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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

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Presentation Transcript
slide2

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.

slide5

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.

slide7

..

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,

+

+

+

..

:

:

:

:

..

..

-

-

NH2, NR2,

:

:

..

-O(CO)CH3

..

..

-

ester

slide8

Compare:

The ring protons in

toluene come at

about 7.2 ppm at

the red line.

Anisole (400 MHz)

2

3

shielded

slide9

THE EFFECT OF CARBONYL SUBSTITUENTS

When a carbonyl group is attached to the ring the

o- and p- 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.

slide10

Compare:

The ring protons in

toluene come at

about 7.2 ppm at

the red line.

Acetophenone (90 MHz)

3

2

3

deshielded

slide12

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

slide15

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 = X

X = Y

X ~ X’

same groups

hydroxyl and amino protons
Hydroxyl and Amino Protons

Hydroxyl and amino protons can appear

almost anywhere in the spectrum (H-bonding).

Carboxylic acid protons generally appear far

downfield near 11 to 12 ppm.

These absorptions are usually broader than

other proton peaks and can often be identified

because of this fact.

slide21

SPIN-SPIN DECOUPLING BY EXCHANGE

In alcohols coupling between the O-H hydrogen and

those on adjacent carbon atoms is usually not seen.

This is due to rapid exchange of

OH hydrogens between the various

alcohol molecules in the solution.

C

O

H

H

In ultrapure alcohols, however,

coupling will sometimes be seen.

R-O-Ha + R’-O-HbR-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)

nmr spectrum of 2 chloropropanoic acid
NMR Spectrum of 2-Chloropropanoic Acid

COOH

3

1

1

~12 ppm

offset = 4.00 ppm