Alkenes - Addition Reactions

Objective

State that in an addition reaction, only one product is formed.

Introduction to Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond ($C=C$). This double bond is the site of characteristic addition reactions.

The presence of the double bond makes alkenes more reactive than alkanes.

Addition Reactions: The Concept

Addition reactions involve the addition of atoms or groups of atoms across the carbon-carbon double bond in an alkene. The double bond breaks, and two new single bonds form.

A key characteristic of addition reactions is that only one product is typically formed. This is because the addition process proceeds in a concerted manner, meaning that the bond breaking and bond forming occur simultaneously.

Mechanism of Addition Reactions

Addition reactions generally follow a concerted mechanism. The π bond in the alkene breaks, and two new σ bonds form with the added reagent. This happens in a single step.

Example: Addition of Hydrogen Halides (HX)

The addition of a hydrogen halide (HX) to an alkene is a classic example of an addition reaction. The reaction follows Markovnikov's rule.

Markovnikov's Rule: In the addition of HX to an alkene, the hydrogen atom adds to the carbon of the double bond that already has more hydrogen atoms, and the halogen atom adds to the other carbon.

Table Summarizing Key Points

Aspect	Description
Reaction Type	Addition Reaction
Reactant	Alkene + Hydrogen Halide (HX)
Product	Haloalkane (addition product)
Number of Products	Only one product is formed.
Mechanism	Concerted (single step)
Rule	Markovnikov's Rule applies.

Why Only One Product?

The addition reaction proceeds through a single, concerted step. There is no intermediate state where multiple products could form. The breaking and forming of bonds occur simultaneously, leading to a single, well-defined product.