IR Spectroscopy: Identifying Functional Groups

Carboxylic Acids vs. Alcohols

• Carboxylic acids have a strong, broad O-H stretch signal between 2500 and 3300 cm⁻¹.
• Carboxylic acids also exhibit a strong C=O stretch signal at approximately 1700 cm⁻¹.
• Alcohols show a strong O-H stretch absorption in the range of 3200 to 3600 cm⁻¹.

Aldehydes vs. Ketones

• Aldehydes and ketones both contain a carbonyl (C=O) functional group.
• The C=O stretch for both aldehydes and ketones appears around 1700 cm⁻¹.
• Aldehydes are distinguished from ketones by the presence of a C-H stretch signal around 2700 cm⁻¹, in addition to the alkane C-H stretch at approximately 2900 cm⁻¹.

Esters vs. Ethers

• Esters have a carbonyl (C=O) stretch at around 1700 cm⁻¹.
• Ethers lack the carbonyl C=O stretch but have a C-O stretch between 1000 and 1150 cm⁻¹.
• Esters exhibit two C-O stretches: one for the sp2 carbon-oxygen bond (1200-1300 cm⁻¹) and another for the sp3 carbon-oxygen bond
• The sp2 C-O stretch in esters has more double bond character due to resonance, leading to a higher wave number compared to the sp3 C-O stretch in ethers.

Primary vs. Secondary Amines

• Primary amines (with two hydrogen atoms attached to the nitrogen) show a double peak signal between 3300 and 3500 cm⁻¹.
• Secondary amines (with one hydrogen atom attached to the nitrogen) exhibit a single peak between 3300 and 3500 cm⁻¹.

Amides

• Amides possess a carbonyl (C=O) stretch near 1700 cm⁻¹.
• Amides also display a signal between 3300 and 3500 cm⁻¹ due to the N-H group.
• Presence of a carbonyl functional group distinguishes them from amines.

Alkanes, Alkenes, and Alkynes

• Alkanes C-H stretch signal is around 2900 cm⁻¹.
• Alkenes C=C signal is a weak to medium signal around 1660 cm⁻¹.
• Alkynes C≡C signal is also weak and appears around 2100 to 2200 cm⁻¹.
• Alkenes C-H stretch appears around 3000 to 3100 cm⁻¹.
• Terminal alkynes C-H stretch display a signal around 3300 cm⁻¹.

Effect of Hybridization on Wave Number

• As the s character of the C-H bond increases, the wave number also increases.
• sp³ C-H stretch < sp² C-H stretch < sp C-H stretch

Alkanes Specific Signals

• CH3 bend has a signal of 1365-1385 cm⁻¹.
• CH, CH2, or CH3 bend varies between 1400 and 1450 cm⁻¹.

Atomic Mass and Wave Number Relationship

• As atomic mass increases, the wave number decreases - inverse relationship.

Bond Strength and Wave Number Relationship

• As bond strength increases, the wave number increases - direct relationship.
• Triple bonds have higher wave numbers than double bonds, and double bonds have higher wave numbers than single bonds.

Conjugation

• Conjugated carbonyl groups absorb energy at a lower wave number compared to non-conjugated carbonyl groups.
• Non-conjugated ketone absorbs around 1720 cm⁻¹.
• Conjugated ketone absorbs around 1680 cm⁻¹.
• Non-conjugated alkene absorbs around 1650 - 1660 cm⁻¹.
• Conjugated alkene absorbs around 1600 cm⁻¹.

Resonance

• Conjugation reduces the wave number for ketones and alkenes because of resonance.
• Conjugated systems have more single bond character, lowering the required energy.