Chapter 8 Flashcards
Délocaliser and localized electrons
Delocalized are shared by three or more atoms
Localized belong to a single atom or are shared by two
Molecule must be planar
When both resonance structures are equally stable, it’s the best option
Hybrid will look like most stable resonance contributor
Benzene
Delocalized électrons, no double or single bond, all 1 1/2 basically
Electrons in pi bonds are the ones that move
6 pi (6 electrons that belong to pi bond are Delocalized)
Benzene
Delocalized électrons, no double or single bond, all 1 1/2 basically
Electrons in pi bonds are the ones that move
6 pi (6 electrons that belong to pi bond are Delocalized)
Molecular orbital
Contruncted by atomic orbital
Cyclooctatetraene
Reacts like alkene
No p orbital overlap because not planar
Cannot be planar due to angle strain
Resonance energy
Energy of contributor is much higher than energy of hybrid of resonnnance (actual structure)
Difference is called resonance energy or délocalisation energy
delocalization energy is the extra stability a compound has as a result of having delocalized electrons.
Stability of resonance contributor
Complète octets are better
Negative charge on most electronegative atom
Positive charge on the l’est electronegative atom
No sperated chatges (no positive and negative if it can be neutral)
Isolated and conjugated diene
Isolated: 2 single bonds separating the double bonds, reacts as 2 separate alkenes,
Conjugated: 1 single bond separating double bonds, electrons more Delocalized, treated resonance energy
Heat of hydrogenation
The smaller the heat of hydrogenation is, the more stable the compound is
Difference in heat of energy between molecule with resonance and one without is the resonance energy and how much more stable it is
Cumulated diene/cumulenes
2 double bonds right next to each other
Creates non superimposable enantiomers, even if they don’t have a chiral center
Stability conjugated isolated cumulated
Conjugated>isolated>cumulated
Conjugated has more delocalization energy than isolated, so more stable
Also bond between the two sp2 carbons is stronger because shorter bonds
Benzyl group and phenyl group
Benzyl: benzene and methyl group
Phenyl: just bezyl group
Benzylic carbon: not part of the benzene
Allylic cation
Allyl with cation, very stable because lots of resonance energy because all resonance structures are equivalently stableo
Ortho para
Ortho: on side of substituent
Para: opposite to it
Order of stability
Benzene, allyl, and tertiary carbocation
Benze and allyl have resonance, making them more stable
Resonance and acidity
More resonance, lower pka, more acidic
Resonance of conjugate base is the important one, so conjugate base is more stable so reaction occurs more
Electronic effects
Electron donation decrease acidity because the conjugate base is less stable
Opposite for electron withdrawal
Electron donation can be done by anything with a lone pair
Electron withdrawing group don’t have a lone pair, and all have a carbon or nitrogen or sulfur with a double bond to oxygen, (or sometimes triple to nitrogen)
Alkyl groups are slightly electron donating by induction
OR are electron donating by resonance
Halogens are electron withdrawing by induction
Aromatic
Benzene very stable because aromatic
Called aromatic, means extra stability than expected, caused bye uninterrupted cloud of pi electrons
Odd number of pairs or pi electrons is what you need to be aromatic (odd number of double bonds + negative charges)
Must be cyclic, planar and every ring atom has a p orbital (sp2 carbon)
How benzene reacts
Electrophilic aromatic Substitution reaction: Replaces one of its H with the electrophile it is reacting with
