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Aryl Halide

When there is a replacement of one or more hydrogen atoms of a hydrocarbon by same number of halogen atoms, it results in the formation of halo compounds.

Hydrocarbons can be aliphatic or aromatic in nature in nature and form respective halo compounds. In alkyl halides also named as haloalkanes, halogen atom attached with a sp3 hybridized carbon atom while in aryl halide halogen atom bonded with a sp2 hybridized carbon atom of aromatic ring.

The carbon-halogen bond in aryl halide is stronger than carbon-halogen bond in alkyl halide due to double bond character in aryl halides. Many organic halogen compounds occur in nature and they are medicinally useful compounds.

 

Aryl Halides Nomenclature

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Haloarenes are also termed as aryl halides in IUPAC as well as common system. There are some rules for nomenclature of haloarenes or aryl halides.

1. If there is only one halogen group bonded directly to benzene ring, it numbered as one position and written as prefix followed with benzene as main ring. For example, Chlorobenzene or aryl chloride,

Chloro Benzene
2. For dihalogen derivatives containing same type of halogens, the prefixes o- (ortho), m-(meta) and p-(para) are used in common system.
3. However in IUPAC nomenclature of dihalogen derivatives, numerical prefixes 1,2;1,3;1,4 are used to assigned the position of halogen.

4. In the presence of two different halogen groups attached to benzene ring, halogen numbered alphabetically and written in the same way. For example, if there are two halogen groups like bromo and Chloro bonded on benzene ring, bromo group will get lowest position and written first as prefixes followed by Chloro group.
Bromo Chloro Benzene
5. In case of parent aromatic hydrocarbon carries a side chain or a substituent, and then numbering of the carbon atoms of the ring begins with the substituent and in case the parent hydrocarbon is benzene, then the numbering begins with halogen group. For example; if there is one Chloro group and one side chain; methyl group bonded on benzene ring, numbering will start from Chloro group and if methylbenzene written as toluene than numbering starts from methyl group followed by Chloro group.
Chloro Toluene
6. In case of polyhalogen derivatives, there is no common name, but only have IUPAC name in which the position of halogens are indicated by Arabic numerals. If halogen groups bonded in alternate manner, instead of Arabic numerals, sym- prefix also used.

Physical Properties of Aryl Halides

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The physical properties of aryl halide are very similar to alkyl halide as both molecules contain carbon-halogen bond. No doubt the bonding as well as polarity of bond is different in both molecules, due to this there physical properties vary from each other.

1. Polarity of carbon-halogen bond

Carbon halogen bond is stronger in haloarene compare to haloalkanes due to resonance in haloarene, which impart partial double bond character to carbon halogen bond. Hence Carbon-halogen bond length is lesser in haloarene compare to haloalkane.

The strength of carbon-halogen bond in aryl halide can be proves by using the bond energies of carbon-hydrogen bond and carbon-halogen bonds in alkyl halide, aryl halide and vinyl halides.

In aryl halides an aromatic sp2 hybridized carbon atom is attached with halogen atom. Since a sp2 hybridized carbon atom is more electronegative compare to sp3 hybridized carbon atom, hence aryl halides are less polar than alkyl halides.
Refractive Index

2. Solubility of aryl halides

Due to less polar nature of aryl halide compare to alkyl halides, they are insoluble in polar solvents like water. Because of high density compare to water, they form a separate lower layer in water. Another reason of insolubility of aryl halides in water is the interaction between molecules. Aryl halides are bulky molecules compare to water and attached with van der wall dispersion force of attractions. While in between water molecules, there is strong hydrogen bonding.

For make aryl halide molecules soluble in water, it would have to break all these hydrogen bonds between water molecules as well as cleavage of van der wall dispersion forces between aryl halide molecules and both processes cost excess of energy. At the same time the force of attraction between water and aryl halide molecules would be van der wall dispersion forces which are weaker than hydrogen bonds, therefore would not release much energy during formation and aryl halide become insoluble in water.

3. Boiling point and melting point

Boiling point and melting point depend upon the polarity and force of attraction between molecules.
  1. The melting point and boiling point of aryl halides are nearly the same as those of haloalkanes with the same number of carbon atoms.
  2. For the same aryl group the melting point and boiling point increases with increasing the size of the halogen group. Hence the order of haloarenes would be
Iodoarenes > Bromoarenes > Chloroarenes > Fluoroarenes > Arenes

Boiling point Melting point
C6H6
353.3 278.7
C6H5F
358
244
C6H5Cl
405
242.4
C6H5Br
429
228
C6H5I
462
233

This is because as the molecular size increases, the number of electrons increases which further increase the van der wall force of attraction between molecules as well as boiling point.
  • For the same halogen group, the melting point and boiling point increases with increases the molecular mass, i.e. size of aryl group.
  • The boiling point of isomeric dihaloarenes is almost same as it depends on the attraction force and molecular mass, but the melting point is different as it depends upon the symmetry of molecule.
  • The melting point of para-isomer is comparatively high than the corresponding ortho and meta-isomers , because of more symmetrical nature and close packing of molecules in crystal lattice which increase the intermolecular force of attraction between molecules.


Sandmeyer's Reaction

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Sandmeyer reaction is used for the preparation of aryl halide by using diazonium compounds as reactant. By using this reaction all the three haloarenes; Chloroarene , Iodoarene and Bromoarene can be prepared.

Benzene diazonium salts are not stable at room temperature and readily decomposed to phenols. Hence they prepared during the reaction in situ with the reaction of aniline and nitrous acid (NaNO2 + HCl) at 273-278 K .

NaNO2 + dilute HCl → HNO2 + NaCl
C6H5NH2 + HNO2 + HCl →
C6H5N2+Cl- + 2H2O

The preparation of diazonium salts form aniline is also called as diazotisation.

Diazotisation
When diazonium salts are treated with cuprous chloride (CuCl) dissolved in hydrochloric acid or cuprous bromide (CuBr) dissolved in Hydrobromic acid (HBr), formed respective haloarene and reaction called as Sandmeyer reaction which named after the Swiss chemist Traugott Sandmeyer.

In reaction mechanism, the halogen attached to copper enters in the benzene ring and form haloarenes. Reaction follows nucleophilic substitution mechanism through the formation of free radicals as intermediates.

For the preparation of aryl iodide, diazonium salts simply heated with aqueous solution of potassium iodide (KI) and form nitrogen gas with potassium chloride as side products

C6H5N2+Cl- +KI (aq) → C6H5I +N2 + KCl

Schienmann Reaction

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For the preparation of Fluorobenzene from diazonium salts, Sandmeyer cannot use. Fluorobenzene prepared by using fluoroborate salts diazonium compounds. This reaction is known a Balz-Schiemann reaction.

Heating of benzene diazonium tetrafluoroborate forms Fluorobenzene with nitrogen gas and boron trifluoride as by-products. Benzene diazonium tetrafluoroborate can be prepared by diazotization of aniline with sodium nitrite and fluoroboric acid (HBF4) at 273-278 K temperature.

C6H5NH2 + NaNO2 + HBF4 C6H5N2+ BF4-
Aniline Sodium Benzene
nitrite diazoniumtetrafluoroborate

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