Q1(b). Carbonium ion and carbocation
Define carbonium ion. Is it identical or different with respect to carbocation? Explain giving suitable examples.
Answer in brief
A carbonium ion is a five-coordinate carbocation with a three-centre two-electron bond (CH₅⁺, 2-norbornyl cation). It is one class of carbocation; the other, the common trivalent R₃C⁺ ion (tert-butyl cation), is a carbenium ion. So the two terms are related but not identical.
Key points
- every carbonium ion is a carbocation, but most carbocations met in SN1, E1 and electrophilic addition are carbenium ions.
- The 2-norbornyl cation is the best known stable carbonium ion: the C1–C6 bonding pair is shared over C1, C2 and C6 (a 3c–2e bond), so C6 is five-coordinate.
- In older texts (and in many exam answers) “carbonium ion” is used loosely for R₃C⁺. Write the modern meaning and mention the old one.
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Introduction. Carbocation is the general name for any cation with a positive charge on carbon. In the modern (IUPAC, Olah) usage it has two classes: the trivalent carbenium ions and the penta- (or tetra-)coordinate carbonium ions. In older books “carbonium ion” meant every carbocation, so the answer depends on which usage is meant.
Definition
A carbonium ion is a carbocation in which the positive carbon is five-coordinate (or bonded to more groups than its normal valence allows). It contains at least one three-centre two-electron (3c–2e) bond. The parent is the methanium ion CH₅⁺, formed when methane is protonated by a superacid (FSO₃H-SbF₅).
Figure: Left: tert-butyl cation (carbenium). Right: methanium ion CH₅⁺ (carbonium); dashed line = the 3c–2e bond.
Same or different?
| Carbenium ion (classical carbocation) | Carbonium ion (non-classical) | |
| Coordination of C⁺ | three | five (or four in bridged ions) |
| Electrons round C⁺ | six (sextet), empty p orbital | eight, but shared in 3c–2e bonds |
| Bonding | ordinary 2c–2e bonds | at least one 3c–2e bond |
| Shape | planar, sp² | trigonal-bipyramid-like or bridged |
| Examples | CH₃⁺, (CH₃)₃C⁺, PhCH₂⁺, tropylium | CH₅⁺, C₂H₇⁺, the 2-norbornyl cation |
- The two words are not identical: every carbonium ion is a carbocation, but most carbocations met in SN1, E1 and electrophilic addition are carbenium ions.
- The 2-norbornyl cation is the best known stable carbonium ion: the C1–C6 bonding pair is shared over C1, C2 and C6 (a 3c–2e bond), so C6 is five-coordinate.
- In older texts (and in many exam answers) “carbonium ion” is used loosely for R₃C⁺. Write the modern meaning and mention the old one.
Conclusion. A carbonium ion is a five-coordinate carbocation with a three-centre two-electron bond (CH₅⁺, 2-norbornyl cation). It is one class of carbocation; the other, the common trivalent R₃C⁺ ion (tert-butyl cation), is a carbenium ion. So the two terms are related but not identical.
Application. Carbonium ions formed by protonation of C–H and C–C σ bonds on acid catalysts (zeolites, superacids) start the catalytic cracking and isomerisation of alkanes in petroleum refining.
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All parts of Question 1
- 1(a) Cycloheptatrienyl (tropylium) bromide: an ionic organic bromide10 marks
- 1(b) Carbonium ion and carbocation10 marks
- 1(c) trans-1,2-Dibromo-1-methylcyclohexane from methylcyclohexane10 marks
- 1(d) Why Grignard reactions cannot occur in cells10 marks
- 1(e) Thermal electrocyclisation of (2E,4Z,6E)-octa-2,4,6-triene10 marks