Polar Covalent Bonds: Acids and Bases

: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

Use the d1/d2 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

Use the electronegativity values shown in Figure 2-2 to rank the following bonds from least polar to most polar: H₃C–Li, H₃C–K, H₃C–F, H₃C–MgBr, H₃C–OH

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:

Ethylene glycol,, may look nonpolar when drawn, but an

internal hydrogen bond results in an electric dipole moment. Explain.

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.

$\left(\mathbf{a}\right){\mathbf{H}}_{\mathbf{2}}{\mathbf{CPCH}}_{\mathbf{2}}\left(\mathbf{b}\right){\mathbf{CHCl}}_{\mathbf{3}}\left(\mathbf{c}\right){\mathbf{CH}}_{\mathbf{2}}{\mathbf{Cl}}_{\mathbf{2}}\left(\mathbf{d}\right){\mathbf{H}}_{\mathbf{2}}{\mathbf{CPCCl}}_{\mathbf{2}}$

Question: Calculate the formal charges on the atoms shown in red.

Question: Calculate the formal charges on the atoms shown in red.

Question: Calculate formal charges for the non-hydrogen atoms in the following molecules:

(a) Diazomethane,H₂C=N=N:

(b) Acetonitrile oxide, H₃C-C$\equiv$N-O

(c) Methyl isocyanides,H₃CN$\equiv$C:

Question: Organic phosphate groups occur commonly in biological molecules. Calculate formal charges on the four O atoms in the methyl phosphate dianion.

                                Methyl phosphate   

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

(a)

(b)

Draw the indicated number of resonance forms for each of the following

species:

(a) The methyl phosphate anion, CHOPO₃^2-(3)

(b) The nitrate anion,NO₃ (3)

(c) The allylcation,H₂CPCHOCH₂⁺(2)

(d) The benzoate anion (4)

Question: 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.

Draw electron-dot structures for the following molecules, indicatingany unshared electron pairs. Which of the compounds are likely to actas Lewis acids and which as Lewis bases?

(a)${\mathbf{AlBr}}_3$ (b)  ${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{NH}}_2$(c) ${\mathbf{BH}}_3$  

(d) HF (e)${\mathbf{CH}}_3{\mathbf{SCH}}_3$ (f)  ${\mathbf{TiCl}}_4$

Question: The amino acid phenylalanine has pK_a=1.83 , and tryptophan has pK_a=1.83 Which is the stronger acid?

Question: Amide ion,H₂N₂, is a much stronger base than hydroxide ion, HO^-. Which is the stronger acid, NH₃ or H₂O? Explain.

Question: What is the K_a of HCN if its pK_a = 9.31?

Will either of the following reactions take place to a significant extent as written, according to the data in Table 2-3?

a)  $\mathbf{HCN}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}\mathbf{C}{{\mathbf{O}}_{\mathbf{2}}}^{\mathbf{-}}{\mathbf{Na}}^{\mathbf{+}}\stackrel{\mathbf{?}}{\mathbf{\to }\mathbf{N}{{\mathbf{a}}^{\mathbf{+}}}^{\mathbf{-}}\mathbf{CN}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}}$                                                        b)  ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{+}{{\mathbf{Na}}^{\mathbf{+}}}^{\mathbf{-}}\mathbf{CN}\stackrel{\mathbf{?}}{\mathbf{\to }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{O}}^{\mathbf{-}}{\mathbf{Na}}^{\mathbf{+}}\mathbf{+}\mathbf{HCN}}$

Ammonia, NH₃, has =36, and acetone has =19. Will the following reaction take place to a significant extent?

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 ${\mathbf{OH}}^{\mathbf{-}}$.

(a) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{,}\mathbf{HN}{\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{,}\mathbf{P}{\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{3}}$ (b) ${\mathbf{H}}_{\mathbf{3}}{\mathbf{C}}^{\mathbf{+}}\mathbf{,}\mathbf{B}{\mathbf{\left(}\mathbf{CH}\mathbf{\right)}}_{\mathbf{3}}\mathbf{,}{\mathbf{MgBr}}_{\mathbf{2}}$

Imidazole forms part of the structure of the amino acid histidine and can act as both an acid and a base.

                           Imidazole                                                Histidine

(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.

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

Vitamin A                                             Vitamin C

(Retinol)                                            (Ascorbic acid)

Identify the acids and bases in the following reactions:

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

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.

Alcohols can act either as weak acids or as weak bases, just as water can. Show the reaction of methanol, , with a strong acid such as HCl and with a strong base such as ${\mathrm{Na}}^{+-}{\mathrm{NH}}_2$.

The  O-H hydrogen in acetic acid is more acidic than any hydrogens. Explain this result using resonance structures.

Question: Phosgene, Cl₂C=O , has a smaller dipole moment than formaldehyde, H₂C=O , even though it contains electronegative chlorine atoms in place of hydrogen. Explain.

Question:Phosgene, Cl₂C=O , has a smaller dipole moment than formaldehyde,H₂C=O  , even though it contains electronegative chlorine atoms in place of hydrogen. Explain.

Question: Methanethiol, CH₃SH , has a substantial dipole moment  $\mathbf{(}\mathit{\mu }\mathbf{=}\mathbf{1}\mathbf{.}\mathbf{52}\mathbf{ }\mathit{D}\mathbf{)}$ eventhough carbon and sulfur have identical electronegativities. Explain.

Question: 

(A) The $H-Cl$ bond length is $136pm$. What would the dipole moment of $H-Cl$ be if the molecule were $100\%$ ionic, $H^{+}C{l}^{-}$?

(B) The actual dipole moment of HCl is $1.08 D$. What is the percent

ionic character of the $H-Cl$ bond?

Question: 

(A) The $H-Cl$ bond length is $136pm$. What would the dipole moment of $H-Cl$ be if the molecule were $100\%$ ionic, $H^{+}C{l}^{-}$?

(B) The actual dipole moment of HCl is $1.08D$. What is the percent

ionic character of the $H-Cl$ bond?

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

(b) 

Question: Calculate the formal charges on the atoms shown in red.

Question: Calculate the formal charges on the atoms shown in red.

Question: Calculate the formal charges on the atoms shown in red.

Use the ${\mathbf{pK}}_{\mathbf{a}}$table in Appendix B to determine in which direction the

Equilibrium is favored.

(b) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{+}\dot{\mathbf{N}}{\mathbf{H}}_{\mathbf{2}}\mathbf{\rightleftarrows }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\dot{\mathbf{O}}\mathbf{+}{\mathbf{NH}}_{\mathbf{3}}$

Use the ${\mathrm{pK}}_{\mathrm{a}}$table in Appendix B to determine in which direction the

Equilibrium is favored.

(c) $\dot{\mathbf{C}}{\mathbf{H}}_{\mathbf{3}}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{NO}}_{\mathbf{2}}\mathbf{\rightleftarrows }{\mathbf{CH}}_{\mathbf{4}}\mathbf{+}\dot{\mathbf{C}}{\mathbf{H}}_{\mathbf{2}}{\mathbf{NO}}_{\mathbf{2}}$

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

(a) 

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

(c)

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

(d) 

Question: 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.

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 5 C, pink 5 O, blue 5 N, ivory 5 H).

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

a)

b)

c) 

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

Question: 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.

a)

b)

c)

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.

Identify the most electronegative element in each of the following molecules: 

(a)  CH₂FCl                                 (b) FCH₂CH₂Br

(c)  HOCH₂CH₂NH₂                    (d)    CH₃OCH₂Li

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)${\mathbf{H}}_{\mathbf{3}}\mathbf{C}\mathbf{-}\mathbf{CI}$ or $\mathbf{CI}\mathbf{-}\mathbf{CI}$

b) ${\mathbf{H}}_{\mathbf{3}}\mathbf{C}\mathbf{-}\mathbf{H}$ or $\mathbf{H}\mathbf{-}\mathbf{CI}$

c)$\mathbf{HO}\mathbf{-}{\mathbf{CH}}_{\mathbf{3}}$ or ${\mathbf{\left(}{\mathbf{CH}}_{\mathbf{3}}\mathbf{\right)}}_{\mathbf{3}}\mathbf{Si}\mathbf{-}{\mathbf{CH}}_{\mathbf{3}}$

d)${\mathbf{H}}_{\mathbf{3}}\mathbf{C}\mathbf{-}\mathbf{Li}$ or $\mathbf{Li}\mathbf{-}\mathbf{OH}$

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