylic acid, Ka " 5.6 # 10"5




2.47 What is the pH of a 0.050 M solution of formic acid, pKa ! 3.75?

2.48 Sodium bicarbonate, NaHCO3, is the sodium salt of carbonic acid (H2CO3), pKa ! 6.37. Which of the substances shown in Problem 2.40 will react signifi-cantly with sodium bicarbonate?

General Problems2.49 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.

C

CC

O O

C

H H

Maleic acid Fumaric acid

OHHO C

CC

O

O

H

H C OH

HO

2.50 Assume that you have two unlabeled bottles, one of which contains phenol (pKa ! 9.9) and one of which contains acetic acid (pKa ! 4.76). In light of your answer to Problem 2.48, suggest a simple way to determine what is in each bottle.

2.51 Identify the acids and bases in the following reactions:

N

H

O

BH3

TiCl4–

–NaH Na++ H2+

BH3+

+

N

H

O

(a) CH3OH CH3OH2

+

H++

(c)

(d)+

OH H

H H

H

H

O

H

H3CC + TiCl4

CH3

O

H3CC

CH3

O(b)


Exercises 71


2.52 ▲ Which of the following pairs represent resonance structures?

(a)+

NCH2 and+

NCH2+

–O

–O

–ONCH3Cand

–O

–O

NH3+

and

+

CNH2

C

+ –ONCH3C

O OH(c) (d)

CH3C and–

O

O(b)

CH2C–

O H

O

2.53 ▲ Draw as many resonance structures as you can for the following species, adding appropriate formal charges to each:

Nitromethane,

Diazomethane,

Ozone, (a)

(c)

(b)+

NH3C+

–O

O–

OOO

+ –NNH2C

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

CH3H3C

H2O + H+

CH3H3CC+

H

A carbocation An alcohol

H OH

C

How can you account for the fact that the following carbocation gives a mixture of two alcohols on reaction with water?

H3CH2O +

H3CC+

H H OH

C

H

CH2C H3C

C

H

H

CH2OHC

H

CH2C

2.55 We’ll see in the next chapter that organic molecules can be classified accord-ing to the functional groups they contain, where a functional group is a collec-tion of atoms with a characteristic chemical reactivity. Use the electronegativity values given in Figure 2.2 on page 35 to predict the direction of polarization of the following functional groups.

COH

C N

NH2

Amide NitrileAlcoholKetone

(a) (b) (c) (d)O

C

O

C




2.56 The azide functional group (Problem 2.55), such as occurs in azidobenzene, contains three adjacent nitrogen atoms. One resonance structures for azido-benzene is shown. Draw three additional resonance structures, and assign appropriate formal charges to the atoms in all four.

NN

N

Azidobenzene

2.57 Phenol, C6H5OH, is a stronger acid than methanol, CH3OH, even though both contain an O ! H bond. Draw the structures of the anions resulting from loss of H! from phenol and methanol, and use resonance structures to explain the difference in acidity.

Methanol (pKa = 15.54)Phenol (pKa = 9.89)

HCO

H H

HH

O


chapter 2 hw (lecture 3) 3 2 acids and bases€¦ · · 2017-10-07chapter 2 hw (lecture 3) 3 ......

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