How Does the Nature of C–X Bond Affect Chemical Reactivity?
The C in the C–X bond is carbon while the halogen is denoted as X. The high electronegativity of halogen makes the electron cloud attract more towards itself and therefore gains a slight negative charge, while the carbon attains a slight positive charge. As the halogens need only one electron to achieve their nearest inert gas configuration, that is, their octet state, so between one carbon and one halogen atom, only one sigma bond is formed. The C–X bond length in haloarenes increases due to the increase in atomic size from fluorine to astatine and the bond dissociation strength decreases.
Haloarenes Nature of C–X Bond
The chemical compounds containing arenes are known as haloarenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. It contains halogen atoms attached to sp2 hybridized carbon atom(s) of an aryl group. The C–X bond's nature depends on both the halogen of the compound and the nature of carbon in the benzene ring. The alphabet "X" generally denotes halogen. Halogens are group 17 elements that have very high electronegativity. Going down the group, the elements are namely, fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The highest electronegativity is of fluorine (F). The elements in this group need one more electron to complete the nearest noble gas configuration.
The carbon in haloarenes which is a 14th group element has lesser electronegativity as compared to that of the halogens which have higher electronegativity.
Salient Points on the Nature of C–X Bonds in Haloarenes
The salient points on the nature of C–X bonds in haloarenes are as follows:
We know that the halogens are more electronegative than carbon and due to this high electronegativity, it makes the electron cloud attract more towards itself and therefore gains a slight negative charge, while the carbon attains a slight positive charge. Thus, the C–X bond in haloarenes is polarised.
Between one carbon atom and one halogen atom, only one sigma bond is formed because halogens need only one electron to reach the nearest inert gas configuration, that is, the octet state.
The atomic size increases from fluorine to astatine; therefore, as a result, the C–X bond length in haloarenes also increases, and the bond dissociation strength decreases.
Moving down the group, the electronegativity decreases, and we know that dipole moment depends on the difference in electronegativity of carbon and halogens. Therefore, the dipole moment also decreases down the group. Although, there is an exception of C–Cl and C–F dipole moments., Cl has less electronegativity than F, but the dipole moment of C–Cl bond is more than C–F.
Solved Examples
1. Which C–X bond is strongest?
Answer: Carbon–fluoride bond.
Except for the carbon–iodine bond, the carbon–halogen bonds are polar, because the electron pair is pulled closer to the halogen atom than the carbon. This is because the halogens are more electronegative than carbon apart from iodine. Fluorine is the most electronegative that pulls the electron pair more strongly than the other halogens. Therefore, the carbon–fluorine bond is the strongest.
2. Which of the following orders is correct regarding the bond enthalpy ε(C−X) in an alkyl halide (RX)?
(a) ε(C−I) < ε(C−Br) < ε(C−Cl)
(b) ε(C−I) < ε(C−Br) > ε(C−Cl)
Answer: Bond enthalpy C−X (where X is Cl, Br, I) decreases with increase in the atomic number of X. Hence, the correct order is option a.
3. What are the factors responsible for low reactivity of aryl halides towards nucleophilic substitution?
Answer: The following factors are responsible for the low reactivity of aryl halides towards nucleophilic substitution:
The bond length of C–X in haloarene is smaller than C–X bond length in alkyl halide.
By any nucleophile, it is difficult to displace halogen in haloarenes.
A nucleophile easily replaces halogen.
Attack of nucleophiles on aryl halides becomes an electron-rich molecule due to the presence of pi bonds.
FAQs on Nature of C–X Bond: Meaning, Characteristics & Examples
1. What is the fundamental nature of the C-X bond in haloalkanes?
The fundamental nature of the carbon-halogen (C-X) bond in haloalkanes is polar covalent. This is because halogen atoms (X) are more electronegative than carbon atoms. The halogen atom pulls the shared pair of electrons towards itself, acquiring a partial negative charge (δ⁻), while the carbon atom acquires a corresponding partial positive charge (δ⁺). The carbon atom in this bond is sp³ hybridised.
2. How do the bond length and bond enthalpy of the C-X bond change for different halogens?
As we move down the halogen group from fluorine to iodine, the atomic size of the halogen atom increases. This leads to a corresponding increase in the C-X bond length.
- Bond Length Trend: C-F < C-Cl < C-Br < C-I
Consequently, the strength of the bond decreases because the orbital overlap becomes less effective with larger, more diffuse halogen p-orbitals. This means the bond dissociation enthalpy decreases down the group. The C-F bond is the strongest, and the C-I bond is the weakest. For more details, you can check these Haloalkanes and Haloarenes Class 12 Important Questions.
3. How does the nature of the C-X bond differ between haloalkanes and haloarenes?
The nature of the C-X bond differs significantly between haloalkanes and haloarenes primarily due to the hybridization of the carbon atom and the effect of resonance.
- Hybridization: In haloalkanes, the carbon is sp³ hybridised, while in haloarenes, it is sp² hybridised. An sp² carbon is more electronegative, which reduces the polarity of the C-X bond in haloarenes.
- Resonance: In haloarenes, the lone pair of electrons on the halogen atom participates in resonance with the benzene ring. This gives the C-X bond a partial double bond character, making it shorter and stronger than the pure single C-X bond in haloalkanes.
- Reactivity: Due to the stronger bond, the C-X bond in haloarenes is much less reactive towards nucleophilic substitution reactions compared to haloalkanes. You can explore this further in the Haloalkanes and Haloarenes Class 12 NCERT Solutions.
4. Why is the dipole moment of chloromethane (CH₃Cl) greater than that of fluoromethane (CH₃F)?
This is an important exception to general trends. The dipole moment (μ) is a product of charge separation (q) and bond distance (d). Although fluorine is more electronegative than chlorine, creating a larger charge separation (q) in the C-F bond, the C-Cl bond is significantly longer (d). The increase in bond distance for C-Cl outweighs the greater charge separation in C-F. As a result, the product of charge and distance (μ = q × d) is slightly greater for chloromethane (1.86 D) than for fluoromethane (1.847 D).
5. What is the effect of resonance on the C-X bond in haloarenes?
In haloarenes, the lone pairs of electrons on the halogen atom are in conjugation with the π-electrons of the aromatic ring. This allows for the delocalisation of electrons through resonance. The key effect is the creation of a partial double bond character in the C-X bond. This makes the bond shorter (e.g., C-Cl bond length is 169 pm in chlorobenzene vs. 177 pm in methyl chloride) and stronger, thus making haloarenes less susceptible to nucleophilic attack.
6. How does the sp² hybridization of carbon in haloarenes affect the C-X bond's reactivity?
The carbon atom bonded to the halogen in a haloarene is sp² hybridised. An sp² orbital has a higher s-character (33.3%) compared to an sp³ orbital (25%). This higher s-character makes the sp² carbon more electronegative and able to hold the bonded electron pair more tightly. This, combined with the partial double bond character from resonance, makes the C-X bond in haloarenes extremely difficult to break, significantly reducing their reactivity in nucleophilic substitution reactions compared to the C(sp³)-X bond in haloalkanes.
7. Does the C-X bond's polarity make the carbon atom electrophilic or nucleophilic?
The polarity of the C-X bond makes the carbon atom electrophilic. Due to the higher electronegativity of the halogen (X), the carbon atom bears a partial positive charge (δ⁺). This electron deficiency makes the carbon atom a target for attack by nucleophiles (electron-rich species). This electrophilic nature of the carbon atom is central to the chemical reactions of Alkyl Halides.
