Download Unit 26 Alkanes and alkenes 26.3 Representing molecular

Unit 26
26.3
Alkanes and alkenes
Representing molecular structures of organic compounds (page 82)
Condensed structural formulae
Remind students that the carbon-carbon multiple bonds must be shown when writing condensed
structural formulae. For example, but-2-ene should be written as
CH3CH=CHCH3, not CH3CHCHCH3.
Beginners may find it easier to write the extended structural formulae as they can count the
number of bonds to avoid mistakes.
26.6
The alkanes (page 86)
Structural isomerism in alkanes
Alkanes are known with up to about 100 carbon atoms in their molecules. In CH4, C2H6 and C3H8,
there is only one possible way of joining the carbon atoms and hydrogen atoms.
However, there are two possible ways of joining the carbon and hydrogen atoms for C4H10.
Their structural formulae are:
Butane and 2-methylpropane are different compounds although they have the same
molecular formula C4H10. This is an example of a phenomenon called structural isomerism.
Structural isomerism is the existence of two or more compounds with the same molecular
formula but different in the way atoms are joined together. The different compounds are called
structural isomers.
Hence butane and 2-methylpropane are structural isomers. The properties of the isomers
are different because their molecular structures are different. For example, the melting point of
butane is -138 oC while that of 2-methylpropane is -159 oC.
26.8
Physical properties of alkanes (page 87)
Volatility, ignition temperature and flash point
The combustion of a fuel requires a suitable fuel/air mixture and sufficient heat to initiate the
reaction. For the fuel/air mixture to burn, the fuel must be a vapour. Therefore how readily a liquid
vaporizes (a property known as volatility) will determine the ease of combustion.
Commonly used fuels will only burn at room temperature if a mixture of their vapours and air
is ignited with a flame, hot filament or a spark. The minimum temperature at which a fuel/air
mixture spontaneously ignites is called the ignition temperature of the fuel.
The lowest temperature at which a volatile fuel produces enough vapour to form an
explosive mixture when it comes in contact with a flame is called the flash point of the fuel.
Fuels with a flash point much higher than room temperature, such as diesel oil, are relatively
safe to handle because even if they are exposed to a flame they will not explode.
However, a fuel with a flash point below room temperature is potentially very dangerous
because a spark or flame can result in a fire or explosion. Natural gas, LPG, petrol and ethanol all
have flash points below room temperature.
Fuel
Ignition temperature (oC)
Flash point (oC)
Methane
630
-188
Ethane
515
-135
Propane
450
-104
Butane
370
-74
Methanol
455
12
Ethanol
363
13
Petrol
390
-43
Kerosene
380
48
The role of a wick
Some less volatile fuels such as kerosene and candle wax require a wick to vaporize the fuel
before it can undergo combustion. The wick, which is made of cotton or some other absorbent
material, soaks up the liquid fuel which then evaporates from the increased surface area
provided by the wick material.
26.14 Cracking of petroleum (page 99)
Methods of cracking
There are two methods of cracking:
1
Thermal cracking Heat the hydrocarbons to about 500 oC at a pressure of around 30
atmospheres. However, this method is largely replaced by other more efficient methods.
2
Catalytic cracking This is the most common method used. A catalyst is used. As the
hydrocarbons pass over the catalyst, cracking occurs. The temperature applied is similar to
that in thermal cracking although the pressure is close to atmospheric pressure. It is also
possible to control the quality of products.
Cracking of decane
Equations describing cracking of decane are for reference only and students need not memorize
them. Tell students that there are many possible ways of cracking a molecule.
26.19 Chemical properties of alkenes (page 104)
Chemistry and Society
Ethene as a fruit ripener
In addition to being a component of petroleum and natural gas, ethene is a hormone that is
produced naturally by plants to control the ripening of fruits, the germination of seeds and the
blossoming of flowers. Fruits and vegetables give off ethene naturally when they start to ripen.
The ripening process can be hastened by placing fruit in a paper bag, thereby surrounding the
fruit with the ethene gas it gives off.
Apply the knowledge of fruit ripening
Food processing companies sometimes add ethene to unripe fruits to allow them to appear on
the market earlier.
Ethene can also be very harmful to many fruits, vegetables, flowers and plants by
accelerating the aging process and decreasing the product quality and shelf life. A number of
precautions are taken by fruit growers to ensure that ethene-sensitive product is not exposed to
ethene:
 ethene-producing items (such as apples, bananas, melons, peaches, pears and tomatoes) are
stored separately from ethene-sensitive ones (such as broccoli, cabbage, cauliflower, leafy
greens, lettuce, etc.);
 engine fumes are kept away from fruits since diesel and gasoline powered engines all produce
ethene in amounts large enough to cause damage to the ethene-sensitive items;
 storage areas are ventilated to remove any ethene from the air.
Question
Suggest how knowledge of the fruit ripening of ethene can contribute to wider consumer choice
with respect to availability of different fruits throughout the year.
Notes for question
Ripe fruit is a popular product. However, it is not easy to arrange. Most fruit is seasonal. Growers
and wholesalers want the season to extend as long as possible to improve total sales. The
retailer wants fruit to look in peak condition, but runs the risk of unsold fruit rotting on the shelf. In
summer, this is a big problem; in winter, getting the fruit to ripen is a bigger problem.
Knowledge of the fruit ripening of ethene can be applied to marketing fruit in the following
ways:
 bananas can be picked green and ripened at their destination using ethene;
 rotting fruit should be removed quickly as it produces extra ethene, thus accelerating ripening
of nearby fruits;
 fruits can be stored for long periods by keeping them at low temperatures and increasing the
carbon dioxide level around them.