Question

Pentene-1 with \(\mathrm{HCl}\) gives (a) 3-chloropentane (b) 2 -chloropentane (c) 1,2 -dichloropentane (d) 1 -chloropentane.

Short answer

The product of the reaction between pentene-1 and HCl, according to Markovnikov's rule, is (b) 2-chloropentane.

Step-by-step solution

Understand the reaction

Identify the type of reaction. Here, pentene-1 (also known as 1-pentene) reacts with hydrochloric acid (HCl), which is a typical addition reaction where HCl adds across the double bond of the alkene.

Apply Markovnikov's rule

Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen atom (H) attaches to the carbon with the most hydrogens already attached, and the halogen (X) attaches to the carbon with fewer hydrogens. In 1-pentene, the double bond is between the first and second carbon.

Predict the product

Following Markovnikov's rule, the hydrogen atom from HCl will add to the first carbon (which has more hydrogens initially) and the chlorine (Cl) will add to the second carbon. The product formed is 2-chloropentane.

Key concepts

Addition Reactions

An addition reaction is a fundamental type of chemical reaction where two or more substances combine to form a single product. This type of reaction is characteristic of alkenes and alkynes, which contain double or triple bonds, respectively. During an addition reaction, the multiple bond between atoms breaks, and new bonds are formed, allowing the added atoms or groups to become part of the resulting molecule.

In the context of alkenes, such as pentene, the double bond is highly reactive and can readily break to allow new elements to attach. The case of pentene reacting with hydrochloric acid (HCl) exemplifies an addition reaction because the HCl molecule adds across the carbon atoms that were previously double-bonded. The products of such reactions depend on the structure of the alkene and the nature of the added molecule.

Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. The double bond consists of one \/\/ sigma (\(\sigma\)) bond and one pi (\(\pi\)) bond. The presence of this double bond provides a site for chemical reactivity, making alkenes far more reactive than their saturated alkane counterparts.

In the given exercise, pentene-1 (or 1-pentene) is an alkene with the double bond located between the first and second carbon atoms, counted from the end of the carbon chain. This positioning of the double bond is crucial as it influences the outcome of addition reactions. Alkenes, due to the position of their double bonds, can form multiple products when they undergo addition reactions. Therefore, understanding the behavior of alkenes and their reactivity patterns is critical for predicting the outcomes of such reactions.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to occur at one direction or position over another leading to the selective formation of one product when multiple outcomes are possible. This concept is exceptionally pertinent in the realm of addition reactions involving alkenes, where the orientation of the added groups in relation to the double bond matters greatly.

Markovnikov's rule is an essential principle related to regioselectivity. It predicts the outcome of the addition of protic acids (such as HCl, HBr, HI) to alkenes. According to this rule, the proton (H+) will add to the carbon with the most hydrogens (and consequently the least substituted carbon), whereas the halide (Cl-, Br-, I-) or other nucleophile will add to the more substituted carbon atom.

For example, in the addition of HCl to 1-pentene, following Markovnikov's rule, the chloride ion attaches to the second carbon, which has fewer hydrogen atoms attached initially, resulting in the formation of 2-chloropentane. This preference occurs because the more substituted carbocation intermediate is typically more stable, leading to the major product of the reaction. Understanding regioselectivity is key to accurately predicting the structural outcomes of addition reactions involving alkenes.