Electromeric Effect in Organic Compounds

Electromeric Effect

• Instantaneous formation of a dipole in an organic compound molecule due to complete transfer of shared pi electron pairs to one of the atoms under the influence of an attacking reagent.
• Observed in organic compounds with at least one multiple bond.
• Temporary effect that remains as long as the attacking reagent is present and exposed to the organic compound.
• Important points:
  • Temporary effect
  • Only occurs in multiple bonds
  • Displacement of pi (π) electrons only
  • Only occurs in the presence of attacking reagent

Types of Electromeric Effect

• Classified into two types: +E effect and -E effect
• Based on the direction of electron pair transfer

+E Effect

• Electron pair of the pi bond is moved towards the attacking reagent
• Observed in the addition of acid to alkenes
• Attacking reagent attaches itself to the atom which obtained an electron pair in the transfer
• Generally observed when the attacking reagent is an electrophile and the pi electrons are transferred towards the positively charged atom
• Example: Protonation of ethene

-E Effect

• Electron pair of the pi bond is moved away from the attacking reagent
• Attacking reagent attaches itself to the positively charged atom in the molecule
• Generally observed when the attacking reagent is a nucleophile and the pi electrons are transferred to the atom which the attacking reagent will not bond with
• Example: Addition of nucleophiles to carbonyl compounds

Aldehydes and Ketones

• Compounds of the general formula RCHO (aldehydes) and RR’ CO (ketones)
• Contain carbonyl group, C=O, which largely determines their chemistry
• Carbonyl group has one hydrogen atom attached to it in aldehydes, and two hydrocarbon groups attached to it in ketones
• Properties:
  • Higher boiling points than alkenes of similar molecular weight due to polarity
  • Lower boiling points than alcohols of similar molecular weight since they are not hydrogen bond donors
  • Considerable solubility in water due to being hydrogen bond acceptors
• Carbonyl carbon is sp2-hybridized, with a bond angle close to 120° (trigonal planar)
• Carbon-oxygen double bond consists of a σ C-O bond and a π C=O bond

This quiz covers the electromeric effect, a temporary dipole formation in organic compounds with multiple bonds, occurring due to pi electron pair transfer.