Hydrocarbons



Carbon Compounds in the Atmosphere

Let's look at the molecules in the atmosphere.



Most of the molecules are either N2, at 78% of the atmospheric gases, or O2, at 21% of the atmospheric gases. All of the others are in small concentrations. These include the carbon oxides, hydrocarbons, and halocarbons (molecules with carbon bonded to F, Cl, Br, or I).

Hydrocarbons

Hydrocarbons are made up of only carbon and hydrogen atoms. The most common is also the smallest, methane. That and other common hydrocarbons are listed below. (Note that a wedge is coming out and a dashed line goes back. This helps to indicate the three dimensional structure of the molecules.)

Methane is the primary constituent of natural gas and it is released into the atmosphere from crude oil production and some industrial activities. More of it is released from biological processes. Soil bacteria and bacteria in the guts of termites and ruminants, like cows produce, produce methane. Other mammals (including humans) produce smaller amounts.



Small amount of methane migrate from the troposphere (layer of atmosphere closest to Earth) to the troposphere but most methane and all of the other hydrocarbon molecules react with HO and O2. After a series of reactions, all hydrocarbons are converted to carbon dioxide and water.

Name/formula Structure Space-filling Model Source
methane

CH4

an alkane
termites, cows, natural gas
ethane

CH3CH3

an alkane
natural gas, decomposing plant material
ethene or ethylene

H2C=CH2

an alkene
ripening fruit
ethyne or acetylene

HCCH

an alkyne
industrial production
propane

CH3CH2CH3

an alkane
natural gas, decomposing plant material
propene

CH3CH=CH2

an alkene
garlic, plant oils, burning refuse


Types of Hydrocarbons

Hydrocarbons that have 4 bonds to every carbon atom are called alkanes. These are the least reactive of the hydrocarbons.

Hydrocarbons that have a double bond between two carbon atoms are called alkenes. These are more reactive than alkanes. Electron-poor molecules react at the pi bond of alkenes.

Hydrocarbons that have a triple bond between two carbon atoms are called alkynes. These are the most reactive. Electron-poor molecules react at the pi bond of alkynes.
Naming



The hybrid orbital model of bonding is a simple and useful way to describe the bonding in hydrocarbons. What orbitals are involved in the bonding between carbon atoms and between carbon and hydrogen atoms in propene?


Professor Patricia Shapley, University of Illinois, 2012