Ch 15: Important Signals in IR Spectroscopy

1500

IR absorption of aromatic ring 1450-1600; 1650-2000 (1500)

Medium

1650

Ethylene - weak/short signal

If connect to 2 hydrogens and 2 carbons - dipole - signal

if conjugated - 1600

if connect to 4 of the same groups - no dipole - signal at 1650

1680-1600

C=N Imine

1680

Amide

1680

Conjugated Ketone

1710

Conjugated Ester

1720

Carbonyl

1720

Ketone

1740

Ester

1760

Carboxylic Acid

1760, 1820

Acid Anhydride

1800

Acid Chloride

2100-2200

Alkyne

2200-3600

Oxygen of an Ester

2230

Nitrile

2750

Aldehyde

2900

Alkane

3100

Alkene (sp2)(vinylic C-H bond)

3200

Secondary Amine

3300

Alkyne (sp)

3350, 3450

Primary Amine

3200-3600

Alcohol

3000

Ether

IR absorption of Alkane (bond to H- sp3)

2900

Medium

IR absorption of Alkene: =C-H (bond to H - sp2)

3100

Medium

IR absorption of Alkyne: =C-H (bond to H - sp)

3300

Strong

IR absorption of Alcohol

3500

1.) O-H bonds participating in H bonding - broad signal - 3200- 3600

2.) no O-H bonds participating in H bonding - narrow signal - 3600

Strong, broad

IR absorption of Carboxylic Acid

1.) O-H with strong H bonding: 2500 - 3600

2.) also accompanied by broad C=O signal: 1700

Strong

Ketone

1720

Conjugated Ketone

broad and strong C=O signal appears at 1680

IR absorption of Ester

1740

Strong

Ketone vs Ester

IR absorption of Aldehyde

1700 - 2700 (2750)

Strong

IR absorption of Conjugated Ester

1710

IR absorption of Primary Amines: N - H2

two signals at 3350 and 3450

Medium

IR absorption of Secondary Amines: N - H

one signal at 3200

weak

IR absorption of Amide

1680

Stong

Diagnostic Region

1500-4000

Fingerprint Region

400-1500

IR absorptions of C - N, C - O, and C - C are located in the _______ region.

fingerprint (1600-400)

IR Single Bond Region Alkane

400-1600

IR Double Bond Region Alkene

1600 - 1850

IR Triple Bond Region Alkyne

2100-2300

Medium

Analyzing IR spectrums:

1.) draw line at 1500

2.) focus on any signals to left of this line (diagnostic) and identify Alkanes (3000); Alkenes (1600+); Alkynes (2200)

-use tables below

- each signal appearing in diagnostic region will have 3 characteristics: wavenumber, intensity, and shape

3.) when looking for X—H bonds - draw line at 3000 and look for signals that appear to left

Calculating HDI

1.) rewrite the molecular formula as if the compound had no elements other than C and H

- Add one H for each halogen

- Ignore all oxygen atoms

- Subtract one H for each nitrogen

2.) determine how many Hs missing

C6H12 - 6 carbon atoms require (2 X 6) + 2 = 14 - missing 2H atoms - one degree of unsaturation for 2 H missing

C4H6 - 4 carbon atoms require (2 X 4) + 2 = 10 - missing 4H - 2 degrees for 4 H missing

HDI of 4 or more indicates:

aromatic ring

HDI = 0

no rings or double bonds

HDI = 1

must have either 1 double bond or 1 ring but NOT both

HDI = 2

then there are a few possibilities: two rings, two double

bonds, one ring and one double bond, or one triple bond

sp bond has shortest bond length

strongest bond

sp3 has longest bond length

weakest bond