Alkanes: Formulas, Structures, and Reactions

Core Concepts

Alkanes are the most basic compounds in Organic Chemistry. However, they are also very important as they form the backbones of many other complex compounds. In this article, you will learn about the structure of alkane, its nomenclature, and some of its reactions.

Topics Covered in Other Articles

Alkane Structure

Alkanes are hydrocarbons (compounds consisting entirely of carbons and hydrogens) whose bonds are all sigma (σ) bonds. That is, the carbons of an alkane form single bonds with each other and with the hydrogen atoms. It is for this reason that alkanes are called saturated hydrocarbons, where the lack of double or triple bonds allows each carbon to bond to the maximum number of hydrogen atoms possible.

general structure of alkanes — The general structure of an alkane

Moreover, alkanes can attach to each other and form branched alkanes; or they can get into a cyclic formation and form cycloalkanes. We will discuss these structures more later in the article.

Quick Facts on Alkanes

Structure: Carbon atoms single-bonded to each other and to hydrogen atoms

General formula: C_nH_2n+2
pH: Neutral
pKa: Above 30
Solubility: Insoluble in water but soluble in nonpolar organic solvents
C–C bond length: 1.53 Å
C–C bond strength: 356 kJ mol^-1
IR Spectroscopy: The C–H bond of an alkane gives a signal at around 2850 – 3000 cm^-1

Alkane Isomerism

Structural Isomers

Alkanes with more than 4 carbons can exist as structural isomers. That is, the same molecular formula can represent many compounds with different structures. One of these structures is a straight-chained alkane (where each carbon can only bond to 2 other carbons maximum), while the rest are branched alkanes (where each carbon can bond to 4 other carbons maximum).

The more carbons an alkane has, the more structural isomers it can have. For example, C₅H₁₂ has 3 isomers, while C₉H₂₀ has 35 isomers.

Conformers

Since alkanes can freely rotate around their C–C bonds, they can exist in 3D space as a variety of shapes (or “conformations”). Think of your arm: You can twist and bend it in any way you like, but it will still be your arm. The structure of your arm doesn’t change, only its shape does. Some alkane conformations are stable (low in energy), while some are unstable (high in energy).

Newman Projection

We have covered this topic extensively in this article: Newman Projections.

Basically, think of the Newman Projection as two ceiling fans sticking on top of each other. The farther the blades are from each other, the more stable the conformation.

Newman projection of ethane — The Newman projection of two different conformations of ethane. The left one is the **eclipsed** conformation, where ethane is the least stable. The right one is the **staggered** conformation, where ethane is the most stable.

Cyclohexane Conformations

Cyclohexanes can twist into many shapes, but perhaps the most well-known one is their chair conformations. This is because the chair conformation is the most stable conformation for cyclohexane. The chair experiences no angle strain, no eclipsing strain, and little steric strain.

Other popular conformations of cyclohexane are (in descending order of stability): Twist boat, Boat, and Half-chair.

cyclohexane conformations — From left to right: Chair, Twist boat, Boat, Half-chair

Alkane Nomenclature

We have covered this topic extensively in this article: Naming Alkanes, Naming Cycloalkanes. Thus, the following information will just be a refresher.

Naming Alkanes

To name an alkane (a pure alkane, without any other functional groups), we first identify the parent chain, which is the longest chain (which consists of the largest number of carbons).
Then, we name the substituents (if there are any) by adding a prefix in front of “-yl”. These prefixes are the same as the prefixes used for naming parent chains (e.g. meth, eth, prop, etc.). Here are some examples: methyl, ethyl, and propyl.
Next, we assign a number to each substituent to indicate their positions within the molecule. We should assign to the first substituent we encounter the lowest number possible. If there is a tie, we move on to assign the second substituent we encounter the lowest number possible (but still higher than the first substituent, of course).
Also, remember to attach another set of prefixes like di, tri, tetra in front of the substituents to indicate how many of that kind of substituent is present on the molecule.
Finally, we attach the suffix “ane” to the end of our molecule’s name.

Naming Cycloalkanes

If the ring is the parent chain (i.e. it has the most carbon atom), then we add the prefix “cyclo” in front of the name of the parent chain.

If the ring is just a substituent (i.e. there exists a longer carbon chain than the ring), then we add the prefix “cyclo” in front of the name of said ring substituent.

Naming Bicyclic Compounds

Bicyclic compounds form when two rings fuse into each other. The two carbons where the rings are fused are called “bridgeheads”. The format for naming a bicyclic compound is this: substituents + bicyclo[a.b.c] + parent name.

To begin, we figure out the parent name by counting the total number of carbons that form our two fused rings. Next, excluding the bridgeheads, we count the number of carbons of one ring, the number of carbons of the other ring, and the number of carbons between the bridgeheads. We then rank those numbers from highest to lowest, which corresponds with the order of a, b, and c (a is highest, c is lowest).

Reactions with Alkanes

General Reaction Trends

Alkanes are quite inert and don’t undergo many reactions. It is for this reason that chemists often omit alkanes when listing functional groups. Nonetheless, the combustion reaction of alkanes is one of the better-known reactions in chemistry, and the halogenation of alkanes can prove useful in synthesizing more complex molecules.

Preparation of Alkanes

From Alkenes

Alkenes can undergo a hydrogenation reaction to give us alkanes. In a hydrogenation reaction, molecular hydrogen (H₂) is added across the double bond of an alkene in the presence of a metal catalyst like Pt, Pd, or Ni. The same reaction can be performed on alkynes to yield the same result, involving the addition of two H₂ molecules.

From Carbonyls

There are two popular methods chemists use to reduce carbonyls (aldehydes and ketones) into alkanes.

The Clemmensen Reduction involves treating the carbonyl with zinc amalgam (zinc whose surface is an alloy of zinc and mercury) under acidic conditions.
The Wolff–Kishner Reduction involves first the formation of a hydrazone by treating the carbonyl with hydrazine (NH₂NH₂) and then the reduction of said hydrazone under basic conditions.

clemmensen reduction, a reaction involving alkanes — Clemmensen Reduction

wolff kishner reduction reaction, a reaction involving alkanes — Wolff–Kishner Reduction

Reactions of Alkanes

Complete Combustion

Complete combustion involves the burning of alkanes in the excess presence of oxygen. More specifically, it involves the combination of alkanes with oxygen gas to produce carbon dioxide and water. The reaction is highly exothermic, releasing heat and light as a result.

Incomplete Combustion

An incomplete combustion reaction is very similar to a complete combustion reaction. They are both exothermic, releasing heat and light. The main difference is that, in an incomplete combustion, alkanes are burned in a limited (insufficient) supply of oxygen gas. The resulting products are water and either carbon monoxide (which is quite toxic) or carbon. To understand why, we must recall that in a combustion reaction, all the bonds of an alkane are broken, and the hydrogen atoms get oxidized first (forming water). Because the supply of oxygen is limited, there is not enough oxygen left for carbon to form carbon dioxide. Therefore, the carbons have to settle for carbon monoxides or just carbons if the supply of oxygen is even more limited.

Incomplete combustion of methane

Incomplete combustion of methane, with an even more limited supply of oxygen. The product formed is soot.

Halogenation

The bromination and chlorination of alkanes are among the most popular free radicals reactions. In this example, we achieve the chlorination of methane by treating methane with Cl₂ in the presence of an energy source like heat or light. This results in one of the hydrogens of methane being replaced with a chlorine atom, forming chloromethane (CH₃Cl). Thus, chemists classify this reaction as a substitution reaction. Moreover, depending on how excess Cl₂ is, chloromethane can undergo further chlorination to form dichloromethane (CH₂Cl₂), chloroform (CHCl₃), or carbon tetrachloride (CCl₄).