“What is gasoline?” is a question with a very short answer: hydrocarbons. “What hydrocarbons are in gasoline?” is a question that requires a considerably longer answer.

Hydrocarbons constitute the majority of the substance of fossil fuels and biofuels as well. More importantly, hydrocarbons are the components — the stuff — that make fossil fuels, and biofuels valuable. Hydrocarbons are the energy source in gasoline, fossil fuels, and biofuels that ignite, combust and burn: oxidize. 

Gasoline is a mixture of hydrocarbons that release energy when they oxidize. Because hydrocarbons release energy when they oxygenize, hydrocarbons are the most valuable source of energy on the planet, at least currently. And of all fossil fuels, gasoline is far and away the most widely consumed. 

Fossil Fuel Types Determined by Mixtures of Different Hydrocarbons

In addition to hydrocarbons being the source of fossil fuel energy, hydrocarbons are also the reason there are different types of fossil fuels. Just as there are different types of fossil fuels and biofuels, there are different types of hydrocarbons. The qualities of each category, class, and specific hydrocarbon determine fossil fuel types. Gasoline, diesel, propane, methane, jet fuel, bunker fuel, fuel oil, ethanol, and biodiesel each have a different combination of hydrocarbons. 

But though there are different categories, classes, and specific hydrocarbons, every hydrocarbon consists of only two types of atoms. 

As the name implies, hydrocarbons are made from bonds between hydrogen and carbon atoms. The bonds between carbon and hydrogen determine hydrocarbon category, class, and type. So too does the number of carbon and hydrogen atoms bonds in a molecule or molecule chain. 

Just as different combinations of hydrocarbons determine fuel type, different carbon and hydrogen bond combinations determine hydrocarbon types.

Fossil Fuel Type Determined by Hydrocarbon Size and Carbon-to-Hydrogen Ratio

The category, class, and combination of hydrocarbons determine fossil fuel type. Fossil fuel type is a measure of two qualifiers: fuel weight and fuel density. Fuel weight and fuel density are different measurements of fuel qualities and both fuel weight and fuel density are consequences of one variable with two variables. 

First, weight and fuel density are consequences of hydrocarbon molecule size. By extension, the structure of hydrocarbon molecule chains plays a role in weight and density. The size, length, and pattern of the hydrocarbons determine the weight and fuel density of a fossil fuel. 

Second, fuel weight and energy density are consequences of the carbon-to-hydrogen ratios of the hydrocarbons molecules in a fossil fuel. The greater the number of carbon atoms in relation to hydrogen atoms, the greater the weight and density of a hydrocarbon.

Hydrocarbon Size and Length and Fossil Fuel Weight and Energy Density

The larger and longer the hydrocarbon molecules in a fossil fuel, the heavier the fossil fuel. The smaller and shorter, the lighter the fossil fuel. As one would expect, gas-state fossil fuels like methane and propane have small, short molecules and molecule chains. Heavy fossil fuels like diesel and bunker fuel contain large, long-chain hydrocarbon molecules. Gasoline is a medium weight of fossil fuel. 

Additionally, the number of hydrogen atoms attached to the carbon molecules in the backbone of a hydrocarbon also plays a role in weight and energy density. Carbon molecules are heavier than hydrogen molecules — every element on the periodic table is heavier than hydrogen. So, the higher the number of carbon atoms in relation to hydrogen atoms in a hydrocarbon, the heavier the hydrocarbon. 

So, the heaviest, most energy-rich hydrocarbons are those that have size and density. The largest and longest and hydrocarbon molecule chains have the greatest weight and density. And, those that have the highest carbon-to-hydrogen ratios have the greatest weight and density. 

Molecular Structure of Gasoline

Gasoline has greater weight and density that gas-state fossil fuels like natural gas — methane — and propane. Fossil fuels like diesel and kerosene have greater weight and density than gasoline. The bulk of gasoline consists of hydrocarbons with, “between 4 and 12 carbon atoms per molecule (commonly referred to as C4-C12).” 

With respect to molecule chain size and length and carbon-to-hydrogen ratio, gasoline is somewhere near the middle of the spectrum of fossil fuels.

Two Categories of Hydrocarbons in Gasoline

There are hundreds of hydrocarbons in gasoline. But, each type of hydrocarbon falls into one of two categories: saturated or unsaturated. 

Saturated hydrocarbons are the most stable. Saturated hydrocarbons are those with carbon backbones with no space to take on more hydrogen or carbon atoms. There are three types of saturated hydrocarbons. They can be linear, branched, or loops.  Branched saturated hydrocarbons that are looped have the name cycloalkanes. 

Like saturated hydrocarbons, unsaturated hydrocarbons can be linear, branched, or loop. But, unsaturated hydrocarbons can easily take on additional hydrogen atoms. Unsaturated hydrocarbons, as a result, are unstable.

Because of their stability, saturated hydrocarbons burn with a clear, clean flame. Unsaturated hydrocarbons burn with a smoky flame and can be toxic.

Types of Saturated Hydrocarbons

There are three types of saturated hydrocarbons in gasoline: alkanes, isos, and cyclic hydrocarbons. Alkanes are saturated hydrocarbons with a continuous, linear chain of carbon atoms that does not branch. Up to three hydrogen atoms can attach to each carbon atom. 

Iso hydrocarbons are saturated hydrocarbon chains with branches. Along the linear chain of carbon atoms in a hydrocarbon, up to three carbon atoms can attach to each carbon atom in the chain. And to the carbon atoms attached to the carbon atoms in the chain, hydrogen atoms can attach. 

The third kind of saturated hydrocarbons in gasoline are cyclic. A cyclic saturated hydrocarbon is one in which the last two carbon atoms at the ends of a hydrocarbon chain bond making a loop. For example, cyclohexane is a looped saturated hydrocarbon chain containing six carbon atoms. 

Classes of Saturated and Unsaturated Hydrocarbons in Gasoline

The two categories of hydrocarbons — saturated and unsaturated — are comprised of two classes each. “Paraffins and naphthenes are classified as saturated hydrocarbons because no more hydrogen can be added to them without breaking the carbon backbone. Aromatics and olefins are classified as unsaturated hydrocarbons. They contain carbon to carbon double bonds or aromatic bonds that can be converted to single bonds by adding hydrogen atoms to the adjacent carbons.” 

Most Common Hydrocarbons in Gasoline

There are between 500 and 1,000 types of hydrocarbons in every fossil fuel. “Gasoline is a complex mixture of over 500 hydrocarbons that may have between 5 to 12 carbons. Alkanes type compounds, either straight chain or branched compounds are present in the greatest amounts. Smaller amounts of alkane cyclic and aromatic compounds are also present.”  There are different amounts of different hydrocarbons in ever fossil fuel. It is the ratio of one type of hydrocarbon to another that determines fossil fuel type. 

 Simply, “Gasoline contains mainly alkanes (paraffins), alkenes (olefins), and aromatics,” according to Advanced Motor Fuels.

Alkanes (Paraffins) Present in Gasoline

The most prevalent hydrocarbons in gasoline, alkanes are saturated hydrocarbons with large reserves of energy. “Alkanes are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) linked exclusively by single bonds. Each carbon atom forms 4 bonds (either C-H or C-C bonds). Each hydrogen atom is connected to a single carbon atom, by an H-C bond.” 

A list of the paraffins in gasoline include:

1. N-butane
2. N-pentane
3. N-hexane
4. N-heptane
5. 2-methylbutane
6. 2,2-dimethylpropane
7. 2,2-dimethylbutane
8. 2,2-dimethyl pentene
9. 2,2,3-trimethyl butane
10. 2,2,4-trimethylpentane (isooctane)

Again, alkanes are extremely stable because they have no space for the addition of more carbon or hydrogen atoms. All alkanes molecule chains have the same basic structure. Alkanes are hydrocarbon molecule chains in which a carbon atom’s bonds are between one or two other carbon atoms and between two or three hydrogen atoms. 

The carbon atoms at the end of an alkane chain have three hydrogen bonds and one carbon bond. The carbon atoms in the middle of an alkane molecule chain have two carbon atom bonds and two hydrogen atom bonds. Some alkane hydrocarbons have branches or loops. But, every carbon atom in all alkanes has four bonds. Every carbon atom either has one carbon bond and three hydrogen bonds or two carbon bonds and two hydrogen bonds. 

Carbon-to-Carbon Bonds Difference Between Alkanes and Alkenes

So, the only difference between different alkanes is the number of carbon-to-carbon bonds. N-butane, for example, has four carbon atoms. Two of the carbon atoms in n-butane — those two at each end — have one carbon bond and three hydrogen bonds. The two carbon atoms in the middle have two carbon bonds and two hydrogen bonds. 

Alkanes constitute roughly 55 percent of the hydrocarbons in gasoline. And, about 17 percent of the alkanes in gasoline are n-alkanes. About 32 percent of the alkanes in gasoline are branched alkanes. And, about 5 percent of the alkanes in gasoline are cycloalkanes. 

The second largest sum of hydrocarbons in gasoline is aromatics. 

Aromatics (Alkynes) Present in Gasoline

Aromatics are unsaturated hydrocarbons. That means aromatics have space to acquire more carbon atoms and more hydrogen atoms. So, aromatics easily transform from one type of hydrocarbon molecule to another easily. As such, aromatics are highly volatile.  

There are far more present, but the most common aromatics in gasoline are:

1. Benzene
2. Toluene or methylbenzene
3. m-xylene or 1,3-dimethylbenzene
4. Ethylbenzene
5. Propylbenzene
6. Isopropylbenzene

Aromatics have a lower energy content than alkanes. While a lower energy content is a negative, aromatics typically have higher octane than alkanes. The higher the octane count in the aromatics in gasoline, the higher the octane count of the gasoline. As a result, the higher the octane in gasoline, the less likely that an engine will knock. 

Knocking — detonation — occurs when different portions of an air-fuel mixture ignite at different times. High octane fuels detonate uniformly in the cylinder of an engine. In the past, lead was the component in gasoline that prevented knocking. Likewise, adding lead to gasoline increases its octane. But in 1997, the addition of lead to gasoline to increase octane count became illegal. Lead is a highly toxic element that damages the atmosphere as well as people, flora, and fauna. 

To produce high octane fuel without adding lead, petroleum producers began increasing the percentage of aromatics in gasoline. So, aromatics are an essential component in gasoline. But, there are drawbacks to aromatics. Aromatics produce combustion residue and toxic emissions. While not as toxic as lead, aromatics are far more toxic than alkanes. 

“Aromatics in gasoline are the new lead,” said Carol Werner, executive director of the Environmental and Energy Study Institute, a Washington-based group that advances policy solutions for clean and sustainable energy. “It’s what keeps me up at night.”

In addition to alkanes and aromatics, gasoline contains a third class of hydrocarbon, alkenes.

Alkenes (Olefins) Present in Gasoline

Alkenes, like aromatics, are unsaturated hydrocarbons. That means they are unstable and — and for lack of a more descriptive term — dirty. Alkenes, typically, have an even higher octane rating than aromatics. And, alkenes are potentially even more toxic. 

“Aromatics [and olefins] in gasoline have high octane numbers. However, aromatics and olefins may worsen engine cleanliness, and also increase engine deposits, which is an important factor for new sophisticated engines and after-treatment devices. Aromatics may lead to carcinogenic compounds in exhaust gases, such as benzene and polyaromatic compounds. Olefins in gasoline may lead to an increase in the concentration of reactive olefins in exhaust gases, some of which are carcinogenic, toxic or may increase ozone-forming potential.”

The question, “what is in gasoline?” is hydrocarbons. However, “what hydrocarbons are in gasoline?” is answerable in several ways. The most specific is alkanes, aromatics, and alkenes. But, that answer begs the questions, “which hydrocarbons in gasoline are most valuable?” 

Alkanes. Alkanes are the hydrocarbons in gasoline that have the greatest weight and density and are the most stable. In other words, alkanes produce the most energy and alkanes oxidize the cleanest. So, the question, “what is the best gasoline produced in the world?” is answerable by determining which regions produce the most crude oil with the highest alkane counts.