Alkane Reactions - Online Tutor, Practice Problems & Exam Prep

• Alkanes are the least reactive hydrocarbons, primarily undergoing only combustion and halogenation reactions.
• Combustion of alkanes involves reacting with oxygen to produce carbon dioxide and water.
• Halogenation is a substitution reaction where a halogen (bromine or chlorine) replaces a hydrogen atom in the alkane, facilitated by heat or UV light (represented as HV).
• During halogenation, diatomic halogen molecule (X2) splits into two atoms; one halogen atom substitutes a hydrogen atom in the alkane, forming an alkyl halide and releasing HX.
• Alkanes can undergo mono-substitution (one hydrogen replaced) or poly-substitution (multiple hydrogens replaced), depending on the specific conditions or requirements of the reaction.

Common Types of Alkane Reactions

Now, when it comes to alkane reactions, realize that alkanes are the least reactive of the hydrocarbons, and in fact they only do. Two major types of reactions are common types of reaction are combustion and halogenation. Now, combustion we know from Gen. Chem. We say here that the simplest type of combustion reactions involve a hydrocarbon reacting with oxygen produce carbon dioxide in water.

Now halogenation is something new. Well, this represents a substitution reaction where bromine or chlorine replaces one of the hydrogens on the alkane. Now here it requires heat or UV light. Here we represent UV light as hν. And what it does here is it it's to break the bond between halogens and that halogen then attaches itself to the alkane.

So if we take a look here, we have alkane halogenation reaction. Here we have our simple alkane in the form of methane. It's reacting with X2X, here again to represent bromine or chlorine. So this is Cl2 or Br2. Through the use of heat or hν ultraviolet light, we split this X2 into two pieces and then what happens is that one of them substitutes out this hydrogen. So now we'd have an X here. Oops, X here and then we still have an additional X hanging around and this H just got kicked out. So they would combine together to give us HX here.

Now when it comes to this, we're going to say that all Keynes can be mono substituted, and that just means only one hydrogen is substituted. Or they can be Poly substituted where more than one hydrogen is substituted O. Pay close attention to the question being asked of you. Are they asking for a mono product or are they asking for a Poly product? If depending on what they ask, you can get different results. In this particular one, we're doing a mono halogenation, we're just replacing one hydrogen with a halogen and in the end we make an alkyl halide as a product.

Alkane Reactions Example

Here it states what are the major products of the following halogenation reaction. Assume monosubstitution. All right. So if we take a look at this, we know that there's ACH₃ here and ACH₃ here. There is ACH₂ here and ACH₂ here. And then here we just have another Chapter 2. Now, you might notice that I'm trying to do some type of symmetry here. This is a pentane chain. It has symmetry in it. I'm showing that there's this red carbon on this end and this red carbon on this end. That's telling me that they are the same meaning.

Remember, in this type of reaction we're going to substitute out one hydrogen for chlorine. If I put the chlorine on either end, it give me the same exact product. Both of them will be called 1 chloral pentane. So let's actually show that, right? So remember this is just substituting a chlorine for one of the hydrogens and if we were to name this, this would be 123-451-2345. We start numbering from the end with the substituent in the form of chlorine. Both of these will be 1 chloral pentane since they give the same name that the same molecule. So just focus only on one of them. This would just be a redundancy.

Same thing with the ones in blue. If I put the chlorine on either one of them, they both would give Me 2 chloral pentane, so only show one of them. So this will be another major product. And then finally the one in the middle. If I put a chlorine on that one, that one's unique because putting one on that one would give me 3 chloral pentane. So here we'd get three major types of products, and and that's the way you're able to determine how many possible products you can make.

Add the chlorine to different positions and then name those molecules that you've created. If they have the same name, they may represent the same molecule. You don't need to show more than one. OK, unique names, equal unique products. OK, so here we'd have three major products from the monochlorination of pentane.

Determine the major product(s) of the following alkane reaction. (Assume monosubstitution.)

• A.
• B.
• C.
• D.