Question

Classify each of the following hydrocarbons as alkanes, alkenes, or alkynes. (a) \(\mathrm{C}_{6} \mathrm{H}_{14}\) (b) \(\mathrm{C}_{3} \mathrm{H}_{4}\) (c) \(\mathrm{C}_{9} \mathrm{H}_{18}\)

Short answer

Question: Classify the following hydrocarbons as alkanes, alkenes, or alkynes: (a) C6H14 (b) C3H4 (c) C9H18 Answer: (a) C6H14 is an alkane. (b) C3H4 is an alkyne. (c) C9H18 is an alkene.

Step-by-step solution

Identify the molecular formula for each compound

We are given the molecular formulas for the three hydrocarbons: (a) \(\mathrm{C}_{6} \mathrm{H}_{14}\) (b) \(\mathrm{C}_{3} \mathrm{H}_{4}\) (c) \(\mathrm{C}_{9} \mathrm{H}_{18}\)

Check if the formula matches the general formula for Alkanes, Alkenes or Alkynes

Now we can simply compare the molecular formulas of the given hydrocarbons to the general formulas of alkanes, alkenes, and alkynes to classify them. (a) For \(\mathrm{C}_{6} \mathrm{H}_{14}\): Alkane: \(C_6H_{2 \times 6 + 2} = C_6H_{14}\) This compound matches the general formula for alkanes, so it is an alkane. (b) For \(\mathrm{C}_{3} \mathrm{H}_{4}\): Alkyne: \(C_3H_{2 \times 3 - 2} = C_3H_{4}\) This compound matches the general formula for alkynes, so it is an alkyne. (c) For \(\mathrm{C}_{9} \mathrm{H}_{18}\): Alkene: \(C_9H_{2 \times 9} = C_9H_{18}\) This compound matches the general formula for alkenes, so it is an alkene.

Write down the classifications

After comparing the molecular formulas to the general formulas, we can classify the hydrocarbons as follows: (a) \(\mathrm{C}_{6} \mathrm{H}_{14}\) is an alkane. (b) \(\mathrm{C}_{3} \mathrm{H}_{4}\) is an alkyne. (c) \(\mathrm{C}_{9} \mathrm{H}_{18}\) is an alkene.

Key concepts

Alkanes

Alkanes are a type of hydrocarbon that are fully saturated, which means they contain only single bonds between carbon atoms. This saturation gives alkanes a stable structure, allowing them to exist as "straight" or "branched" chains. The general formula for alkanes is \( C_nH_{2n+2} \), where \( n \) is the number of carbon atoms in the molecule. Each carbon is connected to four other atoms, typically hydrogen or carbon.

Characteristics of alkanes include:

• Typically have a non-polar nature, which means they are not soluble in water but easily dissolve in organic solvents.
• Have relatively low reactivity due to the strength of the carbon-carbon and carbon-hydrogen bonds.
• Burn in oxygen to release energy, making them useful as fuels. This reaction is known as combustion.

To illustrate, consider hexane (\( C_6H_{14} \)), an example that follows the alkane formula. As an alkane, it is a saturated hydrocarbon and quite stable.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, making them unsaturated. The presence of this double bond introduces more complexity than found in alkanes, influencing their chemical behavior. The general formula for alkenes is \( C_nH_{2n} \). This formula reveals that alkenes have two fewer hydrogen atoms than their alkane counterparts.

Key properties of alkenes include:

• The double bond allows for a degree of reactivity not seen in single bonds, allowing alkenes to participate in addition reactions.
• They can serve as the building blocks for polymers, undergoing processes such as polymerization to form long chains of repeating units.
• Commonly found as gases or low-boiling-point liquids, largely due to the lack of strong intermolecular interactions.

An example of an alkene is nonene (\( C_9H_{18} \)). As predicted by the formula, it contains a double bond, classifying it within the alkenes.

Alkynes

Alkynes differ from both alkanes and alkenes by containing at least one carbon-carbon triple bond, which imparts high reactivity due to the bond strength. The general formula for alkynes is \( C_nH_{2n-2} \). This designation means alkynes have four fewer hydrogen atoms than the corresponding alkanes.

Some important characteristics of alkynes are:

• The triple bond is linear in shape, causing alkynes to have distinct geometries and properties.
• They are highly reactive, capable of undergoing addition reactions similar to alkenes but with different mechanistic pathways.
• Used in a variety of chemical syntheses to introduce functionality or extend carbon chains.

A typical example is propyne (\( C_3H_{4} \)), which aligns with the alkyne formula. The presence of a triple bond lowers hydrogen count and enhances reactivity, categorizing it as an alkyne.