Organic chemistry Part 10: Reactivity and reaction mechanisms

Organic Chemistry PART 10 Introduction to Reaction Mechanisms

Appendix 1 The structure, relative stability and theory of alkyl carbocations encountered in organic reaction mechanisms

See also

Introduction to reaction mechanisms and a glossary of selected terms used in organic chemistry

[Author ©  Dr Phil Brown PhD: Doc Brown's exam revision notes suitable for A level chemistry students of advanced pre-university/college level organic chemistry courses:  relative stability of carbocations  [page updated April 15th 2026 *]

Index of organic chemistry technical terms and mechanism pages

Carbocations and relative stability

This is a detailed discussion that contravenes the previously accepted arguments about the relative stability of carbocations and the relevance to e.g. the S_N1 and S_N2 mechanisms of nucleophilic substitution reactions of haloalkanes

A positive ion derived from an organic molecule where the charge is usually carried by a carbon atom e.g. the ethyl carbocation is CH₃CH₂⁺ 

and note that the three bonds from the carbon carrying the positive charge are in a trigonal planar configuration (2 x C-H and a C-C in this case).   (see mechanisms Part 10.3)

(Note you will come across the same type of positive ion in mass spectrometry)

The stability trend of carbocations is usually quoted and accepted as:

tertiary R₃C⁺  >  secondary R₂CH⁺  >  primary R-CH₂⁺  >  CH₃⁺   (R =alkyl)

which are e.g. can be derived from the corresponding haloalkanes structures shown below, which you come across in studying their nucleophilic substitution reactions.

and this carbocation stability trend is backed up by experimental evidence, but there are several theories explaining the stability trend of these aliphatic carbocations.

In the nucleophilic substitution reactions of haloalkanes, this trend influences both the mode of the mechanism (S_N1 or S_N2) and the speed of the substitution (reaction kinetics can be 1st or 2nd order).

The original inductive effect theory (from the 1950s) argued that the inductive effect (+I) of the alkyl groups causes stabilisation, that is the alkyl groups donate electron charge to the positive carbon of the carbocation, thereby conferring extra stability.

It is argued that this inductive effect increases with increase in the number of electron charge donating alkyl groups attached to the carbon atom carrying the positive charge.

On the carbocation diagram above, the red arrow heads indicate the supposed +I effect of the alkyl groups (here I've restricted the above diagram to methyl groups).

So, positive charge attraction is increasing on the electrons from 1 to 3 C-C sigma bonds, so the positive charge is dispersed across 1 to 3 other carbon atoms (ignoring CH₃⁺) and their hydrogen atoms too!

As a general rule in chemistry, the more dispersed the charge, the more stable is the species carrying the charge (this also applies to the hyperconjugation theory outlined below).

BUT, although the stability trend itself is not being challenged, the +I induction theory is being challenged and another argument is based on the concept of hyperconjugation and should be understood, even at pre-university level.

It is argued that alkyl groups are NOT inductively electron releasing.

Theoretically it has been shown that alkyl groups actually have a small -I effect compared to hydrogen and it should be noted that the electronegativity of carbon is higher than hydrogen, C 2.5 > H 2.2 (Pauling scale) which fits in with a small -I effect of the alkyl groups.

This is quite contrary to the inductive effect argument diagram above.

To outline another explanation of the stability trend, we need to introduce the concept of hyperconjugation.

In organic chemistry, hyperconjugation is described as the delocalization of electrons from sigma (σ) bonds (like C-H or C-C) into adjacent empty or partially filled p-orbitals (in this case) or π orbital systems.

This confers an extra stability on the carbocation species by the 'spreading' or 'delocalising' of the charge and lowering the potential energy of the carbocation.

The simplified diagram below shows this hyperconjugation for alkyl based carbocations

The complete alternative theory requires a knowledge of molecular orbital theory, but the 'pre-university' argument presented here to explain the carbocation stability trend can be summarised as... an increase in C-C or C-H sigma bond electrons available, allows greater hyperconjugation with the unoccupied 2p orbital (i.e. it can become partially occupied), with increase in alkyl groups attached to the positive carbon atom of the carbocation and this hyperconjugation can extend across all the alkyl groups.

Although a more sophisticated theory than the +I theory, both in a sense involve the idea of spreading the positive charge and lowering the electronic potential energy of the carbocation, thereby stabilising it to some extent, so perhaps both arguments are valid?

For further reading see links below, on which I've based the above explanations.

https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/07%3A_Alkenes-_Structure_and_Reactivity/7.10%3A_Carbocation_Structure_and_Stability (good reading on hyperconjugation for both teacher and adventurous students!)

https://pubs.rsc.org/en/content/articlelanding/2025/ob/d4ob01572j (need to have an account and teacher orientated)

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