Question

Which of the following will react with water? (a) \(\mathrm{CHCl}_{3}\) (b) \(\mathrm{Cl}_{3} \mathrm{CCHO}\) (c) \(\mathrm{CCl}_{4}\) (d) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\)

Short answer

(b) \( \mathrm{Cl}_{3} \mathrm{CCHO} \) reacts with water to form chloral hydrate.

Step-by-step solution

Identify Reactivity With Water

To identify which compounds will react with water, we need to consider the types of interactions that are possible between the molecules and water. Most organic molecules with chlorine (Cl) as part of their structure do not react with water because Cl is already a stable atom in these compounds.

Examine (a) \( \mathrm{CHCl}_{3} \)

Chloroform (\( \mathrm{CHCl}_{3} \)) does not react with water under normal conditions. The structure is stable and the chlorine atoms have no significant driving energy to exchange places with water molecules.

Examine (b) \( \mathrm{Cl}_{3} \mathrm{CCHO} \)

This compound is chloral. Chloral reacts with water to form chloral hydrate, a reaction that involves the addition of water to the compound without breaking any carbon-chlorine (C-Cl) bonds. Therefore, (b) \( \mathrm{Cl}_{3} \mathrm{CCHO} \) does react with water.

Examine (c) \( \mathrm{CCl}_{4} \)

Carbon tetrachloride (\( \mathrm{CCl}_{4} \)) is a non-polar compound and does not react with water. The carbon in \( \mathrm{CCl}_{4} \) is fully saturated with chlorine atoms, preventing the reaction with water.

Examine (d) \( \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \)

1,2-Dichloroethane (\( \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \)) does not react with water. It is a stable molecule where chlorine atoms form part of the saturated backbone, providing no driving energy for a reaction with water.

Key concepts

Reactivity with Water

Understanding how certain organic molecules interact with water requires examining their molecular structure and stability. Water is a polar solvent, meaning it has positively and negatively charged sides due to the arrangement of its hydrogen and oxygen atoms. This makes water very effective at interacting with ionic or polar substances. However, not all organic molecules with chlorine atoms will react with water. In general, for an organic molecule to react with water, there must be a driving force, such as the formation of stronger bonds or the release of energy. If a chlorine-containing organic compound is stable and not reactive, it likely won't react with water under normal conditions. For example, chloroform (\(\mathrm{CHCl}_3\)) and carbon tetrachloride (\(\mathrm{CCl}_4\)) do not typically react with water because their chlorine-carbon bonds are stable and immersed in a non-polar environment.

Carbon-Chlorine Bond

The carbon-chlorine (C-Cl) bond is a significant factor in determining the reactivity of chlorine-containing organic molecules. A C-Cl bond is polar because chlorine is more electronegative than carbon, leading to an uneven distribution of electron density. This can influence how a molecule behaves chemically. However, mere polarity isn't enough to make a molecule react with water. The stability and reactivity of a C-Cl bond largely depend on the molecule's context. In \(\mathrm{CHCl}_3\) and \(\mathrm{CCl}_4\), these bonds are relatively inert in the presence of water because the overall molecular structure is stable and does not promote further reactions. Conversely, in a compound like chloral (\(\mathrm{{Cl}_3CCHO}\)), the structure can accommodate a reaction with water, forming chloral hydrate without breaking any C-Cl bonds.

Organic Molecules with Chlorine

Organic molecules containing chlorine atoms display a variety of chemical behaviors based on their specific structures. Chlorine atoms in these molecules typically form strong covalent bonds with carbon, which can lead to stability that resists reactions with common solvents like water. For instance, 1,2-dichloroethane (\(\mathrm{ClCH}_2 ext{--}\mathrm{CH}_2 ext{--Cl}\)) features chlorine atoms bonded along a saturated carbon chain, resulting in a stable molecule that does not interact with water. On the other hand, chloral is more chemically active with water due to its differing structure and reactivity dynamics. It's important to appreciate that while organic chlorine compounds vary in reactivity, the stability of each bond and the overall molecular environment both play crucial roles in determining how these compounds may interact with water or other substances.