Properties and Examples of Aliphatic Compounds
In organic chemistry, hydrocarbons refer to compounds consisting of nothing but carbon and hydrogen. Further, the hydrocarbons get classified into two broad categories - aliphatic hydrocarbons and aromatic hydrocarbons. The term aliphatic comes from the Greek word ‘Aleiphar,’ which means ‘fat.’ It refers to hydrocarbons obtained by the chemical degradation of oils or fats. An aliphatic compound is an organic one. It has carbon and hydrogen joined together in straight, branched chains, or non-aromatic rings.
These compounds may be both saturated (alkanes) and unsaturated (alkenes, alkynes). Most simple aliphatic hydrocarbon is methane, CH4. A majority of aliphatic hydrocarbons are flammable. Ethylene, isooctane, and acetylene are examples of the same. In this article, you can learn about aliphatic hydrocarbons, saturated and unsaturated hydrocarbons, and their properties, etc.
What is Aliphatic Compound?
An aliphatic compound or aliphatic hydrocarbon contains hydrogen and carbon atoms linked together in straight chains. Sometimes, the chains can also occur in branched form or non-aromatic structures. Keep in mind that aliphatic compounds are organic. Apart from the hydrogen, different elements like oxygen, nitrogen, chlorine, and sulfur may get joined with the carbon atoms in the chain.
Halogenated aliphatic hydrocarbons are chemicals where one or more hydrogen atoms get replaced by the halogen. Aliphatic hydrocarbons encompass a diverse group of organic compounds characterized by different single, double, and triple bonds, and an open-chain structure as well. Below is a classification of hydrocarbons; as you can see, there are four types of aliphatic compounds. They are alkanes, alkenes, alkynes, and cyclic or cyclic-alkanes.
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Properties of Aliphatic Hydrocarbons
A great majority of aliphatic compounds are flammable. They often get used as fuel sources like methane, propane, ethylene, acetylene, and so on.
Aliphatic compounds may be cyclic or acyclic. They can have close chains or rings of carbon atoms in their molecules.
When two alkanes have equal molecular mass, the more highly branched alkane has a lower boiling point. The melting points for aliphatic compounds increase with size, but in a less typical manner.
Since the hydrocarbons are nonpolar, they are insoluble in water and other polar solvents. However, they get dissolved in non-polar solvents like benzene and diethyl ether. Also, note that hydrocarbons are less dense than water, and they can stay afloat on water surfaces.
Below you can Find Some Examples and a List of Aliphatic Hydrocarbons
Extraction of Aliphatic Hydrocarbons
Aliphatic compounds get extracted by the process of pressurized fluid extraction (PEE). Organic and aqueous extraction solvents get used in the process of PEE. Water, when converted to steam, can get used to obtaining aliphatic hydrocarbons from solid and semi-solid environmental samples. There has been a minimal usage of aliphatic hydrocarbons as solvents in conventional flame spectrometry. Occasionally, they get used as diluents for other solvents.
Saturated and Unsaturated Hydrocarbons
Aliphatic compounds can be either saturated or unsaturated. Saturated hydrocarbons mostly contain alkanes. They are open-chain hydrocarbons having carbon-carbon single bonds. Mostly, the bond refers to a covalent bond. Such compounds are inert, and they don’t quickly react with acid, bases, or various reagents. When hydrocarbon molecules don’t have any double bonds in them, they become saturated hydrocarbons.
When the hydrocarbon molecules have at least one double bond, you can call them unsaturated. It means those molecules can accept more hydrogen atoms. These hydrocarbons are more reactive than saturated ones because the double bonds are not as strong as single bonds in saturated hydrocarbons. One part of the double bond is easily breakable as compared to breaking an entire single bond.
In saturated hydrocarbons, there are no double bonds. It means that there are adequate hydrogen atoms in the molecule and it has no space for adding more. In unsaturated hydrocarbons, you can see a fewer amount of hydrogen atoms bonding with carbon atoms. Unsaturated hydrocarbons are alkenes and alkynes. They respectively have one C – C double bond and one C – C triple bond.
FAQs on Aliphatic Hydrocarbons
1. What are aliphatic hydrocarbons as per the CBSE Class 11 syllabus?
Aliphatic hydrocarbons are organic compounds made entirely of carbon and hydrogen atoms, arranged in straight chains, branched chains, or non-aromatic rings. Unlike aromatic compounds, they do not contain a benzene ring. They form the basis of organic chemistry and are classified into alkanes, alkenes, alkynes, and their cyclic analogues.
2. What are the main types of aliphatic hydrocarbons, with examples?
Aliphatic hydrocarbons are primarily classified based on the types of carbon-carbon bonds they contain:
- Alkanes: These are saturated hydrocarbons with only single bonds (C-C). Their general formula is CₙH₂ₙ₊₂. Example: Propane (C₃H₈), used in LPG.
- Alkenes: These are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). Their general formula is CₙH₂ₙ. Example: Ethene (C₂H₄), used to make polythene.
- Alkynes: These are unsaturated hydrocarbons with at least one carbon-carbon triple bond (C≡C). Their general formula is CₙH₂ₙ₋₂. Example: Ethyne (C₂H₂), also known as acetylene, used in welding.
- Cycloalkanes: These are saturated hydrocarbons where carbon atoms form a ring. Example: Cyclohexane (C₆H₁₂).
3. What is the key difference between aliphatic and aromatic hydrocarbons?
The primary difference lies in their structure and chemical stability. Aliphatic hydrocarbons have open-chain or non-aromatic cyclic structures. In contrast, aromatic hydrocarbons must contain at least one stable, planar ring of atoms (like a benzene ring) with delocalised pi (π) electrons that follow Hückel's rule (4n+2 π electrons). This special arrangement makes aromatic compounds exceptionally stable compared to aliphatic compounds.
4. Why are these compounds called 'aliphatic'?
The term 'aliphatic' originates from the Greek word 'aleiphar,' which means 'fat' or 'oil.' Early chemists first isolated these types of hydrocarbons through the chemical breakdown of animal fats and plant oils. Their greasy, nonpolar nature is a characteristic property that led to this historical name.
5. Why do branched-chain alkanes have lower boiling points than their straight-chain isomers?
This happens due to differences in intermolecular forces. Straight-chain alkanes are linear and have a larger surface area, allowing for stronger van der Waals forces between molecules. Branching makes a molecule more compact and spherical, which reduces the surface area available for contact. This weakens the intermolecular forces, meaning less energy (and a lower temperature) is needed to separate the molecules and cause boiling.
6. How does the type of bond (single, double, or triple) affect the reactivity of an aliphatic hydrocarbon?
The bond type is the main determinant of reactivity:
- Alkanes (single bonds): They only have strong sigma (σ) bonds, which are difficult to break. This makes them relatively unreactive and they typically undergo substitution reactions only under harsh conditions (e.g., UV light).
- Alkenes and Alkynes (double/triple bonds): They contain weaker pi (π) bonds in addition to sigma bonds. The electron clouds of these π bonds are more exposed, making them susceptible to attack by electrophiles. They readily undergo addition reactions, making them significantly more reactive than alkanes.
7. What are some important real-world applications of aliphatic hydrocarbons?
Aliphatic hydrocarbons are fundamental to modern life and are used in various applications:
- Fuels: Methane (natural gas), propane (LPG), and octane (a component of petrol) are used as major energy sources.
- Industrial Solvents: Hexane and heptane are used as nonpolar solvents for oils and fats.
- Raw Materials: Ethene is the monomer for producing polyethylene, one of the world's most common plastics. Propene is used to make polypropylene.
- Chemical Synthesis: They serve as starting materials for the synthesis of alcohols, plastics, and other organic compounds.
8. How can one chemically test for unsaturation in an aliphatic hydrocarbon?
A simple and common laboratory method is the bromine water test. When an unsaturated hydrocarbon (an alkene or alkyne) is mixed with reddish-brown bromine water (Br₂ in H₂O), it undergoes an addition reaction across the double or triple bond. This consumes the bromine, causing the reddish-brown colour to disappear instantly. A saturated hydrocarbon, like an alkane, will not react, and the colour will persist.
