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2.7: Drawing Resonance Forms

CHEM 231: Organic Chemistry I Textbook · LibreTexts

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    Look back at the resonance forms of the acetate ion and the acetone anion shown in the previous section. The pattern seen there is a common one that leads to a useful technique for drawing resonance forms. In general, any three-atom grouping with a p orbital on each atom has two resonance forms:

    Two resonance forms in a three-atom grouping of X, Y, and Z with one double bond in the structure shown separated by a double-headed arrow.

     

    The atoms X, Y, and Z in the general structure might be C, N, O, P, S, or others, and the asterisk (*) might mean that the p orbital on atom Z is vacant, that it contains a single electron, or that it contains a lone pair of electrons. The two resonance forms differ simply by an exchange in the position of the multiple bond and the asterisk from one end of the three-atom grouping to the other.

    By learning to recognize such three-atom groupings within larger structures, resonance forms can be systematically generated. Look, for instance, at the anion produced when H+ is removed from 2,4-pentanedione by reaction with a base. How many resonance structures does the resultant anion have?

    The chemical reaction of 2,4 – pentanedione with a base.

    The 2,4-pentanedione anion has a lone pair of electrons and a formal negative charge on the central carbon atom, next to a bond on the left. The grouping is a typical one for which two resonance structures can be drawn.

     

    An arrow mechanism of resonance in a 2,4-pentanedione anion.

    Just as there is a bond to the left of the lone pair, there is a second bond to the right. Thus, we can draw a total of three resonance structures for the 2,4-pentanedione anion.

    Three resonance forms of 2,4-pentanedione, separated by two double-headed arrows.

     

    Worked Example \(\PageIndex{1}\): Drawing Resonance Forms for an Anion

    Draw three resonance structures for the carbonate ion, CO32–.

    The chemical structure of a carbonate ion.
    Strategy

    Look for three-atom groupings that contain a multiple bond next to an atom with a p orbital. Then exchange the positions of the multiple bond and the electrons in the p orbital. In the carbonate ion, each singly bonded oxygen atom with three lone pairs and a negative charge is adjacent to the double bond, giving the grouping Condensed structure of O with two nonbonding pairs double-bonded to C, which is single-bonded to another O with three nonbonding pairs and a negative charge..

    Solution

    Exchanging the position of the double bond and an electron lone pair in each grouping generates three resonance structures.

    Three resonance structures of the carbonate ion, featuring the lone pairs and charges on the oxygen atoms in each case and separated by two double-headed arrows.
    Worked Example \(\PageIndex{2}\): Drawing Resonance Forms for a Radical

    Draw three resonance forms for the pentadienyl radical, where a radical is a substance that contains a single, unpaired electron in one of its orbitals, denoted by a dot (·).

    The chemical structure of pentadienyl radical, featuring an unpaired electron on the right-most carbon atom.
    Strategy

    Find the three-atom groupings that contain a multiple bond next to an atom with a p orbital.

    Solution

    The unpaired electron is on a carbon atom next to a bond, giving a typical three-atom grouping that has two resonance forms.

    Two resonance forms of pentadienyl radical, featuring an unpaired electron on the right-most carbon atom in one case and the central carbon atom in the next, separated by a double-headed arrow.

    In the second resonance form, the unpaired electron is next to another double bond, giving another three-atom grouping and leading to another resonance form.

    Two resonance forms of pentadienyl radical, featuring an unpaired electron on the central carbon atom in one case and the left-most carbon atom in the next, separated by a double-headed arrow.

    Thus, the three resonance forms for the pentadienyl radical are:

    All three resonance forms of pentadienyl radical, separated by double-headed arrows.

    Exercises

    Exercise \(\PageIndex{1}\)

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

    1. chemical=
    2. Chemical structures of 2-methyl-4-ethylhexa-1,3-diene on the left and 2-ethyl-4-methylpenta-1,3-diene on the right.
    Answer

    The structures in (a) are resonance forms.

    Exercise \(\PageIndex{2}\)

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

    1. The methyl phosphate anion, CH3OPO32– (3 resonance structures)
    2. The nitrate anion, NO3– (3)
    3. The allyl cation, H2C=CH–CH2+ (2)
    4. The benzoate anion (2)

    The chemical structure of benzoate anion where a carboxylate group COO- is attached to a benzene ring.

    Answer
    1. Three different resonance forms of methyl phosphate anion, featuring a phosphorus atom at the center and separated by double-headed arrows.
    2. Three different resonance forms of nitrate anion featuring a nitrogen atom at the center and separated by two double-headed arrows.
    3. Two different resonance forms of allyl cation separated by one double-headed arrow.
    4. Two different resonance forms of benzoate, resulting in the migration of a negative charge from one oxygen to the other.

    This page titled 2.7: Drawing Resonance Forms was last modified on Thu, 07 Aug 2025 02:29:14 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Sol Parajon Puenzo (Cañada College) via source content that was edited to the style and standards of the LibreTexts platform.