IB Chemistry: Topic 10: Organic Chemistry
Hydrocarbons and an Introduction to Halogenoalkanes
CONTENTS
- General Information
- Aliphatic Hydrocarbons o Alkanes
o Alkenes o Alkynes
o Cyclic Forms
- Aromatic Hydrocarbons
- Introduction to Halogenoalkanes
GENERAL INFORMATION
As their name suggestions, hydrocarbons are organic compounds containing exclusively hydrogen and carbon atoms. Hydrocarbons are divided into aliphatic compounds and aromatic compounds. Aromatic structures contain benzene rings or similar structures, while aliphatic compounds do not incorporate aromatic rings.
When compared to other organic materials, hydrocarbons are relatively unreactive. However, most hydrocarbons are flammable, burning in oxygen to produce carbon dioxide and water vapor. They will also undergo substitution reactions in the presence of ultraviolet light. Hydrocarbons with double bonds will undergo addition reactions.
Gasoline is a mixture of hydrocarbons, with the grade of gasoline based on the octane rating system. Heptane is a hydrocarbon that burns with small explosions, while isooctane burns evenly. The octane rating is the percentage of isooctane in gasoline: for example, a rating of 87 indicates gasoline made of 87% octane and 13% heptane.
ALIPHATIC HYDROCARBONS
Alkanes
The carbon-carbon bonds are all single bonds. Alkanes are saturated— they have the maximum number of hydrogen atoms possible.
With carbon chains of four carbons or more, it is possible to arrange the chains with branches. Compounds with the same molecular formula but different structural formulas are referred to as structural isomers.
IB Chemistry: Topic 10: Organic Chemistry
Hydrocarbons and an Introduction to Halogenoalkanes
Alkenes
There is at least one carbon-carbon double bond. Since there is at least one double bond, alkenes are unsaturated—they do not have the maximum number of hydrogen atoms possible. The double bond(s) may be located in different places.
Alkynes
There is at least one carbon-carbon triple bond. Like alkenes, alkynes are unsaturated.
Cyclic Forms
The hydrocarbons examined so far have been arranged in chains—however, aliphatic hydrocarbons can also be arranged in non-aromatic rings.
Cycloalkanes have the general formula , as compared to the alkanes’ general formula of . The additional carbon-carbon bond reduces the amount of hydrogen atoms. Cycloalkanes are saturated. Examples of unsaturated cyclic hydrocarbons are cyclopentene and cyclobutene.
An alternative way to depict cyclic forms is to use the condensed structure, which uses basic shapes—each vertex represents a carbon atom.
AROMATIC HYDROCARBONS
Aromatic is a term used to refer to the class of compounds containing benzene-like, six-membered rings with three double bonds. The benzene ring contains 6 carbon atoms and 6 hydrogen atoms—one would expect it to have three carbon-carbon double bonds. However, its unexpected stability is due to its resonance structure: its bonds are intermediate between a typical single bond and double bond.
IB Chemistry: Topic 10: Organic Chemistry
Hydrocarbons and an Introduction to Halogenoalkanes
Benzene can be depicted in various ways: as either drawing alternating double bonds or using a hexagon and drawing a circle in the center to represent the delocalized electrons.
Note that aromatic structures can also be fused together, such as for naphthalene.
INTRODUCTION TO HALOGENOALKANES
Halogenoalkanes are molecules in which one or more hydrogen atoms have been replaced by halogen atoms (fluorine, chlorine, bromine, or iodide). Depending on the location of the halogen atom, halogenoalkanes can be primary, secondary, or tertiary.
The boiling point increases as the halide is further down Group 17 (Cl < Br < I). Additionally, the longer the carbon chain, the higher the boiling point.