Nature of C-X Bond in Haloalkanes: Haloalkanes and haloarenes have a special bonding in them wherein the halogen atom, chlorine, bromine, iodine or fluorine is bonded directly to the aliphatic or aromatic carbon. The resultant \({\rm{C – X}}\) bond assumes a special character because of the electronegativity difference between the two atoms in the bond-  carbon and the halogen atom. Due to the electronegativity difference and the nature of the carbon atom- aliphatic or aromatic, the haloalkanes and arenes are highly reactive in nature.

In this article, we will discuss the type of bonding and nature of \({\rm{C – X}}\)  bonding in haloalkanes and haloarenes and their typical characteristics.

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Bonding in Haloalkanes and Haloarenes

Halogens, as we know, are highly reactive elements and are most electronegative too. They, therefore, form a large number of derivatives of organic compounds, which are highly useful in several applications, including synthesis of other organic compounds, industrial applications, etc.  The electronegativity difference between the two atoms makes the nature of the \({\rm{C – X}}\) bond a special study. The reactions of the organic compounds formed by halogens have special significance because of the nature of the \({\rm{C – X}}\) bond. Since the organic compounds formed by halogens are resistant to soil bacteria, they are found in nature too. 

The two types of organic compounds that the halogens form include:

1. Haloalkanes: Replacing a hydrogen atom from aliphatic hydrocarbons with halogens results in haloalkanes. The halogens in haloalkanes are attached to the \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridized carbon atom of an alkyl group. Haloalkanes or alkyl halides, allylic halides, and benzylic halides have a C-X bond with \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridization (halogen bonded to \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) carbon) 
2. Haloarenes: When a hydrogen atom from an aromatic hydrocarbon is replaced by a halogen atom \({\rm{(F,Cl,Br,orI)}}\) gives haloarenes. The halogens in haloarenes are attached to the \({\rm{s}}{{\rm{p}}^2}\) hybridized carbon atom of an aryl group.Vinylic halides and aryl halides have halogens attached to the \({\rm{s}}{{\rm{p}}^2}\)  carbon atom.

The haloalkanes and haloarenes are further classified into mono, di, tri, and poly-halogen compounds and on the nature of the \({\rm{C – X}}\) bond present in them.

Nature of C-X Bond

In both haloalkanes and haloarenes, the carbon atom in the hydrocarbon-aliphatic or aromatic – is bonded with the halogens. Since halogens are highly electronegative, the bond between carbon (less electronegative than halogen) and a halogen atom \({\rm{(F, Cl, Br, or, I)}}\) are affected and is polar.

It is essential to understand the nature of the \({\rm{C – X}}\) bond present before studying the reactions of these compounds since a polar bond can influence the way a compound reacts with other compounds.

Nature of C-X Bond in Haloalkanes

To begin with, a simple haloalkane has the formula: R-X. Carbon from aliphatic hydrocarbon directly bonds to the halogen atom in a covalent bond. 

Since the electronegativity of halogens is much higher than that of carbon, the electron cloud in the \({\rm{C – X}}\) bond shifts closer to the halogen atom. Hence, the halogen in the bond gets a partial negative charge, while the carbon gains a partial positive charge.

Depending upon the type of halogen attached to the haloalkanes, the partial charge on the \({\rm{C – X}}\) bond and the polarity may differ. For instance, when we go down the group, from fluorine to iodine, the atomic radii or the size of the halogens increases. Hence, the bond length between carbon and the halogen increases as we go from \({\rm{C – F}}\) to \({\rm{C – I}}\).

Also, since the electronegativity of the halogens decreases with an increase in size or as we move down the group from fluorine to iodine, the polarity of the bond between Carbon and halogens decreases from \({\rm{F}}\) to \({\rm{I}}\). The bond enthalpies and dipole moments of the Carbon-halogen bond as we move from fluorine to iodine is as follows:

C-X Bond	The bond length in pm	C-X Bond Enthalpies in KJ/mol	Dipole moment in Debye
\(\mathrm{CH}_{3}-\mathrm{F}\)	\(139\)	\(452\)	\(1.847\)
\(\mathrm{CH}_{3}-\mathrm{Cl}\)	\(178\)	\(351\)	\(1.860\)
\(\mathrm{CH}_{3}-\mathrm{Br}\)	\(193\)	\(293\)	\(1.830\)
\(\mathrm{CH}_{3}-1\)	\(214\)	\(234\)	\(1.636\)

Hence, according to the dipole moment values, it is clear that the polarity of the \({\rm{C – X}}\) bond decreases with an increase in the radius of halogens or their size and a decrease in electronegativity. 

Nature of C-X Bond in Haloarenes

Haloarenes have the halogens bonded to \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridized carbon of the benzene ring. The \({\rm{C – X}}\) bond is polar, too, as in haloalkanes. However, the difference between the \({\rm{C – X}}\) bond in haloalkanes and haloarenes is that the \({\rm{C – X}}\) bond acquires a partial double bond character because the partial charge on the halogens is involved in resonance with the ring. 

Hence, the polarity of the \({\rm{C – X}}\) bond in haloarenes is lesser than the haloalkanes, and the bond length of the \({\rm{C – X}}\) is also shorter than the haloalkanes. The reasons are as follows:

1. Haloarenes, due to the ring structure and the partial double bond, are more electron-rich than the haloalkanes.
2. The resonance makes the ring structure attracted to the halogens more strongly than the haloalkanes.

For example, the dipole moment of Chlorobenzene is \({\rm{1}}{\rm{.69D}}\), while that of chloromethane is \({\rm{1}}{\rm{.83D}}\)

Summary

The \({\rm{C – X}}\) bond formed between carbon and halogens in haloalkanes and haloarenes has special significance due to their polar nature. Haloalkanes have \({\rm{s}}{{\rm{p}}^3}\) hybridized carbon attached to the halogens, while haloarenes have \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridized carbon attached to the halogens. The polarity of the \({\rm{C – X}}\) bond in haloalkanes decreases when we move from fluorine to iodine because the atomic size increases and electronegativity decrease from \({\rm{F}}\) to \({\rm{I}}\). The polarity or the dipole moment values show enough indication of the fact. Also, in haloarenes, due to the ring structure and resonance, the \({\rm{C – X}}\) bond gains partial double bond character, and therefore, the bond length becomes shorter. Hence, the polarity of the \({\rm{C – X}}\) bond in haloarenes is lesser than the haloalkanes 

FAQs

Q.1. What type of hybridization is seen in the C-X bond in Haloalkanes and Haloarenes?
Ans: In haloalkanes or alkyl halides, the halogen is attached to the \({\rm{s}}{{\rm{p}}^3}\) hybridized carbon, while in haloarenes, the halogens are attached to the \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridized carbon of the ring structure.

Q.2. What is the nature of the carbon halogen bond?
Ans: The carbon-halogen bond in haloarenes and haloalkanes are polar due to the difference in the electronegativity between the two atoms- carbon and halogens.

Q.3. Why are C-X bonds polar?
Ans: The polarity of the \({\rm{C – X}}\) bond in haloalkanes and haloarenes is because there is a large electronegativity difference between halogens and carbon atoms. When a covalent bond is formed between two atoms with a large electronegativity difference, the bond that is formed is polar in nature. 

Q.4. Which of the two Haloalkane or Haloarene have greater C-X bond length?
Ans: Haloarenes have a shorter bond length due to the fact that the ring structure and the resonance in the ring give the \({\rm{C – X}}\) bond a double bond character. The polarity, therefore, decreases. This can be seen in the fact that the dipole moment of Chlorobenzene is \({\rm{1}}{\rm{.69D}}\), while that of chloromethane is \({\rm{1}}{\rm{.83D}}\).

Q.5. Why do the polarity of the C-X bond decreases as we move down the group? Which is the most polar C-X bond in haloalkanes?
Ans: When we move down a group (halogens), two things happen: atomic size increases and electronegativity decreases. In halogens, too, as we move down from fluorine to iodine, the size increases, and because of this, the electronegativity decreases. Hence, the polarity of the \({\rm{C – X}}\) bond from \({\rm{C – F}}\) to \({\rm{C – I}}\) decreases.
The most polar \({\rm{C – X}}\) bond in haloalkanes, therefore, would be the \({\rm{C – F}}\) bond. It is because fluorine, owing to the smaller size, is most electronegative, therefore increasing the electronegativity difference between C and F and increasing polarity too.

Q.6. What is the hybridization of the C-X bond in Haloarenes?
Ans: Haloarenes, such as chlorobenzene, have the halogens attached to the \({\rm{s}}{{\rm{p}}^2}\) hybridized carbon atom of the ring. 

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