Combustion and Halogenation of Alkanes

Organic Chemistry Lecture 4

• Lecture 4 covers reactions of alkanes, combustion analysis of hydrocarbons, free radical halogenation, reactions of higher alkanes, cycloalkanes, and double bond equivalents.

Reactions of Alkanes

• Alkanes are relatively inert due to non-polar covalent bonds.
• They do not readily react with most acids, bases, or reducing agents.
• Alkanes are frequently used as fuels, undergoing combustion to evolve heat, a process of oxidation where light and heat are released, like in the example of methane reacting with oxygen to produce carbon dioxide, water, and heat (CH4 + 2O2 → CO2 + 2H2O + heat).

Combustion Analysis of Hydrocarbons

• Gay-Lussac's Law states that volumes of gases in a chemical reaction are related numerically, if measured at standard temperature and pressure (STP).
• Avogadro's Law states equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
• Combustion analysis involves reacting a known volume of hydrocarbon with a known volume of oxygen. The volume of products and unreacted oxygen are measured after cooling.
• This method is used to determine the molecular weight of an alkane.
• Products from a hydrocarbon's combustion with oxygen include carbon dioxide and water.

Problem Question Example

• A problem provides a gaseous hydrocarbon with a known volume reacted with a volume of oxygen and results in an amount of residual gas with a given composition that was found to be carbon dioxide.
• The question requires the determination of the hydrocarbon's molecular formula.

Past Paper Question Example (1997)

• A gaseous hydrocarbon (X) with a volume of 15 cm³ is mixed with oxygen to a total volume of 145 cm³.
• After combustion, the residual volume is 100 cm³.
• One-fourth of the gas remains after passing through aqueous sodium hydroxide.
• This problem asks for the molecular formula (given the measurements were done at STP with 10 marks).

Determination of Molecular Formulae using Percent Composition

• Percent composition (by mass) of elements in a sample can be experimentally determined, helping calculate empirical and molecular formulas.
• The empirical formula shows the relative proportions of elements but not the actual number of atoms. Calculating percent composition values is done by multiplying the element's molar mass by the number of atoms of the element, divided by the molar mass of the compound, multiplied by 100%.

Example Problem (December 2000)

• A liquid hydrocarbon (G) is composed of 12.27% hydrogen. Find the molecular formula.

Free Radical Halogenation

• Reaction of Methane with Chlorine involves three steps: initiation, propagation, and termination.
• Free radical halogenation reactions produce different products with varying stability, like 1°, 2°, and 3° methyl, as seen in the product ratios.

Multiple Substitution Reactions

• Illustrates halogenation substitution, including steps of monohalogenation, dihalogenation, trihalogenation, tetrahalogenation.

Reaction with Alkanes (Free Radical Halogenation)

• Reactions are initiated by light or heat; halogen-alkanes are produced.
• The reaction products depend on the stability of radicals (1° < 2° < 3°). Fluorine reactions are explosive; chlorine/bromine reactions require light or heat; iodine is unreactive.

Mechanism of Free Radical Halogenation

• Explains the different steps in initiation, propagation, and termination.

Reaction Selectivity

• Bromine halogenation is more selective than chlorine or fluorine halogenation. Different product yields arise from selective substitutions.

Free Radicals in Diseases

• Free radicals are involved in various diseases, affecting biological substances, including cancer, strokes, Alzheimer's, rheumatoid arthritis, Parkinson's disease, and cataracts.

Past Paper Question (July/August 2004)

• 2-Methylpropane reacted with bromine under light conditions is examined.
• The question requires determining why there's a higher yield of 2-bromo-2-methylpropane by outlining the reaction mechanism, properly annotating each step, all with 15 marks.

Cycloalkanes

• Saturated hydrocarbons with a cyclic structure of carbon atoms.

• They follow the formula CnH2n.

• Named by adding "cyclo" before the corresponding alkane name, e.g., cyclopropane, cyclohexane.

Cis-Trans Isomerism

• Cycloalkanes can exhibit cis-trans isomerism.
• Cis isomers have similar groups on the same side of the ring, while trans isomers have similar groups on opposite sides.

Double Bond Equivalents (DBE)

• DBE provides a measure of unsaturation/saturation in an organic compound.
• It counts double bonds or rings in an organic compound.

Calculation of DBE

• The formula is used to calculate DBE values for given hydrocarbon and halogenated compounds.

Next Lecture:

• Alkenes: bonding, geometric isomerism, and preparation methods.

Test your knowledge on the combustion reactions of alkanes and the halogenation processes. This quiz covers various concepts including gas laws, unreacted reactants, and product yields in hydrocarbon reactions. Perfect for students studying organic chemistry or combustion analysis.