Lect 2

ORGANIC SPECTROSCOPY Chem.345

BY

Dr. Ahmed El Taher

e-mail: taher2211@yahoo.com

ALKANES C–H

occurs around 3000 cm-1

CH2

methylene groups have absorption at 1450 cm-1

CH3

methyl groups have absorption at 1375 cm-1

For cyclic alkanes, as the ring strain increases the frequency of C–H bond is shifted to left i.e. higher frequency

Note: Methyl band is split into two peaks if a geminal dimethyl group is present

ALKENES =C–H

C=C

occurs to the left of 3000 cm-1

occurs near 1660 - 1600 cm-1

Symmetric substituted bonds don’t absorb in the infrared Symmetric disubstituted trans double bonds show weak absorption, cis are stronger

Factors affecting C=C stretching vibration frequency : a) Conjugation Conjugation of C=C provides more single bond character i.e. the force constant (K) decrease and thus the vibration frequency also decrease

b) Ring size Endo – cyclic double bond As the ring size decrease, the C=C vibrates at lower frequency till cyclobutene then the frequency increase again in cyclopropene

Exo – cyclic double bond As the ring size decrease, the C=C vibrates at higher frequency

ALKYNES ≡C–H stretch near 3300 cm-1

C≡C

stretch near 2150 cm-1

Symmetric substituted triple bonds give either no or weak absorption

 ≡ C−H in alkyne is of (sp) hybridization type (50% s -

character) giving the strongest bond, thus vibrates at higher frequency than alkene = C−H and alkane C-H

 Conjugation moves the band of ≡ C−H absorption to the right i.e. lower frequency

AROMATIC HYDROCARBONS =C–H

C=C

stretch is to the left of 3000 cm-1

ring stretch occur in pair at 1600 cm-1 and 1475 cm-1

The out–of–plane C–H bending vibration can be used to assign the position of substitution on the aromatic ring

Monosubstituted ring

ortho–Disubstituted

two strong bands near 690 cm-1 and 750 cm-1

one strong band near 750 cm-1

meta–Disubstituted

para–Disubstituted

three bands near 690 cm-1, 780 cm-1 and 880 cm-1

one strong band in the region from 800 to 850 cm-1

ALCOHOLS AND PHENOLS

O–H

O–H

C–O

Free O–H stretch is a sharp peak at 3650–3600 cm-1

The hydrogen bonded O–H stretch is a broad peak which occurs at 3500-3200 cm-1

stretching occurs in the range 1250-1000 cm-1 1 ry alcohol 2 ry alcohol 3 ry alcohol 1050 cm-1

1100 cm-1

1150 cm-1

phenol 1220 cm-1

 The higher value of phenolic C–O stretching vibration frequency is due to the resonance which gives it more double bond character

Factors affecting O–H stretching vibration frequency : a) H–bonding H–bonding weakens O–H bond, thus shift O–H band to lower frequency as broad band

Intermolecular H–bonding Intramolecular H–bonding Concentration dependent

Concentration independent

As dilution increase, O–H bond As dilution increase, O–H band appears as sharp band shifted to doesn't change its shape or higher frequency position

The O-H stretch region

b) Steric effect As bulkness increase around the O–H group, thus preventing the formation of H–bonding and therefore O–H appears as sharp free band at higher frequency

ETHERS C–O

stretch occurs in the range 1300-1000 cm-1

 Conjugation shift C–O band to higher frequency is due to the resonance which gives it more double bond character

AMINES

N–H

C–N

Stretching occurs in the range 3500-3300 cm-1 Bending occurs in the range 1640-1500 cm-1

stretch at in the range 1350-1000 cm-1

 The C–N absorption occurs at a higher frequency in aromatic amines than aliphatic amines because resonance increases the double-bond character between the ring and the attached nitrogen atom.

NITRILES C≡N

stretch near 2250 cm-1

 Conjugation with double bonds or aromatic rings moves the absorption to a lower frequency.

CARBONYL COMPOUNDS

C=O is sensitive to its environment, each different kind of C=O comes at a different frequency

Factors affecting C=O stretching vibration frequency : Inductive effect( 1)  Electron withdrawing groups strengthen C=O bond, then higher frequency absorption results  Electron donating groups weaken C=O bond, then lower frequency absorption results

R

O

O

C

C

R = Me, Et, etc.

X

X = F, Cl, Br, O

Resonance effect( 2)  +R groups weaken C=O bond, due to increased single bond character and lowering the C=O absorption frequency

Hydrogen bonding effect( 3)  H–bonding weakens the C=O bond, thus lowers its force constant, resulting in a lowering of the frequency

(4) Conjugation effect ď ˝ Conjugation increases the single bond character of C=O, thus lowers its force constant, resulting in a lowering of the frequency

(5) Ring size effect ď ˝ As the ring size decrease (ring becomes more strained) the strength of exocyclic C=O increases due to increasing of (s) character and thus shifts to higher frequency

1. ALDEHYDES and KETONES C=O

Ketone: stretch at 1715 cm-1

C=O

Aldehyde: stretch at 1725 cm-1

C–H

Aldehyde CH: two weak bands at 2750 and 2850 cm-1

 Ketone absorbs at lower frequency than aldehyde because of the electron donating effect of the second alkyl group weakens the C=O bond in ketone and lower the force constant and absorption frequency

Ring size effect

Conjugation effect

2. CARBOXYLIC ACID C=O

O–H

C–O

stretch at 1710 cm-1

The hydrogen bonded O–H stretch is a broad peak which occurs at 3400-2400 cm-1

stretching occurs in the range 1320-1210 cm-1

 Carboxylic acid exist in monomeric form in very dilute solution, and it absorbs higher than ketone because of electron withdrawing effect of (OH) group  In concentrated solution, the acid forms H–bonding. This weakens the C=O bond and lower the stretching force constant (K), resulting in lowering of the C=O frequency

3. ANHYDRIDE C=O

has 2 bands, occurs at 1810 and 1760 cm-1

C–O

stretching occurs in the range 1300-900 cm-1

 Anhydride gives two absorption bands that are due to symmetric and asymmetric stretching vibrations  Conjugation moves the frequency to right while ring strain moves the frequency to left

4. ACID CHLORIDE C=O

occurs at 1800 cm-1

ď ˝ The highly electronegative halogen atom strengthens the carbonyl bond through inductive effect and shift the frequency to higher value

ď ˝ Conjugation moves the frequency to right

5. ESTER C=O

occurs at 1735 cm-1

C–O

Stretch in 2 bands, occurs in the range 1300-1000 cm-1

 Esters have two C–O single bonds, one that is a pure single bond and one that has partial double–bond character

Conjugation effect  Conjugation with α , β – unsaturation or aryl increases the single bond character of C=O, thus lowers its force constant, resulting in a lowering of the frequency

ď ˝ Conjugation with frequency to left

singly

bonded

oxygen

shift

the

Ring size effect ď ˝ Cyclic ester (Lactones) give the expected increase in C=O frequency for decreasing ring size

6. AMIDE C=O

N–H

C–N

occurs at 1690 cm-1

Stretching occurs in the range 3500-3100 cm-1 Bending occurs in the range 1640-1550 cm-1

stretch at 1400 cm-1

ď ˝ Primary amides have 2 bands (NH2) while secondary amides have one band (NH). Tertiary amide will not show NH absorption

Ring size effect ď ˝ Cyclic amides (Lactams) give the expected increase in C=O frequency for decreasing ring size

SUMMARY Ketones are at lower frequency than Aldehydes because of the second electron-donating alkyl group.

Acid chlorides are at higher frequency than ketones because of the electron-withdrawing halide.

Esters are at higher frequencies than ketones due to the Electron - withdrawing oxygen atom. This is more important than resonance with the electron pair on the oxygen.

Amides are at lower frequencies than ketones due to resonance involving the unshared pair on nitrogen. The electron - withdrawing effect of nitrogen is less important than the resonance.

Note the electronegativity difference, O versus N, weights the two factors (resonance/ e-withdrawal) differently in esters than in amides.

Acids are at lower frequency than ketones due to H-bonding.

NITRO COMPOUNDS N=O

has 2 bands, occurs at 1550 and 1350 cm-1

 The nitro group (NO2) gives two strong bands in the infrared spectrum, due to symmetric and asymmetric stretching vibrations.  The nitroso group (R–N=O) gives only one strong band, which appears in the range from 1600 to 1500 cm -1.