Alkanes and sp3 Carbon
A concise Cambridge International AS Level Chemistry revision guide to sp3 hybridised carbon (13.3.2 and 13.3.3): how the four sp3 orbitals form, the tetrahedral shape and 109.5° bond angle, the σ bonds of alkanes with free rotation, and straight-chain, branched and cyclic molecules (13.3.1).
- 13.3.1-a
- 13.3.1-b
- 13.3.1-c
- 13.3.2-c
- 13.3.3-c
What these spec points say
- 13.3.1-a describe organic molecules as straight-chained
- 13.3.1-b describe organic molecules as branched
- 13.3.1-c describe organic molecules as cyclic
- 13.3.2-c describe and explain the shape of, and bond angles in, molecules containing sp3 hybridised atoms
- 13.3.3-c describe the arrangement of σ and π bonds in molecules containing sp3 hybridised atoms
Tetrahedral Carbon and the Zig-Zag Chain
Choose a step to watch four bonding pairs spread to 109.5° and then build, twist and flatten a butane chain. Drag to rotate.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Hybridisation of Carbon: sp³, sp² and sp Orbitals
Choose a step to watch carbon’s 2s and 2p orbitals mix into sp³, sp² or sp hybrids, see the angle between them and which 2p orbitals are left for π bonds.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: The Alkane Family and Shared Pairs
Before you start, check the alkane general formula, what saturated means and what a single covalent bond is.
Why Carbon Hybridises
A carbon atom has the electron configuration 1s2 2s2 2p2, which suggests only two unpaired electrons and two bonds.
In practice carbon always forms four bonds. Cambridge explains this using hybridisation: the 2s orbital and the three 2p orbitals mix to form new orbitals of equal energy, each holding one electron.
In an alkane all four are mixed, giving four sp3 hybrid orbitals.
| Orbitals mixed | Hybrid orbitals formed | Shape of the set | Bond angle | Found in |
|---|---|---|---|---|
| One 2s + three 2p | Four sp3 | Tetrahedral | 109.5° | Alkanes, and any carbon with four single bonds |
| One 2s + two 2p | Three sp2 (one p left over) | Trigonal planar | 120° | Alkenes, carbonyl carbon |
| One 2s + one 2p | Two sp (two p left over) | Linear | 180° | Alkynes, nitriles |
Mixing one 2s and three 2p orbitals gives four sp3 hybrid orbitals pointing to the corners of a tetrahedron.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Key idea: The number of hybrid orbitals equals the number of atomic orbitals mixed. Four orbitals mixed gives four sp3 orbitals, so the carbon makes four σ bonds.
Quick Check: Name the Hybridisation
Five quick questions on carbon atoms in molecules the page does not use.
The Tetrahedral Carbon of Alkanes
Each sp3 orbital overlaps head-on with an orbital on another atom to form a σ bond: with a 1s orbital of hydrogen for a C-H bond, or with an sp3 orbital of another carbon for a C-C bond.
Because the electron density of a σ bond lies along the line between the nuclei, the two ends can rotate freely about the bond.
The four σ bonds repel each other equally and point to the corners of a tetrahedron, so every carbon atom in an alkane is tetrahedral with bond angles of 109.5°.
In propane, for example, both the end carbons and the middle carbon are tetrahedral, which is why the carbon chain zig-zags rather than lying in a straight line.
Every carbon in propane is sp3 hybridised and tetrahedral, and the C-C σ bonds allow free rotation.
Exam sentence: In an alkane each carbon atom is sp3 hybridised; its four sp3 orbitals form four σ bonds arranged tetrahedrally with bond angles of 109.5°, and there is free rotation about each C-C σ bond.
Quick Check: Counting Sigma Bonds
Type a number in each gap; count carbon to carbon and carbon to hydrogen bonds separately.
Quick Check: Two Drawings of Butane
Decide whether the two drawings show one compound or two, and why.
Straight-Chain, Branched and Cyclic Molecules
Cambridge 13.3.1 asks you to describe organic molecules as straight-chained, branched or cyclic. All three are built from sp3 carbons in alkanes; the difference is how the tetrahedral carbons are joined.
| Description | Meaning | Example | General formula (alkanes) |
|---|---|---|---|
| Straight-chain | Every carbon is bonded to at most two other carbons, forming one continuous chain | Hexane | CnH2n+2 |
| Branched | At least one carbon is bonded to three or four other carbons, so side chains come off the main chain | 2-Methylpentane, 2,2-dimethylbutane | CnH2n+2 |
| Cyclic | The carbon atoms form a ring | Cyclohexane | CnH2n |
A “straight” chain is not geometrically straight: the tetrahedral angles make it zig-zag. In cyclohexane the ring is not flat either; it puckers into a chair shape so that every carbon keeps its 109.5° angles.
Exam focus: Straight-chain and branched isomers share a molecular formula; the cyclic alkane with the same number of carbons has two fewer hydrogen atoms because two C-H bonds are replaced by the extra C-C bond that closes the ring.
All bonds in an alkane are σ bonds, so groups rotate freely and different drawings can be one molecule.
Quick Check: Pick the Accurate Statement
In each round, choose the one statement that is accurate.
Common Exam Mistakes
- Saying carbon has four sp3 orbitals because it “needs four bonds”. Give the mechanism: one 2s and three 2p orbitals mix.
- Describing C-C bonds in alkanes as π bonds. All bonds in an alkane are σ bonds.
- Drawing the carbon chain as a straight line and quoting 180°. The chain zig-zags with 109.5° angles.
- Giving cyclohexane the alkane formula C6H14. The ring formula is C6H12.
Exam sentence: sp3 carbon: four hybrid orbitals, four σ bonds, tetrahedral, 109.5°, free rotation.
FAQs
These questions cover the hybridisation points students most often get wrong.
Why does carbon form four bonds if it has only two unpaired electrons?
The 2s and three 2p orbitals mix into four sp3 orbitals of equal energy, each with one electron, so four σ bonds can form.
Are all the bonds in an alkane σ bonds?
Yes. Every C-C and C-H bond in an alkane is a σ bond formed by head-on overlap. There are no π bonds.
Why is the bond angle 109.5° and not 90°?
Four bonding pairs repel equally and get as far apart as possible, which is a tetrahedron with angles of 109.5°.
Is a straight-chain alkane really straight?
No. The tetrahedral angle at each carbon makes the chain zig-zag. Straight-chain only means there are no branches.
Why does cyclohexane have two fewer hydrogens than hexane?
Closing the ring uses one extra C-C bond, so two C-H bonds are lost. Cyclohexane is C6H12; hexane is C6H14.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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