Alkane Reactions - Online Tutor, Practice Problems & Exam Prep

Alkanes, the least reactive hydrocarbons, primarily undergo combustion and halogenation reactions. Combustion involves alkanes reacting with oxygen to produce carbon dioxide and water. Halogenation is a substitution reaction where halogens like bromine or chlorine replace hydrogen atoms in alkanes, requiring heat or UV light to initiate. This process can yield mono or poly substituted products, resulting in alkyl halides. Understanding these reactions is crucial for grasping organic chemistry fundamentals, particularly in the context of hydrocarbons and their derivatives.

Common Types of Alkane Reactions Concept 1

Now when it comes to alkane reactions, realize that alkanes are the least reactive of the hydrocarbons, and in fact, they only do 2 major types of reactions. Our 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 to produce carbon dioxide and 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 is it breaks the bond between halogens, and that halogen then attaches itself to the alkane.

So if we take a look here, we have an alkane halogenation reaction. Here we have our simple alkane in the form of methane, it's reacting with X₂. X here again can represent bromine or chlorine. So this is Cl₂ or Br₂. Through the use of heat or hν ultraviolet light, we split this X₂ into 2 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'd 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 alkanes 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.

So 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 1

Here it states, what are the major products of the following halogenation reaction? Assume monosubstitution. Alright. So if we take a look at this, we know that there's a CH₃ here, and a CH₃ here. There is a CH₂ here, and a CH₂ here. And then here we just have another CH₂. 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 1 hydrogen for chlorine. If I put the chlorine on either end, it gives me the same exact product. Both of them will be called 1-chloropentane. 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 1, 2, 3, 4, 5. We start numbering from the end with the substituent in the form of Chlorine. Both of these will be 1-chloropentane. Since they give the same name, they're 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-chloropentane. 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-chloropentane. So here we'd get 3 major types of products, 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, then they represent the same molecule. You don't need to show more than one. Okay. Unique names equal unique products. Okay? So here we have 3 major products from the monochlorination of pentane.

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

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What are the main types of reactions that alkanes undergo?

Alkanes primarily undergo two main types of reactions: combustion and halogenation. Combustion involves alkanes reacting with oxygen to produce carbon dioxide and water, a reaction commonly known from general chemistry. Halogenation is a substitution reaction where halogens like bromine (Br₂) or chlorine (Cl₂) replace hydrogen atoms in alkanes. This reaction requires heat or UV light to initiate and can result in either mono-substituted or poly-substituted products, leading to the formation of alkyl halides.

How does the halogenation of alkanes occur?

Halogenation of alkanes occurs through a substitution reaction where a halogen (Br₂ or Cl₂) replaces a hydrogen atom in the alkane. This process requires heat or UV light (denoted as hv) to break the bond between the halogen molecules. For example, in the halogenation of methane (CH₄) with chlorine (Cl₂), the reaction produces chloromethane (CH₃Cl) and hydrogen chloride (HCl). The general reaction can be represented as:

${\mathrm{CH}}_4+{\mathrm{Cl}}_2\to {\mathrm{CH}}_3\mathrm{Cl}+\mathrm{HCl}$

What is the difference between mono-substituted and poly-substituted products in alkane halogenation?

In alkane halogenation, a mono-substituted product occurs when only one hydrogen atom in the alkane is replaced by a halogen atom. For example, in the halogenation of methane (CH₄) with chlorine (Cl₂), the product is chloromethane (CH₃Cl). A poly-substituted product occurs when more than one hydrogen atom is replaced by halogen atoms. For instance, further halogenation of chloromethane can produce dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃), and even tetrachloromethane (CCl₄).

Why are alkanes considered the least reactive hydrocarbons?

Alkanes are considered the least reactive hydrocarbons because they have strong C-H and C-C single bonds and lack functional groups that can easily participate in chemical reactions. The bonds in alkanes are relatively stable and require significant energy to break. Additionally, alkanes are non-polar molecules, which makes them less likely to react with polar reagents. This inherent stability is why alkanes primarily undergo only combustion and halogenation reactions under specific conditions such as high temperatures or UV light.

What conditions are required for the halogenation of alkanes?

The halogenation of alkanes requires either heat or ultraviolet (UV) light to initiate the reaction. These conditions are necessary to break the bond between the halogen molecules (e.g., Cl₂ or Br₂), allowing the halogen atoms to react with the alkane. The reaction can be represented as:

${\mathrm{CH}}_4+{\mathrm{Cl}}_2\to {\mathrm{CH}}_3\mathrm{Cl}+\mathrm{HCl}$

Here, the presence of heat or UV light (hv) is crucial to initiate the reaction by breaking the Cl₂ bond.