Question

a. What is a homologous series? b. By what method are straight-chain hydrocarbons named? c. Name the straight-chain alkane with the molecular formula \(\mathrm{C}_{10} \mathrm{H}_{22}\).

Short answer

A homologous series consists of compounds with a similar formula and properties. Straight-chain hydrocarbons use IUPAC naming by counting carbon atoms and adding '-ane.' \( \text{C}_{10}\text{H}_{22} \) is decane.

Step-by-step solution

Understand Homologous Series

A homologous series is a group of organic compounds that have a similar general formula, similar chemical properties, and show a gradation in physical properties (such as boiling point or melting point). Each member of the series differs from the next by a CH2 group.

Naming Straight-Chain Hydrocarbons

Straight-chain hydrocarbons are named using the International Union of Pure and Applied Chemistry (IUPAC) nomenclature system. First, identify the number of carbon atoms in the longest continuous chain. Based on the number of carbon atoms, use the root name (meth-, eth-, prop-, but-, etc.). Then, add the suffix '-ane' to indicate that the compound is an alkane (saturated hydrocarbon).

Identify the Name of the Straight-Chain Alkane

To name the straight-chain alkane with the molecular formula \(\text{C}_{10}\text{H}_{22}\), count the number of carbon atoms in the chain, which is 10. According to IUPAC nomenclature, the root name for 10 carbon atoms is 'dec-.' Adding the suffix '-ane' results in the name 'decane.'

Key concepts

Homologous Series

In organic chemistry, understanding homologous series is fundamental. A homologous series is a family of compounds that have a similar general formula and show a regular pattern in physical properties, such as boiling point and melting point. These compounds exhibit similar chemical properties because they contain the same functional group.

One noticeable feature of a homologous series is that each successive member differs by a \(\text{-CH}_2\) group. For example, in the alkane series, methane (\(\text{CH}_4\)) is followed by ethane (\(\text{C}_2\text{H}_6\)), propane (\(\text{C}_3\text{H}_8\)), and so forth. This regular increase leads to gradual changes in their physical properties.

Moreover, because they share the same functional group, members of a homologous series undergo similar chemical reactions. This makes it easier to predict the behavior of unfamiliar compounds within the series based on known compounds.

Some key characteristics of a homologous series include:

• Similar chemical properties.
• Regular pattern in physical properties.
• Each member differs by a \(\text{-CH}_2\) unit.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) nomenclature is the standardized system used to name organic compounds. Using a consistent system ensures that chemists around the world can understand each other’s work without confusion.

For naming straight-chain hydrocarbons (alkanes), follow these steps:

• Identify the number of carbon atoms in the longest continuous chain.
• Select the corresponding root name based on the number of carbon atoms (e.g., meth- for one carbon, eth- for two, prop- for three, etc.).
• Add the suffix “-ane” to indicate that the compound is an alkane, a type of saturated hydrocarbon.

For example, if you have a hydrocarbon chain comprising five carbon atoms, its name will be “pentane” (pent- + -ane). If there are 10 carbon atoms, such as in the given molecular formula \(\(\text{C}_{10}\text{H}_{22}\)\), the name is “decane” (dec- + -ane).

The IUPAC naming system also includes rules for naming branched hydrocarbons and those with functional groups, but the basic principles of identifying the longest chain and applying the appropriate suffix ensure clarity and consistency in organic chemistry.

Alkanes

Alkanes are a class of hydrocarbons that are saturated, meaning they only have single bonds between carbon atoms. This gives them the general formula \(\(\text{C}_{n}\text{H}_{2n+2}\)\). Because they are saturated, alkanes are relatively unreactive compared to other types of hydrocarbons like alkenes and alkynes.

Alkanes are named using the IUPAC system, where their names end with the suffix “-ane”. They can be found as straight chains or branched chains with varying lengths. The simplest alkane is methane (\(\text{CH}_4\)), followed by ethane (\(\text{C}_2\text{H}_6\)), propane (\(\text{C}_3\text{H}_8\)), and many more.

Here are some important properties and uses of alkanes:

• They are generally non-polar and insoluble in water.
• They have a lower density than water.
• They are used extensively as fuels (e.g., methane in natural gas, butane in lighters).

Alkanes play a significant role in both chemistry and industry. Their straightforward structure makes them a good starting point for studying more complex organic compounds.