Concept explainers
Consider Lewis formulas A, B, and C:
A B C
Are A, B, and C constitutional isomers, or are they resonance contributors?
Which have a negatively charged carbon?
Which have a positively charged carbon?
Which have a positively charged nitrogen?
Which have a negatively charged nitrogen?
What is the net charge on each?
Which is a more stable structure, A or B? Why?
Which is a more stable structure, B or C? Why?
What is the CNN geometry in each according to VSEPR?
Interpretation:
The given statements for the following Lewis formulas are to be determined.
Concept introduction:
Isomers are classified as different compounds having the same molecular formula. Satisfying the octet rule and maximizing the number of bonds usually occur simultaneously.
A more electronegative atom favors a negative charge and a less electronegative atom favors a positive charge. The formal charge of an atom is given by:
Answer to Problem 47P
Solution:
Lewis formulas
Lewis formula
Lewis formula
Lewis formulas
Lewis formulas
Net charge on all Lewis formulas
Lewis formula
Lewis formula
The CNN geometry in Lewis formula
Explanation of Solution
(a) Constitutional isomers or resonance contributors.
In order to be constitutional isomers, Lewis formulas must have the same molecular formula and different connectivity. Given Lewis formulas
Resonance contributors differ only in the distribution of electrons. Given Lewis formulas
Delocalization of electrons is shown using curved arrows as follows:
In Lewis formula
In Lewis formula
(b) Negatively charged carbon.
The formal charge on each carbon atom in all the three given Lewis formulas is as follows:
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon |
Lewis formulas with formal charges are
Hence, Lewis formula
(c) Positively charged carbon.
The formal charge on each carbon atom in all the three given Lewis formulas is as follows:
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon |
Lewis formulas with formal charges are
Hence, Lewis formula
(d) Positively charged nitrogen.
The formal charge on each nitrogen atom in all the three given Lewis formulas is as follows:
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Nitrogen(a) | |||
Nitrogen(b) |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Nitrogen(a) | |||
Nitrogen(b) |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Nitrogen(a) | |||
Nitrogen(b) |
Lewis formulas with formal charges are
Hence, Lewis formulas
(e) Negatively charged nitrogen.
The formal charge on each nitrogen atom in all the three given Lewis formulas is as follows:
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Nitrogen(a) | |||
Nitrogen(b) |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Nitrogen(a) | |||
Nitrogen(b) |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Nitrogen(a) | |||
Nitrogen(b) |
Lewis formulas with formal charges are
Hence, Lewis formulas
(f) Net charge on each.
The formal charge on each atom for all the three given Lewis formulas is as follows:
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
Lewis formula
There is one negative charge on carbon and one positive charge on the adjacent nitrogen; hence, net charge on the Lewis formula
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
Lewis formula
There is one negative charge on the nitrogen atom in the middle and one positive charge on the adjacent nitrogen; hence, net charge on the Lewis formula
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
Lewis formula
There is one positive charge on carbon and one positive charge on one nitrogen atom; hence, net charge on the Lewis formula
(g) The more stable structures.
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
The Lewis formulas A and B with formal charges are
The most stable structure is the contributing structure with the greater number of covalent bonds, which contributes more to the resonance hybrid, as long as the octet rule is not exceeded for second-row elements. The major contributor is the one with the smallest separation of oppositely charged atoms, when two or more structures satisfy the octet rule. The major contributor is the one in which the negative charge resides on the most electronegative atom and the positive charge on the least electronegative element, among the structural formulas that satisfy the octet rule and in which one or more atoms bear a formal charge.
In given Lewis formulas
Out of the given three Lewis formulas, in Lewis formula
Hence, Lewis formula
(h) The more stable structures.
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
Atoms | Valence electrons of neutral atom | Electron count | Formal charge |
Carbon | |||
Hydrogen | |||
Nitrogen(a) | |||
Nitrogen(b) |
Lewis formulas
The most stable structure is the contributing structure with the greater number of covalent bonds, which contributes more to the resonance hybrid, as long as the octet rule is not exceeded for second-row elements. The major contributor is the one with the smallest separation of oppositely charged atoms, when two or more structures satisfy the octet rule. The major contributor is the one in which the negative charge resides on the most electronegative atom and the positive charge on the least electronegative element, among the structural formulas that satisfy the octet rule and in which one or more atoms bear a formal charge.
In Lewis formula
Hence, Lewis formula
(i) CNN geometry.
In Lewis formula
In Lewis formula
In Lewis formula
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Chapter 1 Solutions
ORGANIC CHEMISTRY - CONNECT ACCESS CARD
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