Hydrocarbons are among the simplest, yet most abundant, organic chemicals: as their name suggests, they are composed solely of chains of hydrogen and carbon atoms, although the variation in properties such as chain length, number of bonds, and chain structure mean that hydrocarbons – specifically, alkanes, alkenes and alkynes – are quite diverse.
How are alkanes, alkenes and alkynes different?
The main difference between alkanes, alkenes and alkynes comes from the number of covalent bonds between any two carbon atoms in a hydrocarbon. Alkanes only contain single bonds, while alkenes have at least one double bond, and alkynes at least one triple bond.
What is an Alkane?
Alkanes – also referred to as saturated hydrocarbons – are so named because each carbon atom is bonded to the maximum number of hydrogen atoms.
As covalently-bonded substances, alkanes are generally non-polar. Their weak dipole-dipole bonds are unable to sever the hydrogen bonding of water molecules, and they are therefore not miscible in water. When added to water, alkanes will float to the top, as they are less dense.
Some alkanes – such as the first four species: methane, ethane, propane and butane - are notable for being readily flammable gases. Pentane, a liquid at standard temperature and pressure (STP), is also flammable.
The longer the hydrocarbon chain of an alkane is, the harder it becomes like a solid. Most solid alkanes, however, are soft and waxy.
What is an Alkene?
Alkenes, alongside alkynes, are unsaturated hydrocarbons – because there is at least one double covalent bond between carbon atoms which a hydrogen bond would otherwise take.
An alkene is colorless and odorless - except for ethene (ethylene), which possesses a very mild sweet odor. Like alkanes, they are non-polar substances that only dissolve in similar chemicals, such as benzene. Additionally, they are slightly more reactive as the electrons shared in a double bond can be more easily removed.
The first three alkenes – methene, ethene and propene – are gases. As more carbon atoms are added, an alkene becomes more like a liquid, then a solid. Their boiling and melting points are comparable with alkanes.
What is an Alkyne?
Alkynes are also non-polar unsaturated hydrocarbons, as they possess at least one triple carbon-carbon bond.
Nearly every alkyne has neither color nor odor, with only ethyne (acetylene) having a faint sweet and musky scent. Compared to other hydrocarbons, the concentrated electron density from an alkyne’s triple bonds makes them slightly more soluble in polar solvents. However, they remain insoluble in water.
At STP, ethyne, propyne, and butyne – the first three alkynes – are gaseous. The next eight alkynes are in a liquid state.
Due to their triple bonds, alkynes also possess slightly higher boiling points.
Differences between Alkane, Alkenes and Alkynes
Type of Covalent Bonds
Alkanes, alkenes and alkynes are most easily distinguished through their covalent bonds.
All adjacent carbon atoms in an alkane are connected to each other through single covalent bonds. This frees up each carbon atom to bond with the maximum number of hydrogen atoms, which is why alkanes are known as “saturated hydrocarbons.”
Alkenes and alkynes are unsaturated hydrocarbons. Alkenes have at least one double covalent bond between two carbon atoms, and alkynes have at least one triple bond.
Empirical Formula
Empirical formulae signify how many carbon and hydrogen atoms are present in a particular hydrocarbon.
Alkanes have a chemical formula of CnH2n+2, the formula for alkenes is simply CnH2n, and alkynes have a formula of CnH2n-2. The letter “n” refers to the number of carbon atoms in the molecule.
Boiling Points
In general, alkynes have somewhat higher boiling points compared to alkanes and alkenes. In fact, the simplest alkene – ethene – has a lower boiling point (-103.7 C) than the alkane ethane (-88.6 C), while the alkyne acetylene turns to vapor at -84.0 C.
Reactivity
The three hydrocarbon groups are relatively stable. However, carbon-carbon double bonds and triple bonds are more reactive than hydrogen bonding – in effect, alkynes exhibit higher reactivity than alkenes, and alkenes are slightly more reactive than alkanes.
Solubility
Alkanes, alkenes and alkynes are all non-polar substances that only dissolve in like solvents. Alkanes and alkenes are virtually insoluble in water, although alkynes exhibit very slight solubility in polar substances.
Subgroups
Alkanes can be further grouped into straight-chain alkanes that consist of a linear carbon skeleton, branched alkanes where an atom in the carbon skeleton may be connected to another carbon by a single bond, and cycloalkanes that form a ring structure.
Alkenes with two double bonds are alkadienes (dienes), those with three are alkatrienes (trienes), and so on. Any alkene with multiple double bonds is a polyene. Cycloalkenes are alkenes in the form of a ring.
The alkyne counterparts to the above include polynes and cycloalkynes.
States of Matter
The first four alkanes – methane, ethane, propane and butane – vaporize at below-freezing temperatures, and are thus gaseous at STP. Liquid alkanes range from pentane to hexadecane. Most alkanes longer than hexadecane are waxy solids.
Meanwhile, only the first three alkenes - ethenes, propenes and butenes – are gaseous. Five to fourteen-carbon alkenes are liquid; the rest are solids.
Only ethyne, propyne and but-1-yne are gases. The next eight are liquids, with the heaviest alkynes being solid.
Other Names
Alkanes are also known as paraffins, while the common names for alkenes and alkynes are, respectively, olefins and acetylenes.
Note that the proper use of the term “acetylene” refers to the simplest alkyne, ethyne.
Comparison Chart: Alkanes vs Alkenes vs Alkynes
| Areas | Alkanes | Alkenes | Alkynes |
| Covalent Bonds | Single bonds only | At least one double bond | At least one triple bond |
| Chemical Formula | CnH2n+2 | CnH2n | CnH2n-2 |
| Boiling Points | Methane: -161.6°C Ethane: -88.6°C | Ethene: -103.7°C | Generally higher Ethyne: -84.0°C |
| Reactivity | Stable | Stable | Slightly more reactive |
| Solubility | Only soluble in non-polar solvents | Only soluble in non-polar solvents | Non-polar; slightly soluble in polar solvents |
| Subgroups | Straight-chain, branched, cycloalkanes | Polyenes (e.g. alkadienes), cycloalkenes | Polynes (e.g. alkatriynes), cycloalkynes |
| States of Matter | 1 to 4-carbon: gas 5-16: liquid 17+: solid | 1 to 4-carbon: gas 5-14: liquid 15+: solid | 1 to 3-carbon: gas 4-11: liquid 12+: solid |
| Other Names | Paraffins | Olefins | Acetylenes |
How are Alkanes, Alkenes and Alkynes similar?
Alkanes, alkenes and alkynes are all examples of hydrocarbons – simple compounds that consist solely of carbon and hydrogen atoms.
As such, they share many similarities both physically and chemically, such as their very low boiling points, insolubility and immiscibility to polar solvents, relative chemical stability, and high flammability.
In general, all three hydrocarbons lack any discernible color or odor, apart from exceptions such as ethene and acetylene.
Furthermore, the melting and boiling points of these hydrocarbons increase with the number of carbon atoms present in the compound.
FAQ
Which alkane is found in cigarette lighters?
Lighter fluid is most commonly butane – an alkane with only four carbon atoms.
Butane and other alkanes, such as propane, are flammable and good for heating because they don’t emit smoke while burning.
However, butane is desirable in lighters as the spark from its flint and steel can only create a flame by igniting a gas. Butane is gaseous at temperatures over -0.5 C, but it can turn into a liquid in a slightly pressurized container, making it safer to store.
Why is acetylene considered an excellent fuel?
Ethyne, or acetylene (C2H2), is highly-popular as a fuel source for welding and metal-cutting.
This is because acetylene flame produces the highest temperatures, and it can easily be concentrated to a particular point to minimize wasted heat. As it also needs the least oxygen to combust properly, it is also the most efficient fuel for welding.
An added benefit is that acetylene, like many hydrocarbon gases, is lighter than air; it accumulates far above the floor and away from any fires, reducing the likelihood that it will accidentally ignite. This makes the alkyne a safe, powerful and cost-saving fuel for any welding job.
Conclusion
Alkanes, alkenes and alkynes are colorless and odorless non-polar hydrocarbons.
The key difference between them is that alkanes only contain carbon-carbon single covalent bonds, while alkenes and alkynes respectively have at least one double or triple bond.
Both alkanes and alkenes are stable and virtually insoluble in water, while alkynes are slightly more reactive and can dissolve in polar solvents to a very limited degree. Alkenes and alkynes are also unsaturated hydrocarbons, as at least one carbon atom is no longer bonded to its maximum number of hydrogen atoms.
Alkynes also generally have higher boiling points.
