Carbon
atoms can also form double or triple bonds with one another. Molecules that
have double bonds are known as alkenes, and have names ending in –ene, while those that have
triple bonds are called alkynes, and have names ending in -yne. Molecules that have only
single bonds contain the maximum possible number of hydrogen atoms, and are
therefore described as saturated. Where there are double or triple bonds, there
are fewer places available for hydrogen, so these compounds are described as
unsaturated.
To
give a simple example, ethane has two carbons joined by a single bond, leaving
each able to bond to three hydrogen atoms, so its chemical formula is C2H6 and it is an alkane.
In ethene there is a carbon-carbon double bond, so it can only have four
hydrogens, making it an alkene with the formula C2H4.
Ethyne has a triple bond, giving it the formula C2H2, and
making it an alkyne. This compound is better known as acetylene.
The
carbon atoms can also form rings. Alkanes with rings have names beginning with cyclo-. Therefore, cyclohexane is
an alkane with six carbon atoms joined by single bonds in such a way as to form
a ring. A ring with alternating single and double bonds is also possible, and
is known as a benzene ring. Hydrocarbons containing a benzene ring are known as
aromatic, because many of them are pleasant-smelling.
Some
hydrocarbon molecules have chains that branch. Butane, which normally consists
of a single chain, can exist in a form where one carbon atom is bonded to two
others, forming a branch. These alternative forms of a molecule are known as
isomers. The branched isomer of butane is known as isobutane.
Production
Most
production of hydrocarbons is from fossil fuels: coal, oil, and natural gas,
which are extracted from the ground in quantities of millions of tons per day.
Crude oil is mostly a mixture of many different alkanes and cycloalkanes, with
some aromatic compounds. These can be separated from one another at oil
refineries by distillation, because of their different boiling points. Another
process that is used is known as “cracking”: catalysts are employed to break
some of the larger molecules into smaller ones that are more useful as fuels.
Properties
Generally
speaking, the more complex a hydrocarbon is, the higher its melting and boiling
points. For example, the simpler types, such as methane, ethane, and propane,
with one, two, and three carbons, respectively, are gases. Many forms are
liquids: examples are hexane and octane. Solid forms include paraffin wax — a
mixture of molecules with between 20 and forty carbon atoms — and various
polymers consisting of chains of thousands of atoms, such as polyethylene.
The
most notable chemical properties of hydrocarbons are their flammability and
their ability to form polymers. Those that are gases or liquids will react with
oxygen in the air, producing carbon dioxide (CO2) and water, and
releasing energy in the form of light and heat. Some energy must be supplied to
start the reaction, but once started, it is self-sustaining: these compounds
will burn, as illustrated by lighting a gas hob with a match or spark. Solid
forms will also burn, but less easily. In some cases, not all the carbon will
form CO2; soot and smoke can be produced by some types when they
burn in air, and in an insufficient supply of oxygen, any hydrocarbon can
produce the toxic, odorless gas, carbon monoxide (CO).
Uses
The flammability of hydrocarbons makes
them very useful as fuels, and they are the primary energy source for today’s
civilization. Worldwide, most electricity is generated by the burning of these
compounds, and they are used to propel practically every mobile machine: cars,
trucks, trains, planes, and ships. They are also used in the manufacture of
many other chemicals and materials. Most plastics, for example, are hydrocarbon
polymers. Other uses include
solvents, lubricants and propellants for aerosol cans.
Problems
with Fossil Fuels
Hydrocarbons
have been a very successful fuel source over the last two hundred years or so,
but there are increasing calls to scale back their use. Their combustion
produces smoke and soot, causing serious pollution problems in some areas. It
also produces large amounts of CO2. There is widespread agreement
among scientists that increasing levels of this gas in the atmosphere are
helping to trap heat, raising global temperatures and changing the Earth’s
climate.
In
addition, fossil fuels will not last
forever. Burning fuel at the current rate, oil could run out in less than a
century and coal in several centuries. All of this has led to calls to develop
renewable energy sources such as solar and wind power, and the construction of
more nuclear power plants, which produce zero CO2 emissions. In 2007, the Nobel Peace Prize was
awarded to former US Vice President Al Gore and the UN’s Intergovernmental
Panel on Climate Change for their work in confirming and spreading the message
that the combustion of hydrocarbons is largely responsible for global warming.
SOURCE/REFERENCE:
Walang komento:
Mag-post ng isang Komento