Alkynes and Nitriles: sp Carbon
A concise Cambridge International AS Level Chemistry revision guide to sp hybridised carbon (13.3.2 and 13.3.3): how two sp orbitals and two unhybridised p orbitals give a triple bond made of one σ and two π bonds, the linear 180° shape around C≡C and C≡N, and how sp, sp2 and sp3 carbons compare.
Forming sp Hybrid Orbitals
When a carbon atom forms a triple bond, only the 2s orbital and one 2p orbital mix. This gives two sp hybrid orbitals pointing in opposite directions, 180° apart, and leaves two unhybridised 2p orbitals at right angles to each other and to the sp orbitals.
| Feature | sp carbon | Compare sp2 | Compare sp3 |
|---|---|---|---|
| Orbitals mixed | One 2s + one 2p | One 2s + two 2p | One 2s + three 2p |
| Hybrid orbitals | Two sp | Three sp2 | Four sp3 |
| Unhybridised p orbitals | Two | One | None |
| σ bonds formed | Two | Three | Four |
| π bonds formed | Two | One | None |
| Shape and angle | Linear, 180° | Trigonal planar, 120° | Tetrahedral, 109.5° |
Key idea: Each unhybridised p orbital makes one π bond, so an sp carbon with two p orbitals left over can make two π bonds: the second and third bonds of a triple bond.
Quick Check: Counting σ and π Bonds
Five quick questions on molecules that do not appear on this page.
The Triple Bond in Ethyne
In ethyne, HC≡CH, each carbon is sp hybridised. One sp orbital overlaps head-on with the other carbon to form the C-C σ bond, and the other sp orbital overlaps with a hydrogen 1s orbital to form the C-H σ bond. The two remaining p orbitals on each carbon overlap sideways with their partners on the other carbon to form two π bonds, one above and below the molecule and one in front and behind.
The C≡C bond therefore contains one σ bond and two π bonds. Because the two sp orbitals point in opposite directions, the H-C-C bond angle is 180° and ethyne is a linear molecule. The π bonds prevent rotation about the triple bond.
Bonding in Ethyne, C2H2: sp Hybrids, Sigma and Pi Bonds
Drag to rotate, scroll or pinch to zoom. Step through the bonding one layer at a time, from the sp hybrid orbitals on each carbon to the finished triple bond. The side view looks straight down the molecule and shows both pi bonds at right angles.
Hybrid orbitals
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Bonding in Hydrogen Cyanide, HCN: sp Hybrids, Sigma and Pi Bonds
Drag to rotate, scroll or pinch to zoom. Build the molecule one layer at a time, from the sp hybrid orbitals on carbon and nitrogen through to the finished triple bond and the lone pair. The side view looks straight down the molecule and shows both pi bonds at right angles.
Hybrid orbitals
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Hybridisation of Carbon: sp³, sp² and sp Orbitals
Drag to rotate, scroll or pinch to zoom. Switch between the three states to see how mixing the 2s and 2p orbitals changes the shape around the carbon nucleus, and which 2p orbitals are left unhybridised.
sp³ hybridisation
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Exam sentence: In ethyne each carbon is sp hybridised: the sp orbitals form two σ bonds at 180°, and the two unhybridised p orbitals on each carbon form two π bonds, so the molecule is linear.
Quick Check: Atoms on a Straight Line
Decide how far along the chain the linear shape of a triple bond reaches.
Quick Check: Cis and Trans in an Alkyne
Decide whether a blocked rotation is enough on its own to give geometrical isomers.
The Nitrile Group
The same bonding occurs in a nitrile, R-C≡N. The nitrile carbon is sp hybridised: one sp orbital bonds to the alkyl group, the other forms the C-N σ bond, and the two p orbitals form two π bonds with the nitrogen atom. The nitrogen is also sp hybridised, with its lone pair in the sp orbital pointing away from the carbon.
The C-C≡N unit is linear, with a bond angle of 180° at the nitrile carbon. In ethanenitrile, CH3CN, the methyl carbon is sp3 and tetrahedral while the nitrile carbon is sp and linear, so one small molecule contains both kinds of carbon.
| Molecule | Carbon atom | Hybridisation | Shape around it | Angle |
|---|---|---|---|---|
| Ethanenitrile, CH3CN | CH3 carbon | sp3 | Tetrahedral | 109.5° |
| Ethanenitrile, CH3CN | CN carbon | sp | Linear | 180° |
| Propyne, CH3C≡CH | Both triple-bond carbons | sp | Linear | 180° |
| Propene, CH3CH=CH2 | Both double-bond carbons | sp2 | Trigonal planar | 120° |
Exam focus: Cambridge asks for the shape and bond angle around a named atom, so identify the hybridisation of that atom only: count its σ bonds and lone pairs (two gives sp, three gives sp2, four gives sp3).
Quick Check: Hybridisation Along a Chain
Work along the formula one carbon atom at a time.
Quick Check: Two Nitrogen Atoms
Drag the words and numbers into place to compare the two nitrogen atoms of one molecule.
Common Exam Mistakes
- Saying a triple bond is three σ bonds or three π bonds. It is one σ bond and two π bonds.
- Quoting 120° for an alkyne carbon. Two σ bonds and no lone pairs give 180°.
- Forgetting that the p orbitals must be at right angles to each other, so the two π bonds are in perpendicular planes.
- Assigning one hybridisation to a whole molecule. Each carbon atom is hybridised according to its own bonds.
Exam sentence: sp carbon: two hybrid orbitals, two σ bonds, two π bonds from two unhybridised p orbitals, linear, 180°.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Free Radical Substitution FAQs
These questions summarise the core exam points on alkane substitution and free radicals.
Why are alkanes usually unreactive?
Alkanes are usually unreactive because they contain strong C-C and C-H bonds. These bonds require a large amount of energy to break.
What condition is needed for alkanes to react with chlorine?
Ultraviolet light is needed. UV light provides enough energy to break the Cl-Cl bond by homolytic fission, forming chlorine radicals.
What is a free radical?
A free radical is a species with an unpaired electron. The unpaired electron is usually represented using a dot, such as Cl•.
Why is the reaction called substitution?
It is called substitution because a hydrogen atom in the alkane is replaced by a halogen atom.
Why can a mixture of products form?
After the first substitution, the haloalkane product can undergo further substitution. This can replace more hydrogen atoms and produce a mixture of chlorinated products.
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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