http://www.ck12.org Chapter 9. Chemistry of Carbon
Isomers usually have somewhat different properties. For example, straight-chain molecules generally have higher
boiling and melting points than their branched-chain isomers. The boiling and melting points ofiso-butane are -12°C
and -160°C, respectively. Compare these values with the boiling and melting points of butane inTable9.2. Do these
two compounds follow the general trend?
Cycloalkanes
Ring-shaped alkanes are called cycloalkanes. They usually contain just five or six carbon atoms because larger rings
are not very stable. However, rings can join together to create larger molecules consisting of two or more rings.
Compared with the straight- and branched-chain alkanes, cycloalkanes have higher boiling and melting points.
Unsaturated Hydrocarbons
Unsaturated hydrocarbonscontain at least one double or triple bond between carbon atoms. As a result, the
carbon atoms are unable to bond with as many hydrogen atoms as they would if they were joined only by single
bonds. This makes them unsaturated with hydrogen. Unsaturated hydrocarbons are classified on the basis of their
bonds as alkenes, alkynes, or aromatic hydrocarbons.
Alkenes
Unsaturated hydrocarbons that contain at least one double bond are calledalkenes. The name of a specific alkene
always ends in –ene, with a prefix indicating the number of carbon atoms.Figure9.10 shows the structural formula
for the smallest alkene. It has just two carbon atoms and is named ethene. Ethene is produced by most fruits and
vegetables. It speeds up ripening and also rotting.Figure9.11 shows the effects of ethene on bananas.
FIGURE 9.10
Ethene is the smallest alkene.Like alkanes, alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes
can also form isomers, or compounds with the same atoms but different shapes. Generally, the physical properties
of alkenes are similar to those of alkanes. Smaller alkenes, such as ethene, have relatively high boiling and melting
points. They are gases at room temperature. Larger alkenes have lower boiling and melting points. They are liquids
or waxy solids at room temperature.
Alkynes
Unsaturated hydrocarbons that contain at least one triple bond are calledalkynes. The name of specific alkynes
always end in –yne, with a prefix for the number of carbon atoms.Figure9.12 shows the smallest alkyne, called