Which of the following has the shortest C-Cl bond?
(a) Cl-CH=CH-CH3
(b) Cl-CH=CH-NO2
(c) Cl-CH=CH-OC

Q: Why does resonance make the bond shorter?

Resonance delocalizes the lone pair on chlorine into the C-Cl bond, giving it partial double bond character. Double bonds are shorter and stronger than single bonds.

Q: Doesn't chlorine have a -I effect? Why is it donating electrons?

Yes, chlorine has a -I (inductive) effect, but it also has a +R (resonance) effect because of its lone pairs. When attached to a conjugated system, the +R effect often dictates the bond order, even if the overall electron density is pulled via induction.

Q: Why doesn't the methoxy group (-OCH3) shorten the bond as much?

The methoxy group is an electron-donating group (+R effect). It pushes electron density into the double bond, which opposes the electron donation from chlorine. This cross-conjugation reduces the double bond character of the C-Cl bond.

Q: How do we know -NO2 is stronger than other groups?

The -NO2 group has both a strong -I effect and a strong -R effect because nitrogen is bonded to two highly electronegative oxygen atoms. This makes it one of the most powerful electron-withdrawing groups in organic chemistry.

Q: What is the difference between -R and -M effects?

They are essentially the same. -R stands for Resonance effect, and -M stands for Mesomeric effect. Both describe the withdrawal of electron density through a conjugated pi system.

Question

Which of the following has the shortest $C-Cl$ bond?

Options:

(A) $Cl-CH=CH-CH_3$

(B) $Cl-CH=CH-NO_2$

(C) $Cl-CH=CH-OCH_3$

(D) $Cl-CH=CH_2$

Sigma Percentile · Accepted Answer

### The Anatomy of a Bond

Imagine a tug-of-war between two atoms sharing a pair of electrons. In a simple single bond, the electrons are shared relatively equally, and the bond has a standard, predictable length. But what happens when the molecules aren't so simple? What if there's a network of alternating single and double bonds? This is where the magic of **resonance** comes into play.

In our problem, we are asked to find the molecule with the shortest carbon-chlorine ($C-Cl$) bond. At first glance, all four options feature a chlorine atom attached to a carbon-carbon double bond. This specific arrangement is known as a vinylic chloride system. Because the chlorine atom possesses lone pairs of electrons, it isn't just a passive bystander; it can actively participate in the electron dance of the adjacent double bond.

### The Power of Resonance

To understand bond length, we must understand bond order. A pure single bond is the longest, a double bond is shorter, and a triple bond is the shortest. If a single bond can somehow acquire **partial double bond character**, it will shrink in length.

How does it acquire this character? Through resonance! The lone pair on the chlorine atom can delocalize, moving towards the adjacent carbon atom to form a temporary double bond.

$$:\ddot{Cl}-CH=CH-R \longleftrightarrow :\overset{+}{Cl}=CH-\overset{-}{C}H-R$$

The more stable this resonance structure is, the more time the molecule spends in this state, and the greater the double bond character of the $C-Cl$ bond. Therefore, our goal is to find the molecule that encourages this resonance the most.

### The Nitro Group's Pull

Let's evaluate the substituent $R$ at the other end of the double bond in each option:
- In option (a), we have a methyl group ($-CH_3$), which is electron-donating via the $+I$ effect. It pushes electrons into the system, opposing the flow from chlorine.
- In option (c), we have a methoxy group ($-OCH_3$), which is strongly electron-donating via the $+R$ effect. This creates a severe clash of electron flow (cross-conjugation), drastically reducing the resonance from chlorine.
- In option (d), we just have a hydrogen atom, which is neutral.
- But in option (b), we have a **nitro group ($-NO_2$)**.

The nitro group is an absolute powerhouse. It is one of the strongest electron-withdrawing groups in organic chemistry, exerting both a strong $-I$ (inductive) and a powerful $-R$ (resonance) effect.

### The Final Verdict

When chlorine tries to donate its lone pair into the double bond, the nitro group acts like a vacuum, eagerly pulling those electrons all the way across the molecule.

$$:\ddot{Cl}-CH=CH-NO_2 \longleftrightarrow :\overset{+}{Cl}=CH-CH=N(O^-)_2$$

This extended, highly favorable conjugation means the resonance structure where $C-Cl$ is a double bond is exceptionally stable. Consequently, the $C-Cl$ bond in $Cl-CH=CH-NO_2$ possesses the **maximum double bond character** among all the choices.

More double bond character directly translates to a shorter, stronger bond. Thus, the molecule with the shortest $C-Cl$ bond is undoubtedly the one with the nitro group.

Animated Solution for Chemistry - Organic Chemistry: Which of the following has the shortest $C - C l$ bond?

Select Answer:

Visualized Solution

$A na l y z in g t h e O pt i o n s$

$B o n d L e n g t h an d R eso nan ce$

$E v a l u a t in g S u b s t i t u e n t E f f ec t s$

$R eso nan ce in O pt i o n (b)$

$C o n c l u s i o n$

$F oo d f or T h o ug h t$

The Sigma Insight: Bond Fission, Electronic Displacement and Hyperconjugation

The Anatomy of a Bond

The Power of Resonance

The Nitro Group's Pull

The Final Verdict

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Animated Solution for Chemistry - Organic Chemistry: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Which of the following has the shortest C−Cl bond?