Question

Explain why (i) the dipole moment of chlorobenzene is lower than that of cyclohexyl chloride? (ii) alkyl halides, though polar, are immiscible with water? (iii) Grignard reagents should be prepared under anhydrous conditions?

Short answer

(i) Chlorobenzene has lower dipole moment due to electron delocalization. (ii) Alkyl halides are immiscible in water due to weak dipole interactions compared to water’s hydrogen bonds. (iii) Grignard reagents require anhydrous conditions to prevent decomposition by water.

Step-by-step solution

Understanding Dipole Moments

The dipole moment is a measure of the separation of positive and negative charges in a molecule. Cyclohexyl chloride has a higher dipole moment than chlorobenzene because, in chlorobenzene, the delocalization of electrons due to the aromatic system reduces the electron-withdrawing ability of the chlorine atom. As a result, chlorobenzene has a lower dipole moment.

Intermolecular Forces and Polarity

Alkyl halides are polar molecules due to the presence of a significant dipole from the carbon-halogen bond. However, they are immiscible with water because the dipole-dipole interactions between alkyl halides are weaker than the strong hydrogen-bonding interactions between water molecules. The energy required to break the water hydrogen bonds is not compensated by the weaker dipole interactions with alkyl halides.

Water's Effect on Grignard Reagents

Grignard reagents are highly reactive organometallic compounds that act as strong bases. If water is present, it will react with the Grignard reagent, resulting in its decomposition into an alkane and losing its reactivity. Therefore, Grignard reagents must be prepared and handled under strictly anhydrous, or water-free, conditions to avoid this side reaction.

Key concepts

Dipole Moments

Dipole moments occur in molecules where there is an uneven distribution of electric charge. This happens because different atoms in a molecule have different electronegativities, pulling the shared electrons unevenly and creating positive and negative charge regions. In cyclohexyl chloride, the carbon-chloride bond has a simple polar character, meaning the electron cloud is more substantially pulled towards chlorine. In contrast, chlorobenzene's situation is a bit more unique. Chlorine is attached to a benzene ring, which is an aromatic system.
This type of system allows for delocalization of electrons, which diminishes the strict polar character you see in simple systems. Consequently, chlorobenzene has a lower dipole moment than cyclohexyl chloride because the aromatic ring decreases the ability of the chlorine atom to withdraw electrons effectively. In simple terms, cyclohexyl chloride has no electron-sharing ring to dilute its polarity, making its dipole moment more pronounced.

Alkyl Halides

Alkyl halides are a class of compounds where a halogen atom, such as chlorine, bromine, or iodine, is bonded to an alkyl group. These compounds are polar due to the significant dipole moment originating from the carbon-halogen bond. Nonetheless, you'd find them immiscible with water. This might seem counterintuitive initially, but it all boils down to intermolecular interactions.
Water molecules exhibit strong hydrogen bonding—a very intense form of dipole-dipole interaction—whereas the dipole-dipole interactions in alkyl halides are noticeably weaker. When you mix alkyl halides with water, the energy required to break the strong hydrogen bonds between water molecules is not compensated for by the establishment of weaker interactions with the alkyl halides.

• This means that alkyl halides will separate from water rather than mix with it because the system prefers to maintain the strong, stable hydrogen bonds already present in water.

Grignard Reagents

Grignard reagents are special and highly versatile compounds used widely in organic synthesis. They are made by reacting an alkyl or aryl halide with magnesium, and they act as strong bases and nucleophiles. Because of their high reactivity, Grignard reagents are particularly sensitive to moisture.
When Grignard reagents come into contact with water, they react aggressively to form an alkane and a hydroxide magnesium salt, effectively rendering them inactive. This sensitivity to water means they must be produced under strictly anhydrous conditions.

• "Anhydrous conditions" effectively means everything is dry, and care is taken to keep water away during the preparation and use of these reagents.
• In essence, Grignard reagents need a water-free environment to maintain their reactivity for subsequent chemical reactions, as even a small amount of water can spoil their intended activity.

Understanding this aspect helps preserve their efficacy in synthesizing different organic molecules.