HS Chemistry - Essential Functional Groups

Alkanes

Overview of The Page

This page will cover:

• What are alkanes?
• How are alkanes named?
• What reactions do alkanes participate in?

Alkanes are the simplest kind of organic molecules, made only of Hydrogen atoms and Carbon atoms, and containing only single bonds. This page will cover some important information about alkanes.

Straight-chained & Branched Alkanes

Straight-chained alkanes are alkanes with only one chain, while branched alkanes are alkanes with many chains. Both contain only Hydrogen atoms, Carbon atoms, and single bonds. They are also both said to be of the same homologous series, as all straight-chained and branched alkanes have the same algebraic formula, C_nH_2n+2, which means that an alkane with n Carbon atoms contains 2n + 2 Hydrogen atoms.

As all the bonds in an alkane are single bonds, all the Carbon atoms have 4 bonds, which gives them a tetrahedral geometry. Because of this, alkanes have the maximum number of Hydrogen atoms possible, as each pair of atoms only has one bond between them. We say alkanes are saturated.

As straight-chained and branched alkanes are part of the same homologous series, they display similar physical and chemical trends. In particular, the longer the main chain of the alkane molecule, the higher its viscosity and boiling point.

This is often used when cracking them. Alkanes are often found in crude oil, and to separate them, thermal and catalytic cracking are used:

• Thermal cracking is when crude oil is boiled to different temperatures. Since different alkanes have different boiling points, with smaller alkanes having lower boiling points, boiling the crude oil to different temperatures only boils some alkanes, which can then be extracted. Repeating this multiple times allows the alkanes in the crude oil to be separated. Steam is used to initiate the process.

• Catalytic cracking is when a catalyst is used to initiate and expedite the process, rather than steam. Otherwise, it is functionally the same as thermal cracking.

Both of these processes fall under the category of fractional distillation - distilling repeatedly at multiple temperatures to separate the hydrocarbons in crude oil.

Cycloalkanes

Cycloalkanes are alkanes whose Carbons are connected to form a ring. Examples include cyclopropane $whose skeletal formula is shown below$:

And cyclohexane $whose skeletal formula is shown below$:

Again, as in all skeletal formulas, each unlabeled point represents a Carbon atom, and the Hydrogen atoms are all implied.

In cycloalkanes, each Carbon atom, being connected to two other Carbon atoms, can only be bonded to two other Hydrogen atoms. Thus, cycloalkanes form their own homologous series, and have their own separate general formula, C_nH_2n

Given how cycloalkanes have a cyclical shape, naming them also becomes tricky. You can't start at an end of the chain, because there is no end. Therefore, when choosing the numbering to use, use the numbering that ends up with the lowest sum for the positions of the locants $branches$.

Additionally, when naming cycloalkanes, halogens do not take precedence over alkyl groups in the prefix portion of the name.

Naming Alkyl Groups

Given how there are many different types of alkyl groups, and they will become increasingly more common in later units, we should introduce the alkyl naming system at this point.

Recall how methane and ethane cannot have any branches $methane because there is only one Carbon atom, and ethane because the Carbon atoms can rotate around the single bonds$. We also mentioned how side chains in an organic molecule can have their own side chains.

Similarly, methyl and ethyl side chains can only be connected to the main chain in a straight line, for much the same reason. However, propyl and butyl side chains, having more than 2 Carbon atoms, can be connected to the main chain in various ways. We note them in the prefix as follows:

• If the propyl and butyl side chains are connected to the main chain in a straight line $that is, the side chain's 1^st^ Carbon atom is connected to the main chain$, they are noted as "propyl" or "butyl". The example shown below has a propyl chain boxed in red, identifiable as a propyl chain because it's the first Carbon atom in the side chain that is connected to the main chain.

  

• If the propyl and butyl side chains are connected to the main chain from their 2^nd Carbon atom $rather than their 1^st^ Carbon atom$, they are noted as "isopropyl" or "sec-butyl". The example shown below has a sec-butyl chain boxed in red, identifiable as a sec-butyl chain because it's the second Carbon atom in the side chain that is connected to the main chain.

  

• If the side chain looks like a methylpropane group, as shown below:

  

  It is noted as "isobutyl".

• If the side chain looks like a dimethylpropane group, as shown below:

  

  It is noted as "tert-butyl".

Note: The definitions for naming alkyl groups are more general than what is shown here, and this pattern extends to longer chains like pentyl and hexyl groups. However, that won't be covered here. Additionally, the examples above are only meant to show what different alkyl groups look like. For example, the following molecule was used to show what an isobutyl group would look like:

In reality, this molecule would be named 2,4-dimethylheptane, not 2-isobutylpentane, because the longest Carbon chain consists of 7 Carbon atoms $since Carbon-Carbon single bonds are rotatable$. However, the shape of the chain boxed in red is the general shape of an isobutyl side chain. Similarly, the other examples are only meant to outline the general shapes of the mentioned alkyl groups, and those specific examples may in fact be named differently.

Alkane Reactions

Alkanes contain only Carbon and Hydrogen atoms. Since the difference between Carbon's electronegativity $2.5$ and Hydrogen's electronegativity $2.1$ is 0.4, this means that alkanes are non-polar. Combined with the fact that alkanes consist of only single bonds, it turns out that alkanes are mostly unreactive $aka inert$. However, there are two main alkane reactions: combustion and substitution reactions with halogens.

• Combustion:

  • When fossil fuels are burned, the alkanes inside them undergo a combustion reaction. There are two types of combustion for alkanes:

    • Complete combustion of alkanes occurs when there are enough or excess Oxygen molecules $O~2~$ present. When the temperature becomes high enough for combustion, the alkane's bonds break. The Hydrogen atoms react with the O₂ molecules to produce H₂O, and the Carbon atoms react with the O₂ molecules to produce CO₂. An example of this is when a campfire is burning; since excess O₂ is present, the alkanes completely combust.

    • Incomplete combustion of alkanes occurs when there aren't enough Oxygen molecules $O~2~$ present. When the temperature becomes high enough for combustion, the alkane's bonds break. The Hydrogen atoms then react with the O₂ molecules to produce H₂O. However, now there aren't enough O₂ molecules left for Carbon to react with and produce CO₂. Instead, the Carbon atoms react with the O₂ molecules to produce CO. An example of this is inside a car's Internal Combustion Engine $ICE$; since there isn't enough O₂ inside the ICE for the alkanes to completely combust, incomplete combustion occurs.

  • The high temperatures of ICEs also reacts Nitrogen gas $N~2~$ with O₂ to produce Nitrogen monoxide through the reaction:

    N₂ + O₂ ⇌ 2NO

    • This poses a problem. Carbon monoxide, if inhaled for too long, can bond with the hemoglobin in the blood, halting a person's ability to intake Oxygen and posing a deadly threat to them. Nitrogen monoxide, on the other hand, is a serious air pollutant that can lead to nitric acid dissolved in rain.

    • To prevent this from occurring, cars use catalytic converters to oxidize CO and reduce NO. The reaction is:

      2CO + 2NO → 2CO₂ + N₂

      It requires a catalyst, like Al₂O₃ or rare earth metals like Palladium $Pd$, to occur, and oxidizes CO into CO₂ while reducing NO into N₂. While CO₂ is an air pollutant and a greenhouse gas, it is not toxic the way CO is.

• Substitution Reactions With Halogens:

  • Alkanes are normally unreactive, and therefore, in order to react, they either need to be raised to very high temperatures to break the intramolecular bonds $which is what happens in combustion$ or they need to be attacked by a very reactive particle $which is what happens in substitution reactions with halogens$.

  • The substitution reaction of alkanes with halogens is also known as a free-radical substitution, as it involves free radicals. As with any reaction involving free radicals, it undergoes the three steps of initiation, propagation, and termination.

    1. Initiation: In the presence of UV light, a halogen molecule $for example, Br~2~$ will undergo homolytic fission, producing two Bromine atoms, each with 7 electrons. Each of these Bromine atoms is a free radical, so it is incredibly reactive.

       

    2. Propagation: One of the Bromine free radicals will react with an alkane $for example, ethane$.

       

       As it’s a free radical, the Bromine will bond with a Hydrogen atom from the ethane molecule, forming HBr. The Bromine is no longer a free radical. However, without the Hydrogen atom, the ethane molecule becomes an ethyl free radical.

       

       The propagation can stop here, but it does not have to. The ethyl free radical could react with a Bromine molecule:

       

       And the propagation process would repeat over again. The Bromine free radical could attack this molecule again $which would lead to another one of the Hydrogen atoms being substituted for a Bromine atom$, or it could attack another molecule.

    3. Termination: A Bromine free radical will react with the ethyl free radical to form C₂H₅Br. When two free radicals react, the result is a single molecule - the free radicals have been eliminated.

       

       Depending on how many propagation steps the ethyl free radical went through, it could have 1, more than 1, or even all of its Hydrogen atoms replaced by Bromine atoms.