Question

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

Answer 1

Hint: A hint for each part of the question.
Think about what the mesomeric effect is and how it works in stabilising/destabilising aryl halides.
Water possesses very strong hydrogen bonding between its atoms.
What happens when Grignard reagents react to moisture? How does this affect their usability?

Step-by-Step Solution:
Let us go through each of these questions one at a time and arrive at an eventual conclusion.

A) The Cl-atom in chlorobenzene is linked to a \[s{{p}^{2}}\] hybridized carbon atom while in cyclohexyl chloride, the Cl-atom is linked to a \[s{{p}^{3}}\] hybridized carbon atom. Now, the \[s{{p}^{2}}\] hybridized carbon has more s-character than the \[s{{p}^{3}}\] hybridized carbon atom. Therefore, the former is more electronegative than the latter making the density of electrons of C - Cl bond near the Cl-atom less in chlorobenzene than in cyclohexyl chloride.
Moreover, the - R effect of the benzene ring of chlorobenzene decreases the electron density of the C - Cl bond near the Cl-atom. As a result, the polarity of the C - Cl bond in chlorobenzene decreases. Hence, the dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.

B) As the hint states, to be miscible with water the solute-water force of attraction must be stronger than the solute-solute and water-water forces of attraction.
Now, alkyl halides are polar molecules and so held together by dipole-dipole interactions. Similarly, strong hydrogen bonding exists between the water molecules. Therefore, the new force of attraction that exists between the alkyl halides and water molecules is weaker than the alkyl halide-alkyl halide and water-water forces of attraction. Therefore, even though alkyl halides are polar, they are still immiscible with water.

C) Grignard reagents should be prepared under anhydrous conditions because Grignard reagents are highly reactive. In the presence of moisture, they react to give alkanes.
\[\begin{align}
  & \text{ }RMgX\;\;\;\;+\text{ }{{H}_{2}}O\text{ }\;\;\;\longrightarrow \text{ }R\text{ }-\text{ }H\text{ }+\text{ }Mg\left( OH \right) \\
\end{align}\]
$\text{(Grignard}\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\text{ (Alkane)}$
$\text{reagent)}$

Note: The knowledge of the effect of mesomeric effect and the order of stability depending on different constituents is very important to solve the first part of the question.
Remember how strong Hydrogen bonds really are and what effects they can have on any possible solute when trying to solve the second part.
And finally, for the third part, be very mindful of the formula of the Grignard reagent when trying to come up with the chemical equation which governs the final reaction.