Intro to Addition Reactions: Study with Video Lessons, Practice Problems & Examples

Alkenes and alkynes participate in addition reactions, where pi bonds break to form new sigma bonds. The three main types are halogenation, hydrogenation, and hydrohalogenation. Halogenation adds halogens, creating dihalides; hydrogenation adds hydrogens, converting alkenes to alkanes; and hydrohalogenation introduces hydrogen and a halogen, forming alkyl halides. Each pi bond requires one mole of reagent, while triple bonds need two. Understanding these reactions is crucial for grasping organic chemistry concepts, particularly in the context of hydrocarbons and their transformations.

Addition Reactions Concept 1

Now we can say here that alkenes and alkynes undergo addition reactions. Now what exactly is an addition reaction? Well, this is the addition of atoms to pi bonds, which results in double or triple bonds breaking. So double or triple bond breakage. Here we're going to say that Pi bonds are broken, and new sigma bonds are formed. Here we're going to say there are 3 major types of addition reactions, and they are halogenation, hydrogenation, and hydrohalogenation. So here we take a look at additional reactions, we have here halogenation and hydrogenation. Halogenation just means that we're adding halogens to our Pi bonds. If we take a look here, we have our alkene, we have a carbon here and a carbon here, remember carbon must make 4 bonds. What happens is we break one of these Pi bonds, and we use that bond to help us attach these 2 halogens to my used-to-be alkene. In the process, we make what's called a dihalide. In hydrogenation, we're adding hydrogens to our Pi bond. So here, these 2 hydrogens get added, we still have these other hydrogens attached, so they're still there. We're more focused on what’s our new overall compound. So adding hydrogens here through hydrogenation creates what? An alkane. Then if we go to hydrohalogenation, we put this a bit different because we're not adding the same two groups to the double bond. Here we have our alkene and we're using HX. So here we have hydrogen and a halogen, the halogen here is bromine or chlorine. What happens is that we have the hydrogen and halogen adding to create an alkyl halide. Later on, we'll learn specific rules to tell us which double bonded carbon gets the hydrogen and which one gets the halogen. But for right now just realize that hydrohalogenation adds a hydrogen, which is the hydro part, and a halogen, which is the halogen part, to create an alkyl halide. Now, here we're going to say, we have 1 mole of reagent needed for every Pi bond. So if we had a triple bond, we'd need 2 moles of the reagent. An alkene only has 1 Pi bond, so we only need 1 mole. Now, remember, a double bond is composed of 1 sigma bond and 1 Pi bond. A triple bond consists of 1 sigma bond and 2 Pi bonds. The Sigma bond is always going to be there. It's just the number of Pi bonds that are increasing. Right? So just remember, we have these three types of additional reactions that are pretty common amongst alkene species and alkyne species.

Intro to Addition Reactions Example 1

Which of the following represents an addition reaction? So remember, at this point we're talking about addition reactions in respect to alkenes and alkynes. We're adding things to these alkenes and alkynes by sacrificing their π bonds. If we take a look at the first one, we have a π bond here, but it's not of an alkene or an alkyne, and it's still there at the end, so this would not be our type of addition reaction. Next, we have an alcohol, and we're using some type of acid and heat, and it actually creates an alkene. Remember, addition reactions were getting rid of the π bond to add things to our structure. The answer is c, because in c, what do we have? We have an alkene, we're adding hydrochloric acid to it. We see that an H got added here, and a chlorine got added to the other alkene carbon. This represents hydrohalogenation. This would be our type of addition reaction we've covered so far. Alright. So here our final answer will be option c.

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

Here’s what students ask on this topic:

What are the main types of addition reactions in organic chemistry?

The main types of addition reactions in organic chemistry are halogenation, hydrogenation, and hydrohalogenation. In halogenation, halogens (like Cl₂ or Br₂) are added to the pi bonds of alkenes or alkynes, resulting in dihalides. Hydrogenation involves the addition of hydrogen (H₂) to pi bonds, converting alkenes to alkanes. Hydrohalogenation adds a hydrogen atom and a halogen atom (HX, where X is a halogen like Cl or Br) to the pi bond, forming alkyl halides. Each pi bond requires one mole of reagent, while triple bonds need two moles.

How does halogenation of alkenes work?

In halogenation, halogens such as Cl₂ or Br₂ are added to the pi bonds of alkenes. The process involves breaking the pi bond and forming new sigma bonds with the halogen atoms. For example, when an alkene reacts with Br₂, the double bond breaks, and each carbon atom of the former double bond forms a new bond with a bromine atom, resulting in a dihalide. This reaction is useful for synthesizing compounds with halogen substituents.

What is the difference between hydrogenation and hydrohalogenation?

Hydrogenation and hydrohalogenation are both addition reactions but differ in the reagents and products. Hydrogenation involves adding hydrogen (H₂) to the pi bonds of alkenes or alkynes, converting them into alkanes. In contrast, hydrohalogenation involves adding a hydrogen atom and a halogen atom (HX, where X is a halogen like Cl or Br) to the pi bond, forming alkyl halides. Hydrogenation results in saturated hydrocarbons, while hydrohalogenation produces halogen-substituted hydrocarbons.

Why do triple bonds require two moles of reagent in addition reactions?

Triple bonds consist of one sigma bond and two pi bonds. In addition reactions, each pi bond requires one mole of reagent to break and form new sigma bonds. Therefore, a triple bond, having two pi bonds, needs two moles of reagent to fully react. For example, in the hydrogenation of an alkyne, two moles of H₂ are required to convert the alkyne into an alkane, breaking both pi bonds in the process.

What is the product of hydrogenation of an alkene?

The product of hydrogenation of an alkene is an alkane. During hydrogenation, hydrogen (H₂) is added to the pi bond of the alkene, breaking the double bond and forming new sigma bonds with the hydrogen atoms. This process converts the unsaturated alkene into a saturated alkane, which has only single bonds between carbon atoms.