Question

problem 2-17 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 oh⁻. (a) ch₃ch₂oh, hn(ch₃)₂, p(ch₃)₃ (b) h₃c⁺, b(ch₃)₃, mgbr₂

Explanation:

Part (a): Lewis Bases Reacting with HCl

Lewis bases donate an electron pair. HCl is a Lewis acid (accepts electron pair, via \( \text{H}^+ \) or \( \text{Cl}^- \), but here \( \text{H}^+ \) is the electrophile).

1. \( \boldsymbol{\text{CH}_3\text{CH}_2\text{OH}} \) (Ethanol)

• The O atom in \( \text{OH} \) has lone pairs.
• Curved arrow: From O’s lone pair to \( \text{H}^+ \) (from HCl).
• Reaction: \( \text{CH}_3\text{CH}_2\text{O}^\text{:}\text{H} + \text{H}^+

ightarrow \text{CH}_3\text{CH}_2\text{OH}_2^+ \) (protonated ethanol).

2. \( \boldsymbol{\text{HN(CH}_3\text{)}_2} \) (Dimethylamine)

• The N atom has lone pairs.
• Curved arrow: From N’s lone pair to \( \text{H}^+ \) (from HCl).
• Reaction: \( \text{H}\text{:N(CH}_3\text{)}_2 + \text{H}^+

ightarrow \text{H}_2\text{N}^+(CH_3)_2 \) (protonated dimethylamine).

3. \( \boldsymbol{\text{P(CH}_3\text{)}_3} \) (Trimethylphosphine)

• The P atom has lone pairs (group 15, 3 bonds + 1 lone pair).
• Curved arrow: From P’s lone pair to \( \text{H}^+ \) (from HCl).
• Reaction: \( \text{:P(CH}_3\text{)}_3 + \text{H}^+

ightarrow \text{HP}^+(CH_3)_3 \) (protonated trimethylphosphine).

Part (b): Lewis Acids Reacting with \( \boldsymbol{\text{OH}^-} \)

Lewis acids accept an electron pair. \( \text{OH}^- \) is a Lewis base (donates electron pair).

1. \( \boldsymbol{\text{H}_3\text{C}^+} \) (Methyl Carbocation)

• Carbocations have an empty \( p \)-orbital (electron-deficient).
• Curved arrow: From \( \text{OH}^- \)’s lone pair to the empty orbital of \( \text{C}^+ \).
• Reaction: \( \text{H}_3\text{C}^+ + \text{:OH}^-

ightarrow \text{CH}_3\text{OH} \) (methanol).

2. \( \boldsymbol{\text{B(CH}_3\text{)}_3} \) (Trimethylborane)

• B is group 13, has 3 bonds (6 valence electrons, electron-deficient).
• Curved arrow: From \( \text{OH}^- \)’s lone pair to B’s empty orbital.
• Reaction: \( \text{B(CH}_3\text{)}_3 + \text{:OH}^-

ightarrow \text{B(CH}_3\text{)}_3\text{OH}^- \) (or forms a coordinate bond, \( \text{B(CH}_3\text{)}_3\text{:OH}^- \)).

3. \( \boldsymbol{\text{MgBr}_2} \) (Magnesium Bromide)

• \( \text{Mg}^{2+} \) is electron-deficient (loses 2 electrons, has empty orbitals).
• Curved arrow: From \( \text{OH}^- \)’s lone pair to \( \text{Mg}^{2+} \).
• Reaction: \( \text{MgBr}_2 + 2\text{:OH}^-

ightarrow \text{Mg(OH)}_2 + 2\text{Br}^- \) (or stepwise, with \( \text{OH}^- \) donating to \( \text{Mg}^{2+} \)).

Summary of Curved Arrows:

• Lewis Bases (a): Arrows start at lone pairs (O, N, P) and point to \( \text{H}^+ \) (from HCl).
• Lewis Acids (b): Arrows start at \( \text{OH}^- \)’s lone pair and point to electron-deficient centers (empty orbitals in \( \text{C}^+ \), B, or \( \text{Mg}^{2+} \)).

(Note: For precise curved arrow drawings, represent the electron flow as described, with arrows originating from lone pairs (for bases) or the base’s lone pair (for acids) and terminating at the electrophilic/electron-deficient site.)

Answer:

Part (a): Lewis Bases Reacting with HCl

Lewis bases donate an electron pair. HCl is a Lewis acid (accepts electron pair, via \( \text{H}^+ \) or \( \text{Cl}^- \), but here \( \text{H}^+ \) is the electrophile).

1. \( \boldsymbol{\text{CH}_3\text{CH}_2\text{OH}} \) (Ethanol)

• The O atom in \( \text{OH} \) has lone pairs.
• Curved arrow: From O’s lone pair to \( \text{H}^+ \) (from HCl).
• Reaction: \( \text{CH}_3\text{CH}_2\text{O}^\text{:}\text{H} + \text{H}^+

ightarrow \text{CH}_3\text{CH}_2\text{OH}_2^+ \) (protonated ethanol).

2. \( \boldsymbol{\text{HN(CH}_3\text{)}_2} \) (Dimethylamine)

• The N atom has lone pairs.
• Curved arrow: From N’s lone pair to \( \text{H}^+ \) (from HCl).
• Reaction: \( \text{H}\text{:N(CH}_3\text{)}_2 + \text{H}^+

ightarrow \text{H}_2\text{N}^+(CH_3)_2 \) (protonated dimethylamine).

3. \( \boldsymbol{\text{P(CH}_3\text{)}_3} \) (Trimethylphosphine)

• The P atom has lone pairs (group 15, 3 bonds + 1 lone pair).
• Curved arrow: From P’s lone pair to \( \text{H}^+ \) (from HCl).
• Reaction: \( \text{:P(CH}_3\text{)}_3 + \text{H}^+

ightarrow \text{HP}^+(CH_3)_3 \) (protonated trimethylphosphine).

Part (b): Lewis Acids Reacting with \( \boldsymbol{\text{OH}^-} \)

Lewis acids accept an electron pair. \( \text{OH}^- \) is a Lewis base (donates electron pair).

1. \( \boldsymbol{\text{H}_3\text{C}^+} \) (Methyl Carbocation)

• Carbocations have an empty \( p \)-orbital (electron-deficient).
• Curved arrow: From \( \text{OH}^- \)’s lone pair to the empty orbital of \( \text{C}^+ \).
• Reaction: \( \text{H}_3\text{C}^+ + \text{:OH}^-

ightarrow \text{CH}_3\text{OH} \) (methanol).

2. \( \boldsymbol{\text{B(CH}_3\text{)}_3} \) (Trimethylborane)

• B is group 13, has 3 bonds (6 valence electrons, electron-deficient).
• Curved arrow: From \( \text{OH}^- \)’s lone pair to B’s empty orbital.
• Reaction: \( \text{B(CH}_3\text{)}_3 + \text{:OH}^-

ightarrow \text{B(CH}_3\text{)}_3\text{OH}^- \) (or forms a coordinate bond, \( \text{B(CH}_3\text{)}_3\text{:OH}^- \)).

3. \( \boldsymbol{\text{MgBr}_2} \) (Magnesium Bromide)

• \( \text{Mg}^{2+} \) is electron-deficient (loses 2 electrons, has empty orbitals).
• Curved arrow: From \( \text{OH}^- \)’s lone pair to \( \text{Mg}^{2+} \).
• Reaction: \( \text{MgBr}_2 + 2\text{:OH}^-

ightarrow \text{Mg(OH)}_2 + 2\text{Br}^- \) (or stepwise, with \( \text{OH}^- \) donating to \( \text{Mg}^{2+} \)).

Summary of Curved Arrows:

• Lewis Bases (a): Arrows start at lone pairs (O, N, P) and point to \( \text{H}^+ \) (from HCl).
• Lewis Acids (b): Arrows start at \( \text{OH}^- \)’s lone pair and point to electron-deficient centers (empty orbitals in \( \text{C}^+ \), B, or \( \text{Mg}^{2+} \)).

(Note: For precise curved arrow drawings, represent the electron flow as described, with arrows originating from lone pairs (for bases) or the base’s lone pair (for acids) and terminating at the electrophilic/electron-deficient site.)