Haloalkanes Haloarenes - Meaning, Classification, Properties, FAQs

Halogen Containing Organic Compounds:

Halogen atom containing the organic compound which is used as the solvent specially for the relatively non-polar compound. These are also used as initiating material for synthesis of wide range of organic compound. If the halogen atom derivative that is chemistry haloalkanes and haloarenes are also commercially important as dry-cleaning agents, refrigerants, drugs, and the alkyl halide are used in insecticides.

Haloalkanes

Classification of haloalkanes:

Haloalkanes and haloarenes Flow Chart

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Monohalogen derivatives: When two or more than two hydrogen atoms of aliphatic hydrocarbon which are being replaced by the corresponding number of the halo alkane atom for haloalkanes examples chlorine, bromine, or iodine, thus the resulting compounds are known as halogen derivatives of alkanes or haloalkanes. Monohalogen derivatives can be seen in both haloalkanes and haloarenes. These are represented as R-X, where R is Alkyl group and X=Br, Cl, I. The general formula for monohalogen derivatives is (C_nH_2n+1 X).

R-H+X₂→R-X+HX where R-H is Aliphatic Hydrocarbon and R-X is Alkyl Halide.

For haloalkanes examples: CH₃+Cl₂→CH₃Cl+HCl. All the saturated aliphatic hydrocarbons are called alkanes (C_nH_2n+2). Also, the halogen atoms contain halogen atoms which are attached to the sp³ hybridized carbon atoms.

1. Primary Alkyl Halide: Alkyl halide which have halogen atom bonded with primary carbon atom are known as primary alkyl halide. The general formula is given by R-CH₂-X.

2. Secondary Alkyl Halide: Alkyl halide which have halogen atom bonded with secondary carbon atom are known as secondary alkyl halide. The general formula is given by R-(R')CH-X.

3. Tertiary Alkyl Halide: Alkyl halide which have halogen bonded with primary carbon atom are known as primary alkyl halide. The general formula is given by R-(R')C(R")-X.

• Polyhalogen derivatives: Similarly, when more than one atom of hydrogen of alkanes are substituted by the other corresponding number of halogen atom the following compound is called as polyhalogen derivatives of alkanes. Similarly, Polyhalogen derivatives can be seen in both haloalkanes and haloarenes. Further on basis of number of halogen present in molecule are classified as follow as:

1. Dihalogen derivatives: Two hydrogen atoms of alkane which is substituted by the two halogens which forms the following compound, and it is called as Dihalogen derivatives of alkane. The general formula is given by C_nH_2nX₂. Dihalogen derivatives are further classified into:

2. Vicinal Dihalides: Two halogen atoms which are present on adjacent carbon atom. E.g., Ethylene dichloride.

3. Geminal Dihalides: Two halogen atoms which are present on same carbon atom. E.g., Ethylidene dichloride.

4. Trihalogen derivatives: When three hydrogen of alkane which is substituted by the other three halogen atom following compound is known as the Trihalogen derivatives of the alkane. The general formula is given by C_nH_2n-1 X₃.

5. Tetrahalogen derivatives: When four hydrogen of alkane which is substituted by other four halogen atom following compound is known as the Tetrahalogen derivatives of alkane. The general formula is given by C_nH_2n-2X₄.

Halogenation of Alkanes: Replacement of one or more than hydrogen of alkanes by halogen for haloalkanes examples Chlorination, Bromination, Iodination of alkanes. Halogenation reaction is chain reaction which proceeds by the formation of free radical. Also, the halogenation reaction is for both haloalkanes and haloarenes.

This halogenation continues till every hydrogen in alkane is replaced by halogen.

1. Chlorination: When alkanes are heated with chlorine in the presence of UV light or at high temperature so that it forms alkyl chlorides. Generally, chlorination is given by

R-H+Cl₂+UV Light UV Light→R-Cl+HCl

2. Bromination: When alkanes are heated with bromine in the presence of aluminium tribromide (anhy. AlBr3) with corresponding formation of alkyl bromide. Bromination is given by

R-H+Br₂+anhy. AlBr₂anhy. AlBr₂→R-Br+HBr

3. Iodination: When alkane is treated with iodine the following product formed is alkyl iodide. However, Hydroiodic acid is more stronger reducing agent which also reduces alkyl iodide back to alkane by making iodination reaction a reversible reaction. The reaction here needs to be processed out in presence of oxidizing agent haloalkanes examples mercuric acid, Iodic acid, dil. Nitric acid, etc. This oxidizing agent prevents backward reaction by decomposing the hydroiodic acid (HI) formed. The general representation is given by

R-H+I₂→R-I+HI

Markownikoff’s Rule: When unsymmetrical haloalkenes are treated with unsymmetrical reagent which is HX then the negative part of reagent is (-X) which get added to the unsaturated carbon of double bond this contain less number of hydrogen. The general formula is given by

R-CH-CH_{2Unsymmetrical alkene}+HX_{Halogen Acid}→RCHXCH_{3Major Product (2-Haloalkane)}

Anti-Markownikoff’s Rule: When unsymmetrical haloalkenes are treated with unsymmetrical reagent which is HX in the presence of peroxide (Na₂O₂) then the negative part of reagent is (-X) which get added to the unsaturated carbon of double bond this contain more number of hydrogen. The general formula is given by

R-CH=CH_{2Unsymmeterical Alkene}+HBr_{Unsymmetrical Reagent}Peroxide→RCH₂CH₂Br_{Major Product (1-Bromoalkane)}

The most important thing is that peroxide effect is shown only and only by the HBr meanwhile HCl and HI always is added according to Markownikoff’s rule.

Related Topics Link

• Markovnikov Rule
• Enantiomers
• Wurtz Reaction
• SN1 Reaction Mechanism
  

Halogen Reactions:

Finkelstein Reaction: The Finkelstein reaction is used for preparation of alkyl iodide. Alkyl bromide/chloride are heated with sodium iodide solution in Acetone(dry) and forms alkyl iodide. NaCl and NaBr are way lesser soluble in Acetone(dry) and it gets precipitated further these precipitates is removed by the filtration process along with the backward reaction is prevented.

R-X_{Alkyl halide}+NaI_{Sodium Iodide}Dry acetone→RI_{Alkyl Iodide}+NaX_{Sodium Halide}

Swarts Reaction: Similarly, the alkyl fluorides which can prepare by the Swarts reaction method by the reaction of Mercurous fluoride, Silver fluoride, Cobalt fluoride, or Antimony fluoride on alkyl chloride/bromide.

2R-X_{Alkyl Halide}+Hg₂F_{2Mercurous Fluoride}2RF_{Alkyl Fluoride (Fluoroalkane)} +Hg₂X₂

Substitution Reaction: When an atom or group of atoms are substituted or replaced from substrate the number of another atoms or groups are known as substitution reaction.

R-X_{Alkyl Halide}+Y^-_NucleophileR-Y_{Substituted Alkane}+X^-_{Halide Ion}

Where Y^- is nucleophile like OH^-, CN^-, NH₂^-, R-O^-, RCOO^-, I^-, etc.

Elimination Reaction: A reaction in which two atoms are removed from the adjacent carbon atom in a molecule which forms an unsaturated compound which is known an elimination reaction.

Williamson’s synthesis of Ethers: Alkyl halide is heated with the alkali alkoxide to give respective ether. In this reaction, halide group undergoes substitution with an alkoxy group (-O-R'). For example, sodium alkoxide can be prepared by action of sodium metal on alcohol.

R-X_{Alkyl Halide}+NaOR'_{Sodium Alkoxide}Heat→R-O-R_ether'+NaX

Physical Properties and Chemical Properties of Haloalkanes:

1. The lower elements are gases at the room temperature and the higher elements are solids or liquids.

2. The more volatile halogen compound is more sweet smell it has.

3. The boiling point of the Alkyl halide is always greater than respective hydrocarbons.

4. Since the polarity of Carbon-halogen bond along with dipole-dipole-London force and Van der Waal’s between the molecules of haloalkanes are stronger thus there is increase in boiling point.

Haloarenes

Haloarenes definition:

What is Haloarenes?

When one or more than one hydrogen atoms of an aromatic hydrocarbon is substituted by the respective number of halogen atom and the major product form from the reaction is called as haloarenes. The general formula for haloarenes meaning is given by Ar-X, where Ar is aryl group and X is halogen i.e., F, Br, Cl, I. Aromatic hydrocarbons are known as arenes. Also, Haloarenes contain halo alkane atoms attached to sp² hybridized carbon atom.

Classification of Halogen derivatives:

haloalkanes and haloarenes flow chart

Monohaloarenes: When one hydrogen atom of arene is replaced by one halogen the following compound formed is known as monohaloarenes. Example: Chlorobenzene, Bromobenzene.

1. Dihaloarenes: When two hydrogen atoms of arene is replaced by two halogen the following compound formed is known as Dihaloarenes. Example: 1,2-Dichlorobenzene, 1,4-Dibromobenzene.

2. Polyhaloarenes: When two or more hydrogen atom of arene is replaced by same number of halogens, the following compound formed is known as Polyhaloarenes. Example: 1,3,5-Tribromobenzene (Trihaloarene), 1,2,3,5-Tetrachlorobenzene (Tetrahaloarene).

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• NCERT solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes
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• NCERT notes Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes

Nomenclature of Haloarenes:

1. Aryl halides are named by prefixing the 'halo' to name of parent aromatic hydrocarbon.

2. For Dihalogen derivative, the prefixes o-, m-, p- are used in common system but in IUPAC system, numerical prefix (1, 2) (1, 3) and (1, 4) are respectively used.

3. In case, parent aromatic hydrocarbon carries a side chain or substituent, then numbering of carbon atoms begins with carbon atom attached to halo atom.

4. However, polyhalogen derivatives do not have common names (Except three groups in symmetrical position, then suffix `sym' is used) but only have IUPAC names in which positions of halogens are indicated by Arabic numerals.

Nature of Carbon-Halogen bond in Haloarenes:

In haloarenes, lone pairs of electrons on the halogen atom is in conjugation with the electrons of benzene ring. In case of chlorobenzene, lone pair of electrons from chlorine atom as well as six electrons from carbon atoms of ring are thereby associated to all seven atoms (six carbon atoms and one halogen atom). The delocalization of electrons gives the double bond character to the C—X bond.

Polyhalogen Compounds: Carbon compound which have more than one halogen atom it is known as polyhalogen compounds. Some of the Haloarenes example: p,p’-Dichlorodiphenyltrichloroethane (DDT); Freons (CCl₂F₂); Iodoform (CHI₃); Tetrachloromethane CCl₄; Trichloromethane (CHCl₃); Dichloromethane (CH₂Cl₂).

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NCERT Chemistry Notes:

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