Aliphatic Hydrocarbons

Overview and Definition

Aliphatic hydrocarbons are organic compounds consisting only of carbon and hydrogen in structures that are not aromatic. Their carbon atoms are arranged in:

• open chains (straight or branched), and/or
• non‑aromatic rings (e.g., cycloalkanes)

They are subdivided according to the type of carbon–carbon bonds:

• Alkanes – only single C–C bonds (saturated)
• Alkenes – at least one C=C double bond (unsaturated)
• Alkynes – at least one C≡C triple bond (unsaturated)

A single molecule can, in principle, contain several double and/or triple bonds; as long as no aromatic system (such as a benzene ring) is present, it is still classified as aliphatic.

Typical general formulas for simple, non‑cyclic aliphatic hydrocarbons:

• Alkanes (unbranched): $C_nH_{2n+2}$
• Alkenes with one double bond (unbranched): $C_nH_{2n}$
• Alkynes with one triple bond (unbranched): $C_nH_{2n-2}$

For cyclic variants (like cycloalkanes), the general formulas differ; these are discussed in more detail in the later subchapters.

Structural Features of Aliphatic Hydrocarbons

Open-Chain (Acyclic) vs. Cyclic

Open‑chain (acyclic) aliphatic hydrocarbons:

• Carbon atoms connected to form a chain
• The chain can be:
• straight (unbranched): each internal carbon is bonded to two other carbons
• branched: at least one carbon is connected to three or four other carbons

Cyclic aliphatic hydrocarbons (also called alicyclic hydrocarbons):

• Carbon atoms form a ring structure, but without aromaticity
• Examples: cycloalkanes such as cyclohexane
• Their bonding situation is analogous to alkanes or alkenes, but in a ring

Saturated vs. Unsaturated

• Saturated aliphatic hydrocarbons:
• Only single bonds between carbon atoms
• Each carbon has as many hydrogens as possible (subject to the valence of carbon)
• Example: alkanes and cycloalkanes
• Unsaturated aliphatic hydrocarbons:
• Contain one or more C=C double bonds and/or C≡C triple bonds
• Fewer hydrogens than the corresponding saturated compound with the same number of carbons
• Examples: alkenes, alkynes, cycloalkenes

This distinction is important because saturation strongly influences reactivity and typical reaction types (e.g., substitution vs. addition).

Types of Isomerism Relevant for Aliphatic Hydrocarbons

Within aliphatic hydrocarbons, several forms of structural isomerism and stereoisomerism occur; the general concept of isomerism is discussed elsewhere. Here, the focus is on the types that typically appear in this compound class.

Chain (Constitutional) Isomerism

Constitutional isomers differ in the way atoms are connected. For aliphatic hydrocarbons, chain isomerism is especially common:

• Same molecular formula, different arrangement of the carbon skeleton
• Example for alkanes:
• $C_4H_{10}$ can be:
• an unbranched chain (n‑butane)
• a branched chain (2‑methylpropane)

Similar chain isomerism is found in longer-chain alkenes and alkynes.

Position Isomerism (Location of Multiple Bonds or Substituents)

In unsaturated aliphatic hydrocarbons, the position of the double or triple bond can vary:

• Example: $C_4H_8$ alkenes (ignoring cyclic structures for the moment) may have the double bond between different pairs of carbon atoms

If substituents are present (e.g., in substituted alkanes), their position along the chain can also give rise to position isomers.

Geometric (cis–trans / E–Z) Isomerism in Alkenes

For alkenes with restricted rotation around the C=C bond:

• If each double‑bonded carbon has two different substituents, two arrangements are possible:
• cis (or Z): selected substituents on the same side
• trans (or E): selected substituents on opposite sides

These are often different compounds with distinct physical and chemical properties, despite having the same connectivity and formula.

(Other types of stereoisomerism are treated in more detail in the chapter on isomerism.)

Physical Properties – General Trends

While exact values depend on the particular substance, aliphatic hydrocarbons show characteristic trends in physical properties with changing chain length, degree of branching, and presence of multiple bonds.

Intermolecular Forces and Polarity

• The C–H and C–C bonds have low polarity
• Overall molecules are typically nonpolar or only very weakly polar
• Dominant intermolecular forces: London dispersion forces (a type of van der Waals force)
• Consequences:
• Very low solubility in water
• Good solubility in many organic solvents and in each other
• Low boiling and melting points compared with polar compounds of similar molar mass

Influence of Chain Length

For homologous series (e.g., n‑alkanes):

• Increasing the number of carbon atoms:
• Increases molar mass
• Increases surface area
• Strengthens dispersion forces

Therefore, with increasing chain length:

• Boiling points and melting points increase
• At room temperature:
• Short chains (e.g., $\leq C_4$ for alkanes) are gases
• Medium chains are liquids
• Long chains (waxes) are solids

Similar trends are observed for alkenes and alkynes with comparable chain lengths.

Influence of Branching

For isomeric alkanes (same formula, different branching):

• More highly branched isomers:
• Are more compact and spherical
• Have smaller contact area between neighboring molecules
• Show weaker dispersion forces

Result:

• More branching → lower boiling point (for the same molecular formula)

Branching also influences melting points and densities, though the relationship is less straightforward.

Influence of Double and Triple Bonds

Compared with the corresponding saturated chain:

• Unsaturation (C=C or C≡C) changes:
• Electron distribution
• Molecular geometry (e.g., planar arrangement around double bonds)
• Boiling points of simple alkenes/alkynes are often similar to or slightly lower than those of the corresponding alkanes with the same carbon count
• The main impact of multiple bonds is usually on chemical reactivity, discussed in more depth in the subchapters on saturated and unsaturated hydrocarbons.

Occurrence and Sources

Aliphatic hydrocarbons occur widely in nature and in technical processes. Only a few common examples are mentioned here to illustrate their relevance; more detailed application contexts are treated elsewhere.

In Nature

• Natural gas:
• Mainly low‑molecular‑mass alkanes (methane, ethane, propane, butane)
• Largely of biogenic or thermogenic origin (decomposition of organic material over geological timescales)
• Petroleum (crude oil):
• Complex mixture of many hydrocarbons
• Contains:
• Alkanes and cycloalkanes (major fraction)
• Alkenes are typically minor or formed during processing
• Serves as a raw material for fuels and numerous chemical products
• Biological lipids (e.g., waxes):
• Often contain long‑chain aliphatic hydrocarbons or their derivatives
• Provide water‑repellent coatings on plant leaves and animal fur or feathers
• Volatile organic compounds (VOCs) from organisms:
• Some are aliphatic (e.g., small alkenes) and participate in ecological signaling or atmospheric chemistry

Technical and Everyday Occurrence

• Fuels:
• Gasoline, diesel, and kerosene are rich in alkanes and cycloalkanes of various chain lengths
• Liquefied petroleum gas (LPG) largely consists of propane and butane
• Solvents:
• Many paint thinners, degreasers, and cleaning agents contain aliphatic hydrocarbons (e.g., hexane, heptane mixtures)
• Plastics and synthetic materials:
• Many polymers are produced from simple aliphatic monomers (e.g., ethene → polyethylene, propene → polypropylene)

General Reactivity Characteristics

Specific reactions and mechanisms are discussed in the separate chapters on saturated and unsaturated chain hydrocarbons. Here, only the broad differences in reactivity within aliphatic hydrocarbons are outlined.

Saturated Aliphatic Hydrocarbons (Alkanes, Cycloalkanes)

• Contain only C–C and C–H single bonds
• These bonds are relatively strong and nonpolar
• As a result:
• Low chemical reactivity under mild conditions
• Typical reactions include:
• Combustion
• Substitution reactions under more energetic conditions (e.g., halogenation with UV light)

Unsaturated Aliphatic Hydrocarbons (Alkenes, Alkynes, Cycloalkenes)

• Contain at least one multiple bond (C=C or C≡C)
• Pi (π) electrons in multiple bonds are more accessible and more reactive
• This leads to:
• Higher reactivity than alkanes
• Characteristic addition reactions:
• Addition of hydrogen (hydrogenation)
• Addition of halogens, hydrogen halides, water, etc.
• Possibility of polymerization (e.g., alkenes forming polymers)

The presence and number of multiple bonds critically determine the type and speed of reactions that aliphatic hydrocarbons undergo.

Environmental and Safety Aspects (Overview)

Aliphatic hydrocarbons are widely used and handled; knowing their general environmental and safety implications is important, although detailed environmental chemistry is treated elsewhere.

Flammability and Explosion Risk

• Most low‑molecular‑mass aliphatic hydrocarbons:
• Are highly flammable
• Form explosive mixtures with air in certain concentration ranges
• Liquefied gases (e.g., propane–butane mixtures) carry specific handling and storage requirements

Health and Toxicological Considerations

• Short‑term exposure to high concentrations of vapors:
• Can cause dizziness, headaches, and central nervous system depression
• Some aliphatic hydrocarbons (especially certain solvents) can:
• Defat the skin on prolonged contact
• Irritate the respiratory tract

Many simple aliphatic hydrocarbons have relatively low acute toxicity compared with more reactive or functionalized compounds; however, safe handling guidelines must always be observed.

Environmental Persistence

• Many aliphatic hydrocarbons:
• Are poorly soluble in water
• Spread on water surfaces (e.g., oil films)
• Undergo slow degradation by environmental and microbial processes
• In large quantities, they can:
• Harm aquatic and terrestrial ecosystems
• Contribute to air pollution, especially when incompletely combusted

Detailed degradation pathways and environmental effects are addressed in environmental chemistry.

Summary of Key Characteristics

• Composition: Only carbon and hydrogen; no aromatic rings
• Structure: Open chains (straight or branched) and non‑aromatic rings
• Classification: Saturated (alkanes, cycloalkanes) vs. unsaturated (alkenes, alkynes, cycloalkenes)
• Isomerism: Chain, position, and geometric isomerism are especially important
• Physical behavior: Nonpolar, hydrophobic, low boiling points that increase with chain length, decreased boiling point with increasing branching
• Reactivity:
• Saturated: relatively unreactive; mainly combustion and substitution under specific conditions
• Unsaturated: significantly more reactive; typical addition and polymerization reactions
• Importance: Major components of natural gas and crude oil; fundamental raw materials for fuels, solvents, and polymers, and widely present in biological and environmental systems.

The following subchapters will examine saturated and unsaturated chain hydrocarbons in more detail, including their specific structures, nomenclature, and characteristic reactions.