Question

The addition of \(\mathrm{HCl}\) to \(3,3,3\) -trichloropropene gives: (a) \(\mathrm{Cl}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) (b) \(\mathrm{Cl}_{2} \mathrm{CHCH}(\mathrm{Cl}) \mathrm{CH}_{2} \mathrm{Cl}\) (c) \(\mathrm{Cl}_{2} \mathrm{CHCH}_{2} \mathrm{CHCl}_{2}\) (d) \(\mathrm{Cl}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\)

Short answer

The product is (b) \(\mathrm{Cl}_{2} \mathrm{CHCH}(\mathrm{Cl}) \mathrm{CH}_{2} \mathrm{Cl}\).

Step-by-step solution

Identify the Reactants

The reactants given are 3,3,3-trichloropropene (an alkene with a double bond) and hydrochloric acid (HCl). The double bond is between the second and third carbon atoms in the propene molecule.

Determine Markovnikov's Rule Application

Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen (H) will attach to the carbon with the most hydrogen atoms already present, and the halogen (Cl) will attach to the carbon with fewer hydrogen atoms.

Add HCl by Markovnikov's Rule

Following Markovnikov’s rule, the hydrogen from HCl will add to the carbon (C-2 in this case) with more hydrogens, while the chlorine will add to the carbon with fewer hydrogens (C-3).

Write the Product

According to Markovnikov’s rule, the reaction results in the formation of the product with chlorine on the more substituted carbon atom, which here is the terminal C-3 of trichloropropene. Option (b) matches this analysis.

Key concepts

Electrophilic Addition

Electrophilic addition is a fundamental reaction mechanism in organic chemistry that often involves the interaction between an electron-rich and an electron-poor species. In this process, a species with a strong affinity for electrons—an electrophile—attacks an unsaturated compound, typically an alkene or alkyne.

• The electron-rich double bond in an alkene serves as a target for the electrophile. This bond is composed of \( ext{sp}^2\) hybridized carbon atoms, making it more reactive.
• Upon initial attack, the electrophile forms a bond with one of the carbon atoms in the double bond, resulting in a carbocation intermediate, a positively charged and highly reactive species.
• Subsequently, a nucleophile, which has a high electron density, attacks this carbocation, completing the addition.

In the reaction between 3,3,3-trichloropropene and hydrochloric acid (HCl), the HCl molecule acts as an electrophile. The hydrogen is attracted to the electron-rich double bond, initiating the electrophilic addition.

Hydrochlorination

Hydrochlorination involves the addition of hydrochloric acid (HCl) to unsaturated organic compounds like alkenes. In this reaction, both components of HCl, namely hydrogen (H) and chloride (Cl), add across the double bond of the alkene.
Key aspects to note are:

• Following Markovnikov’s Rule, the hydrogen atom will attach to the carbon in the double bond that originally has more hydrogen atoms. This is the result of stabilizing the intermediate carbocation that forms momentarily during the reaction.
• The chlorine atom will, therefore, add to the other carbon atom that carries fewer hydrogens, making the final product more stable through this selective addition.

In our original exercise, when hydrochloric acid reacts with 3,3,3-trichloropropene, the hydrogen will primarily bond to the second carbon (C-2), while chlorine will attach to the third carbon (C-3). The outcome is a distinct molecule with chlorine at the more substituted site in line with Markovnikov’s Rule.

Alkene Reactivity

The reactivity of alkenes is largely defined by the presence of their carbon-carbon double bond. This double bond is not only a site of lower energy due to electron-rich \(\pi \) electrons but also a site ready for chemical attack, primarily by electrophiles.
Here's why alkenes are particularly reactive:

• The \(\pi \) bonds present in the double bond of an alkene are less stable than \(\sigma \) bonds, making them relatively easier to break and more reactive.
• Alkenes are unsaturated, meaning they have additional capacity for bonding that saturated alkanes do not possess. This gives them the potential for forming new molecules by addition reactions.
• Substituents attached to the double-bonded carbons in alkenes can influence reactivity. The more substituted an alkene, the more stable and faster the reaction rate, due to better stabilization of the carbocation intermediate.

In the given exercise, 3,3,3-trichloropropene's reactivity with HCl is a testament to the alkene's ability to easily form a new product through the hydrochlorination process.