Out of Chlorobenzene and Cyclohexyl chloride, which one is more reactive towards nucleophilic substitution rea

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Q. Out of Chlorobenzene and Cyclohexyl chloride, which one is more reactive towards nucleophilic substitution reaction and why?

CBSECBSE Class XII Board 2019Subjective· 1mImportance★★★★★

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Concept understanding — Dipole Moment Trends

Dipole Moment Trends

A dipole moment measures how unevenly the electron density is shared across a bond or molecule — it is the product of the magnitude of separated charge and the distance between the charge centres, and it has a direction (from the positive end to the negative end, by convention pointing toward the more electronegative atom).

What Decides a Bond's Dipole Moment

Two things, working together, not just one:

Electronegativity difference — a bigger gap between the two bonded atoms means a more polarised bond, in principle a bigger dipole.
Bond length — dipole moment is charge × distance, so even a modestly polarised bond can have a sizeable moment if the atoms are far apart, and a strongly polarised but very short bond can have a smaller moment than you'd expect.

These two factors often pull in OPPOSITE directions down a group, which is why dipole-moment trends are frequently non-monotonic even when electronegativity itself decreases smoothly.

The Carbon–Halogen Trend

Going down Group 17 (F → Cl → Br → I), electronegativity decreases steadily, which would suggest the C–X dipole moment should decrease steadily too. But bond length increases at the same time (the halogen atom gets bigger), which pushes the dipole moment back up. The two effects partially cancel, so the C–X dipole moment does NOT fall in a simple, uniform way down the group — it is often close to level from C–Cl to C–I, or even shows a local maximum, rather than tracking electronegativity alone. Always check real values rather than assuming "more electronegative always means bigger dipole."

Molecular Dipole Moment: Vector Addition, Not Just Bond Polarity

A molecule's OVERALL dipole moment is the vector sum of all its individual bond dipoles — geometry matters as much as bond polarity.

$CCl_4$ (tetrahedral, symmetric): four polar C–Cl bonds, but their vectors cancel exactly by symmetry, so the molecule has ZERO net dipole moment despite having polar bonds.
$CHCl_3$ (one bond replaced by C–H): the symmetry is broken, the vectors no longer cancel, and the molecule has a real net dipole moment. …

Why this formula?

Great! Let's build a deep, concept-first understanding of Dipole Moment Trends — not just the formula, but why it works the way it does.

1. What is a Dipole Moment?

A dipole moment arises when there is a separation of charge within a molecule — one end is slightly positive ( $\delta^+$ ), the other slightly negative ( $\delta^-$ ). This is called a polar bond.

The dipole moment is a vector quantity: it has both magnitude and direction.

2. The Key Formula

The magnitude of the dipole moment ( $\mu$ ) for a pair of charges is given by:

\mu = q \times d

Where:

$q$ = magnitude of charge separation (in coulombs, but often in Debye units)
$d$ = distance between the centers of positive and negative charge (in meters or Ångströms)

Why this formula?

It comes directly from the definition: dipole moment measures how much the charges are "pulled apart" and how big those charges are. If you double the charge or double the separation, the dipole moment doubles. It's a product, not a sum.

3. Why the Formula Makes Physical Sense

Imagine a simple diatomic molecule like HCl:

The H atom has $\delta^+$
The Cl atom has $\delta^-$

The dipole moment points from positive to negative (by convention in chemistry, from $\delta^+$ to $\delta^-$ ).

If the bond length $d$ increases, the charges are farther apart → larger $\mu$ .
If the electronegativity difference increases → larger $q$ → larger $\mu$ .

So the formula $\mu = q \times d$ captures both the "how much charge" and "how far apart" factors.

4. The Vector Nature — Why Direction Matters

In molecules with more than two atoms, dipole moments add as vectors, not scalars.

For example, in water ( $H_2O$ ):

Each O–H bond has its own dipole moment (pointing from H to O).
The molecule is bent (~104.5°), so the two bond dipoles partially cancel but not completely.

The net dipole moment is:

\mu_{\text{net}} = \sqrt{\mu_1^2 + \mu_2^2 + 2\mu_1\mu_2\cos\theta}

Where $\theta$ is the angle between the two bond dipoles.

Why this formula?

It's just the law of vector addition (parallelogram law). If the dipoles point in exactly opposite directions ( $\theta = 180^\circ$ ), they cancel completely. If they point in the same direction ( $\theta = 0^\circ$ ), they add fully.

5. Trends You Can Now Explain

Trend	Why? (Based on formula)
Larger bond length → larger $\mu$	$d$ increases in $\mu = q \times d$

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Cyclohexyl chloride is far more reactive toward nucleophilic substitution than chlorobenzene because the C–Cl bond in chlorobenzene has partial double-bond character from resonance, making it stronger and less susceptible to attack, while the benzene ring's electron density also deactivates the carbon toward nucleophiles.

Why Reactivity Differs: The Role of Resonance and Hybridization

Nucleophilic substitution requires a nucleophile to attack an electrophilic carbon and displace the leaving group (here, chlorine). The ease of this process depends critically on two factors: how accessible the carbon is to attack, and how readily the C–Cl bond can break.

In cyclohexyl chloride, we have a straightforward $sp^3$ -hybridized carbon bonded to chlorine. The C–Cl bond is a pure single bond, relatively long and weak. The carbon is tetrahedral, sterically accessible, and the chlorine can leave as $\text{Cl}^-$ without much electronic resistance from the rest of the molecule.

In chlorobenzene, the situation changes dramatically. The chlorine is attached directly to a benzene ring, and this connectivity introduces resonance stabilization that fundamentally alters the C–Cl bond.

Step-by-Step Analysis

1. Resonance in Chlorobenzene

The lone pairs on chlorine can delocalize into the aromatic $\pi$ -system of benzene. We can draw resonance structures where the lone pair on Cl forms a $\pi$ -bond with the ring, placing negative charge on ortho and para positions:

\ce{C6H5-Cl <-> C6H5^{+}=Cl^{-}}

This resonance gives the C–Cl bond partial double-bond character. A bond with double-bond character is shorter, stronger, and much harder to break than a pure single bond.

Important

The C–Cl bond length in chlorobenzene (~169 pm) is significantly shorter than in alkyl chlorides (~177 pm), confirming the partial double-bond character from resonance.

2. Bond Strength Comparison

Because of this resonance stabilization, the bond dissociation energy of the C–Cl bond in chlorobenzene is higher than in cyclohexyl chloride. Breaking a stronger bond requires more energy, making the substitution reaction slower.

3. Electronic Effects on the Carbon Center

In chlorobenzene, the carbon attached to Cl is $sp^2$ -hybridized (part of the aromatic ring). The benzene ring is electron-rich due to the delocalized $\pi$ -electrons. This electron density reduces the electrophilicity of the carbon bearing chlorine—it's less attractive to an incoming nucleophile.

In cyclohexyl chloride, the $sp^3$ carbon has no such electron cloud shielding it. The inductive electron-withdrawing effect of chlorine makes the carbon reasonably electrophilic.

4. Steric and Geometric Factors …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set D1 markMCQ
$Q. Which of the following has zero dipole moment?$
(a) CH3Cl
(b) CCl4
(c) CH2Cl2
(d) CHCl3
›Reveal solutionSolution

Symmetric tetrahedral CCl4 has cancelling bond dipoles -> net dipole moment = 0.

Dipole moment depends on both bond polarity and molecular symmetry. In CCl4 the four C-Cl bonds are identical and arranged tetrahedrally (109.5deg). Their bond dipole vectors are equal in magnitude and point symmetrically, so they cancel completely and the net dipole moment is zero.

…

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CBSE 2023Set F1 markMCQ
$Q. Which of the following compounds has zero dipole moment?$
(a) CH3Cl
(b) CHCl3
(c) CCl4
(d) CH2Cl2
›Reveal solutionSolution
In CCl4 four identical C-Cl bond dipoles point to the corners of a regular tetrahedron and cancel exactly, so the net dipole moment is zero.

Dipole moment is a vector sum of the individual bond dipoles.

CCl4 : regular tetrahedral, four equivalent C-Cl bonds symmetrically arranged → vector sum = 0 → non-polar, mu = 0. …
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CBSE 2023Set ANNUAL1 markMCQ
$Q. Which one of the following has the highest dipole moment?$
(a) CHCl₃
(b) CH₃Cl
(c) CH₂Cl₂
(d) CCl₄
›Reveal solutionSolution

Among $\text{CHCl}_3, \text{CH}_3\text{Cl}, \text{CH}_2\text{Cl}_2, \text{CCl}_4$ , methyl chloride ( $\text{CH}_3\text{Cl}$ ) has the largest net dipole moment.

Dipole moment depends on both bond polarity and molecular symmetry. $\text{CCl}_4$ is perfectly tetrahedral and symmetric, so its C–Cl bond dipoles cancel exactly, giving $\mu = 0$ . As Cl atoms are progressively replaced by less electronegative H atoms, the molecule becomes less symmetric and the resultant dipole increases; $\text{CH}_3\text{Cl}$ (only one C–Cl bond, three C–H bonds reinforcing the same direction) has the largest measur …

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CBSE 2019Set 56/2/11 mark
$Q. Out of Chlorobenzene and Cyclohexyl chloride, which one is more reactive towards nucleophilic substitution reaction and why?$
›Reveal solutionSolution

Cyclohexyl chloride is far more reactive toward nucleophilic substitution than chlorobenzene because the C–Cl bond in chlorobenzene has partial double-bond character from resonance, making it stronger and less susceptible to attack, while the benzene ring's electron density also deactivates the carbon toward nucleophiles.

Why Reactivity Differs: The Role of Resonance and Hybridization

Nucleophilic substitution requires a nucleophile to attack an electrophilic carbon and displace the leaving group (here, chlorine). The ease of this process depends critically on two factors: how accessible the carbon is to attack, and how readily the C–Cl bond can break.

In cyclohexyl chloride, we have a straightforward $sp^3$ -hybridized carbon bonded to chlorine. The C–Cl bond is a pure single bond, relatively long and weak. The carbon is tetrahedral, sterically accessible, and the chlorine can leave as $\text{Cl}^-$ without much electronic resistance from the rest of the molecule.

In chlorobenzene, the situation changes dramatically. The chlorine is attached directly to a benzene ring, and this connectivity introduces resonance stabilization that fundamentally alters the C–Cl bond.

Step-by-Step Analysis

1. Resonance in Chlorobenzene

The lone pairs on chlorine can delocalize into the aromatic $\pi$ -system of benzene. We can draw resonance structures where the lone pair on Cl forms a $\pi$ -bond with the ring, placing negative charge on ortho and para positions:
$\ce{C6H5-Cl <-> C6H5^{+}=Cl^{-}}$
This resonance gives the C–Cl bond partial double-bond character. A bond with double-bond character is shorter, stronger, and much harder to break than a pure single bond.

Important

The C–Cl bond length in chlorobenzene (~169 pm) is significantly shorter than in alkyl chlorides (~177 pm), confirming the partial double-bond character from resonance.

2. Bond Strength Comparison

Because of this resonance stabilization, the bond dissociation energy of the C–Cl bond in chlorobenzene is higher than in cyclohexyl chloride. Breaking a stronger bond requires more energy, making the substitution reaction slower.

3. Electronic Effects on the Carbon Center

In chlorobenzene, the carbon attached to Cl is $sp^2$ -hybridized (part of the aromatic ring). The benzene ring is electron-rich due to the delocalized $\pi$ -electrons. This electron density reduces the electrophilicity of the carbon bearing chlorine—it's less attractive to an incoming nucleophile.

In cyclohexyl chloride, the $sp^3$ carbon has no such electron cloud shielding it. The inductive electron-withdrawing effect of chlorine makes the carbon reasonably electrophilic.

4. Steric and Geometric Factors …
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Q.Out of Chlorobenzene and Cyclohexyl chloride, which one is more reactive towards nucleophilic substitution reaction and why?

Why Reactivity Differs: The Role of Resonance and Hybridization

Step-by-Step Analysis

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