Combustion and Halogenation of Alkanes

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Questions and Answers

What type of reaction is combustion of alkanes classified as?

Hydrolysis reaction

Oxidation reaction (correct)

Neutralization reaction

Reduction reaction

Which law states that equal volumes of gases at the same temperature and pressure contain the same number of molecules?

Boyle's Law

Charles's Law

Avogadro's Law (correct)

Gay-Lussac's Law

In the combustion analysis of hydrocarbons, what happens to unreacted oxygen after the reaction?

It contributes to the formation of water.

It is measured after cooling. (correct)

It remains unchanged.

It is converted into carbon dioxide.

What product is formed when methane reacts with chlorine through free radical halogenation?

Chloromethane Signup and view all the answers

How many moles of oxygen are required to completely combust one mole of a hydrocarbon represented as CxHy?

$x + y/4$ Signup and view all the answers

What ratio is typically considered when analyzing the reactions of higher alkanes during halogenation?

Radical stability ratio Signup and view all the answers

When performing combustion analysis, what is the purpose of passing the gas mixture through aqueous sodium hydroxide?

To absorb carbon dioxide. Signup and view all the answers

What is indicated by the volume decrease observed after the combustion of a hydrocarbon in a closed system?

Complete reaction Signup and view all the answers

What is a characteristic of bromine in terms of selectivity compared to chlorine and fluorine?

Bromine is more selective than chlorine and fluorine. Signup and view all the answers

Which of the following reactions yields a higher product yield?

Bromination of hydrocarbons. Signup and view all the answers

What diseases are believed to be contributed to by free radicals?

Mostly age-related diseases. Signup and view all the answers

What is the role of free radicals in biological systems?

They are thought to damage biological substances. Signup and view all the answers

In the bromine reaction shown, which compound appears as a product when hydrogen reacts with bromine?

BrCH3. Signup and view all the answers

Which property affects the selectivity of bromine more than chlorine?

Bond strength. Signup and view all the answers

What type of reaction typically occurs when bromine reacts with alkane?

Radical substitution. Signup and view all the answers

What is a common misconception about the selectivity of bromine compared to chlorine?

Bromine is less reactive than chlorine. Signup and view all the answers

What is the purpose of determining the percent composition of a compound?

To calculate the empirical and molecular formula of the compound Signup and view all the answers

In the formula for percent composition, what does 'n' represent?

The number of moles of the element per mole of compound Signup and view all the answers

What type of reaction is free radical halogenation of alkanes?

A substitution reaction Signup and view all the answers

Which of the following describes the initiation step in free radical halogenation?

Absorption of heat or light to break bonds Signup and view all the answers

What is the result of dihalogenation in alkane halogenation reactions?

Two halogen atoms are substituted on the same molecule Signup and view all the answers

What differentiates more stable free radicals from less stable ones?

The branching of the carbon chain Signup and view all the answers

In the context of free radical halogenation, which halogen is most reactive?

Fluorine Signup and view all the answers

Which reaction type involves the replacement of two hydrogen atoms with chlorine in a single reaction?

Dihalogenation Signup and view all the answers

During the termination step of free radical reactions, what occurs?

Two free radicals combine to form a stable molecule Signup and view all the answers

When calculating the empirical formula from percent composition, which of the following steps is first?

Convert percent to grams Signup and view all the answers

Study Notes

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.

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Description

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.