The structure and reactivity of many compounds in organic chemistry are greatly dictated by the presence of bulky groups or constituents in the molecule. This is called steric hindrance. It arises because of inter-electronic repulsions due to spatial crowding amongst bulky groups. Using steric factors, we can conclude that trans-2-butene is more stable than cis-2-butene.
Steric Hindrance in Organic Chemistry
Stability of Intermediates
Carbocations
The stability of carbocations can be explained by the Inductive Effect.
CH3++CH2(CH3) +CH(CH3)2+C(CH3)3, i.e., Methyl Primary (1°) Secondary (2°) Tertiary(3°)
We know that alkyl groups of organic chemistry are +I groups, that is, they release electrons through the sigma bonds.
Since the carbon is deficient in electrons, we can say that as the number of methyl groups increases, the stability of the carbocation increases, as the electropositive carbon is satiated by the electrons given by the methyl groups via the +I effect
Therefore, the stability order of carbocations in organic chemistry is of the order 3° > 2° > 1° > Methyl carbocation.
Hyperconjugation
When a C-H σ-bond is in conjugation with a carbocation, this effect is observed. A carbocation has a vacant p-orbital. The bonded σ-electron pair of the C-H bond is displaced towards the vacant p-atomic orbital. This increases the electron density in the empty p-AO.
It is, therefore, a resonance effect, where a C-H bond breaks and the σ-electron pair is delocalised to the vacant p-AO of the carbonation. Since the bond between C and H is broken, it is also called ‘no bond resonance’. It is also referred to as the Baker-Nathan effect.
The more the number of α-hydrogens in a carbonation, the more hyperconjugated structures would be possible. The more the number of structures, the more the stability of the species.
Stability of Carbanions
The stability of carbanions can be explained using the inductive effect.
CH3 — CH2(CH3) — CH(CH3)2 — C(CH3)3 i.e. Methyl Primary (1°) Secondary (2°) Tertiary (3°)
Since alkyl groups are electron-releasing by nature through induction, we can say that the more the number of methyl groups attached to the carbon having a negative charge, the less its stability would be.
This is because the carbon already has a negative charge, to which the methyl groups push electrons via induction. This results in inter-electronic repulsions and destabilises the species.
Therefore, the stability order for carbanions is as follows:
3°< 2°< 1°< Methyl carbanion
Stability of Free Radicals
The stability of free radicals in organic chemistry follows the same trend as that of carbocations.
CH3–CH2(CH3)–CH(CH3)2–C(CH3)3 i.e. Methyl Primary (1°) — Secondary (2°) — Tertiary(3°)
Therefore, the stability order of free radicals is of order: 3° > 2° > 1° > methyl carbocation.
This can be explained with the help of the Hyperconjugation that we saw, but there would be an overlap between the σ-bond of C-H and the odd electron in the p-orbital of carbon.
Note: If there is a possibility of resonance, then that would make it more stable. This is because resonance affects the entire structure.
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