8.1.3 Structural Formulae for Alkynes

Introduction

In the previous sections, we learned about alkanes (single bonds only) and alkenes (at least one double bond). Now we turn to alkynes, which contain at least one carbon-carbon triple bond.

Alkynes are unsaturated hydrocarbons because they contain a triple bond. This triple bond is the key feature that distinguishes alkynes from alkanes and alkenes.

This section explains how to draw structural formulas for alkynes.

Definition

Alkynes: Hydrocarbons which have at least one triple bond between carbon atoms are called alkynes.

General Formula: C_nH_2n-2

Where:

- n = number of carbon atoms

In simple words: Alkynes are hydrocarbons with at least one triple bond between two carbon atoms.

Main Idea

• Alkynes have at least one carbon-carbon triple bond.
• They have the general formula C_nH_2n-2.
• The simplest member of the alkyne family is ethyne (also called acetylene).
• In ethyne, the two carbon atoms share three pairs of electrons.
• This means the carbon atoms are joined by a triple bond.
• Triply bonded carbon atoms form a single bond with one other atom.

Core Concept

Key point: Alkynes contain a carbon-carbon triple bond and have the general formula C_nH_2n-2.

Detailed Explanation

What are Alkynes?

Hydrocarbons which have at least one triple bond between carbon atoms are called alkynes.

Those with one triple bond have the general formula: C_nH_2n-2

Key point: The triple bond is the defining feature of alkynes.

Ethyne (Acetylene)

Ethyne, also called acetylene, is the simplest member of the alkyne family.

Structure:

• In ethyne, the two carbon atoms share three pairs of electrons.
• This means the carbon atoms are joined by a triple bond.

Question from the source: How many hydrogen atoms can share electrons with each triply-bonded carbon atom?

Answer: Each triply-bonded carbon atom can share electrons with one hydrogen atom.

Structure of ethyne:

H - C ≡ C - H

Condensed formula: HC ≡ CH

Key point: Ethyne has a triple bond between the two carbon atoms and one hydrogen atom attached to each carbon.

Drawing Structural Formulas for Alkynes

Steps:

• Write the chain of carbon atoms based on the stem name.
• Place the triple bond at the correct position (if the name includes a number).
• Attach the correct number of hydrogen atoms to each carbon atom so that each carbon has four bonds.
• For the condensed formula, group the atoms attached to each carbon.

Key point: Follow the same rules as for alkanes and alkenes, but include the triple bond.

Intermediate Level

Drawing structural formulas for alkynes:

• Write the chain of carbon atoms based on the stem name.
• Place the triple bond at the correct position.
• Attach the correct number of hydrogen atoms to each carbon atom so that each carbon has four bonds.
• For the condensed formula, group the atoms attached to each carbon.

Examples:

Ethyne: HC ≡ CH

Propyne: CH₃C ≡ CH

But-1-yne: HC ≡ C - CH₂ - CH₃

But-2-yne: CH₃C ≡ CCH₃

Key point: The position of the triple bond is indicated by a number in the name.

Solved Example

Example 8.3: Draw Structural and Condensed Formulas for But-1-yne and But-2-yne

Problem Solving Strategy:

i) But- means four carbon atoms.

ii) Ending -yne indicates an alkyne.

iii) Write a string of four carbon atoms and give a number to each carbon atom from one side.

iv) The number written in the name indicates the position of the triple bond. Make a triple bond between this carbon atom and the next one.

v) Attach the required number of hydrogen atoms to the carbon atoms to give each carbon atom four bonds.

vi) For the normal or condensed structural formula, write each carbon atom's set of hydrogen atoms next to the carbon.

Solution for But-1-yne:

Chain: C¹ - C² - C³ - C⁴

Triple bond between C1 and C2: C¹ ≡ C² - C³ - C⁴

• C1 is triple-bonded to C2, so it needs one hydrogen atom.
• C2 is triple-bonded to C1, so it needs one hydrogen atom.
• C3 is single-bonded to C2 and C4, so it needs two hydrogen atoms.
• C4 is single-bonded to C3, so it needs three hydrogen atoms.

Displayed formula: H - C ≡ C - CH₂ - CH₃

Condensed formula: HC ≡ C - CH₂ - CH₃

Solution for But-2-yne:

Chain: C¹ - C² - C³ - C⁴

Triple bond between C2 and C3: C¹ - C² ≡ C³ - C⁴

• C1 is single-bonded to C2, so it needs three hydrogen atoms.
• C2 is triple-bonded to C3 and single-bonded to C1, so it needs one hydrogen atom.
• C3 is triple-bonded to C2 and single-bonded to C4, so it needs one hydrogen atom.
• C4 is single-bonded to C3, so it needs three hydrogen atoms.

Displayed formula: CH₃ - C ≡ C - CH₃

Condensed formula: CH₃C ≡ CCH₃

Summary Table

Important Points

• - Alkynes = hydrocarbons with at least one carbon-carbon triple bond.
• - General formula: C_nH_2n-2.
• - Simplest alkyne: Ethyne (H - C ≡ C - H), also called acetylene.
• - Triply bonded carbon atoms form a single bond with one other atom.
• - Each carbon atom in a triple bond still forms four bonds total.
• - The triple bond is the key feature of alkynes.
• - Alkynes are unsaturated hydrocarbons.
• - The position of the triple bond is indicated by a number in the name.

Remember

• - Alkynes = hydrocarbons with a triple bond.
• - General formula: C_nH_2n-2.
• - Simplest alkyne: Ethyne (H - C ≡ C - H).
• - Triple bond = three pairs of shared electrons.
• - Each carbon in a triple bond forms 4 bonds total.
• - Triple bond = site of addition reactions.
• - Alkynes are unsaturated.
• - Position of triple bond is indicated by a number.
• - Examples:
• - Ethyne: HC ≡ CH
• - Propyne: CH₃C ≡ CH
• - But-1-yne: HC ≡ C - CH₂ - CH₃
• - But-2-yne: CH₃C ≡ CCH₃

Common Mistakes

Quick Revision

Structural Formulae for Alkynes:

Hydrocarbons which have at least one triple bond between carbon atoms are called alkynes. Those with one triple bond have the general formula C_nH_2n-2.

Ethyne, also called acetylene is the simplest member of the alkyne family. In ethyne, the two carbon atoms share three pairs of electrons. This means the carbon atoms are joined by a triple bond.

• Structure of ethyne:
• H - C ≡ C - H

Example 8.3: Draw structural and condensed formulas for But-1-yne and But-2-yne.

• Problem Solving Strategy:
• i) But- means four carbon atoms.
• ii) Ending -yne indicates an alkyne.
• iii) Write a string of four carbon atoms and give number to each carbon atom from one side.
• iv) The number written in the names indicates the position of the triple bond. Make a triple bond between this carbon atom and next to it.
• v) Attach the required number of hydrogen atoms to the carbon atoms to give each carbon atom four bonds.
• vi) For the normal or condensed structural formula write each carbon atom's set of hydrogen atoms next to the carbon.

Solution:

• But-1-yne:
• C¹ ≡ C² - C³ - C⁴
• HC ≡ C - CH₂ - CH₃

• But-2-yne:
• C¹ - C² ≡ C³ - C⁴
• CH₃ - C ≡ C - CH₃

60-Second Revision

• Alkynes = hydrocarbons with a triple bond.
• General formula: C_nH_2n-2.
• Simplest alkyne: Ethyne (HC ≡ CH).
• Triple bond = three pairs of shared electrons.
• Each carbon in a triple bond forms 4 bonds total.
• Triple bond = site of addition reactions.
• Alkynes are unsaturated.
• Position of triple bond is indicated by a number.
• Examples:
• Ethyne: HC ≡ CH
• Propyne: CH₃C ≡ CH
• But-1-yne: HC ≡ C - CH₂ - CH₃
• But-2-yne: CH₃C ≡ CCH₃
• ⚠️ Warning: alkynes are unsaturated hydrocarbons — they contain at least one carbon-carbon triple bond.