Intro to Addition Reactions: Videos & Practice Problems

Alkenes and alkynes are known for undergoing addition reactions, where atoms are added to pi bonds, resulting in the formation of new sigma bonds as double or triple bonds are broken. There are three main types of addition reactions: halogenation, hydrogenation, and hydrohalogenation. Halogenation involves adding halogens to the pi bonds, forming dihalides. Hydrogenation adds hydrogen atoms, converting alkenes into alkanes. Hydrohalogenation introduces a hydrogen and a halogen to an alkene, yielding alkyl halides. The number of moles of reagent required for these reactions corresponds to the number of pi bonds present; each pi bond requires one mole of reagent. Remember, a double bond consists of one sigma and one pi bond, while a triple bond consists of one sigma and two pi bonds, with the sigma bond being a constant feature.

Addition Reactions

Alkenes and alkynes are known to undergo addition reactions, which involve the addition of atoms to pi bonds, leading to the breaking of double or triple bonds. In these reactions, pi bonds are broken, and new sigma bonds are formed. There are three primary types of addition reactions: halogenation, hydrogenation, and hydrohalogenation.

Halogenation involves the addition of halogens to the pi bonds of alkenes. For instance, when an alkene with two carbon atoms is treated with halogens, one of the pi bonds is broken, allowing the halogens to attach to the carbon atoms, resulting in a compound known as a dihalide.

Hydrogenation, on the other hand, is the process of adding hydrogen atoms to the pi bond of an alkene. This reaction transforms the alkene into an alkane, as the addition of hydrogen saturates the molecule, maintaining the other hydrogen atoms already present.

Hydrohalogenation is slightly different, as it involves the addition of a hydrogen atom and a halogen atom (HX, where X can be bromine or chlorine) to the double bond. This reaction produces an alkyl halide. The specific distribution of hydrogen and halogen across the double-bonded carbons will be determined by certain rules that will be explored later.

It is important to note that one mole of reagent is required for each pi bond present. For example, a triple bond, which contains two pi bonds, would necessitate two moles of reagent, while an alkene with a single pi bond requires only one mole. A double bond consists of one sigma bond and one pi bond, while a triple bond comprises one sigma bond and two pi bonds. The sigma bond remains constant, with the number of pi bonds varying based on the type of bond present.

In summary, these three types of addition reactions—halogenation, hydrogenation, and hydrohalogenation—are fundamental processes that occur in alkenes and alkynes, facilitating the transformation of these unsaturated hydrocarbons into more saturated forms.

Intro To Addition Reactions Example

In organic chemistry, an addition reaction is a process where two or more reactants combine to form a single product, typically involving alkenes and alkynes. These reactions are characterized by the breaking of π (pi) bonds in the unsaturated hydrocarbons, allowing new atoms or groups to be added to the molecule.

For instance, consider the reaction of an alkene with hydrochloric acid (HCl). In this case, the alkene undergoes hydrohalogenation, where the π bond is broken, and a hydrogen atom (H) and a chlorine atom (Cl) are added across the double bond. This results in the formation of a haloalkane. The general representation of this reaction can be expressed as:

Alkene + HCl → Haloalkane

In contrast, if a reaction retains the π bond or involves the formation of an alkene from an alcohol, it does not qualify as an addition reaction. Therefore, when identifying addition reactions, it is crucial to ensure that the π bond is sacrificed and new atoms are incorporated into the structure.

In summary, the correct representation of an addition reaction among the options provided is when an alkene reacts with hydrochloric acid, resulting in the addition of H and Cl across the double bond, exemplifying the concept of hydrohalogenation.

How many moles of reagent are needed for the addition of the following alkyne?

• A. 2 moles
• B. 3 moles
• C. 1 mole
• D. 4 moles

Answer: (A)

Here’s what students ask on this topic:

What is an addition reaction in organic chemistry?

An addition reaction in organic chemistry is a type of chemical reaction where two or more molecules combine to form a larger molecule. This process involves the addition of atoms or groups of atoms to a molecule, typically across a double or triple bond in an unsaturated compound, such as an alkene or alkyne. During the reaction, the pi bond (π bond) in the unsaturated compound breaks, and new sigma bonds (σ bonds) are formed, resulting in a saturated molecule.

For example, in the hydrogenation of ethene (ethylene), hydrogen molecules add across the carbon-carbon double bond of ethene to produce ethane, a saturated hydrocarbon. This reaction is typically catalyzed by metals such as palladium, platinum, or nickel.

Addition reactions are fundamental to organic synthesis and are widely used in the production of polymers, pharmaceuticals, and many other chemical products. They are characterized by their regioselectivity (where the new bonds form) and stereoselectivity (the spatial arrangement of the new atoms), which are critical factors in determining the properties of the resulting compound.

What is an addition reaction in which two halogen atoms are added to an unsaturated organic molecule to produce a saturated alkyl halide containing two halogens?

An addition reaction where two halogen atoms are added to an unsaturated organic molecule, resulting in a saturated alkyl halide containing two halogens, is known as a halogenation reaction. Specifically, when the reaction involves adding two identical halogen atoms, it's referred to as vicinal dihalogenation.

In this process, an alkene or alkyne, which are types of unsaturated hydrocarbons with double or triple carbon-carbon bonds respectively, react with a diatomic halogen molecule such as chlorine (Cl₂) or bromine (Br₂). The pi bond (π bond) of the unsaturated hydrocarbon is broken, and each halogen atom forms a single bond (σ bond) with each of the two carbon atoms that were originally part of the double or triple bond. This results in a vicinal (neighboring) dihalide, where the term 'vicinal' refers to the halogen atoms being attached to adjacent carbon atoms in the resulting saturated molecule. This reaction is typically characterized by its regioselectivity and can be used to test for the presence of unsaturation in a molecule, as the color of the halogen will typically disappear upon reaction.