CHAPTER 11: Unit 6. Chemical Reactions: Alkanes & Cycloalkanes

Learning ObjectiveIdentify the main chemical properties of alkanesAlkanes are the only family of organic compounds that has no functional groups, so alkanes undergo few reactions.Two important reactions that saturated hydrocarbons undergo are combustion and halogenation.  Combustion is a type of oxidation reaction. Whether an organic compound can undergo oxidation or reduction can be determined by looking at the nature of the C-H and C-O bonds in the product.Alkane molecules are nonpolar and therefore generally do not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents. Consider butane as an example:Neither positive ions nor negative ions are attracted to a nonpolar molecule. In fact, the alkanes undergo so few reactions that they are sometimes called paraffins, from the Latin parum affinis, meaning “little affinity.”1) Nothing happens when alkanes are merely mixed with oxygen (O2) at room temperature, but when a flame or spark provides the activation energy, a highly exothermic combustion reaction proceeds vigorously. For methane (CH4), the reaction is as follows:CH4 + 2O2 → CO2 + 2H2O + heatIf the reactants are adequately mixed and there is sufficient oxygen, the only products are carbon dioxide (CO2), water (H2O), and heat—heat for cooking foods, heating homes, and drying clothes. Because conditions are rarely ideal, however, other products are frequently formed. When the oxygen supply is limited, carbon monoxide (CO) is a by-product:2CH4 + 3O2 → 2CO + 4H2OThis reaction is responsible for dozens of deaths each year from unventilated or improperly adjusted gas heaters. (Similar reactions with similar results occur with kerosene heaters.)

2) Halogenation is a substitution reaction in which one of more hydrogen atoms of the hydrocarbon are replaced by hydrogen atoms. This is an example of a substitution reaction.

A substitution is a reaction in which an atom is replaced by another atom or a group of atoms.

Methane        heat or light           Chloromethane

In next step, CH2Cl2, CHCl3 and CCl4 are formed along with the main products.Halogenation usually results in mixture of products rather than single product. More than one product is obtained because more than one hydrogen atom can be halogenated. Amount of halogen Cl2, Br2 and reaction condition can provide proper yield of the desired product.

PRACTICE PROBLEMS:

Write the balanced equation for the combustion of Pentane.

Homework Exercises

1. Why do alkanes usually not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents?
2. Write an equation for the complete combustion of methane (CH₄, the main component of natural gas).
3. What is the most important reaction of alkanes?
4. Name some substances other than oxygen that react readily with alkanes.

Answers

1. Alkanes are nonpolar; they do not attract ions.

3. combustion