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Haloarenes are the chemical compounds that have arenes in them. Here one or more hydrogen atoms bonded to an aromatic ring are replaced by halogens. The bond thus formed is a C-X bond, this bond is dependent on the nature of carbon in the aromatic ring and the halogen which is attached. Here the halogens are generally denoted by an ‘X’ and the Carbon is denoted by ‘C’. These compounds are very commonly used in daily life. The C–X bond length in haloarenes increases owing to the increase in atomic size from fluorine to astatine and the bond dissociation strength decreases.
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Key Takeaways: Haloarenes, Haloalkanes, C-X Bond, Halogens, Hydrocarbons, Aryl Halide, Alkyl Halide, Compounds, Bond Dissociation, Carbon, Fluorine
Nature of C-X Bond in Haloarenes
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As we have discussed above, haloarenes can be defined as those chemical compounds where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. These compounds contain halogen atoms attached to sp2 hybridized carbon atom(s) of an aryl group. The nature of the C–X bond depends on the halogen of the compound as well as the nature of carbon in the benzene ring.
Halogens are Group 17 elements in the periodic classification of elements and have very high electronegativity such as fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Fluorine (F) has the highest electronegativity out of all these elements. The elements in Group 17 need one more electron to complete the nearest noble gas configuration.
Carbon in haloarenes is a Group 14 element and has lesser electronegativity as compared to that of the halogens which have higher electronegativity.
The Carbon-fluoride bond has the strongest C-X bond. Because the size of the halogen atom decreases the further one goes down in the periodic table iodine is the largest atom and fluorine is the smallest one. Some examples of C-X bonds include CH3-Cl, CH3-F, CH3-l.
Nature of C-X Bond in Haloarenes
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Key Features of the Nature of C–X Bonds in Haloarenes
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Here are some of the salient features of the nature of C-X Bonds in Haloarenes:
- Due to halogens being more electronegative than carbon, C-X bonds are polarized in haloarenes, the halogens also attract more electrons due to high electronegativity of the hydrogen. And so it also gains a slightly negative charge and carbon obtains a slightly positive charge as well.
- Halogens need only one electron to achieve the nearest noble gas configuration and so only one sigma bond is hence formed in between only one carbon and a halogen atom.
- The C-X bond length in halogen increases from fluorine and astatine as the atomic size increases from fluorine to astatine.
- The electronegative difference between carbon and halogens determines the dipole moment. Electronegativity of the halogen and dipole moment are directly proportional as the former decrease also leads to the latter’s decrease. However, an exception to this rule is the C-CI and CF dipole moment.
Group 7- Halogens
Things to Remember
- Haloarenes can be defined as those chemical compounds where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens.
- In the C-X bond, C denotes Carbon while X denotes Halogen. One of the strongest C-X bonds is the carbon-fluoride bond.
- Because the halogens are more electronegative than carbon, C-X bonds are polarized in haloarenes. The halogens also attract more electrons owing to the high electronegativity of the hydrogen. And it also gains a slightly negative charge and carbon obtains a slightly positive charge as well.
- While haloalkanes have a halogen atom connected with Sp3 hybridized carbon atom(s) attached, haloarene has sp2 hybridized carbon atom(s) attached from the aryl group.
- The C-X bond length in halogen increases from fluorine and astatine as the atomic size increases from fluorine to astatine.
- The electronegative difference between carbon and halogens determines the dipole moment. Both of these are also directly proportional as the former's decrease also leads to the latter decrease.
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Sample Questions
Ques. What are halogens? (3 Marks)
Ans. Halogens are non-metals. Halogens also refer to Group 17 which is a group that forms with six other non-metallic elements which are as follows:
- Fluorine
- Chlorine
- Bromine
- Iodine
- Astatine
- Tennessine
Ques. Explain the hybridization of the C atom in the C-X bond in Haloarenes? (3 Marks)
Ans. Hybridization is of three types, namely, sp3, sp2, sp. In haloarenes and haloalkanes, the hybridization is on the basis of single or multiple bonds. In haloarenes (or aryl halides) the atoms are attached to sp2 hybridized carbon of the benzene ring. And because of resonance, there are partial double bond characteristics present in the C-X bond and so in haloarenes the carbon atoms which are attached to the Sp2 hybridized. In benzene, each carbon is sp2 hybridized.
Ques. Differentiate between Haloalkanes and Haloarenes. (5 Marks)
Ans. The difference between Haloalkanes and Haloarenes is stated below:
- Haloalkanes
- It is an aliphatic compound that contains halogen.
- Examples of haloalkanes include Methyl chloride and Ethyl bromide.
- Haloalkanes derive from open-chain hydrocarbons.
- Haloalkanes do not have fragrant rings.
- Haloalkanes are a group of organic compounds comprising alkane compounds and halogens.
- Haloarenes
- It is a fragrant compound that contains halogen.
- Haloarenes are a group of organisms that consist of aromatic compounds which have one or two halogens atoms.
- Haloarenes have fragrant rings.
- Haloarenes derive from aromatic hydrocarbons.
- Examples of haloarenes include chlorobenzenes, bromobenzene.
Ques. Explain the process of nomenclature of halogenated compounds. (5 Marks)
Ans. The International Union of Pure and Applied Chemistry (IUPAC) has named all organic compounds. The process of nomenclature of halogenated compounds includes:
- Firstly, the longest carbon chain is found.
- This longest carbon chain number in terms of the number of carbon atom(s) to which halogens attached are numbered the lowest.
- Next, these atoms are labelled with greek words as prefixes (which include di, tetra and tri), this is used to number the identical halogen atoms that are attached to a carbon atom.
- If the halogen atoms attached are different, they shall be numbered in alphabetical order.
- Before the name of the parent hydrocarbon, the position of the halogen atom is denoted by mentioning the name and position of the halogen.
Ques. Which bond is the most polar and why? (3 Marks)
Ans. In terms of most polar bonds, fluorine and iodine form the most polar bond. This is because fluorine has the highest electronegativity and iodine is the least electronegative out of Group 17, which is a group that formed with six other non-metallic elements some of which include astatine (At), fluorine (F), Chlorine (Cl), bromine (Br). Of this group, as mentioned above fluorine is the highest electronegative while iodine is its least counterpart. And due to this, the electronegativity difference is the highest in the fluorine and iodine bond (I-F). Hence, the I-F bond is the most polar.
Ques. State which is Aryl halide and Alkyl Halide of the mentioned components. (3 Marks)
(1)Methyl chloride
(2)Ethyl bromide
(3)Chlorobenzenes
(4)Bromobenzene
(5)Iodobenzene
(6)2-Chlorotoluene
Ans. Here is the answer for the same:
- Methyl chloride - Alkyl halide
- Ethyl bromide - Alkyl halide
- Chlorobenzenes - Aryl halide
- Bromobenzene - Aryl halide
- Iodobenzene - Aryl halide
- 2-Chlorotoluene - Aryl halide
Ques. Is the C-X bond ionic or covalent? (3 Marks)
Ans. C-X bonds are essentially nonmetals that form covalent bonds. A covalent bond is formed when the atoms mutually share one or more pairs of atoms. This bond is formed when the electronegative difference between the two atoms is very small. And hence, the C-X bond formed by sharing one atom each mutually is a covalent and not an ionic bond.
Ques. Why are Haloarenes less reactive to nucleophilic substitution? (3 Marks)
Ans. Haloarenes (aryl halide) acquire a partial bond characteristic in a C-X bond, this is because of resonance. However, the C-X bond in Haloalkanes (alkyl halide) is purely a single bond. Hence, this leads to Haloarenes (aryl halide) having more bond strength than that of Haloalkanes (alkyl halide).
Haloarenes has sp2 hybridized carbon atoms attached to it, while haloalkanes have sp3 hybridized carbon atoms. Therefore, Haloarenes (sp2) have greater s-character and hence are more electronegative. Due to this reason, haloarenes (aryl halide) react less to nucleophilic substitution than haloalkanes (alkyl halide).
Ques. Why are aryl halides more stable than alkyl halides? (3 Marks)
Ans. The best way to know about the stability of aryl halides and alkyl halides is the nucleophilic substitution method, wherein, alkyl halides may undergo reaction substitution in the nucleophilic substitution. However, aryl halide in the same conditions will not undergo reaction substitution. Aryl halides are usually more stable than alkyl halides in nucleophilic substitution. However, one also needs to consider the various ways in which stability outcomes can be different.
Ques. What is the difference between bond polar in C-X bond in haloarenes and haloalkanes? (3 Marks)
Ans. The polarity of C-X in haloarenes is much shorter than haloarenes and the strength of the haloalkanes C-X bond is higher than that of haloarenes. The reason for this is that the double bond and the ring structure result in the haloarenes being electron-rich. Because the resonance makes the ring structure of the C-X bond to be more attracted to the halogens in comparison to haloalkanes.
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