C5. Conjugation Flashcards
what is conjugation?
a phenomenon that occurs when p-orbitals of adjacent atoms overlap, allowing for the delocalisation of electrons across multiple atoms. Electrons are delocalised throughout the extended pi bond (one note for diagram)
what does conjugation produce?
-enhances stability
-changes to structure e.g creates planarity (when sterically allowed)
-different reactivity (to non- conjugated systems)
-characteristic spectrospcopic behaviour: absorption of ultraviolet/ visible light (e.g creation of colour)
what does conjugation start and end with?
starts and ends with a double bond with alternating double and single bonds
Describe conjugation without alternating bonds
a lone pair adjacent to a double bond can be (but is not necessarily) conjugated
Describe the effect of conjugation on bond lengths
extended pi bond system causes the C-C bond to have partial double bond characteristics making it shorter than you would expect.
C=C bond length in ethene is 133pm but the C=C bond length in butadiene is 134pm. The C-C bond length in ethane is 153 pm and in butadiene, it is 146pm.
Describe conjugation in amides?
nitrogen in amides can change its hybridisation to sp2 and put its lone pair into a p-orbital. Conjugation is stabilising, lowering the energy level of the molecule (one note for the structure)
why so alternating double/ single bonds have conjugation? why are amides conjugated?
they both have a continuous, uninterrupted run of three or more atoms each with an unhybridised p orbital. This means that sp2 or sp hybridization is a requirement for conjugation
How do sp3 carbon atoms interrupt conjugation?
they can create multiple conjugated systems
How does conjugation relate to resonance?
lewis structure does not describe conjugation. Resonance hybrid shows electron delocalisation, extended pi bond and resonance stabilisation (one note for diagram)
