Question

problem 2-9 which of the following pairs of structures represent resonance forms, and which do not? explain. (a) chemical structure and chemical structure (b) chemical structure and chemical structure problem 2-10 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) chemical structure of benzoate anion

Explanation:

Problem 2 - 9

Part (a)

Resonance forms differ only in the distribution of electrons (π electrons or lone pairs), not in the position of atoms. In the first structure, the positive charge is on the carbon attached to the benzene ring, and in the second, the double bond is between the benzene ring - attached carbon and the adjacent carbon, with the positive charge on the benzene ring. The atoms are in the same positions, and the difference is in electron (and charge) distribution. So, these are resonance forms.

Part (b)

In the first structure, the double bonds are in positions \( \ce{H2C = C - C = C - CH3} \) (with substituents \( \ce{CH3} \) and \( \ce{CH2CH3} \) and \( \ce{H} \)), and in the second, the double - bond positions are changed such that the carbon - atom connectivity is altered (the terminal carbons have different bonding; in the first, one terminal is \( \ce{H2C =} \) and in the second, one terminal is \( \ce{= CH2} \) but the central carbon - carbon double - bond pattern is different). Resonance forms must have the same atomic connectivity. So, these are not resonance forms.

Problem 2 - 10

Part (a): Methyl phosphate anion, \( \ce{CH3OPO3^{2 - }} \)

The methyl phosphate anion has a central \( \ce{P} \) atom bonded to \( \ce{O - CH3} \) and three \( \ce{O} \) atoms (two of which carry a negative charge in the overall anion).

• Resonance Form 1: \( \ce{CH3O - P( = O)( - O^ - )( - O^ - )} \) (the double bond is between \( \ce{P} \) and one of the non - methyl - attached \( \ce{O} \) atoms, and two \( \ce{O} \) atoms have a negative charge).
• Resonance Form 2: \( \ce{CH3O - P( - O^ - )( = O)( - O^ - )} \) (the double bond is between \( \ce{P} \) and the second non - methyl - attached \( \ce{O} \) atom, with the same charge distribution logic).
• Resonance Form 3: \( \ce{CH3O - P( - O^ - )( - O^ - )( = O)} \) (the double bond is between \( \ce{P} \) and the third non - methyl - attached \( \ce{O} \) atom).

Part (b): Nitrate anion, \( \ce{NO3^ - } \)

The nitrate anion has a central \( \ce{N} \) atom bonded to three \( \ce{O} \) atoms.

• Resonance Form 1: \( \ce{O = N( - O^ - )( - O^ - )} \) (double bond between \( \ce{N} \) and one \( \ce{O} \), two \( \ce{O} \) with negative charge).
• Resonance Form 2: \( \ce{O( - O^ - ) = N( - O^ - )} \) (double bond between \( \ce{N} \) and the second \( \ce{O} \)).
• Resonance Form 3: \( \ce{O( - O^ - )( - O^ - ) = N} \) (double bond between \( \ce{N} \) and the third \( \ce{O} \)).

Part (c): Allyl cation, \( \ce{H2C = CH - CH2^ + } \)

The allyl cation has a conjugated system.

• Resonance Form 1: \( \ce{H2C = CH - CH2^ + } \) (the double bond is between the first and second carbon, positive charge on the third carbon).
• Resonance Form 2: \( \ce{H2C^ + - CH = CH2} \) (the double bond is between the second and third carbon, positive charge on the first carbon).

Part (d): Benzoate anion

The benzoate anion has a benzene ring attached to a \( \ce{CO2^ - } \) group.

• Resonance Form 1: The double bond in the \( \ce{CO2^ - } \) is between the carbon and one of the oxygen atoms, and the negative charge is on the other oxygen: \( \ce{C6H5 - C( = O)( - O^ - )} \) (with the benzene ring's π - electron system in conjugation).
• Resonance Form 2: The double bond in the \( \ce{CO2^ - } \) is between the carbon and the other oxygen atom, and the negative charge is on the first oxygen: \( \ce{C6H5 - C( - O^ - )( = O)} \) (again, with the benzene ring's π - electron system in conjugation).

Final Answers (Summarized)

Problem 2 - 9

(a) These are resonance forms (electron/charge distribution changes, atomic positions same).
(b) These are not resonance forms (atomic connectivity changes).

Problem 2 - 10

(a) Three resonance forms as described for \( \ce{CH3OPO3^{2 - }} \).
(b) Three resonance forms as described for \( \ce{NO3^ - } \).
(c) Two resonance forms as described for \( \ce{H2C = CH - CH2^ + } \).
(d) Two resonance forms as described for the benzoate anion.

Answer:

In the first structure, the double bonds are in positions \( \ce{H2C = C - C = C - CH3} \) (with substituents \( \ce{CH3} \) and \( \ce{CH2CH3} \) and \( \ce{H} \)), and in the second, the double - bond positions are changed such that the carbon - atom connectivity is altered (the terminal carbons have different bonding; in the first, one terminal is \( \ce{H2C =} \) and in the second, one terminal is \( \ce{= CH2} \) but the central carbon - carbon double - bond pattern is different). Resonance forms must have the same atomic connectivity. So, these are not resonance forms.

Problem 2 - 10

Part (a): Methyl phosphate anion, \( \ce{CH3OPO3^{2 - }} \)

The methyl phosphate anion has a central \( \ce{P} \) atom bonded to \( \ce{O - CH3} \) and three \( \ce{O} \) atoms (two of which carry a negative charge in the overall anion).

• Resonance Form 1: \( \ce{CH3O - P( = O)( - O^ - )( - O^ - )} \) (the double bond is between \( \ce{P} \) and one of the non - methyl - attached \( \ce{O} \) atoms, and two \( \ce{O} \) atoms have a negative charge).
• Resonance Form 2: \( \ce{CH3O - P( - O^ - )( = O)( - O^ - )} \) (the double bond is between \( \ce{P} \) and the second non - methyl - attached \( \ce{O} \) atom, with the same charge distribution logic).
• Resonance Form 3: \( \ce{CH3O - P( - O^ - )( - O^ - )( = O)} \) (the double bond is between \( \ce{P} \) and the third non - methyl - attached \( \ce{O} \) atom).

Part (b): Nitrate anion, \( \ce{NO3^ - } \)

The nitrate anion has a central \( \ce{N} \) atom bonded to three \( \ce{O} \) atoms.

• Resonance Form 1: \( \ce{O = N( - O^ - )( - O^ - )} \) (double bond between \( \ce{N} \) and one \( \ce{O} \), two \( \ce{O} \) with negative charge).
• Resonance Form 2: \( \ce{O( - O^ - ) = N( - O^ - )} \) (double bond between \( \ce{N} \) and the second \( \ce{O} \)).
• Resonance Form 3: \( \ce{O( - O^ - )( - O^ - ) = N} \) (double bond between \( \ce{N} \) and the third \( \ce{O} \)).

Part (c): Allyl cation, \( \ce{H2C = CH - CH2^ + } \)

The allyl cation has a conjugated system.

• Resonance Form 1: \( \ce{H2C = CH - CH2^ + } \) (the double bond is between the first and second carbon, positive charge on the third carbon).
• Resonance Form 2: \( \ce{H2C^ + - CH = CH2} \) (the double bond is between the second and third carbon, positive charge on the first carbon).

Part (d): Benzoate anion

The benzoate anion has a benzene ring attached to a \( \ce{CO2^ - } \) group.

• Resonance Form 1: The double bond in the \( \ce{CO2^ - } \) is between the carbon and one of the oxygen atoms, and the negative charge is on the other oxygen: \( \ce{C6H5 - C( = O)( - O^ - )} \) (with the benzene ring's π - electron system in conjugation).
• Resonance Form 2: The double bond in the \( \ce{CO2^ - } \) is between the carbon and the other oxygen atom, and the negative charge is on the first oxygen: \( \ce{C6H5 - C( - O^ - )( = O)} \) (again, with the benzene ring's π - electron system in conjugation).

Final Answers (Summarized)

Problem 2 - 9

(a) These are resonance forms (electron/charge distribution changes, atomic positions same).
(b) These are not resonance forms (atomic connectivity changes).

Problem 2 - 10

(a) Three resonance forms as described for \( \ce{CH3OPO3^{2 - }} \).
(b) Three resonance forms as described for \( \ce{NO3^ - } \).
(c) Two resonance forms as described for \( \ce{H2C = CH - CH2^ + } \).
(d) Two resonance forms as described for the benzoate anion.