Alkane Reactions

The saturated hydrocarbons referred to as alkanes because of their low polarity and not very reactive. 

Reactions of Alkanes

Concept: Major Reactions of Alkanes

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Video Transcript

Hey guys! In this new video, we're going to take a look at alkane reactions. We’re going to say alkanes are referred to as paraffin, which is derived from Latin, meaning “little affinity”. Here when I say little affinity, that means that they're not very reactive that's because alkanes are just hydrocarbons. Alkanes have no pi bonds, no electronegative elements anywhere, so it's nonpolar. Its main intermolecular force is London dispersion, a non-polar force. When it comes to chemistry and reactions, reactions happen best when there is some polarity in the compound.
We’re going to say that alkanes really only undergo two major types of reactions, one that we've seen from the very beginning of chemistry – combustion. Remember in combustion, our alkane or hydrocarbon reacts with O2 to produce CO2 and water. We could have methane gas reacting with O2 gas to produce H2O liquid plus CO2 gas. Here we just need to balance it. We have four hydrogens here, but only two here so we’d throw a 2. Then we’d have 2 times one, that’s 2 oxygens plus another 2 here is 4 so I’d throw a 2 here. That one, we’re accustomed to see.
The new one that's connected to Organic Chemistry, it's called radical catalyzed halogenation. What in the world does that mean? In radical catalyzed halogenation, we have an alkane, a simple alkane. Let's say we have that methane. What we do with this methane is we use X2 over heat or light, and not just any type of light, UV light. This UV light can be represented by a symbol HV. When you see HV, it just means UV light. When I say X2, X equals Cl or Br.
What's happening here is that we're transforming our alkane into a new compound. In this reaction, what happens is a hydrogen is replaced with a halogen. One of these hydrogens, one of the four hydrogens, will get replaced with let's say Cl and we get this new compound. This compound now is polar because it has chlorines involved. Chlorine is very electronegative. The thing about this is that this reaction can continue forth even more. We could bring in another mole of X2, so more heat, so more light, and that would replace another hydrogen. We could keep bringing in more X2, more Cl2, more heat, more light to replace another and then do it one more time.
At the end, what we'll have is CCl4 as our compound. You go from methane to methyl chloride to methylene chloride. Remember I told you earlier that CH2 is called methylene. Methylene chloride to chloroform to finally, carbon tetrachloride. This reaction is just a way of replacing hydrogens with halogens. You could either replace just one or you just do mono halogenation or you could just pump in a ton of Cl2 or Br2, a ton of heat, a ton of light and replace all the hydrogens on the carbons with halogens. That's much harder to do. Usually, this reaction only replaces one hydrogen with a halogen. But this would be the organic type of reaction that we’re accustomed to see. Combustion is a basic type, more Gen Chem-specific. This here is more Organic-specific. Remember, these are the two major reactions that any alkane can undergo.

One of the major reactions of alkanes is one you should be familiar with: Combustion

The other alkane reaction is a new one called Radical Catalyzed Halogenation, in which a halogen replaces one of the hydrogens in the alkane. 

Free Radical Chain Reactions

Concept: Understanding Free Radical Chain Reactions

6m
Video Transcript

Hey guys! Let's continue in this new video on talking about radical catalyzed halogenation. Free radical chain reaction or radical catalyzed halogenation, all its really doing here, we're going to say under this reaction a hydrogen from an alkane is replaced by a halogen. The halogen that we usually use is either bromine or chlorine. Iodine is too slow and fluorine is way too reactive. If fluorine tried to form a bond, the compound would actually explode.
We're going to say here that in this radical chain reaction, alkanes will react with diatomic halogens, so Cl2 or Br2 in the presence of heat or UV light. We're going to say here that when it comes to this process, there are three basic steps to a radical chain reaction. The first step is initiation. Here, a stable compound undergoes homolytic cleavage. Homolytic cleavage just means, let's say we're dealing with Br. Remember Br halogens when not in the center makes single bonds to one another. In homolytic cleavage, the bond between them breaks evenly, each of them brought to the table one electron. In homolytic cleavage, that bond breaks and each halogen leaves with the electron it came in with. We go from having zero radicals to having two radicals at the end.
What's a radical? A radical is when you have an electron that is not paired. It's just a lone electron. Where's our radical electron? This is our radical electron and this is our radical electron. Electrons normally are found paired up like all these examples here, they're paired up. But a radical reaction, a radical itself doesn't have an electronic to pair up with. It’s just by itself. These are extremely dangerous and very reactive compounds. You hear talks in the news about free radicals how they damage your skin. This is what they're talking about. The sun actually because of the UV rays, UVA and UVB, cause a mutation on your skin where you create free radicals on the surface. This can lead to cancers. It can lead to a lot of different maladies. This is what they're talking about.
In propagation, also called the chain reaction step, so when you hear talks of chain reactions, they’re talking about the propagation step. One of these radicals here basically reacts with our alkane CH4. We have a radical, a stable compound. They react together to form a new radical and a new stable compound. Basically what happens here is that Br comes in and basically kicks out one of the hydrogens.
Now it's CH3Br plus the H I kicked out.
We created a new normal compound and a new radical, that is propagation. In this step, this can happen multiple times. We can say that this happens multiple times until every single hydrogen would be replaced with a Br. It's hard to do this and usually we just do one monohalogenation. We just usually replace one hydrogen with one halogen. But it is possible to try to replace every single hydrogen here with a Br to wind up with CBr4 at the end. But here if that did happen, your professor would make it clear. They would say, “Replace all the hydrogens with halogens.”
Then finally we have the last step of a radical reaction which is called termination. Termination is the opposite of initiation. In initiation, we have one compound breaking up into two radicals. In termination, we have the opposite. Two radicals combine together to form a new stable compound. This H radical that got ejected and remember, we only use one of these Br radicals. There's still another one floating around. That H radical and that Br radical could somehow find each other and form HBr. You have two radicals here, each with one unpaired electron that desperately needs to be paired up. What do they do? They just meet up and have their electrons form a bond to one another, and so now they're stable.
These are the steps of any radical reaction – initiation, propagation, termination. It's important to know what their terms mean and also what's most important, what does each one forms. In initiation we form two radicals. In propagation we form a radical and a new compound. In termination we get rid of all radicals to create one new stable compound. Just remember this.
On the next page, we’ll take a look at different types of radical chain reactions. Remember, all that happens here is we're going to do monohalogenation on the following questions. We're going to replace a hydrogen with one halogen based on the number of types of hydrogens present. Take a look at the next questions and then we'll go over them together. Click on the next video and see how I tackle each one.

In a free radical chain reaction an alkane reacts with Br2 or Cland UV light or heat. 

Example: Determine the major product(s) of the following reaction. 

2m
Video Transcript

Hey guys! Let's take a look at the examples dealing with radical chain reactions. Here I say: Determine the major product or products of the following reaction. Here what we’re going to see is we're just going to do monohalogenation. We're just replacing one hydrogen. Both of these are CH3, so they're both the same, meaning that I could replace an H from this end or an H from this end and it would be the same exact thing.
Here we’d only have one product. I decided to replace it from the end on the right. But again, if we did it on the one on the left, it’d be the same exact thing. In this reaction, we replaced one hydrogen with a halogen. That H that got replaced combines with the extra Br that’s hanging around. Technically those will be your two products at the very end. Remember in monohalogenation, we’re looking for the different types of hydrogens present and replacing them with a halogen.
In this one, I want you guys to attempt this one. Look to see how many different types of hydrogens are present. If there's more than one type of hydrogen present, that means you're going to make a mixture of products, more than one possible final product. If that does exist, show me what those products would look like. Come back, take a look and see how I tackled this question. But first, attempt it on your own.

Example: Determine the major product(s) of the following reaction. 

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