CARBOXYLIC ACIDS. Saturated carboxylic show a weak absorption band near 200 nm resulting the forbidden n → TT * transition. The position of the bond undergoes a small bathochromic shift with an increase in chain length. This band is of little diagnostic value. (α,β-Unsaturated acids) display a strong k-band characteristic of the conjugated system. The attachment of an electronegative or Alkyl group on the α-carbon produces a bathochromic shift
ESTERS AND LACTONES. Esters and sodium salts carboxylic acids show absorption at wavelengths and intensities comparable to the parent acid. Conjugated unsaturated lactones display spectra similar to unsaturated esters.
Benzene Chromophore Benzene displays 3 absorption bands: E1.. at 184 nm (ɛ max 60,000), E2…at 204 nm (ɛ max 7900), B B..band at 256 nm (ɛ max 200).
These bands originate from TT → TT * transitions. The intense band near 180 nm results from an allowed transition, whereas the weaker bands near 200 and 260 nm result from forbidden transitions in the highly symmetrical benzene molecule. Benzene in the vapor phase or in non polar solvent shows B-band(256nm) characterized by fine structures which destroys by more polar solvent or upon substitution.
Substitution of alkyl groups on the benzene ring produces a bathochromic shift of the B-band 256nm, but the effect of alkyl substitution upon the E-bands is not clearly fined. The bathochromic shift is attributed to hyperconjugation in which the σ-electrons of an alkyl C—H bond participate in resonance with the ring. The methyl group is more effective in hyperconjugation than other alkyl groups.
The addition of a second alkyl group to the molecule is most effective in producing a red shift if it is p- position. The para- isomer absorbs at the longest with the largest ɛ max. The ortho isomer generally absorbs at the shortest wavelength with reduced ɛ max. This effect is attributed to steric interactions between the ortho substituent, which effectively reduce hyperconjugation. Substitution on the benzene ring of auxochrcmic (OH, NH 2, etc.) shifts the E- and B-bands to longer wave lengths, frequently with intensification of the B- band and loss of its fine structure, because of n-π.
Conversion of a phenol to the corresponding anion results in a bathochromic shift of the E 2 - and B-bands and an increase in ɛ max because the nonbonding electrons in the anion are available for interaction with the π-electron system of the ring. When aniline is converted to the anilinium cation, the pair of nonbonding electrons of aniline is no longer available for interaction with the π-electrons of the ring, and a spectrum almost identical to that of benzene results.
Interaction between the nonbonding electron pair(s) of a heteroatom attached to the ring and the π-electrons of the ring is most effective when the p orbital of the nonbonding electrons is parallel to the π orbitals of the ring. Thus, bulky substitution in the ortho position of molecules such as N,N-dimethylaniline causes a hypsochromic shift in the E2 -band, accompanied by a marked reduction in ɛ max.
N,N-Dimethylaniline λmax =251nm ɛ max Methyl-N,N-dimethylanilineλ max =248nm ɛ max =6350 When auxochromic groups appear on the same ring as the chromophore, both groups influence the absorption. The influence is most pronounced when an electron donating group and electron attracting group are para to one another (complementary substitution. Table XXIV).
The red shift and increase in intensity of the K-band are related to contributions of the following polar resonance forms:
Biphenyl is the parent molecule of a series of compounds in which two aromatic rings are in conjugation. Resonance energy is at a maximum when the rings are coplanar and essentially zero when the rings are at 90° to one another.