AROMATIC RINGS BENZENE RING HYDROGENS Ring current causes

AROMATIC RINGS

BENZENE RING HYDROGENS Ring current causes protons attached to the ring to appear in the range of 7 to 8 ppm. An anisotropy diagram (next slide) shows the origin of the effect. Protons in a methyl or methylene group attached to the ring appear in the range of 2 to 2. 5 ppm.

MONOSUBSTITUTED RINGS

ALKYL-SUBSTITUTED RINGS In monosubstituted rings with an alkyl substitutent all ring hydrogens come at the same place in the NMR spectrum. R = alkyl (only) Apparently the ring current equalizes the electron density at all the carbons of the ring and, therefore, at all of the hydrogen atoms.

NMR Spectrum of Toluene 5 3

SUBSTITUENTS WITH UNSHARED PAIRS Electronegative elements with unshared pairs shield the o- and p- ring positions, separating the hydrogens into two groups. . . unshared pair Electron-donating groups shield the o-, p- positions due to resonance (see below). . . X =. . OH, OR, . . NH 2, NR 2, . . -O(CO)CH 3. . ester : . . : + : - : : . . - +

Anisole (400 MHz) Compare: The ring protons in toluene come at about 7. 2 ppm at the red line. 2 3 shielded

THE EFFECT OF CARBONYL SUBSTITUENTS When a carbonyl group is attached to the ring the o- protons are deshielded by the anisotropic field of C=O Only the o- protons are in range for this effect. The same effect is sometimes seen with C=C bonds.

Acetophenone (90 MHz) 3 Compare: The ring protons in toluene come at about 7. 2 ppm at the red line. 2 3 deshielded

para -DISUBSTITUTED RINGS

para-Disubstitution 1, 4 -Disubstituted benzene rings will show a pair of doublets, when the two groups on the ring are very different an example: 1 -iodo-4 -methoxybenzene

NMR Spectrum of 1 -iodo-4 -methoxybenzene 3 CHCl 3 impurity 2 2

NMR Spectrum of 1 -bromo-4 -ethoxybenzene 3 4 2

THE p-DISUBSTITUTED PATTERN CHANGES AS THE TWO GROUPS BECOME MORE AND MORE SIMILAR All peaks move closer. Outer peaks get smaller …………………. . … and finally disappear. Inner peaks get taller……………. and finally merge. all H equivalent X=Y X ~ X’ X=X same groups

NMR Spectrum of 1 -amino-4 -ethoxybenzene 3 4 2 2

NMR Spectrum of p-Xylene (1, 4 -dimethylbenzene) 6 4

HYDROXYL AND AMINO PROTONS

Hydroxyl and Amino Protons Hydroxyl and amino protons can appear almost anywhere in the spectrum (H-bonding). These absorptions are usually broader than other proton peaks and can often be identified because of this fact. Carboxylic acid protons generally appear far downfield near 11 to 12 ppm.

NMR Spectrum of Ethanol 3 2 1

SPIN-SPIN DECOUPLING BY EXCHANGE In alcohols coupling between the O-H hydrogen and those on adjacent carbon atoms is usually not seen. C O This is due to rapid exchange of OH hydrogens between the various alcohol molecules in the solution. H H In ultrapure alcohols, however, coupling will sometimes be seen. R-O-Ha + R’-O-Hb R-O-Hb + R’-O-Ha The exchange happens so quickly that the C-H group sees many different hydrogens on the O-H during the time the spectrum is being determined (average spin = 0)