The carbocation stability order is from tertiary the most stable of all to methyl carbocation the least stable. The stability is explained by the electron-donating ability of methyl groups attached to an electrophilic carbon. The substituents attached to the positively charged carbon formed partial bonds with the carbon known as hyperconjugation.

carbocation stability

Rearrangement of carbocation

The carbocation rearranges in a direction that lead to more stable carbocation. The origin of migrating group and destination are adjacent carbons

For example, hydrogen migrates to adjacent carbon. The primary cation is converted to a secondary carbocation as show in the figure below:

Hydride shift

Hydride shift mostly occurs during the dehyration of primary alcohols. When 1-butanol is dehydrated with concentrated H2SO4, only 1-butene is expected but only 10-12% of is produced with 2-butene 90%.

Stability of cyclic carbocations

The stability of cyclic carbocation is somewhat similar to open chain in the case of cycloakane but in cycloalkenes there is effect of conjugation prdominates.

For example, cycloheptatrienyl cation commonly known as tropylium ion is a stable carbocation due to six pi electron conjugated system. Therefore it possess the special stability due to aromatic conjugated resonance of pi electrons.

Carbocation stability determination through a mass spectrum

During fragmentation of the alkyl chain in mass spectrometry, it has been observed probability of formation of the stable carbocation is greater as show in the mass spectrum belowThe mass  spectrum of decene depicts the cleavage occurring at the points where stable fragments are formed.

Concepts Berg

What makes a carbocation?

The heterolytic cleavage of an organic molecule forms carbocation. 

What determines whether or not a carbocation will be stable?

The number of electron-donating groups attached to a carbon atom with a positive charge determines its stability.

Are allylic carbocations more stable than tertiary?

Yes, the allylic carbocations are more stable than tertiary. This is because of the electron donation due to resonance.

How does conjugation affect stability?

The more the conjugation more will the stability of the carbocation. Tropylium ion has resonance and it has a 6 electron system making it relatively more stable.

What is the stability of the Carbanion?

The stability pattern of carbanion is opposite to the carbocation. The negative charge on the carbon is stabilized here by electron-withdrawing groups as well as conjugation attached will make it more stable.

For example, the Chloroethnanate ion is more stable than the ethanoate ion, because chlorine 

Is electron withdrawing group by inductive effect.

Which carbocation is least stable?

Methyl carbocation is the least stable of normal open chain cations. However, the carbocations that have electron-withdrawing groups attached to them are relatively highly unstable.

Why are carbocations unstable?

Carbocation stability is relative to carbocations and they are really difficult to separate. Even the most stable carbocation is more reactive then many organic compounds. It is unstable because of its incomplete octet such the it has 6 electrons in the valence shell and seek electron pair.

Which is the most stable known carbocation?

Tertiary carbocations are most stable among the carbocation.

How to compare the stability of carbocation?

The stability of carbocations is studied by calculated their life span.

How do I compare the stability of carbocation using +I and -I effect?

The substituents attached to carbocations donate or withdraw the charge density by inductive effect. For example, methyl groups are electron donating groups, therefore, stabilizes the carbocation.

Is a carbocation more stable than a nitrogen cation (such as NH2 or NO2)?

Tertiary carbocations are relatively more stable the nitrogen cations and can be separated during a reaction.

But WHY do carbocations rearrange?

Carbocation rearranges to a more stable configuration such as primary to secondary and secondary to tertiary. For example, in a primary carbocation if there is a possibility of shifting a methyl group and hydride it will definitely do so.

why tertiary carbocation is more stable?

A tertiary carbocation is relatively more stable because three methyl or alkyl groups are attached to it. These groups donate electron density to electron-deficient carbon by hyper conjugation.