Greetings to everyone! Haloalkanes, those intriguing compounds with halogen buddies hitching rides on carbon atoms, are like the undercover heroes of organic chemistry. These are also known as alkyl halides and are a class of organic compounds characterized by the presence of halogen atoms (fluorine, chlorine, bromine or iodine). They have halogen atom or atoms attached to a sp3 hybridized carbon atom of an alkyl group. They're found in everything from medicines to plastics, quietly playing crucial roles in countless everyday items. Today, we shall learn about their properties and see some of their methods of preparations.
Properties of Haloalkanes
Physical properties
Alkyl halides have a higher boiling point than those of hydrocarbons with comparable molecular mass. This is due to the strong intermolecular attraction resulting from the polarity of the alkyl halide. For the same alkyl group, the boiling point (bp) of alkyl halides decreases in the order R-I > R-Br > R-Cl > R-F. This is because as the size and mass of halogen atoms decrease, the van der Waals forces decreases. In polyhalogen compounds, the boiling point depends on the molecular weight and varies in the following order: CCl4 > CHCl3 > CH2Cl2 > CH3Cl. The boiling points of isomeric haloalkanes decrease as the number of side chains increases.
Isomeric dihalobenzenes have nearly the same boiling points. However, due to symmetrical structure of the para isomer, they get packed more compactly and hence have higher melting points than the ortho and the meta isomer.
Haloalkanes are not soluble in water as they are unable to form H-bonds.
Chemical Properties
Haloalkanes are no wallflowers when it comes to chemical reactions. They're game for swaps, kicks, and turns, participating in a dance of substitutions, eliminations, and cozying up to metals to form special compounds. Their behavior depends on who they're dancing with and where they are, making them the life of the organic chemistry party.
The C-X bond haloalkanes is polar with partial positive charge on carbon and partial negative charge on halogen (halogens are more electronegative than carbon). Any nucleophile stronger than the nucleophilicity of halide ion can attack at the electropositive carbon atom causing nucleophilic substitution reactions.
Being weak bases, halide ions are stronger bases and hence are good leaving groups. Thus they can undergo elimination reactions with a strong base.
Methods of Preparations
Halogenation of Alkanes: One of the primary methods for preparing haloalkanes is the halogenation of alkanes. This process involves the substitution of one or more hydrogen atoms in an alkane molecule with halogen atoms in the presence of a halogenating agent such as chlorine or bromine. The reaction is typically carried out under UV light or heat to initiate the substitution reaction.
Hydrohalogenation of Alkenes: Haloalkanes can also be synthesized via the addition of hydrogen halides (HCl, HBr, HI) to alkenes, a process known as hydrohalogenation. In this reaction, the pi bond of the alkene is broken, and the halogen and hydrogen atoms are added across the double bond to form a haloalkane. The reaction follows Markovnikov's rule, where the hydrogen atom adds to the carbon atom with the most hydrogen substituents.
From Alcohols: Haloalkanes can also be synthesized from alcohols through substitution reactions. In this method, an alcohol reacts with a hydrogen halide (HCl, HBr, HI) in the presence of a Lewis acid catalyst such as zinc chloride (ZnCl2) to form a haloalkane and water. The reaction follows an SN1 or SN2 mechanism depending on the structure of the alcohol and reaction conditions.
From Carboxylic Acids: Haloalkanes can be obtained from carboxylic acids through the reaction with thionyl chloride (SOCl2) or phosphorus pentachloride (PCl5). These reagents convert the carboxylic acid into an acyl chloride intermediate, which then undergoes nucleophilic substitution to form the corresponding haloalkane.
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