Ch 3 · Shapes of organic molecules; σ and π bonds · AS & A Level Chemistry 9701

Topic 3 of 4 in An introduction to AS Level organic chemistry — AS & A Level Chemistry 9701 — 58 lessons.

Introduction

The three-dimensional shape of an organic molecule is not arbitrary: it follows directly from the type of hybridisation at each carbon atom and from electron pair repulsion. Understanding sigma and pi bonds and the hybrid orbitals that produce them explains bond angles, the planarity of alkenes, and ultimately the reactivity of double and triple bonds towards electrophiles.

Key ideas — Chain shapes: straight-chained, branched and cyclic

Organic molecules can be classified by the arrangement of their carbon skeleton. A straight-chained (unbranched) molecule has all its carbon atoms joined in a single continuous chain, e.g. butane. A branched molecule has one or more side chains attached to the main chain, e.g. 2-methylpropane. A cyclic molecule has its carbon atoms joined in a closed ring, e.g. cyclohexane.

🧑‍🏫 Teacher's explanation

Think of organic molecules like building with LEGO bricks. A straight-chain molecule is like a single long train of bricks connected end-to-end. A branched molecule is like that same train, but with a few extra bricks sticking out from the sides. A cyclic molecule is like taking that train and snapping the front to the back to form a ring.

📖 Definitions

🔍 Worked example — Easy

Identify the type of chain structure for the following three molecules: (i) CH_3CH_2CH_2CH_3, (ii) CH(CH_3)_3, and (iii) C_5H_10 (where the carbons form a ring).

🔍 Worked example — Standard

Explain why ethene, C_2H_4, is a planar molecule.

🔍 Worked example — Exam-style

A molecule has the formula C_4H_10. It is not straight-chained and has only one branch. Draw/describe its structure and determine if it is cyclic.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — sp3 hybridisation and tetrahedral shape

Where a carbon atom forms four single (sigma) bonds, as in alkanes, its one 2s and three 2p orbitals mix to form four identical sp3 hybrid orbitals. These four orbitals repel each other equally and arrange themselves as far apart as possible in three dimensions, giving a tetrahedral shape with bond angles of 109.5^°. Each sp3 orbital forms one sigma (σ) bond by head-on, end-to-end overlap with an orbital on the neighbouring atom.

🧑‍🏫 Teacher's explanation

Imagine you are holding four long, identical balloons tied together at their ends. Because the balloons want to stay as far away from each other as possible to minimize crowding, they naturally push out into a 3D shape where they are spread evenly in all directions. This is exactly what electrons do around a carbon atom.

📖 Definitions

🔍 Worked example — Easy

State the hybridization and the bond angle of the carbon atom in methane, CH_4.

🔍 Worked example — Standard

Calculate the total number of σ bonds in ethane, C_2H_6.

🔍 Worked example — Exam-style

A molecule has a central carbon atom bonded to four chlorine atoms. Describe the hybridization of the carbon and the shape of the molecule.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — sp2 hybridisation and the planar C=C bond

Where a carbon atom forms a double bond, as in alkenes, only its 2s orbital and two of its three 2p orbitals hybridise, forming three sp2 hybrid orbitals. These three orbitals lie in a single plane at 120^° to each other, each forming a sigma bond, so the atoms directly bonded to an sp2 carbon are arranged with trigonal planar geometry. The remaining, unhybridised 2p orbital on each sp2 carbon points perpendicular to this plane; sideways overlap of the two unhybridised p orbitals on adjacent carbons forms a pi (π) bond, a region of electron density above and below the plane of the sigma bonds. A carbon–carbon double bond is therefore one sigma bond plus one pi bond.

🧑‍🏫 Teacher's explanation

Imagine you are building a model kit. In a single bond (like ethane), the carbon atom uses all its 'arms' to reach out in three dimensions, like a tripod. But in a double bond (like ethene), the carbon atom decides to flatten itself out. It uses three of its orbitals to form a flat, triangular shape, and then it uses its leftover 'extra' orbital to reach up and down, perpendicular to that flat triangle.

📖 Definitions

🔍 Worked example — Easy

Identify the number of σ and π bonds in a molecule of ethene, ceC2H4.

🔍 Worked example — Standard

Describe the hybridization and bond angle of the carbon atoms in ethene, ceC2H4.

🔍 Worked example — Exam-style

A molecule has a ceC=C bond. If the π bond is broken, how many σ bonds would remain in the molecule? (Assume the molecule is ethene, ceC2H4)

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — sp hybridisation and linear triple bonds

Where a carbon atom forms a triple bond, as in alkynes, only its 2s orbital and one 2p orbital hybridise, forming two sp hybrid orbitals arranged at 180^° to each other (linear geometry), each forming a sigma bond. The two remaining unhybridised 2p orbitals on each sp carbon are perpendicular to each other and to the sp orbitals; sideways overlap of these with the corresponding orbitals on the adjacent carbon forms two pi bonds at right angles to one another. A carbon–carbon triple bond is therefore one sigma bond plus two pi bonds, giving a linear arrangement of the directly bonded atoms with bond angles of 180^°.

🧑‍🏫 Teacher's explanation

Imagine you are building a model of a molecule using sticks and balls. In a triple bond, the carbon atom doesn't just use its standard 'spherical' s orbital or its 'dumbbell-shaped' p orbitals as they are. Instead, it mixes them together to create new, specialized 'hybrid' orbitals that are much better at reaching out to other atoms.

📖 Definitions

🔍 Worked example — Easy

How many sigma (σ) and pi (π) bonds are present in a molecule of ethyne, C_2H_2?

🔍 Worked example — Standard

Describe the geometry and bond angle of the carbon atoms in propyne, CH_3C≡ CH.

🔍 Worked example — Exam-style

A molecule contains a carbon atom bonded to three other atoms via one sigma bond and two pi bonds. What is the bond angle between the atoms involved in the sigma bonds?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the hybridisation of the carbon atom involved in the triple bond. [1] (b) Describe the shape of the C≡ C unit and state the bond angle. [2] [3 marks]

Key ideas — Comparing sigma and pi bonds

A sigma bond forms by direct, head-on orbital overlap along the axis joining the two nuclei, allowing free rotation about the bond and giving substantial electron density concentrated between the nuclei, which makes it relatively strong. A pi bond forms by sideways overlap of parallel p orbitals above and below (or in front of and behind) the sigma bond axis; because this overlap is weaker and the electron density is exposed above and below the internuclear axis, a pi bond is weaker than a sigma bond and is the site of attack by electrophiles in reactions such as the electrophilic addition of alkenes.

🧑‍🏫 Teacher's explanation

Think of a sigma (σ) bond like two people shaking hands firmly; their hands are directly between them, creating a strong, direct connection that allows them to rotate their bodies around that axis. A pi (π) bond, however, is like two people standing side-by-side and holding their arms out parallel to each other; they are connected, but the connection is much weaker and more 'exposed' to the outside world.

Ch 3 · Shapes of organic molecules; σ and π bonds · AS & A Level Chemistry 9701

All chapters in An introduction to AS Level organic chemistry