Draw the resonance structures of the following compounds: (i) CH2=CH—Cl (chlorine bears three lone pairs) (ii)

NCERT Exemplar · Q42

Q. Draw the resonance structures of the following compounds:

(i) CH2=CH—Cl (chlorine bears three lone pairs)

(ii) CH2=CH—CH=CH2

(iii) CH2=CH—CHO

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Concept understanding — Resonance Structures Drawing

Resonance Structures: What They Are and How to Draw Them

Let's start with a simple question. When you draw a molecule like ozone ( $O_3$ ), you might put a double bond between the central oxygen and one of the end oxygens, and a single bond to the other. But experiments show both O–O bonds are identical — same length, same strength. So which drawing is correct?

Neither single drawing is correct. The real molecule is a hybrid of both possibilities. That's the core idea of resonance.

The Intuition: A Musical Analogy

Think of a chord played on a piano. A C major chord is made of three notes: C, E, G. No single note is the chord — the chord is the blend of all three. Similarly, a resonance hybrid is the blend of all valid Lewis structures (called resonance contributors or canonical forms) for a molecule. The real molecule is not flipping between these forms; it exists as a single, stable average.

Note

Resonance structures are not in equilibrium. The molecule does not switch from one form to another. It is a single structure that is the weighted average of all contributors.

The Precise Definition

Resonance structures are two or more Lewis structures that differ only in the placement of electrons (pi bonds and lone pairs), never in the positions of atoms. The real molecule is described by a resonance hybrid — a superposition of all contributors.

Rules for Valid Resonance Structures

Atoms never move. Only electrons (pi bonds, lone pairs, and sometimes sigma bonds in special cases) change positions.
The total number of electrons stays the same. You are just redistributing them.
Each structure must obey the octet rule (for second-period elements) and have valid formal charges.
All structures must have the same net charge and the same number of unpaired electrons (if any).

How to Draw Resonance Structures: A Step-by-Step Method

Let's use the carbonate ion ( $CO_3^{2-}$ ) as our example.

Step 1: Draw the best Lewis structure

Start with the skeleton: carbon in the center, three oxygens around it. Count valence electrons: C has 4, each O has 6, plus 2 for the charge = $4 + 18 + 2 = 24$ electrons. Place bonds and lone pairs to satisfy octets. You'll get one structure with a C=O double bond and two C–O single bonds, each single-bonded oxygen carrying a negative charge.

Step 2: Identify movable electrons

Look for pi bonds (double or triple bonds) and lone pairs that are adjacent to pi bonds or to atoms with an empty p orbital. In carbonate, the C=O pi bond and the lone pairs on the negatively charged oxygens are the movable parts.

Step 3: Push electrons using curved arrows

An arrow starts at the electron source (a pi bond or lone pair) and points to where the electrons go (to form a new pi bond or to become a lone pair). In carbonate:

Take the pi bond from C=O and push it to become a lone pair on that oxygen.
Simultaneously, take a lone pair from a negatively charged oxygen and push it to form a new C=O pi bond.

Step 4: Draw the new structure

After pushing, you get a second structure where the double bond is on a different oxygen. Repeat to get the third structure (all three oxygens take turns being double-bonded).

Tip

Always check that the total number of electrons and the net charge remain unchanged after each arrow push. A common mistake is to accidentally add or remove electrons.

Common Patterns to Recognize

Pattern	Example	What moves
Allylic system	$CH_2=CH-CH_2^+$	Pi bond shifts, positive charge moves
Conjugated diene	$CH_2=CH-CH=CH_2$	Pi bonds shift (less common in neutral molecules)
Carbonyl group	$R_2C=O$	Lone pair from O forms pi bond, pi bond becomes lone pair
Benzene ring	$C_6H_6$	Alternating double bonds shift around the ring

The Most Common Mistake Beginners Make

Breaking sigma bonds. Remember: sigma bonds (single bonds between atoms) never break in resonance. Only pi bonds and lone pairs move. If you find yourself moving an atom or breaking a single bond, you are drawing a different molecule (a constitutional isomer), not a resonance structure. …

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Resonance structures show the delocalisation of electrons through π-systems and lone pairs. For vinyl chloride, 1,3-butadiene, and acrolein, we draw all valid structures by moving π-electrons and lone pairs while keeping the atomic framework fixed. The true structure is a hybrid of all resonance forms.

Why Resonance Structures Matter

When atoms share electrons in π-bonds or possess lone pairs adjacent to π-systems, those electrons aren't locked in place. They can delocalise across multiple atoms, spreading charge and stabilising the molecule. Resonance structures are our way of representing this delocalisation on paper. Each structure is a snapshot of one possible electron arrangement; the real molecule is a weighted average—a resonance hybrid—of all valid forms.

The key rules: move only electrons (never atoms), obey the octet rule where possible, and use curved arrows to show electron flow. Structures with more bonds, fewer charges, and negative charge on more electronegative atoms contribute more to the hybrid.

(i) CH₂=CH—Cl (Vinyl Chloride)

1. Draw the main Lewis structure

Start with the conventional structure: a C=C double bond and chlorine attached to one carbon with three lone pairs.

\ce{CH2=CH-Cl}

The chlorine has lone pairs in $p$ -orbitals that can overlap with the adjacent π-system.

2. Generate the resonance structure

Push one lone pair from chlorine into the C–Cl bond region, simultaneously pushing the π-electrons of the C=C bond onto the terminal carbon:

\ce{CH2=CH-Cl} \longleftrightarrow \ce{^{-}CH2-CH=\overset{+}{Cl}}

In the second structure, the C–Cl bond becomes a double bond, the original C=C becomes a single bond, and the terminal carbon bears a negative charge while chlorine carries a positive charge.

3. Evaluate contribution

The first structure (no charges) contributes far more to the hybrid because placing a positive charge on the highly electronegative chlorine is energetically unfavourable. Still, the second structure explains why the C–Cl bond in vinyl chloride has partial double-bond character and is shorter than a typical C–Cl single bond.

Tip

Halogens attached to sp²-hybridized carbons can donate lone pairs into the π-system, creating partial double-bond character. This is why vinyl halides are less reactive toward nucleophilic substitution than alkyl halides.

(ii) CH₂=CH—CH=CH₂ (1,3-Butadiene)

1. Draw the main Lewis structure

Two isolated double bonds separated by a single bond:

\ce{CH2=CH-CH=CH2}

The two π-systems are conjugated—separated by one single bond—allowing electron delocalisation.

2. Generate resonance structures

Push the electrons from one π-bond toward the centre, and simultaneously push the other π-bond's electrons outward:

\ce{CH2=CH-CH=CH2} \longleftrightarrow \ce{^{-}CH2-CH=CH-\overset{+}{CH2}}

In the second structure, the terminal carbons bear formal charges (negative on C-1, positive on C-4), and the central C–C bond becomes a double bond while the outer bonds become single.

By symmetry, you can also write:

\ce{CH2=CH-CH=CH2} \longleftrightarrow \ce{\overset{+}{CH2}-CH=CH-^{-}CH2}

3. Interpret the hybrid

The resonance hybrid shows that all three C–C bonds have partial double-bond character. The central "single" bond is shorter than a typical C–C single bond (~1.48 Å vs. 1.54 Å), and the molecule is planar to maximize $p$ -orbital overlap.

Important

Conjugated dienes are more stable than isolated dienes by about 15–20 kJ/mol due to resonance delocalisation. This stabilisation is measurable through heats of hydrogenation.

(iii) CH₂=CH—CHO (Acrolein)

1. Draw the main Lewis structure

A vinyl group attached to an aldehyde:

$\ce{CH2=CH-CHO}$ …

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Q.Draw the resonance structures of the following compounds:

Concept understanding — Resonance Structures Drawing

Resonance Structures: What They Are and How to Draw Them

The Intuition: A Musical Analogy

The Precise Definition

Rules for Valid Resonance Structures

How to Draw Resonance Structures: A Step-by-Step Method

Step 1: Draw the best Lewis structure

Step 2: Identify movable electrons

Step 3: Push electrons using curved arrows

Step 4: Draw the new structure

Common Patterns to Recognize

The Most Common Mistake Beginners Make

Resonance Structures

(i) CH₂=CH—Cl

(ii) CH₂=CH—CH=CH₂ (1,3-butadiene)

(iii) CH₂=CH—CHO …

Why Resonance Structures Matter

(i) CH₂=CH—Cl (Vinyl Chloride)

(ii) CH₂=CH—CH=CH₂ (1,3-Butadiene)

(iii) CH₂=CH—CHO (Acrolein)

Q.Draw the resonance structures of the following compounds:

Why Resonance Structures Matter

(i) CH₂=CH—Cl (Vinyl Chloride)

(ii) CH₂=CH—CH=CH₂ (1,3-Butadiene)

(iii) CH₂=CH—CHO (Acrolein)

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