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B2.1 · Use functional-group, hydrocarbon, isomer, and polymer terminology

Learn to use functional-group, hydrocarbon, isomer, and polymer terminology through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

SCH4U study topic B2.1: functional groups, hydrocarbons, isomers, and polymers

Two clear liquids can look almost identical but behave differently because their particles are built differently. A small change in how atoms are connected can change a compound’s chemical family. Organic chemistry uses shared terms to describe these patterns. First recall that a molecule is a group of atoms joined by chemical bonds, and a structural formula shows how those atoms are connected. This lesson uses that idea to build four useful terms: hydrocarbon, functional group, isomer, and polymer.

What you will learn

Define hydrocarbon and distinguish saturated from unsaturated hydrocarbons.
Identify common functional groups from their characteristic atoms or bonds.
Explain what makes two compounds isomers.
Use monomer, polymer, and repeating-unit terminology accurately.

1. Hydrocarbons: compounds made only of carbon and hydrogen

A hydrocarbon is a compound that contains only carbon and hydrogen atoms. Methane, found in natural gas, is a familiar example. Its formula is

\mathrm{CH_4}

. Hydrocarbons can have different sizes and shapes, but they all meet the same definition: no atoms of other elements are part of the compound.

A carbon atom can form four bonds. In a hydrocarbon, carbon atoms may be joined by single, double, or triple bonds. A saturated hydrocarbon has only single bonds between its carbon atoms. An unsaturated hydrocarbon has at least one carbon–carbon double or triple bond. These terms describe the bonds in the carbon framework; they do not describe how much liquid or gas is present.

The names of some hydrocarbon families signal the type of bond present. An alkane has only carbon–carbon single bonds, an alkene has a carbon–carbon double bond, and an alkyne has a carbon–carbon triple bond. For example, ethane is an alkane, ethene is an alkene, and ethyne is an alkyne. The ending helps identify the family. These names are useful labels, not separate definitions of what a hydrocarbon is.

Hydrocarbons contain carbon and hydrogen only.
Saturated hydrocarbons have only carbon–carbon single bonds.
Unsaturated hydrocarbons have at least one carbon–carbon double or triple bond.

2. Functional groups: recognizable patterns in molecules

Many organic compounds contain atoms besides carbon and hydrogen. A functional group is a specific atom or group of atoms in a molecule that helps identify its chemical family. The rest of the molecule may be a carbon-and-hydrogen chain, but the functional group is the feature used to classify it.

An alcohol contains a hydroxyl group, written

\mathrm{-OH}

, attached to a carbon framework. A carboxylic acid contains a carboxyl group, written

\mathrm{-COOH}

. An amine contains a nitrogen atom in its characteristic group; a simple example is

\mathrm{-NH_2}

. An ester includes the pattern

\mathrm{-COO-}

between carbon-containing parts of the molecule.

Carbon–carbon multiple bonds can also be used as identifying features in this vocabulary. An alkene has a carbon–carbon double bond, while an alkyne has a carbon–carbon triple bond. A carbonyl group is a carbon atom double-bonded to oxygen, written

\mathrm{C=O}

. It occurs in more than one family, including aldehydes and ketones. To distinguish those families, look at where the carbonyl group appears in the structure: an aldehyde has it at the end of the carbon chain, while a ketone has it within the chain.

When identifying a functional group, inspect the bonds and atoms shown in the structural formula. Do not classify a molecule by its name alone if the structure is available. One molecule can contain more than one recognizable feature, so state the feature that supports your classification.

A functional group is a recognizable atom or group of atoms used to identify a chemical family.
Hydroxyl is $\mathrm{-OH}$ ; carboxyl is $\mathrm{-COOH}$ ; carbonyl is $\mathrm{C=O}$ .
Use the displayed structure to identify the group and its position.

3. Isomers: same formula, different structures

A molecular formula tells how many atoms of each element are present. It does not show how those atoms are connected. Isomers are compounds with the same molecular formula but different arrangements of atoms. Because the arrangements differ, the compounds are distinct substances.

For example, the molecular formula

\mathrm{C_4H_{10}}

can represent a chain of four carbon atoms or a structure with a three-carbon main chain and a carbon branch. Both structures have four carbon atoms and ten hydrogen atoms, but the connections differ. They are structural isomers, meaning their atoms are connected in different ways.

To decide whether two structures are isomers, compare their molecular formulas first. If the formulas differ, they are not isomers. If the formulas match, compare the connections between atoms. Identical connections drawn in a different orientation are not a new isomer; the drawing has only been turned or rearranged on the page. Different connections are required.

The word isomer describes the relationship between compounds. It is not the name of a functional group or a type of bond. Keeping those categories separate makes structural comparisons clearer.

Isomers have the same molecular formula.
Their atoms are connected differently.
A rotated or differently positioned drawing does not by itself make a new isomer.

4. Polymers: large molecules built from repeating units

A polymer is a large molecule made from many smaller repeating sections. A monomer is a small molecule that can form part of a polymer. The repeating unit is the section of the polymer structure that appears again and again. These words describe size and structure, not a functional group.

Many everyday materials are polymers. Polyethylene, used in some plastic bags and containers, has a long carbon chain with repeating sections. Its repeating unit is represented as

\mathrm{[-CH_2-CH_2-]}

. The brackets and subscript

n

\mathrm{[-CH_2-CH_2-]_n}

indicate that this section is repeated many times. The symbol

n

does not give a particular count unless one is specified.

Use polymer terminology carefully. The polymer is the complete large molecule; the repeating unit is the recurring section within it; a monomer is a smaller molecule associated with forming a polymer. Do not call every long molecule a polymer just because it contains many atoms. The key feature is the repetition of structural units.

A polymer is a large molecule containing repeating units.
A monomer is a smaller molecule that can form part of a polymer.
The subscript $n$ represents many repeated units, not a stated exact number.

Worked example

Classifying structures and checking for isomers

Compound A has the structure

\mathrm{CH_3CH_2OH}

. Compound B has the structure

\mathrm{CH_3OCH_3}

. Identify the functional group or family shown in each, find each molecular formula, and decide whether A and B are isomers.

Inspect the atom connections
In A, the oxygen is part of an $\mathrm{-OH}$ group attached to the carbon chain. That is a hydroxyl group, so A is an alcohol. In B, the oxygen is between two carbon-containing parts, and there is no $\mathrm{-OH}$ group. B is an ether; it does not have the alcohol functional group.
Count atoms in each structure
A contains two carbon atoms, six hydrogen atoms, and one oxygen atom. B also contains two carbon atoms, six hydrogen atoms, and one oxygen atom. The molecular formulas therefore match.
$\mathrm{C_2H_6O}$
Compare the structures
Although the molecular formulas are the same, the oxygen atoms are connected differently. In A, oxygen is bonded to hydrogen and to a carbon. In B, oxygen is bonded to two carbons. The compounds are structural isomers.

Answer: A is an alcohol with formula

\mathrm{C_2H_6O}

. B is an ether with formula

\mathrm{C_2H_6O}

. They are structural isomers because their formulas match but their atom connections differ.

Check: The conclusion uses both required checks: equal molecular formulas and different connections. Equal formulas alone would not be enough if the structures were identical.

Common mistakes and how to avoid them

Calling any compound with carbon an organic hydrocarbon.

Correction: A hydrocarbon contains only carbon and hydrogen. A compound with oxygen or nitrogen is not a hydrocarbon.

Calling two compounds isomers because their names or shapes look different.

Correction: First confirm that the molecular formulas match. Then confirm that the atom connections differ.

Treating the hydroxyl group and the oxygen atom in an ether as the same functional group.

Correction: A hydroxyl group is

\mathrm{-OH}

. In an ether, the oxygen is bonded between carbon-containing parts and is not part of an

\mathrm{-OH}

group.

Using monomer, repeating unit, and polymer as interchangeable words.

Correction: A monomer is a small molecule, a repeating unit is the recurring section, and a polymer is the large molecule made of repeated sections.

Lesson summary

Hydrocarbons contain only carbon and hydrogen; saturated ones have only carbon–carbon single bonds.
A functional group is a recognizable atom or group of atoms used to identify a chemical family.
Isomers have the same molecular formula but different atom connections.
A polymer is a large molecule with repeating units; a monomer is a smaller molecule associated with forming it.

Check your understanding

Question 1

Which statement correctly defines a hydrocarbon?

A compound containing only carbon and hydrogen
A compound containing carbon and oxygen only
A large molecule made from repeating units
Any compound with a carbon–carbon double bond

Show answer and explanation

A compound containing only carbon and hydrogen

A hydrocarbon contains carbon and hydrogen only. A double bond is not required.

Question 2

Two compounds have the same molecular formula but different atom connections. What is the correct term for their relationship?

They are functional groups.
They are structural isomers.
They are repeating units.
They are saturated hydrocarbons.

Show answer and explanation

They are structural isomers.

Structural isomers share a molecular formula but differ in how their atoms are connected.

Question 3

A long molecule contains the same structural section repeated many times. Which term describes the whole molecule?

Monomer
Hydrocarbon
Polymer
Hydroxyl group

Show answer and explanation

Polymer

The whole large molecule is a polymer. The recurring section is its repeating unit.

Key terms

Hydrocarbon: A compound containing only carbon and hydrogen.
Functional group: A recognizable atom or group of atoms used to identify an organic chemical family.
Isomers: Compounds with the same molecular formula but different atom arrangements.
Monomer: A small molecule that can form part of a polymer.
Polymer: A large molecule made from many repeating units.
Repeating unit: The section of a polymer structure that occurs repeatedly.
Saturated hydrocarbon: A hydrocarbon with only single bonds between carbon atoms.
Unsaturated hydrocarbon: A hydrocarbon with at least one carbon–carbon double or triple bond.

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