Answer to Question #136500 in Organic Chemistry for Haseeb

Question

Acetone shows maximum absorbance at 279nm in hexane where in water its λmax is 265nm Justif

Answer 1

An electron in a bonding σ orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo σ → σ *transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to σ → σ *transitions are not seen in typical UV-Vis. spectra (200 - 700 nm)

n → σ * Transitions

Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n → σ *transitions. These transitions usually need less energy than σ → σ * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n → σ *peaks in the UV region is small.

n → π* and π → π* Transitions

Most absorption spectroscopy of organic compounds is based on transitions of n or π electrons to the π * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the π electrons.

The type of transition is responsible for a band at λmax = 279 nm in absorption spectrum of acetone. In case of n → π* transitions, the polar solvents form hydrogen bonds with the ground state of polar molecules more readily than with their excited states. Therefore, in polar solvents the energies of electronic transitions are increased.

acetone in n-hexane λ max at 279nm. . Solvent wavelength (nm) Water 205 Methanol 210 Ethanol 210 Ether 210 . It should be less polar so that it has minimum interaction with the solute molecules