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Chapter 2: Polar Covalent Bonds; Acids and Bases
19 problems · 37 figures2-1 §2.1 source ↗
Which element in each of the following pairs is more electronegative?
(a)
Li or H
(b)
B or Br
(c)
Cl or I
(d)
C or H
Answer (4 parts)
(a)
H
(b)
Br
(c)
Cl
(d)
C
2-2 §2.1 source ↗
Use the δ+/δ– convention to indicate the direction of expected polarity for each of the bonds indicated.
(a)
H3C–Cl
(b)
H3C–NH2
(c)
H2N–H
(d)
H3C–SH
(e)
H3C–MgBr
(f)
H3C–F
Answer (6 parts)
(a)
(b)
(c)
(d)
(e)
(f)
2-3 §2.1 source ↗
Use the electronegativity values shown in Figure 2.3 to rank the following bonds from least polar to most polar: H3C–Li, H3C–K, H3C–F, H3C–MgBr, H3C–OH
Electronegativity generally increases from left to right across the periodic table and decreases from top to bottom. The values are on an arbitrary scale, with F = 4.0 and Cs = 0.7. Elements in red are the most electronegative, those in yellow are medium, and those in green are the least electronegative.
Answer
H3C−OH < H3C−MgBr < H3C−Li = H3C−F < H3C−K
2-4 §2.1 source ↗
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:
Answer
The nitrogen is electron-rich, and the carbon is electron-poor.
2-5 §2.2 source ↗
Ethylene glycol, HOCH2CH2OH, may look nonpolar when drawn, but an internal hydrogen bond between the two –OH groups results in a dipole moment. Explain.
Answer
The two C–O dipoles cancel because of the symmetry of the molecule:
2-6 §2.2 source ↗
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)
H2C = CH2
(b)
CHCl3
(c)
CH2Cl2
(d)
H2C = CCl2
Answer (4 parts)
(a)
(b)
(c)
(d)
2-7 §2.3 source ↗
Calculate formal charges for the nonhydrogen atoms in the following molecules:
(a)
(b)
(c)
Answer (3 parts)
(a)
For carbon: FC = 4 − 8/2 − 0 = 0 For the middle nitrogen: FC = 5 − 8/2 − 0 = +1 For the end nitrogen: FC = 5 − 4/2 − 4 = −1
(b)
For nitrogen: FC = 5 − 8/2 − 0 = +1 For oxygen: FC = 6 − 2/2 − 6 = −1
(c)
For nitrogen: FC = 5 − 8/2 − 0 = +1 For the triply bonded carbon: FC = 4 − 6/2 − 2 = −1
2-8 §2.3 source ↗
Organic phosphate groups occur commonly in biological molecules. Calculate formal charges on the four O atoms in the methyl phosphate dianion.
Answer
2-9 §2.6 source ↗
Which of the following pairs of structures represent resonance forms, and which do not? Explain.
(a)
(b)
Answer
The structures in (a) are resonance forms.
2-10 §2.6 source ↗
Draw the indicated number of resonance forms for each of the following substances:
(a)
The methyl phosphate anion, CH3OPO32– (3)
(b)
The nitrate anion, NO3– (3)
(c)
The allyl cation, H2C = CH–CH2+ (2)
(d)
The benzoate anion (2)
Answer (4 parts)
(a)
(b)
(c)
(d)
2-11 §2.7 source ↗
Nitric acid (HNO3) reacts with ammonia (NH3) to yield ammonium nitrate. Write the reaction, and identify the acid, the base, the conjugate acid product, and the conjugate base product.
Answer
2-12 §2.8 source ↗
The amino acid phenylalanine has pKa = 1.83, and tryptophan has pKa = 2.83. Which is the stronger acid?
Answer
Phenylalanine is stronger.
2-13 §2.8 source ↗
Amide ion, H2N–, is a much stronger base than hydroxide ion, HO–. Which is the stronger acid, NH3 or H2O? Explain.
Answer
Water is a stronger acid.
2-14 §2.9 source ↗
Will either of the following reactions take place to a significant extent as written, according to the data in Table 2.3?
(a)
(b)
| Acid | Name | pKa | Conjugate base | Name | ||
|---|---|---|---|---|---|---|
  | CH3CH2OH | Ethanol | 16.00 | CH3CH2O– | Ethoxide ion |   |
| H2O | Water | 15.74 | HO– | Hydroxide ion | ||
| HCN | Hydrocyanic acid | 9.31 | CN– | Cyanide ion | ||
| H2PO4– | Dihydrogen phosphate ion | 7.21 | HPO42– | Hydrogen phosphate ion | ||
| CH3CO2H | Acetic acid | 4.76 | CH3CO2– | Acetate ion | ||
| H3PO4 | Phosphoric acid | 2.16 | H2PO4– | Dihydrogen phosphate ion | ||
| HNO3 | Nitric acid | –1.3 | NO3– | Nitrate ion | ||
| HCl | Hydrochloric acid | –7.0 | Cl– | Chloride ion |
Answer
Neither reaction will take place.
2-15 §2.9 source ↗
Ammonia, NH3, has pKa ≈ 36, and acetone has pKa ≈ 19. Will the following reaction take place to a significant extent?
Answer
Reaction will take place.
2-16 §2.9 source ↗
What is the Ka of HCN if its pKa = 9.31?
Answer
Ka = 4.9 × 10–10
2-17 §2.11 source ↗
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 OH–.
(a)
CH3CH2OH, HN(CH3)2, P(CH3)3
(b)
H3C+, B(CH3)3, MgBr2
Answer (2 parts)
(a)
(b)
2-18 §2.11 source ↗
Imidazole, which forms part of amino acid histidine, 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.
Answer (2 parts)
(a)
(b)
2-19 §2.12 source ↗
Of the two vitamins A and C, one is hydrophilic and water-soluble while the other is hydrophobic and fat-soluble. Which is which?
Answer
Vitamin C is water-soluble (hydrophilic); vitamin A is fat-soluble (hydrophobic).
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Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.