Saturated & Unsaturated Hydrocarbons - Tenth Grade Science
Hydrocarbons are the parent organic compounds consisting exclusively of carbon and hydrogen atoms, serving as the foundational framework from which all other functionalized organic molecules are derived. Petroleum, natural gas, gasoline, diesel, and paraffin wax are composed of complex mixtures of hydrocarbons refined from fossil reserves. Based on the bonding nature between carbon atoms, hydrocarbons are classified into saturated hydrocarbons, containing exclusively single carbon-carbon bonds, and unsaturated hydrocarbons, containing one or more double or triple carbon-carbon bonds. Saturated and unsaturated hydrocarbons exhibit contrasting chemical stabilities, addition behaviors, and combustion profiles. In this chapter, you will investigate the structural characteristics of alkanes, alkenes, and alkynes, explore structural isomerism and cyclic ring systems, analyze the organizing principles of homologous series, and examine diagnostic tests for unsaturation.
Classification of Hydrocarbons: Saturated versus Unsaturated
Hydrocarbons are categorized into structural families based on the saturation state of their carbon skeletons.
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| TAXONOMY OF ALIPHATIC HYDROCARBONS |
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| HYDROCARBONS |
| / \ |
| SATURATED UNSATURATED |
| | / \ |
| ALKANES ALKENES ALKYNES |
| (C - C) (C = C) (C === C) |
| CnH2n+2 CnH2n CnH2n-2 |
| |
| Geometry: Tetrahedral Trigonal Planar Linear |
| Hybridization: sp3 sp2 sp |
| Bond Angle: 109.5 deg 120 deg 180 deg |
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Saturated Hydrocarbons: Alkanes
A saturated hydrocarbon is a compound wherein all carbon-carbon bonds are single covalent $\sigma$-bonds, maximizing the hydrogen atom capacity of the carbon skeleton. Saturated open-chain hydrocarbons are designated as alkanes, or historically as paraffins (from Latin *parum affinis*, indicating low chemical reactivity).
The general molecular formula for acyclic alkanes is:
$$\text{C}*n\text{H}*{2n+2} \quad (\text{where } n = 1, 2, 3, \dots)$$
The simplest alkane is methane ($\text{CH}*4$, $n=1$), followed by ethane ($\text{C}_2\text{H}_6$, $n=2$), propane ($\text{C}_3\text{H}_8$, $n=3$), and butane ($\text{C}_4\text{H}*{10}$, $n=4$). In all alkanes, every carbon atom is $sp^3$ hybridized, forming four covalent $\sigma$-bonds directed toward the vertices of a regular tetrahedron with bond angles of $109.5^\circ$. Because all valence requirements are satisfied by single bonds, alkanes are chemically unreactive at room temperature, resisting attack by strong acids, alkalis, and oxidizing agents.
Unsaturated Hydrocarbons: Alkenes and Alkynes
An unsaturated hydrocarbon is a compound containing at least one carbon-carbon double bond or triple bond within its molecular structure. Because multiple bonds replace hydrogen atoms, these compounds contain fewer hydrogen atoms than the corresponding saturated alkanes:
Alkenes (Olefins): Acyclic hydrocarbons possessing at least one carbon-carbon double bond ($-\text{C}=\text{C}-$). The double bond consists of one strong $\sigma$-bond formed by end-to-end $sp^2$ hybrid orbital overlap, and one weaker $\pi$-bond formed by lateral overlap of unhybridized $p$-orbitals. The general molecular formula for alkenes with one double bond is:
$$\text{C}*n\text{H}*{2n} \quad (\text{where } n = 2, 3, 4, \dots)$$
The simplest alkene is ethene ($\text{C}_2\text{H}_4$, traditionally ethylene), followed by propene ($\text{C}_3\text{H}_6$) and butene ($\text{C}_4\text{H}_8$). The double-bonded carbons are $sp^2$ hybridized with trigonal planar geometry and bond angles of $120^\circ$.
Alkynes (Acetylenes): Acyclic hydrocarbons containing at least one carbon-carbon triple bond ($-\text{C}\equiv\text{C}-$). The triple bond comprises one strong $\sigma$-bond and two perpendicular $\pi$-bonds. The general molecular formula for alkynes with one triple bond is:
$$\text{C}*n\text{H}*{2n-2} \quad (\text{where } n = 2, 3, 4, \dots)$$
The simplest alkyne is ethyne ($\text{C}_2\text{H}_2$, commonly acetylene), followed by propyne ($\text{C}_3\text{H}_4$) and butyne ($\text{C}_4\text{H}_6$). The triple-bonded carbons are $sp$ hybridized with linear geometry and bond angles of $180^\circ$.
Because the $\pi$-bonds in alkenes and alkynes have exposed electron clouds above and below the internuclear axis, unsaturated hydrocarbons are chemically reactive, readily undergoing addition reactions to relieve unsaturation.
Structural Isomerism in Hydrocarbons
Isomerism is the phenomenon wherein two or more chemical compounds share the identical molecular formula but possess different structural arrangements of atoms, resulting in distinct physical and chemical properties.
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| STRUCTURAL ISOMERS OF BUTANE (C4H10) |
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| NORMAL BUTANE (n-Butane) ISOBUTANE (2-Methylpropane) |
| Linear Unbranched Branched Chain |
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| H H H H H H H |
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| H - C - C - C - C - H H - C - C - C - H |
| | | | | | | | |
| H H H H H H-C-H H |
| | |
| Boiling Point: -0.5 deg C H |
| Boiling Point: -11.7 deg C |
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Chain Isomerism in Alkanes
In alkanes, structural isomerism emerges when the carbon skeleton can be configured into linear or branched arrangements. Methane ($\text{CH}_4$), ethane ($\text{C}_2\text{H}_6$), and propane ($\text{C}_3\text{H}_8$) possess only one possible structural arrangement; they have no isomers.
Butane ($\text{C}*4\text{H}*{10}$) is the first alkane to exhibit structural chain isomerism, existing as two distinct isomers: Unbranched $n$-butane (butane), featuring a continuous four-carbon chain with a boiling point of $-0.5^\circ\text{C}$. Branched isobutane (2-methylpropane), featuring a branched three-carbon chain with a methyl group on the central carbon, exhibiting a lower boiling point of $-11.7^\circ\text{C}$ due to reduced surface contact area and weaker van der Waals forces.
Pentane ($\text{C}*5\text{H}*{12}$) possesses three chain isomers: $n$-pentane, isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane). As the number of carbon atoms increases, the number of possible isomers grows rapidly: hexane ($\text{C}*6\text{H}*{14}$) has five isomers, decane ($\text{C}*{10}\text{H}*{22}$) has seventy-five isomers, and eicosane ($\text{C}*{20}\text{H}*{42}$) has over 366,000 theoretical isomers.
Cyclic and Aromatic Hydrocarbons
Carbon chains can also link end-to-end to form closed ring structures, classified as cyclic hydrocarbons.
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| CYCLIC HYDROCARBON STRUCTURES |
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| CYCLOHEXANE (C6H12) | BENZENE (C6H6) |
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| Saturated Alicyclic Ring | Unsaturated Aromatic Ring |
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| H2C --- CH2 | HC ==== CH |
| / \ | / \ |
| H2C CH2 | HC CH |
| \ / | \ / |
| H2C --- CH2 | HC ==== CH |
| | |
| All C-C bonds are single (sp3). | Alternating double/single bonds; resonant |
| Formula matches alkene: CnH2n. | delocalized 6 pi-electron aromatic ring. |
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Cycloalkanes: Saturated Rings
Cycloalkanes are closed ring structures composed exclusively of single-bonded $sp^3$ carbons. The general molecular formula for monocyclic cycloalkanes is:
$$\text{C}*n\text{H}*{2n} \quad (\text{where } n = 3, 4, 5, \dots)$$
Because forming a ring requires closing two chain ends, cycloalkanes have two fewer hydrogen atoms than their corresponding open-chain alkanes, giving them the same empirical formula as alkenes. Cyclohexane ($\text{C}*6\text{H}*{12}$) adopts puckered non-planar conformations (primarily the chair conformation) to relieve ring strain and maintain tetrahedral $109.5^\circ$ bond angles.
Benzene: Aromatic Delocalization
Benzene ($\text{C}_6\text{H}_6$) is the parent aromatic hydrocarbon. It consists of a planar hexagonal ring of six $sp^2$ hybridized carbon atoms, each bonded to one hydrogen atom. While historically drawn as alternating single and double bonds (the Kekulé structure), experimental X-ray diffraction confirms that all six carbon-carbon bonds in benzene are identical in length ($139\text{ pm}$), intermediate between a single bond ($154\text{ pm}$) and a double bond ($134\text{ pm}$). The six unhybridized $p$-electrons overlap continuously around the ring, forming a delocalized $\pi$-electron cloud above and below the molecular plane. This resonance stabilization makes benzene unusually stable, resisting standard addition reactions and favoring electrophilic substitution.
The Homologous Series Framework
A homologous series is a family of structurally related organic compounds that share the same functional group, follow a common general molecular formula, and exhibit regular gradations in physical properties and identical chemical behaviors.
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| ALKANE HOMOLOGOUS SERIES PROGRESSION |
+---------+-----------------+-------------------+-------------------+---------------+
| Member | Molecular Form. | Condensed Struct. | Incremental Diff. | Boiling Point |
+---------+-----------------+-------------------+-------------------+---------------+
| Methane | CH4 | CH4 | Base member | -161.5 deg C |
| Ethane | C2H6 | CH3-CH3 | + CH2 (14 u) | -88.6 deg C |
| Propane | C3H8 | CH3-CH2-CH3 | + CH2 (14 u) | -42.1 deg C |
| Butane | C4H10 | CH3-(CH2)2-CH3 | + CH2 (14 u) | -0.5 deg C |
| Pentane | C5H12 | CH3-(CH2)3-CH3 | + CH2 (14 u) | +36.1 deg C |
+---------+-----------------+-------------------+-------------------+---------------+
Defining Characteristics of a Homologous Series
Characteristic 1: Any two successive members of a homologous series differ in their molecular formula by exactly one methylene group ($-\text{CH}_2-$).
Characteristic 2: Any two successive members differ in molecular mass by exactly 14 atomic mass units ($12\text{ u for carbon} + 2 \times 1\text{ u for hydrogen}$).
Characteristic 3: All members of a given series share the same general molecular formula (e.g., $\text{C}*n\text{H}*{2n+2}$ for alkanes, $\text{C}*n\text{H}*{2n}$ for alkenes, $\text{C}*n\text{H}*{2n+1}\text{OH}$ for alcohols).
Characteristic 4: All members of a series display similar chemical properties because they possess the same functional group and bond types.
Characteristic 5: Physical properties—including melting point, boiling point, density, and viscosity—exhibit a regular, gradual progression as molecular mass increases. As carbon chain length grows, increased surface area strengthens intermolecular van der Waals forces, raising melting and boiling points.
Diagnostic Tests for Unsaturation
Unsaturated hydrocarbons can be distinguished experimentally from saturated hydrocarbons using diagnostic chemical tests.
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| CHEMICAL TESTS FOR UNSATURATION |
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| BROMINE WATER TEST | BAEYER'S TEST (Cold Alkaline KMnO4) |
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| Reagent: Aqueous reddish-brown | Reagent: Alkaline potassium permanganate |
| bromine solution (Br2 in water). | (Vivid purple/magenta solution). |
| | |
| Saturated Alkane: | Saturated Alkane: |
| No reaction; solution remains | No reaction; solution remains vivid purple. |
| reddish-brown in darkness. | |
| | Unsaturated Alkene / Alkyne: |
| Unsaturated Alkene / Alkyne: | Purple color discharged; brown precipitate of |
| Reddish-brown color is rapidly | MnO2 forms as the hydrocarbon oxidizes into |
| decolored: | a vicinal diol: |
| C2H4 + Br2 -> CH2Br-CH2Br | C2H4 + H2O + [O] -> CH2OH-CH2OH |
| (1,2-Dibromoethane is colorless) | (Ethane-1,2-diol is colorless) |
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The Bromine Water Decolorization Test
Bromine water is an aqueous solution of molecular bromine possessing a distinct reddish-orange color. When an unsaturated hydrocarbon like ethene is bubbled through bromine water, electrophilic addition occurs across the double bond, converting the unsaturated compound into colorless 1,2-dibromoethane:
CH2=CH2 + Br2(aq) [Red-Brown] -> CH2Br-CH2Br(l) [Colorless]
Saturated hydrocarbons like methane or ethane do not react with bromine water in the absence of ultraviolet radiation, leaving the reddish-brown color unchanged.
Baeyer's Reagent Test
Baeyer's reagent is a cold, dilute, alkaline solution of potassium permanganate ($\text{KMnO}_4$), which displays a vivid purple color. When an alkene or alkyne is shaken with Baeyer's reagent, the double bond is oxidized to a vicinal diol (glycol), while permanganate ($\text{Mn}^{7+}$) is reduced to manganese dioxide ($\text{MnO}_2$, a brown precipitate):
CH2=CH2 + H2O + [O] --[Alkaline KMnO4]--> CH2OH-CH2OH (Ethylene Glycol)
The discharge of the purple color confirms the presence of carbon-carbon unsaturation.
Chapter Exercises
Exercise 1: An open-chain hydrocarbon X has the molecular formula $\text{C}_4\text{H}_8$. Determine whether X is saturated or unsaturated, write the condensed structural formulas and IUPAC names for all possible structural isomers of X, and describe a chemical test to confirm its unsaturation.
Exercise 2: Explain why boiling points increase progressively from methane ($-161.5^\circ\text{C}$) to $n$-pentane ($+36.1^\circ\text{C}$), but decrease when moving from $n$-pentane ($+36.1^\circ\text{C}$) to its branched structural isomer neopentane ($+9.5^\circ\text{C}$).
Exercise 3: Write the general molecular formulas for alkanes, alkenes, alkynes, and cycloalkanes, and determine the molecular formulas for an alkane containing 18 carbon atoms and an alkyne containing 10 carbon atoms.
Solutions and Explanations
Solution 1: The hydrocarbon X has the molecular formula $\text{C}*4\text{H}_8$, matching the general formula $\text{C}_n\text{H}*{2n}$ ($n=4$). Because it is an open-chain hydrocarbon, it contains one carbon-carbon double bond and is therefore an unsaturated alkene. The structural isomers of open-chain $\text{C}_4\text{H}_8$ are: first, but-1-ene: $\text{CH}_2=\text{CH}-\text{CH}_2-\text{CH}_3$; second, but-2-ene: $\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3$; and third, 2-methylprop-1-ene: $\text{CH}_2=\text{C}(\text{CH}_3)-\text{CH}_3$. To confirm its unsaturation, shake a sample of the gas with reddish-brown bromine water. The rapid decolorization of the bromine solution from reddish-brown to clear and colorless confirms the presence of carbon-carbon double bond unsaturation.
Solution 2: Alkanes are non-polar molecules held together in liquid phase by weak intermolecular London dispersion forces. The strength of these dispersion forces is directly proportional to molecular surface area and the total number of polarizable electrons. Moving from methane ($\text{CH}*4$) to $n$-pentane ($\text{C}_5\text{H}*{12}$), each successive methylene group adds two electrons and extends the carbon chain, increasing molecular surface area and strengthening intermolecular attractions. Consequently, progressively higher thermal kinetic energy is required to vaporize the liquid, raising the boiling point. Conversely, when comparing the structural isomers $n$-pentane and neopentane (2,2-dimethylpropane), both share the identical molecular formula ($\text{C}*5\text{H}*{12}$) and electron count. However, $n$-pentane is an extended linear chain with high surface contact area, whereas neopentane is highly branched and adopts a compact, nearly spherical geometry. The spherical shape reduces the contact surface area between adjacent molecules, weakening intermolecular dispersion forces and lowering the boiling point from $+36.1^\circ\text{C}$ for $n$-pentane to $+9.5^\circ\text{C}$ for neopentane.
Solution 3: The general molecular formulas are: alkanes: $\text{C}*n\text{H}*{2n+2}$; alkenes (with one double bond): $\text{C}*n\text{H}*{2n}$; alkynes (with one triple bond): $\text{C}*n\text{H}*{2n-2}$; and monocyclic cycloalkanes: $\text{C}*n\text{H}*{2n}$. For an alkane containing 18 carbon atoms ($n = 18$), the number of hydrogen atoms is $2(18) + 2 = 36 + 2 = 38$, giving the molecular formula $\text{C}*{18}\text{H}*{38}$ (octadecane). For an alkyne containing 10 carbon atoms ($n = 10$), the number of hydrogen atoms is $2(10) - 2 = 20 - 2 = 18$, giving the molecular formula $\text{C}*{10}\text{H}*{18}$ (decyne).