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Hydrocarbon structures and isomers

Hydrocarbon structures and types of isomerism (structural isomers, cis/trans isomers, and enantiomers).


Even though you likely see gasoline-powered vehicles everyday, you rarely see what gasoline itself looks like! To the naked eye, gasoline is a pretty uninteresting yellowish-brown liquid. At the molecular level, though, gasoline is actually made up of a striking range of different molecules, most of them hydrocarbons (molecules containing only hydrogen and carbon atoms).
Some of the hydrocarbons in gasoline are small and contain just four carbon atoms, while others are much larger and have up to twelve carbons. Some hydrocarbons form straight lines, while others have a branched structure; some have only single bonds, while others have double bonds; and still others contain rings. While the different hydrocarbons in gasoline often have very different properties, such as melting point and boiling point, they all produce energy when they’re burned in an engine.

Hydrocarbons are diverse!

As the gasoline example shows, hydrocarbons come in many different forms. They may differ in length, be branched or unbranched, form linear or ring shapes (or both), and include various combinations of single, double and triple carbon-carbon bonds. Even if two hydrocarbons have the same molecular formula, their atoms may be connected or oriented in different ways, making them isomers of one another (and sometimes giving the two molecules very different properties).
Each of these structural features can influence the three-dimensional shape, or molecular geometry, of a hydrocarbon molecule. In the context of large biological molecules such as DNA, proteins, and carbohydrates, structural differences in the carbon skeleton often affect how the molecule functions.

Branching, multiple bonds, and rings in hydrocarbons

Hydrocarbon chains are formed by a series of bonds between carbon atoms. These chains may be long or short: for instance, ethane contains just two carbons in a row, while decane contains ten. Not all hydrocarbons are straight chains. For example, while decane’s ten carbon atoms are lined up in a row, other hydrocarbons with the same molecular formula (C10H22) have shorter primary chains with various side branches. (In fact, there are 75 possible structures for C10H22!)
Hydrocarbons may contain various combinations of single, double, and triple carbon-carbon bonds. The hydrocarbons ethane, ethene, and ethyne provide an example of how each type of bond can affect the geometry of a molecule:
Ethane: tetrahedral organization of bond substituents about the carbon atoms.
Ethene: planar structure due to the presence of the double bond.
Ethyne: linear structure due to the presence of the triple bond.
Image credit: image modified from "Carbon: Figure 2," by OpenStax College, Biology (CC BY 3.0).
  • Ethane (C2H6), with a single bond between the two carbons, adopts a two-tetrahedron shape (one tetrahedron about each carbon). Importantly, rotation occurs freely about the carbon-carbon bond.
  • In contrast, ethene (C2H4), with a double bond between the two carbons, is planer (all of its atoms lie in the same plane). Furthermore, rotation about the carbon-carbon double bond is restricted. This is a general feature of carbon-carbon double bonds, so anytime you see one of these in a molecule, remember that the portion of the molecule containing the double bond will be planar and unable to rotate.
  • Finally, ethyne (C2H2), with a triple bond between the two carbons, is both planar and linear. As with the double bond, rotation is completely restricted about the carbon-carbon triple bond.
An additional structural feature that is possible in hydrocarbons is a ring of carbon atoms. Rings of various sizes may be found in hydrocarbons, and these rings may also bear branches or include double bonds. Certain planar rings with conjugated atoms, like the benzene ring shown below, are exceptionally stable. These rings, called aromatic rings, are found in some amino acids as well as in hormones like testosterone and estrogens (the primary male and female sex hormones, respectively).
Organic molecules with ring structures: cyclopentane, cyclohexane, benzene, and pyridine.
Image credit: OpenStax Biology.
Some aromatic rings contain atoms other than carbon and hydrogen, such as the pyridine ring shown above. Due to their additional atoms, these rings are not classified as hydrocarbons. You can learn more about aromatic compounds in the aromatic compounds chemistry topic.


The molecular geometries of hydrocarbons are directly related to the physical and chemical properties of these molecules. Molecules that have the same molecular formula but different molecular geometries are called isomers. There are two major classes of isomers: structural isomers and stereoisomers.

Structural isomers

Example of structural isomers: butane and isobutane.
Image modified from "Carbon: Figure 4," by OpenStax College, Biology (CC BY 3.0).
In structural isomers, the atoms in each isomer are connected, or bonded, in different ways. As a result, structural isomers often contain different functional groups or patterns of bonding. Consider butane and isobutane, shown above: both molecules have four carbons and ten hydrogens (C4H10), but butane is linear and isobutane is branched. As a result, the two molecules have different chemical properties (such as lower melting and boiling points for isobutane). Because of these differences, butane is typically used as a fuel for cigarette lighters and torches, whereas isobutane is often employed as a refrigerant or as a propellant in spray cans.


In stereoisomers, the atoms in each isomer are connected in the same way but differ in how they are oriented in space. There are many types of stereoisomers, but they can all be sorted into one of two groups: enantiomers or diastereomers. Enantiomers are stereoisomers that are non-superimposable mirror images of each other (“non-superimposable” means that the two molecules cannot be perfectly aligned one on top of the other in space). Enantiomerism is often seen in molecules containing one or more asymmetric carbons, which are carbon atoms that are attached to four different atoms or groups.
Examples of enantiomers: two forms of CHFClBr (with hydrogen and the halogens bonded to a central asymmetric carbon). The two are non-superimposable mirror images of one another.
Image modified from "Carbon: Figure 4," by OpenStax College, Biology (CC BY 3.0).
The molecules above are an example of an enantiomer pair. Both have the same molecular formula and are made up of a chlorine, a fluorine, a bromine and a hydrogen atom bonded to a central carbon atom. However, the two molecules are mirror images of one another, and if you try to place them on top of each other, you’ll find that there’s no way to make them fully line up. Enantiomers are often compared to a person's right and left hands, which are also mirror images that cannot be superimposed.
Most amino acids, the building blocks of proteins, contain an asymmetric carbon. Below, you can see space-filling models of the two enantiomers of the amino acid alanine. Historically, enantiomers in biology were distinguished using the prefixes L and D, and biologists often still use this terminology for amino acids and sugars. However, in the wider world of chemistry, the D/L system has been replaced by another naming system, the R/S system, which is more precise and can be applied to all enantiomers. You can learn more about enantiomers and the R/S naming system in the organic chemistry section.
Image of the L and D isomers of alanine. The two are made up of the same atoms, but are non-superimposable mirror images of one another.
Image modified from "Carbon: Figure 6," by OpenStax College, Biology (CC BY 3.0).
The difference between a pair of enantiomers may seem very small. In some cases, though, two enantiomers may have very different biological effects. For example, the D form of the drug ethambutol is used to treat tuberculosis, while the L form actually causes blindness!1 Additionally, there are many cases where only one enantiomer is produced by the body or found in nature. For example, typically only the L forms of amino acids are used to make proteins (although the D forms of amino acids are occasionally found in the cell walls of bacteria). Similarly, the D enantiomer of the sugar glucose is the main product of photosynthesis, while the L form is rarely seen in nature.
Remember that all stereoisomers can be classified as either enantiomers or diastereomers. Diastereomers are any stereoisomers that are not enantiomers. One common example of a diastereomer is a cis-trans isomer. Cis-trans isomers can occur when atoms or functional groups are situated on either end of a rigid carbon-carbon bond, such as a double bond. In this case, restricted rotation about the double bond means that the atoms or groups attached to either end can exist in one of two possible configurations. If either carbon is attached to two of the same atoms or groups, then this won't matter; however, if both carbons are attached to two different atoms or functional groups, then two different arrangements are possible.
Example of cis-trans isomers: cis-2-butene and trans-2-butene.
Image modified from "Carbon: Figure 4," by OpenStax College, Biology (CC BY 3.0).
For example, in 2-butene (C4H8), the two methyl groups (CH3) can occupy different positions relative to the double bond central to the molecule. If the methyl groups are on the same side of the double bond, this is called the cis configuration of 2-butene; if they are on opposite sides, this is the trans configuration.
In the trans configuration, the carbon backbone is more or less linear, whereas in the cis configuration, the backbone contains a bend, or kink. (Some ring-shaped molecules can also have cis and trans configurations, in which attached atoms are trapped on the same or on opposite sides of the ring, respectively)
In fats and oils, long carbon chains called fatty acids often contain double bonds, which can be in either the cis or trans configuration (shown below). Fatty acids that contain cis double bonds are typically oils at room temperature. This is because the bends in the backbone caused by cis double bonds prevent the fatty acids from packing tightly together. In contrast, fatty acids with trans double bonds (popularly called trans fats), are relatively linear, so they can pack tightly together at room temperature and form solid fats.
Trans fats are linked to an increased risk of cardiovascular disease, so many food manufacturers have eliminated their use in recent years. Fats with trans double bonds are found in some types of shortening and margarine, while fats with cis double bonds may be found in oils, such as olive oil and canola oil. See the article on lipids to learn more about the different types of fats.

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  • aqualine ultimate style avatar for user dropcentral
    The second picture in the article, showing various ring formations, and how they compare includes pyridine. From what I have previously seen here on Khan Academy, it was my understanding that one aspect of hydrocarbons is that they are comprised of chains made up of only carbon and hydrogen. Because of this, wouldn't pyridine be discluded from the hydrocarbon classification because it contains an atom of nitrogen? Thank you much for the clarification!
    (18 votes)
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    • piceratops ultimate style avatar for user Darmon
      Good question! Pyridine is not a hydrocarbon, as hydrocarbons can only contain hydrogen and carbon atoms, by definition. I think the point he was trying to make by showing that molecule is that hydrocarbons can take on ring-shaped forms to form more diverse molecules, which could potentially include other atoms. I hope that makes things more clear! :)
      (20 votes)
  • male robot johnny style avatar for user Tattvam Nair
    how many carbons can be bonded in chain ( maximum) ?
    (4 votes)
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  • piceratops sapling style avatar for user haekele
    What can determine an atoms stability?I mean that it was written that Benzene is exceptionally stable. I am speaking general.
    (4 votes)
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    • piceratops tree style avatar for user Yasmeen.Mufti
      Stability suggests than an atom is very unreactive. For this to happen, the atom needs to be completely neutral. For example, Neon is a nobel gas that rarely reacts with anything else because it has a fully complete outer shell of electrons (which is what every atom "wants".) Sodium however has only one electron in its outer shell, so it would "like" to donate that electron to get rid of it. Chlorine has seven outer electron, so it wants to gain an electron to be stable. Therefore, Sodium and Chlorine easily react together to make table salt.
      (7 votes)
  • piceratops ultimate style avatar for user Maria Annica
    In the paragraph after the heading Cis-trans (geometric) isomers, it says that linear molecules have single carbon-carbon bonds and can rotate freely. This kinda conflicts with the video that goes with this pdf because I thought linear carbon-carbon bonds were triple bonded and does not allow rotation. Only tetrahedrons can do that. Is this a mistake?
    (4 votes)
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    • female robot grace style avatar for user tyersome
      Linear is being used two different ways here, which I agree is a bit confusing.

      The first use doesn't really mean linear in a geometric sense, but it is intended to mean "unbranched and not circular".

      Note that this is analogous to how we often use line — e.g. "The line for tickets was so long it went around the block!".
      (4 votes)
  • duskpin ultimate style avatar for user jeffreyjayreed1971
    How do we know what shape these molecules take and second - How can we control, or is it possible to control the shape of the molecules?
    (3 votes)
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    • piceratops seed style avatar for user RogerP
      The configuration of the bonds around each carbon can be easily determined - this is known as the geometrical configuration and depends on the electron hybridisation (sp, sp2, sp3) of the carbon and other atoms. What can be harder to predict is the overall shape of a large organic molecule because there is rotation around single bonds so a large number of different shapes are possible and a molecule can cycle through all the different configurations, although some will be energetically more stable than others.

      Interactions between different parts of the molecule (e.g., hydrogen bonds, ionic interactions, lone pair repulsions, steric hindrance, etc) are responsible for some configurations being more energetically favourable than others.
      (4 votes)
  • blobby green style avatar for user Hazel Kim
    I don't get what planar is. If you mean that the molecule is symmetric when you divide it into half, why isn't Ethane planar?
    (2 votes)
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    • aqualine seedling style avatar for user Rose
      planar or trigonal planar, it's due to hybridization, an alpha carbon in ethene has a hybridization of sp2 due to three sigma bond pairs resulting in in 2-dimensional geometry(the shape of molecule doesn't have height),there's an angle of 120 between the bond pairs ,
      whereas an alpha carbon in ethane forms four sigma bonds resulting in sp3 hybridization and 3-dimensional geometry(the shape of molecule has height now), that's why ethane is tetrahedral not planar.
      (4 votes)
  • blobby green style avatar for user Stephen Waltz
    Is the psa isomers reference Prostate Specific Antigen? If not, what is it?
    (3 votes)
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  • leaf grey style avatar for user TR
    How can you tell what the structure is if is so small ?
    (3 votes)
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  • blobby green style avatar for user Mathivanan Palraj
    Why cannot we superimpose enantiomers of L and D? In the example, carbon with F, H, Br, and Cl, if you superimpose back to back it exactly aligns.
    (1 vote)
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  • aqualine ultimate style avatar for user George Trigonis
    hey! I got a bit confused when it comes to the rotation of the tetrahedral, the planar, and the linear models. So what is correct? The tetrahedral has possible rotations around the bond when the other two do not? Or does the triple bond of the linear model prevent more possible movements when compared to the planar model?
    (2 votes)
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    • aqualine seedling style avatar for user Rose
      yeah, you are quite right, the single bond between ethane let's it rotate however the molecule wants but in case of ethene and ethyne which have 1 pi bond and 2 pi bonds in addition to 1 sigma respectively these pi bonds restrict their moment of rotation that's why only ethane is able rotate freely whereas other two can't.
      (3 votes)