Intro to Addition Reactions - Online Tutor, Practice Problems & Exam Prep

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

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 π 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 three 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 four bonds.

What happens is we break one of these Pi bonds and we use that bond to help us attach these two 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 π bond. So here these two hydrogens get added. We still have these other hydrogens attached. So they're still there, but 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 one mole of reagent needed for every π bond. So if we had a triple bond, we'd need 2 moles of the reagent. An alkene only has one π bond, so we only need one mole.

Now remember a double bond is composed of 1 Sigma bond and one π bond. A triple bond consists of 1 Sigma bond and 2 π bonds. The Sigma bond is always going to be there, it's just the number of π bonds that are increasing, right? So just remember we have these three types of addition reactions that are pretty common amongst alkene species and alkyne species.

Intro To Addition Reactions Example

Which of the following represents an addition reaction? So remember, at this point we're talking about addition reaction in respect to alkenes and alkynes. We're adding things to these alkenes and alkynes by sacrificing their Pi bonds.

If we take a look at the first one, we have a 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 Pi 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 additional reaction we've covered so far, right? 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 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.