Saturated Chain Hydrocarbons

Introduction and Scope

In the context of aliphatic hydrocarbons, saturated chain hydrocarbons are those in which all carbon–carbon bonds are single bonds and the carbon atoms are arranged in open chains (not in rings). They form a very important and conceptually simple family of organic compounds and serve as a basis for understanding more complex structures and reactions.

In this chapter we focus on:

• What distinguishes saturated chain hydrocarbons from other hydrocarbons.
• Their structural types (unbranched and branched).
• Their general formulas and homologous series.
• Typical physical properties.
• Typical reactions and chemical behavior that are characteristic for this class.

General aspects of formulas, structures, and basic organic nomenclature are handled in other chapters and will only be used here to the extent necessary.

Definition and Classification

Saturation and Chain Structure

A hydrocarbon is called saturated when:

• Every carbon atom has formed four single covalent bonds (to C or H).
• There are no C=C double bonds and no C≡C triple bonds.

A hydrocarbon is called a chain (acyclic, open-chain) hydrocarbon when:

• The carbon atoms are connected in an open sequence.
• There is no closed ring (that would be a cycloalkane and is treated elsewhere).

Combining both:

• Saturated chain hydrocarbons are acyclic hydrocarbons with only C–C single bonds and C–H bonds.

Alkanes as Saturated Chain Hydrocarbons

The main family of saturated chain hydrocarbons is the alkanes. When we refer to alkanes in this chapter, we mean specifically acyclic (non-cyclic) alkanes.

Characteristics of alkanes:

• Composition: carbon and hydrogen only.
• C atoms: sp³-hybridized, approximately tetrahedral geometry.
• Bonding: only σ-bonds (sigma bonds) between C–C and C–H.
• General name pattern: names ending in -ane (methane, ethane, propane, …).

Cycloalkanes (ring alkanes) are generally discussed separately and are not the focus here.

Structural Types

Unbranched (Normal) Alkanes

Unbranched saturated chain hydrocarbons are often called:

• n-alkanes (normal alkanes), or
• straight-chain alkanes.

In these compounds:

• Each carbon (except the terminal carbons) is bonded to two neighboring carbon atoms and to two hydrogen atoms.
• Terminal carbons are bonded to one carbon and three hydrogens.

Examples of unbranched alkanes:

• Methane: 1 C atom.
• Ethane: 2 C atoms.
• Propane: 3 C atoms.
• Butane: 4 C atoms.
• Pentane: 5 C atoms.

Higher unbranched alkanes continue the same pattern.

Branched Alkanes

As the number of carbon atoms increases (from C₄ onward), branched saturated chain hydrocarbons become possible.

Key points:

• Branched alkanes have at least one carbon atom connected to three or four other carbon atoms.
• They are structural isomers of the corresponding unbranched alkanes (same molecular formula, different connectivity).
• Branching patterns strongly influence physical properties (e.g. melting and boiling points) and somewhat affect chemical reactivity (e.g. combustion behavior, selectivity in substitution reactions).

Example:

• C₄H₁₀:
• n-butane (unbranched),
• isobutane (2-methylpropane, branched).

A detailed treatment of isomerism is provided in the chapter on the concept of isomerism; here we only note that branching increases rapidly with chain length.

General Formulas and Homologous Series

General Molecular Formula for Acyclic Saturated Hydrocarbons

For a saturated, acyclic hydrocarbon with $n$ carbon atoms, the molecular formula is:

$$
\mathrm{C_nH_{2n+2}} \quad (n \ge 1)
$$

This reflects:

• Each internal C bonded to 2 H (plus 2 C).
• Each terminal C bonded to 3 H (plus 1 C).

Examples:

• Methane: $\mathrm{CH_4}$ (n = 1)
• Ethane: $\mathrm{C_2H_6}$ (n = 2)
• Propane: $\mathrm{C_3H_8}$ (n = 3)

This formula holds for both unbranched and branched acyclic alkanes. Cycloalkanes obey a different general formula (treated elsewhere).

Homologous Series

The alkanes form a homologous series:

• Each successive member differs from the previous one by a $\mathrm{CH_2}$ unit.
• This stepwise increase in chain length leads to a gradual change in physical properties (e.g. boiling point, melting point, density).

If we list some unbranched alkanes:

• Methane $\mathrm{CH_4}$
• Ethane $\mathrm{C_2H_6}$
• Propane $\mathrm{C_3H_8}$
• Butane $\mathrm{C_4H_{10}}$
• Pentane $\mathrm{C_5H_{12}}$
• Hexane $\mathrm{C_6H_{14}}$

each one contains one more $\mathrm{CH_2}$ group than the previous.

This regularity is important later when relating structure to physical properties and when predicting formulas.

Structural Features and Bonding

Bond Angles and Geometry

Because the carbon atoms are sp³-hybridized:

• Each carbon center in a saturated chain hydrocarbon has approximately tetrahedral geometry.
• The ideal bond angle at each carbon is about $109.5^\circ$.

Consequences:

• Even “straight-chain” alkanes are actually zig-zag chains in three dimensions; they are not linear in the strict geometric sense.
• Rotation around C–C single bonds is largely unrestricted at room temperature, so many conformations are possible.

C–C and C–H Bonds

The key bonds in saturated chain hydrocarbons are:

• C–C single bonds:
• Strong σ-bonds.
• Relatively non-polar.
• C–H bonds:
• Also σ-bonds.
• Weakly polarized (C slightly more electronegative than H), but overall nearly non-polar.

Because of this:

• Alkanes are generally chemically rather unreactive compared to many other organic compounds.
• They do not have functional groups in the sense of strongly reactive structural units; the C–H and C–C σ-bonds are comparatively inert.

Physical Properties

Aggregation State and Boiling Points

The boiling points of unbranched saturated chain hydrocarbons:

• Increase with rising chain length (higher molecular mass, larger surface).
• For very low molecular mass:
• Methane, ethane, propane, and butane are gases at room temperature.
• Intermediate chain lengths (about C₅–C₁₆):
• Liquids at room temperature.
• Higher members:
• Waxy or solid substances.

Main reasons for the trend:

• Larger molecules have more electrons and larger surface areas.
• This strengthens London dispersion forces (a type of van der Waals interaction).
• Stronger intermolecular forces require more energy (higher temperatures) for molecules to escape into the gas phase.

Influence of Branching

Branching affects boiling and melting points:

• Branched alkanes have lower boiling points than their unbranched isomers (for the same molecular formula).
• Explanation:
• Branching makes molecules more compact and reduces the effective surface area for intermolecular contact.
• Weaker dispersion forces lead to lower boiling points.

For melting points:

• The picture can be more complex, but generally:
• Highly symmetrical molecules pack better in the solid state and may have unusually high melting points.
• Many branched alkanes are less symmetrical and can have lower melting points than their unbranched analogues.

Solubility

Due to their non-polar nature:

• Saturated chain hydrocarbons are insoluble or only very slightly soluble in water, which is polar.
• They are soluble in non-polar or weakly polar organic solvents, such as hexane itself, other alkanes, or organic solvents like benzene and many ethers.

A useful rule of thumb: “like dissolves like”—non-polar substances dissolve well in non-polar solvents.

Density and Appearance

For typical alkanes:

• Their density is lower than that of water; they float on water.
• Many liquid alkanes are:
• Colorless.
• Have low viscosity at moderate chain lengths.
• Many solid alkanes (higher homologues, paraffins) are:
• White or translucent.
• Waxy.

Chemical Behavior and Typical Reactions

General Inertness

Saturated chain hydrocarbons are often described as chemically inert under mild conditions:

• They do not react with strong acids, strong bases, or oxidizing agents under conditions where many other organic compounds would.
• They are resistant to many chemical reagents at room temperature.

This apparent inertness, however, is relative. Under more energetic conditions (e.g. heat, ultraviolet light, catalysts), saturated chain hydrocarbons do undergo characteristic reactions.

Combustion

One of the most important reactions of saturated chain hydrocarbons is combustion:

• In the presence of sufficient oxygen, complete combustion leads to:
• Carbon dioxide: $\mathrm{CO_2}$
• Water: $\mathrm{H_2O}$
• For a general alkane $\mathrm{C_nH_{2n+2}}$, the ideal complete combustion equation is:

$$
\mathrm{C_nH_{2n+2} + \left(\frac{3n+1}{2}\right) O_2 \rightarrow n\,CO_2 + (n+1)\,H_2O}
$$

Example (methane):

$$
\mathrm{CH_4 + 2\,O_2 \rightarrow CO_2 + 2\,H_2O}
$$

Characteristics:

• Combustion is strongly exothermic (releases a large amount of energy).
• Many saturated chain hydrocarbons are used as fuels:
• Methane (natural gas).
• Propane and butane (liquefied petroleum gas).
• Mixtures of higher alkanes (components of gasoline, diesel, kerosene).

If oxygen supply is insufficient, incomplete combustion occurs, forming carbon monoxide $\mathrm{CO}$ or even solid carbon (soot), which has significant health and environmental relevance.

Substitution Reactions with Halogens

Another characteristic reaction of saturated chain hydrocarbons is radical substitution with halogens (especially chlorine and bromine) under the influence of light or heat.

General form (for a simple alkane):

$$
\mathrm{RH + X_2 \xrightarrow{h\nu \ \text{or} \ \Delta} RX + HX}
$$

where:

• $\mathrm{RH}$ is the alkane,
• $\mathrm{X_2}$ is a halogen (Cl₂, Br₂),
• $\mathrm{RX}$ is the haloalkane,
• $\mathrm{HX}$ is the hydrogen halide (HCl, HBr).

Example with methane and chlorine:

$$
\mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl}
$$

Features:

• The reaction proceeds via a radical mechanism (formation of reactive intermediates with unpaired electrons).
• Multiple substitutions can occur, leading to a mixture of products (e.g. $\mathrm{CH_3Cl}$, $\mathrm{CH_2Cl_2}$, $\mathrm{CHCl_3}$, $\mathrm{CCl_4}$ from methane).
• The position of substitution in higher alkanes depends on the type of hydrogen (primary, secondary, tertiary) and the relative reactivity of different C–H bonds.

These halogenation reactions are an important way to transform relatively unreactive saturated chain hydrocarbons into more reactive derivatives (haloalkanes), which can then be further transformed.

Cracking and Reforming (Overview)

In industrial chemistry, saturated chain hydrocarbons (especially those derived from petroleum) are subject to processes such as:

• Cracking:
• Breaking long-chain alkanes into shorter-chain alkanes and alkenes.
• Achieved by heat, sometimes in the presence of catalysts.
• Purpose: adjust the distribution of molecular sizes for fuel applications (e.g. produce more gasoline-range molecules).
• Reforming and Isomerization:
• Reorganizing the structure of alkanes (e.g. converting straight-chain alkanes to branched ones or to aromatic compounds).
• Goal: improve fuel quality (e.g. higher octane number).

The detailed discussion of these industrial processes appears in chemical engineering and petroleum processing chapters; here we simply note that saturated chain hydrocarbons are the primary feedstock.

Natural Occurrence and Uses

Sources

Saturated chain hydrocarbons occur predominantly in:

• Natural gas:
• Mainly methane, with smaller amounts of ethane, propane, and butane.
• Petroleum (crude oil):
• Complex mixture containing many different alkanes (as well as cycloalkanes and aromatic compounds).
• Biogenic sources:
• Some are produced in smaller amounts by biological activity (e.g. biogas from anaerobic decomposition).

They may also appear in:

• Wax components of plants and animals.
• Certain protective and energy-storage materials in organisms.

Uses

Important uses include:

• Energy carriers:
• Heating (methane, propane, butane).
• Fuels (gasoline, diesel, jet fuel—mixtures rich in alkanes).
• Chemical raw materials:
• Starting materials for the synthesis of many other organic compounds.
• Solvents:
• Non-polar alkanes such as hexane are used as solvents and extraction agents.
• Lubricants and waxes:
• Higher alkanes and mixtures of them serve as lubricants, greases, and paraffin waxes.

Their relative chemical inertness and non-polarity also shape their role in the environment and in technological applications (for instance, as hydrophobic coatings or components of insulating materials).

Summary

Saturated chain hydrocarbons (acyclic alkanes):

• Contain only C–C single and C–H bonds and follow the general formula $\mathrm{C_nH_{2n+2}}$.
• Exist as unbranched and branched structures, with branching giving rise to structural isomerism.
• Form a homologous series where each member differs by a $\mathrm{CH_2}$ unit.
• Are non-polar, low-density substances whose boiling and melting points increase with molecular size and are influenced by branching.
• Are relatively inert chemically but participate in important reactions such as combustion and halogen substitution.
• Occur mainly in natural gas and petroleum and are central as fuels and as feedstocks for the chemical industry.

This understanding provides a foundation for studying unsaturated hydrocarbons, aromatic compounds, and functionalized organic molecules, which introduce more reactive structural features.