CBSE Class 12 Chemistry-Alkyl Halides & Haloarenes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts will help the student to understand the chapter and score better marks in the examinations.
Class XII: Chemistry
Chapter 10: Haloalkanes and Haloarenes
1. Nature of C-X bond in alkyl halides: X is more electronegative than carbon. So, the C-X bond is polarized with C having a partial positive charge and X having a partial negative charge
2. Common and IUPAC names:-
Chemical formula Common Name IUPAC Name
CH2Cl2 Methylene chloride Dichloro methane
CHCl3 Methylene chloride Trichloro methane
CCl4 Carbon tetrachloride Tetracholro methane
CH3CH(Cl)CH3 Isopropyl chloride 2-Chloro propane
CH3CH(Cl)CH2CH3 Chloride sec-butyl 2-Chloro butane
CH2=CHCl Vinyl chloride Chloroethene
C6H5Cl Benzyl chloride Chlorobenzene
a) Chain isomerism
b) Position isomerism
6. Physical properties of haloalkanes: a. Solubility. Although haloalkanes are polar in nature, yet they are practically very slightly soluble in water. In order for a haloalkane to dissolve in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between water molecules. However Haloalkanes are not able to form hydrogen bonds with water and therefore, less energy is released when new attractions are set up between the haloalkane and the water molecules because these are not as strong as the original hydrogen bonds in water molecules. As a result, solubility of haloalkanes in water is low.
- Simple fluoro and chloroalkanes are lighter than water while bromides and polychloro devrivatives are heavier than water.
- With the increase in number of carbon atoms, the densities go on increasing.
- With the increase in number of halogen atoms, the densities go on increasing.
- The densities increase in the order: Fluoride < chloride < bromide < iodide
The density also increases with increasing number and atomic mass of the halogen.
c. Method and Boiling points:
Molecules of organic halogen compounds are generally polar. Due to the polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole – dipole and van der Waals) between the molecules are stronger in halogen derivatives of alkanes. As a result melting and boiling points of chlorides, bromides and iodides are considerably higher than those of the parent hydrocarbon of comparable molecular mass.
- Amongst themselves, the following trends are observed:
- For the same alkyl group the boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl > RF where R is an alkyl group. This is because with the increase in the size of the halogen, the magnitude of van der Waals force increase.
- In general, the boiling points of chloro, bromo and iodo compounds increase with increase in the number of halogen atoms.
- For the same halogen atom, the boiling points of haloalkanes increase with increase in the size of alkyl groups.
- For isomeric alkyl halides, the boiling points decrease with branching. This is because branching of the chain makes the molecule more compact and, therefore,decrease the surface area. Due to decrease in surface area, the magnitude of van der Waals forces of attraction decreases and consequently, the boiling points of the branched chain compound is less than those of the straight chain compounds.
7. Physical Properties of Haloarenes:
a. These are generally colourless liquids or crystalline solids.
b. These are heavier than water.
c. Melting and boiling points of haloarenes:
• Melting and boiling points of haloarenes are nearly the same as those of alkyl halides containing the same number of carbon atoms.
• The boiling points of monohalogen derivatives of benzene are in the order: iodo > bromo > chloro > fluoro
• For the same halogen atom, the melting and boiling points increase as the size of the aryl group increases.
• The melting point of para isomer is quite higher than that of ortho or meta isomers.
This is due to the fast that is has symmetrical structure and there fore, its molecules can easily pack loosely in the crystal lattice. As a result intermolecular forces of attraction are stronger and therefore, greater energy is required to break its lattice and it melts at higher temperature.
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