Chemistry: Alkanes, Alkenes, and Alkynes

Understanding Chemistry: Alkanes, Alkenes, and Alkynes

Carbon can form four bonds, while hydrogen forms one. This simple rule helps students check whether a hydrocarbon structure makes sense. In a chain, each carbon must reach a total of four bonds when all its links to nearby atoms are counted.

Organic chemists use this rule constantly when drawing structures. Names give clues too. The beginning of a name tells the number of carbons in the main chain.

Meth means one, eth means two, prop means three, and but means four. The ending identifies the bond family. Learning names becomes easier when students first count the longest chain, then number it from the end nearest an important multiple bond.

Molecules with the same numbers of carbon and hydrogen atoms can still be different substances. These are called isomers. A carbon chain may be straight or branched, which can change boiling point and melting point.

For molecules with a double bond, the location of that bond matters. A double bond near one end of a chain creates a different compound from a double bond near the middle. Some alkenes have another kind of isomerism because groups on either side of the double bond cannot freely turn.

Groups can be arranged on the same side or on opposite sides. This difference affects the molecule's shape and sometimes its biological effects.

Multiple bonds are useful reaction sites because new atoms can attach across them. In hydrogenation, hydrogen is added to a multiple bond. This process can turn an alkene into a more saturated product.

In hydration, the added parts come from water, producing an alcohol under suitable conditions. Polymer making is another important example. Many small alkene molecules can link into long chains, creating materials such as polyethylene.

The carbon double bonds open during this process, then new single bonds connect the repeating units. This explains why a small change in bonding can lead to materials with very different properties.

Hydrocarbons store chemical energy in their bonds. When they burn with enough oxygen, the products are carbon dioxide and water, and energy is released as heat or light. This is why natural gas, petrol, and candle wax can act as fuels.

With too little oxygen, burning can produce carbon monoxide or tiny carbon particles called soot. Carbon monoxide is dangerous because it interferes with oxygen transport in the blood.

When balancing combustion reactions, count carbon atoms first, then hydrogen atoms, then adjust oxygen last. Students should pay close attention to whether a displayed structure shows every hydrogen, since line structures often leave carbon atoms and their attached hydrogens unstated.