Carbon Carbon Double Bond as Substituents
In the earlier chapters, we studied how a carbon–carbon double bond in an alkene acts as a reactive site for addition reactions. But a double bond does more than just take part directly in reactions. It also affects reactions happening at other parts of the molecule. In this chapter, we study the double bond as a substituent and how it changes reactivity.
Why Double Bonds Influence Reactions Elsewhere
A double bond has a special feature called conjugation. This means the π (pi) electrons can overlap with other orbitals in nearby parts of the molecule, allowing electron sharing. This electron delocalization affects stability, reaction paths, and speeds. To understand this, we also use resonance.
Free-Radical Halogenation in Alkenes: Substitution vs Addition
Take the example of propene (CH2=CH–CH3):
- The double bond can undergo electrophilic addition (like ethene).
- In the presence of peroxides, it can undergo free-radical addition with HBr.
- The methyl group attached to the double bond modifies how fast these reactions happen and in which direction (orientation).
However, the methyl group itself can also undergo reactions typical of alkanes, such as free-radical substitution. So the molecule has two possible reaction sites: the double bond and the methyl group.
Controlling Where the Reaction Happens
The site of halogen attack depends on reaction conditions:
- High temperature or UV light → promotes free-radical substitution (on the alkyl part).
- Low temperature, no light → promotes addition across the double bond.
At around 500–600°C, propene reacts with chlorine mainly to give allyl chloride (CH2=CH–CH2Cl), which is a substitution product.
Why Addition Does Not Occur at High Temperature
Halogen atoms do add to double bonds, but at high temperature the first radical intermediate breaks apart before the next step can occur, preventing addition. Instead, substitution dominates.
NBS and Allylic Bromination
N-Bromosuccinimide (NBS) is used to selectively brominate alkenes at the allylic position. It works by keeping a low, steady concentration of bromine in the reaction mixture. This allows allylic substitution without competing addition to the double bond.
Orientation and Reactivity of Allylic and Vinylic Positions
The double bond strongly affects reactivity:
- Vinylic hydrogens (attached directly to double-bonded carbons) are very hard to replace.
- Allylic hydrogens (on the carbon next to the double bond) are very easy to abstract.
Experiments show that the ease of forming radicals follows this order:
Allyl > 3° > 2° > 1° > CH3• > Vinyl
This means:
- The most stable radical is the allyl radical (due to resonance).
- The least stable is the vinylic radical.
Why Allyl Radicals Are So Stable
An allyl radical has resonance structures. The unpaired electron can shift between two positions. This delocalization makes the radical stable, and therefore easier to form.
Allylic Rearrangements
Because the allyl radical is resonance-stabilized, reactions often lead to mixtures of products. For example, when 1-octene is brominated using NBS:
- 3-bromo-1-octene forms (expected product)
- But major product is 1-bromo-2-octene (rearranged)
This is called allylic rearrangement. No atoms actually move; only the double bond shifts position because of the resonance forms of the allyl radical.
Summary
- A carbon–carbon double bond can act as a substituent, influencing reactivity elsewhere.
- Allylic positions are highly reactive toward radical substitution.
- Vinylic positions are very unreactive.
- Allyl radicals are stable due to resonance.
- NBS is used for selective allylic bromination.
- Allylic rearrangement occurs because the allyl radical has multiple resonance forms.
Detailed Notes:
For PDF style full-color notes, open the complete study material below:
PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ CARBON-CARBON DOUBLE BOND AS SUBSTITUENTS.