Alkane Reactions: Videos & Practice Problems

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

Alkanes, the least reactive hydrocarbons, primarily undergo two significant types of reactions: combustion and halogenation. Combustion involves a hydrocarbon reacting with oxygen, resulting in the formation of carbon dioxide and water. This fundamental reaction is well-known from general chemistry.

Halogenation, on the other hand, is a substitution reaction where a halogen, such as bromine (Br₂) or chlorine (Cl₂), replaces one of the hydrogen atoms in the alkane. This process requires heat or ultraviolet (UV) light, denoted as HV, to break the bond between the halogen molecules. For instance, in a halogenation reaction involving methane (CH₄), the alkane reacts with a halogen molecule (X₂). The UV light facilitates the dissociation of X₂ into two halogen atoms, allowing one halogen atom to substitute a hydrogen atom in the alkane, resulting in the formation of an alkyl halide (R-X) and hydrogen halide (HX) as a byproduct.

Alkane halogenation can be classified into mono-substitution, where only one hydrogen is replaced, or poly-substitution, where multiple hydrogens are substituted. It is crucial to understand the context of the question being asked, as the desired product can vary significantly based on whether mono or poly substitution is specified. In cases of mono halogenation, the outcome is a single alkyl halide product, highlighting the importance of clarity in reaction conditions and desired results.

Alkane Reactions Example 1

In the halogenation reaction of pentane, specifically focusing on monosubstitution, we can identify the major products formed when chlorine replaces hydrogen atoms. Pentane, a five-carbon alkane, exhibits symmetry, which simplifies the analysis of potential substitution sites.

When considering the structure of pentane, we can denote the carbon chain as follows: CH₃-CH₂-CH₂-CH₂-CH₃. The two terminal carbons (red carbons) are equivalent, meaning that substituting a chlorine atom at either end will yield the same product, which is named 1-chloropentane. This is because the numbering of the carbon chain starts from the end closest to the substituent, leading to the same designation regardless of which terminal carbon is substituted.

Next, we examine the two internal CH₂ groups (blue carbons). Substituting chlorine at either of these positions will result in the same product, 2-chloropentane. Again, since both substitutions yield the same name, we only need to represent one of them.

Finally, substituting chlorine at the central carbon (the third carbon) produces a unique product, 3-chloropentane. This substitution is distinct from the others, as it results in a different molecular structure.

In summary, the major products from the monochlorination of pentane are 1-chloropentane, 2-chloropentane, and 3-chloropentane. The key takeaway is that when determining the products of a halogenation reaction, identical names indicate identical molecules, allowing for a streamlined representation of the products formed.

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

• A.
• B.
• C.
• D.

Answer: (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.