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Nature of C-X bond and Physical Properties, Reaction with PCl5, PCl3, SOCl2 and HX is considered one of the most asked concept.
47 Questions around this concept.
For the reaction
The correct statement is
Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge. As we go down the group in the periodic table, the size of halogen atom increases. Fluorine atom is the smallest and iodine atom is the largest. Consequently, the carbon-halogen bond length also increases from to
.
Physical properties of Haloalkanes and Haloarenes:
(1) These are less soluble in but more soluble in Organic solvents
(2) Their density follows the order:
Iodide > Bromide > Fluoride > Chloride
(3) Their boiling point follows the order:
Iodide > Bromide > Fluoride > Chloride
(4) Boiling point of Isomeric haloalkanes decreases with the increase in branching
(5) Boiling point of isomeric dihalobenzene is nearly the same and follows the order
para > ortho > meta
The silver(I) salts of carboxylic acids react with halogens to give unstable intermediates which readily decarboxylate thermally to yield alkyl halides. The reaction is believed to involve homolysis of the C-C bond and a radical chain mechanism. In this reaction, ester is formed as a by-product. The reaction occurs as follows:
For example:
Mechanism
Chain Initiation
Chain Propagation
Chain Termination
(major)
(minor)
(minor)
For example:
The rate of reaction with changing the alkyl group (R) in the above reaction varies as
It is to be noted that with , silver salt of carboxylic acid gives ester as the main product instead of alkyl iodide.
For example
The reaction occurs as follows:
The reaction occurs as follows
Finkelstein Reaction
Finkelstein's reaction is a method of preparation of alkyl iodides from alkyl chlorides or alkyl bromides. In this reaction, alkyl chlorides or bromides are treated with NaI in the presence of acetone to form alkyl iodides. The reaction occurs as follows:
We use NaI because it is soluble in acetone as it is covalent in nature. All other sodium halides are ionic in nature and thus not soluble.
For example:
Swarts Reaction
Halide exchange is also used for the preparation of alkyl fluorides by Swarts Reaction. Alkyl chloride/bromide is heated in presence of AgF, Hg2F2, CoF2 or SbF3 to give alkyl fluoride.
For example:
The reaction of alcohols ROH with PCl5 and PCl3 yields an alkyl halide RCl. The reactions of alcohols with PCl5, PCl3 and SOCl2 occurs as follows:
POCl3 and H3PO3 are generated in liquid phase and hence they are very hard to separate while SO2 and HCl are gases and thus they are easy to remove. Hence, for chlorination, we always use SOCl2 as the best option among the given reagents.
Mechanism
The reactions occurs as follows:
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