Electronic Effects Flashcards
-I effect
electron withdrawing inductive effect
- group attached to C is more electronegative than C
- partial positive charge formed on C
+I effect
electron donating inductive effect
- group attached to C is less electronegative than C
- partial negative charge formed on C
Functional groups with -I effect
C-X
C-CX₃
C=O
C-OH
C(=O)H
C(=O)OH
C(=O)R
C-NO₂
Functional groups with +I effect
C-Li
C-Mg
C-CH₃
C-R (alkyl group)
Inductive Effects
The way in which differences in electronegativity polarise electron density in the sigma framework of a molecule.
Reactivity
The movement of electron density to break weak bonds and form new stronger bonds.
In heterolytic bond cleavage, the bonding pair moves onto the…
… more electronegative atom.
The transistion state is…
… the least stable structure in a reaction step.
Hammond Postulate states…
… the transition state of a rection will be similar to the thing it is closest in energy to.
The transisiton state is often similar to the reactive intermediate.
Principle of energetically accessible intermediates
The more stable / low energy a reactive intermedtae, the lower the energy barrier to a reaction. The reaction proceeds more rapidly.
Mesomeric Effects
The movement of electron density within the pi (double bond and lone pair) system of a molecule via resonance conjugation.
Between inductive and mesomeric effects, which has the greatest importance and why?
Mesomeric effects have greater importance.
The electrons in the pi system are more mobile so can move over larger distances.
-M effect
Electron withdrawing effect.
-M groups ‘accept’ pairs of electrons
+M effect
Electron donating effect
+M groups can donate a pair of electrons
Groups with -M effect
Ketone (R-(C=O)-R)
Cyanide (-C≡N)
Nitro (-NO₂)
Groups containing double / triple bonds, particularly to electronegative atoms.
Groups with +M effects
-OH (hydroxyl)
-O⁻
-O-R (ether)
-S-R (thioether)
-NR₂ (amine)
-Cl
-Br
Groups with singly bonded atoms holding a lone pair, or groups with a C=C pi bond.
Which groups can be +M or -M, depending on what they are bonded to?
Alkenyl (R₂C=C(R)-) and phenyl (C₆H₅).
Which effect is responsible for the +I inductive effect of alkyl groups?
Hyperconjugation - bonding electrons from adjacent C-H bonds partially overlap with empty p orbital of carbocation. Partial resonance conjugation donates some electron density to the empty orbital.
