Infrared Spectroscopy

What is IR?

spectrum that allows us to detect functional groups in a compound

Does a molecule have to be polar or non polar to be detected in IR?

A molecule must be polar and have covalent bonds. There needs to be a dipole moment in the molecule

What does the absorption of IR cause?

Molecular vibration. Bond lengths are stretched and bond angles are bent. 

Sp3?

slightly below 3000

sp2?

slightly above 3000 (found in alkenes and aromatics)

When given a formula, what’s the first step to figuring out the structure (functional group) from an IR spectrum

Find the degrees of unsaturation

What do the degrees of unsaturation tell us?

How many rings or pi bonds are present in a compound

Double bonds

1600-1850

INTENSE peaks at this region

Triple bonds

2100-2300

Z-H bonds 

2700-4000

C=O

Just above 1700cm. Tends to be the more intense signal

C=O signals lowered by?

Lowered by conjugation. Attached to benzene or alkene will be found 20-40cm lower

C=C

Around 1650cm (narrow and weak)

C=O

1720cm (broad and strong)

OH (alcohol)

~3500 (very broad)

What is a cause of broad peaks? (referring to OH)

Hydrogen bonding will make a peak very broad. It creates a wide distribution of hydrogen bond strengths and lengths within a sample. The extend of the hydrogen bonding can also affect the broadening of a peak. The more e-neg the atom is (O>N), its more likely to have a broader peak.

Amine peaks? (NH)

primary amine will have two signals/peaks, secondary amine will have one signal/peak

Aromatic

small but equal and consistent consistent peaks (1650-2000)

C=C

Strong peak C=C (sp2 also must be present)

OH (H bonded)

VERY broad peak 3550-3200

C=O

Strong intense peak 1650- 1820 (check chart for specific functional groups)

carboxyllic acid

~ 1715 (along C=O peak)

Amide (primary)

3350-3500 (fairly broad)

Amide (secondary)

3350-3500 (fairly broad) TWO identical small peaks at the end (each represent the amount of hydrogens bonded to nitrogen)

C-N

1000-1200 (fair peak)