Chemical Properties: Combustion, Oxidation, Addition & Substitution - Tenth Grade Science
Organic carbon compounds undergo diverse chemical transformations that provide primary energy sources for transportation and heating, as well as chemical pathways for synthesizing industrial plastics, solvents, and pharmaceuticals. Unlike inorganic ionic reactions, which occur rapidly in aqueous solution via electrostatic ion pairing, organic reactions involve the cleavage and reforming of directional covalent bonds, typically requiring specific activation energies, catalysts, or electromagnetic radiation. The chemical reactivity of carbon compounds is categorized into four primary reaction classes: combustion, controlled oxidation, addition, and substitution. In this chapter, you will investigate the thermodynamics of complete versus incomplete combustion, explore selective oxidation of alcohols to carboxylic acids, analyze catalytic addition across unsaturated bonds during industrial hydrogenation, and master the free-radical halogenation substitution mechanisms of saturated alkanes.
Combustion Transformations and Thermochemistry
Combustion is an exothermic redox reaction in which a carbon compound reacts rapidly with atmospheric oxygen, releasing thermal energy and light.
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| COMPLETE VERSUS INCOMPLETE COMBUSTION |
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| COMPLETE COMBUSTION (Excess Air) | INCOMPLETE COMBUSTION (Limited Air) |
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| Hydrocarbon + Excess O2 -----> | Hydrocarbon + Limited O2 -----> |
| CO2(g) + H2O(g) + Maximum Heat | CO(g) + C(s) (Soot) + H2O(g) + Reduced Heat |
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| Clean, non-luminous blue flame. | Smoky, luminous yellow flame. |
| Complete oxidation of all carbon. | Toxic carbon monoxide gas and carbon black. |
| Example: | Example: |
| CH4 + 2O2 -> CO2 + 2H2O + 891 kJ | 2CH4 + 3O2 -> 2CO + 4H2O + Reduced Heat |
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Complete Combustion of Saturated Hydrocarbons
When saturated hydrocarbons (alkanes) burn in an excess supply of oxygen, all carbon atoms are oxidized to carbon dioxide, while all hydrogen atoms are oxidized to water vapor:
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) + 891 kJ/mol
C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(g) + 2220 kJ/mol
Saturated hydrocarbons produce a clean, non-luminous blue flame because the carbon-to-hydrogen ratio is relatively low, permitting complete oxidation without thermal cracking into soot. The large negative enthalpy of combustion ($\Delta H < 0$) makes alkanes effective domestic and industrial fuels.
Incomplete Combustion and Environmental Hazards
If the air supply is restricted, incomplete combustion occurs. Insufficient oxygen leaves carbon partially unoxidized, generating toxic carbon monoxide gas ($\text{CO}$) and fine particles of unburned elemental carbon, observed as black soot:
CH4(g) + O2(g) [Restricted] -> C(s) [Soot] + 2H2O(g)
Carbon monoxide is a hazardous, odorless, colorless gas. When inhaled, it binds to hemoglobin in red blood cells with an affinity approximately two hundred times greater than oxygen, forming carboxyhemoglobin, which inhibits oxygen delivery to vital tissues and can cause asphyxiation.
Unsaturated hydrocarbons (alkenes and alkynes) contain a higher percentage of carbon by mass than saturated alkanes. When burned in open air, ambient oxygen diffusion is insufficient to fully oxidize the high carbon content, resulting in a luminous yellow flame accompanied by heavy black smoke and soot deposits. Complete combustion of ethyne requires enrichment with pure oxygen, as utilized in oxy-acetylene welding torches to generate temperatures exceeding $3000^\circ\text{C}$:
2C2H2(g) + 5O2(g) -> 4CO2(g) + 2H2O(g) + Immense Heat
Controlled Chemical Oxidation
While combustion destroys the carbon skeleton to produce carbon dioxide and water, controlled oxidation uses specific chemical oxidants to transform functional groups while preserving the carbon chain.
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| OXIDATION PATHWAYS OF PRIMARY ALCOHOLS |
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| [Alcohol] [Aldehyde] [Carboxylic Acid]|
| R-CH2-OH --[+[O]]--> R-CHO --[+[O]]--> R-COOH |
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| Reagents: Alkaline Potassium Permanganate (KMnO4 + KOH + Heat) |
| Acidified Potassium Dichromate (K2Cr2O7 + H2SO4 + Heat) |
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Oxidation of Ethanol to Ethanoic Acid
When primary alcohols are warmed with strong chemical oxidizing agents, they undergo progressive oxidation:
CH3CH2OH(l) --[Alkaline KMnO4 or Acidified K2Cr2O7, Heat]--> CH3COOH(aq)
In this reaction, alkaline potassium permanganate ($\text{KMnO}_4$) or acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) acts as an oxidizing agent, transferring nascent oxygen to the organic substrate:
When ethanol reacts with acidified potassium dichromate, the solution changes from bright orange (dichromate ion, $\text{Cr}_2\text{O}_7^{2-}$, with chromium in the $+6$ oxidation state) to deep green (chromium(III) ion, $\text{Cr}^{3+}$). This color change serves as a diagnostic test for primary alcohols and forms the chemical basis of breathalyzer tests.
Addition Reactions of Unsaturated Hydrocarbons
Addition reactions occur when incoming atoms or groups add across the carbon-carbon multiple bonds of an unsaturated compound, breaking weaker $\pi$-bonds to produce a single saturated product without eliminating any atoms.
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| MECHANISM OF CATALYTIC ADDITION |
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| H H H H |
| \ / | | |
| C = C + H - H --[Ni, 200 C]--> H - C - C - H |
| / \ | | |
| H H H H |
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| Ethene (Unsaturated) Ethane (Saturated) |
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Catalytic Hydrogenation of Vegetable Oils
Industrial hydrogenation transforms liquid unsaturated vegetable oils into semi-solid fats (such as margarine or vanaspati ghee). Natural vegetable oils contain long hydrocarbon chains with multiple double bonds (polyunsaturated fatty acids), which keep the oil liquid at room temperature.
When heated to approximately $200^\circ\text{C}$ in the presence of a finely divided transition metal catalyst—such as nickel ($\text{Ni}$), palladium ($\text{Pd}$), or platinum ($\text{Pt}$)—hydrogen gas adds across the double bonds:
Vegetable Oil (Liquid, Unsaturated) + H2 --[Ni Catalyst, 200 deg C]--> Saturated Fat (Solid)
The nickel catalyst provides an active surface where hydrogen molecules dissociate into atomic hydrogen, lowering the reaction activation energy and facilitating addition across the $\pi$-bonds. From a nutritional perspective, consuming saturated and trans fats generated by industrial partial hydrogenation is associated with increased serum LDL cholesterol and cardiovascular health risks.
Halogen Addition across Multiple Bonds
Halogens add across alkenes and alkynes without requiring a catalyst. Bubbling ethene through a solution of bromine dissolved in carbon tetrachloride causes bromine atoms to add across the double bond:
CH2=CH2 + Br2 -> CH2Br-CH2Br (1,2-Dibromoethane)
The rapid decolorization of the reddish-brown bromine solution confirms the addition reaction and serves as an analytical test for unsaturation.
Substitution Reactions of Saturated Hydrocarbons
Because saturated alkanes contain only single $\sigma$-bonds, they cannot undergo addition reactions. Instead, they react through substitution, wherein one or more hydrogen atoms on the alkane are sequentially replaced by other atoms or groups.
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| PHOTOCHEMICAL CHLORINATION OF METHANE |
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| Reaction: CH4 + Cl2 --[Ultraviolet Light (h*nu)]--> CH3Cl + HCl |
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| FREE-RADICAL CHAIN REACTION MECHANISM: |
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| 1. Initiation: Cl - Cl --[UV Light]--> 2 Cl* (Homolytic Cleavage) |
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| 2. Propagation: Cl* + CH4 -----> *CH3 + HCl |
| *CH3 + Cl2 -----> CH3Cl + Cl* |
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| 3. Termination: Cl* + Cl* -----> Cl2; *CH3 + *CH3 -----> CH3-CH3 |
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Free-Radical Halogenation Mechanism
Under ambient indoor conditions in the dark, methane and chlorine gas do not react. However, upon exposure to diffuse sunlight or ultraviolet radiation ($h\nu$), chlorination proceeds rapidly via a free-radical chain mechanism:
Initiation: Ultraviolet photons supply the activation energy required to homolytically cleave the covalent bond of a chlorine molecule, producing two neutral chlorine free radicals, each possessing an unpaired electron:
$$\text{Cl}-\text{Cl} \overset{h\nu}{\longrightarrow} 2\text{Cl}^\bullet$$
Propagation: A chlorine radical collides with a methane molecule, abstracting a hydrogen atom to form hydrogen chloride and generating a methyl free radical ($\text{CH}_3^\bullet$). The methyl radical then attacks an intact chlorine molecule to form chloromethane and regenerate a chlorine radical:
$$\text{Cl}^\bullet + \text{CH}_4 \longrightarrow \text{HCl} + \text{CH}_3^\bullet$$
$$\text{CH}_3^\bullet + \text{Cl}_2 \longrightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet$$
The regenerated chlorine radical propagates the chain reaction. If chlorine is supplied in excess, substitution continues sequentially, replacing all four hydrogen atoms on the carbon:
CH4 + Cl2 --[UV]--> CH3Cl + HCl (Chloromethane)
CH3Cl + Cl2 --[UV]--> CH2Cl2 + HCl (Dichloromethane)
CH2Cl2 + Cl2 --[UV]--> CHCl3 + HCl (Trichloromethane / Chloroform)
CHCl3 + Cl2 --[UV]--> CCl4 + HCl (Tetrachloromethane / Carbon Tetrachloride)
Termination: The chain reaction ends when two radicals collide and combine, eliminating free radicals from the reaction mixture:
$$\text{Cl}^\bullet + \text{Cl}^\bullet \longrightarrow \text{Cl}_2, \quad \text{CH}_3^\bullet + \text{Cl}^\bullet \longrightarrow \text{CH}_3\text{Cl}, \quad \text{CH}_3^\bullet + \text{CH}_3^\bullet \longrightarrow \text{CH}_3-\text{CH}_3$$
Chapter Exercises
Exercise 1: An organic compound A with the molecular formula $\text{C}_2\text{H}_6\text{O}$ produces brisk effervescence of hydrogen gas when treated with sodium metal. When compound A is heated with alkaline potassium permanganate, it oxidizes to compound B, which turns blue litmus paper red. When A is warmed with B in the presence of concentrated sulfuric acid, a sweet-smelling compound C is formed. Identify compounds A, B, and C, and write balanced chemical equations for all described reactions.
Exercise 2: Contrast the combustion characteristics of cooking gas (liquefied petroleum gas, predominantly butane) with that of kerosene fuel in a wick stove, and explain why domestic gas stoves feature adjustable air inlet holes.
Exercise 3: Write the stepwise chemical equations for the complete photochemical chlorination of ethane ($\text{C}_2\text{H}_6$) in the presence of excess chlorine gas under ultraviolet light, identifying all mono- and di-substituted products formed in the initial steps.
Solutions and Explanations
Solution 1: Compound A with molecular formula $\text{C}_2\text{H}_6\text{O}$ is ethanol ($\text{CH}_3\text{CH}_2\text{OH}$). Alcohols react with electropositive sodium metal to liberate hydrogen gas: $2\text{CH}_3\text{CH}_2\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{CH}_2\text{ONa} + \text{H}_2\text{(g)}$. Heating ethanol with alkaline potassium permanganate oxidizes it into ethanoic acid ($\text{CH}_3\text{COOH}$), compound B: $\text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$. Ethanoic acid is a carboxylic acid that turns blue litmus paper red. When ethanol (A) is warmed with ethanoic acid (B) in the presence of concentrated sulfuric acid (acting as an acid catalyst and dehydrating agent), an esterification reaction occurs, producing the sweet-smelling ester ethyl ethanoate ($\text{CH}_3\text{COOCH}_2\text{CH}_3$), compound C: $\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$.
Solution 2: Cooking gas (LPG, predominantly butane, $\text{C}*4\text{H}*{10}$) is a saturated gaseous alkane that mixes uniformly with atmospheric air prior to combustion. When ignited in a properly adjusted burner, it undergoes complete combustion, producing a clean, non-luminous blue flame that yields maximum heat output without depositing soot on cookware. Conversely, kerosene in a wick stove is a complex mixture of higher liquid hydrocarbons ($\text{C}*{10}$ to $\text{C}*{16}$) with higher carbon content. The liquid fuel must be drawn up porous wicks and vaporized by burner heat. If air circulation is limited, incomplete combustion occurs, producing a luminous yellow flame and soot deposits on cookware. Domestic gas burners feature adjustable air inlet ports (air collars) to control the flow of primary air into the gas stream. If these air vents become clogged with grease or cooking spillage, the air-to-fuel ratio drops, causing incomplete combustion characterized by yellow, smoky flames and soot accumulation, signaling that the vents need cleaning.
Solution 3: Photochemical chlorination of ethane ($\text{C}_2\text{H}_6$) proceeds via free-radical substitution in the presence of ultraviolet light ($h\nu$). In the first step, one hydrogen atom is replaced to form chloroethane: $\text{C}_2\text{H}_6 + \text{Cl}_2 \overset{h\nu}{\longrightarrow} \text{C}_2\text{H}_5\text{Cl} + \text{HCl}$. In the second step, substitution of a second hydrogen atom yields two structural isomers of dichloroethane: 1,1-dichloroethane ($\text{CH}_3-\text{CHCl}_2$) through substitution on the same carbon, and 1,2-dichloroethane ($\text{CH}_2\text{Cl}-\text{CH}_2\text{Cl}$) through substitution on the adjacent carbon: $\text{C}_2\text{H}_5\text{Cl} + \text{Cl}_2 \overset{h\nu}{\longrightarrow} \text{C}_2\text{H}_4\text{Cl}_2 + \text{HCl}$. Continued reaction with excess chlorine replaces remaining hydrogens sequentially, producing 1,1,1-trichloroethane and 1,1,2-trichloroethane ($\text{C}_2\text{H}_3\text{Cl}_3$), 1,1,1,2-tetrachloroethane and 1,1,2,2-tetrachloroethane ($\text{C}_2\text{H}_2\text{Cl}_4$), pentachloroethane ($\text{C}_2\text{HCl}_5$), and finally fully substituted hexachloroethane ($\text{C}_2\text{Cl}_6$).