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Why are saturated hydrocarbons less reactive than unsaturated hydrocarbons?
They contain oxygen.
They are aromatic.
They have double bonds.
Their single bonds are strong.
Their single bonds are strong.
Saturated hydrocarbons (alkanes) contain only strong CтИТC and CтИТH sigma (╧Г) bonds, which require high energy to break. In contrast, unsaturated hydrocarbons (alkenes and alkynes) contain ╧А (╧А) bonds, which are weaker and more accessible to electrophilic attack, making them significantly more reactive.
Saturated hydrocarbons (alkanes) contain only strong CтИТC and CтИТH sigma (╧Г) bonds, which require high energy to break. In contrast, unsaturated hydrocarbons (alkenes and alkynes) contain ╧А (╧А) bonds, which are weaker and more accessible to electrophilic attack, making them significantly more reactive.
Think of saturated hydrocarbons as a sturdy puzzle with all pieces locked together, while unsaturated hydrocarbons are like a puzzle with 'loose' pieces sticking out, making it much easier for something else to grab onto them.
SAT = Stable, All-Tied (sigma bonds); UNSAT = Unstable, Needs-Action (pi bonds).
E╧ГтАЛ>E╧АтАЛ тАФ The bond dissociation energy of a sigma bond is greater than that of a ╧А bond.
CnтАЛH2n+2тАЛ тАФ General formula for saturated alkanes.
CnтАЛH2nтАЛ тАФ General formula for simple alkenes.
In alkanes, the stable sp3 hybridization creates a non-polar, tightly held electron density between carbons. Unsaturated compounds possess sp2 or sp hybridization, where p-orbitals overlap side-ways to form ╧А-bonds. The electron density of ╧А-bonds lies above and below the molecular plane, making these electrons highly susceptible to being donated to electrophiles, thus facilitating addition reactions.
Alkanes undergo substitution reactions under harsh conditions (e.g., UV light).
Alkenes and alkynes undergo addition reactions under mild conditions.
The electron-rich pi bond acts as a nucleophile.
Sigma bonds have maximum orbital overlap, leading to higher stability.
Saturated hydrocarbons are stable, making them ideal fuels and inert solvents.
Low reactivity limits their use in synthesizing complex molecules directly without radical initiation.
Paraffin wax (saturated) used in candles and coating.
Vegetable oil hydrogenation (unsaturated to saturated) in food industry.
| Feature | Saturated | Unsaturated |
|---|---|---|
Bond type | Sigma bonds only | Sigma and Pi bonds |
Bond energy of CтИТC single bond is approx 347┬аkJ/mol.
Option A is wrong as oxygen is not a default component of hydrocarbons.
Option B is wrong because alkanes are generally aliphatic, not aromatic.
Option C is wrong because unsaturated compounds contain double bonds, which actually increases their reactivity.
D is correct тАФ Their single bonds are strong and require high energy to break, preventing easy chemical reactions.
Always remember that in organic chemistry, electron density distribution dictates reactivity; where there is a pi-bond, there is an electron-rich site prone to reaction.
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