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B3.3 · Explain substitution, addition, elimination, oxidation, esterification, and hydrolysis

Learn to explain substitution, addition, elimination, oxidation, esterification, and hydrolysis through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

SCH4U study topic B3.3: recognizing what changes in organic molecules

A reaction can change a substance’s smell or other observable properties. The molecules have changed, but their atoms have not vanished. Organic reactions rearrange atoms and bonds to make different molecules. Before naming a reaction, review a few structural features: an alkane has carbon–carbon single bonds; an alkene has a carbon–carbon double bond; an alcohol contains an –OH group; and a carboxylic acid contains a –COOH group. A functional group is a recognizable group of atoms that helps determine how a molecule reacts. Compare the starting molecules with the products to see which atoms or groups are added, removed, or replaced.

What you will learn

Explain substitution, addition, and elimination by describing how atoms or groups change in a molecule.
Explain oxidation as a change that commonly involves gaining oxygen or losing hydrogen.
Explain esterification and hydrolysis using the roles of an alcohol, a carboxylic acid, and water.
Use balanced equations and changes in bonds to distinguish the six reaction types.

1. Start with what changes

A structural formula shows how atoms are connected. A bond is a connection between atoms. In an organic reaction, some connections change as atoms are rearranged into new molecules. The reaction name describes the main pattern of change.

In a substitution reaction, one atom or group in a molecule is replaced by another atom or group. For example, a hydrogen atom in methane can be replaced by chlorine. The carbon framework remains intact; the replacement is the defining change.

In an addition reaction, atoms or groups add across a carbon–carbon double bond. The double bond becomes a single bond, and each of its carbon atoms forms a new bond to an added atom or group. The alkene becomes a product with single bonds between those carbon atoms.

In an elimination reaction, atoms or groups are removed from a molecule and a carbon–carbon double bond forms. This contrasts with addition: addition uses a double bond as a place to join atoms, while elimination forms a double bond as atoms or groups leave.

Substitution replaces one atom or group with another.
Addition joins atoms or groups across a carbon–carbon double bond.
Elimination removes atoms or groups and forms a carbon–carbon double bond.

2. Oxidation in organic chemistry

Oxidation is a reaction pattern in which a substance loses electrons. In common Grade 12 organic examples, you can often recognize oxidation by a change in the carbon compound: it gains oxygen, loses hydrogen, or both. This is a useful way to identify familiar examples, but not every oxidation has to show the same visible change.

For example, ethanol can be oxidized to ethanal. At the particle level, the organic molecule loses hydrogen overall. It does not gain an oxygen atom: ethanol and ethanal each contain one oxygen atom. The oxygen represented by [O] is supplied by an oxidizing substance, and it appears in the water product in this equation.

The symbol [O] is course-level shorthand for oxygen supplied by an oxidizing substance. It is not a complete formula for that substance. In the equation, the shorthand helps show that ethanol loses hydrogen as it changes to ethanal, while water forms as another product.

Do not confuse oxidation with addition. Addition refers to atoms or groups joining across a carbon–carbon double bond. Oxidation refers to the overall oxidation change; in these common organic examples, gaining oxygen or losing hydrogen is a useful clue.

\mathrm{CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O}

Common organic oxidation examples often involve gaining oxygen, losing hydrogen, or both.
In ethanol changing to ethanal, the organic molecule loses hydrogen; its oxygen count stays at one.
[O] represents oxygen supplied by an oxidizing substance.

3. Esterification and hydrolysis

An ester is an organic compound with a characteristic –COO– connection between its carbon-containing parts. Many esters have noticeable odours, sometimes described as fruity. An observation such as an odour can help connect a reaction to everyday experience, but the reaction type is identified from the molecules and how they change.

Esterification is the reaction of a carboxylic acid with an alcohol to form an ester and water. A carboxylic acid contains a –COOH group, and an alcohol contains an –OH group. In the overall reaction, parts of these reactants join to make the ester, and water is also formed.

Hydrolysis is a reaction in which water helps split a compound into smaller products. For ester hydrolysis, an ester reacts with water to form a carboxylic acid and an alcohol. The word hydrolysis signals that water participates in breaking apart the compound. It is not enough for water to be present; the reaction description includes chemical change and the resulting products.

The equations show the overall patterns in opposite directions. Each equation conserves the number of each kind of atom. Conditions are not included because the focus is on identifying and explaining the reaction types.

\mathrm{CH_3COOH + CH_3CH_2OH \rightarrow CH_3COOCH_2CH_3 + H_2O}

Esterification: a carboxylic acid and an alcohol form an ester and water.
Hydrolysis of an ester: an ester and water form a carboxylic acid and an alcohol.
Atoms are rearranged; they are not created or destroyed.

4. Compare structures and check equations

A reliable method is to compare reactants with products. First identify the functional groups. Then check whether a group has been replaced, whether a double bond has formed or disappeared, and whether water or oxygen appears in the overall change. These clues help you name the main reaction pattern.

Keep the six patterns distinct. Replacement points to substitution. Joining across a double bond points to addition. Removal that produces a double bond points to elimination. Gaining oxygen or losing hydrogen in a common organic example points to oxidation. A carboxylic acid and an alcohol forming an ester and water point to esterification. An ester reacting with water to form a carboxylic acid and an alcohol points to hydrolysis.

A balanced equation has the same number of each type of atom on both sides. Structural formulas help show changes in connections, while counting atoms checks conservation. Never change subscripts to balance an equation, because changing a subscript changes the identity of a molecule. If needed, use coefficients in front of formulas.

Compare both the bonds and the groups in the reactants and products.
Use the product pattern to distinguish reactions that may look similar.
Check that each element has the same atom count on both sides.

Worked example

Identify and verify an ester reaction

Ethanol and ethanoic acid react to form ethyl ethanoate and water. Identify the reaction type, explain the molecular change, and check whether the equation is balanced.

Recognize the reactants and product
Ethanol is an alcohol, and ethanoic acid is a carboxylic acid. Ethyl ethanoate is an ester, and water is also a product. This combination identifies the reaction as esterification.
Describe the molecular change
The alcohol and carboxylic acid join to form the ester, and water is produced. This is not hydrolysis: in hydrolysis, water would be a reactant, and the ester would split into a carboxylic acid and an alcohol.
Check conservation of atoms
Count each element in the structural equation. The left side has four carbon atoms, eight hydrogen atoms, and three oxygen atoms. The ester and water on the right have the same totals. Therefore, the equation is balanced with coefficients of one. No calculation units or rounding are needed for this atom count.
$\mathrm{CH_3COOH + CH_3CH_2OH \rightarrow CH_3COOCH_2CH_3 + H_2O}$

Answer: This is esterification. A carboxylic acid and an alcohol form an ester and water. The equation is balanced.

Check: There are four carbon atoms, eight hydrogen atoms, and three oxygen atoms on each side.

Common mistakes and how to avoid them

Calling every reaction that forms a double bond an addition reaction.

Correction: If atoms or groups leave and a carbon–carbon double bond forms, the pattern is elimination. Addition uses a double bond as the place where atoms or groups join.

Describing esterification as an ester reacting with water.

Correction: A carboxylic acid and an alcohol form an ester and water in esterification. An ester reacting with water to form an acid and an alcohol is hydrolysis.

Saying ethanol gains oxygen when it changes to ethanal in the equation shown.

Correction: Ethanol and ethanal each contain one oxygen atom. The organic molecule loses hydrogen overall; the oxygen supplied by [O] appears in the water product.

Treating [O] as a complete molecular formula for a reactant.

Correction: In this course-level equation, [O] is shorthand for oxygen supplied by an oxidizing substance.

Lesson summary

Substitution replaces one atom or group with another.
Addition joins atoms or groups across a carbon–carbon double bond.
Elimination removes atoms or groups and forms a carbon–carbon double bond.
Common organic oxidation involves gaining oxygen, losing hydrogen, or both; ethanol changing to ethanal involves loss of hydrogen overall.
Esterification joins a carboxylic acid and an alcohol to form an ester and water.
Hydrolysis uses water to split an ester into a carboxylic acid and an alcohol.

Check your understanding

Question 1

An alkene reacts and its carbon–carbon double bond becomes a single bond as two atoms attach to the carbon atoms. Which reaction pattern is shown?

Substitution
Addition
Elimination
Hydrolysis

Show answer and explanation

Addition

The atoms attach across the alkene double bond. That is the defining pattern of addition.

Question 2

Which reactants and products describe hydrolysis of an ester?

A carboxylic acid and an alcohol form an ester and water.
An ester and water form a carboxylic acid and an alcohol.
An alkene and hydrogen form an alkane.
One atom in a molecule is replaced by another.

Show answer and explanation

An ester and water form a carboxylic acid and an alcohol.

Hydrolysis uses water to split an ester into a carboxylic acid and an alcohol.

Question 3

In the common organic change from ethanol to ethanal, what happens to the organic molecule?

It gains an oxygen atom.
It loses hydrogen overall.
It gains a carbon–carbon double bond.
It is split by water into an acid and an alcohol.

Show answer and explanation

It loses hydrogen overall.

Ethanol and ethanal each contain one oxygen atom. The organic molecule loses hydrogen overall, while water forms as a product.

Key terms

Bond: A connection between atoms in a molecule.
Functional group: A recognizable group of atoms in a molecule that helps determine its reactions.
Carboxylic acid: An organic compound containing a –COOH group.
Ester: An organic compound containing a characteristic –COO– connection between its carbon-containing parts.
Hydrolysis: A reaction in which water helps split a compound into smaller products.
Oxidation: A reaction pattern involving loss of electrons; common organic examples often involve gaining oxygen or losing hydrogen.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation B3.3. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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