Organic Chemistry - Sepectroscopy

Chapter 11 MCAT

Spectroscopy

measures the energy differences between possible states of molecular system by determining the frequencies of electromagnetic radiation absorbed by the molecules

Infrared spectroscopy

Measures the molecular vibrations, bond stretching, bending, combination

Fingerprint region

more complex vibrations patterns caused by motion of molecule as a whole seen in 1500 to 400 cm-1 range

What is IR spectra blotted as

percent transmittance

Alkanes wavenumber (C-H)

2800-3000

Alkanes wavenumber (C-C)

1200

Alkenes Wavenumber (=C-H)

3080-3140

Alkenes wave number (C=C)

1645

Alkynes wavenumber (=C-H)

3300

Alkyne wavenumber (C=C)

2200

Aromatic wavenumber (C-H)

2900-3100

Aromatic wavenumber (C-C)

1475-1725

Alcohol wavenumber (O-H)

3100-3500 Broad

Ether wavenumber

1050-1150

Aldehyde Wavenumber (O)C-H

2700-2900

Aldehyde wavenumber C=O

1700-1750

Ketones wavenumber (C=O)

1700-1750

Carboxylic acid wavenumber (C=O)

1700-1750

Carboxylic acid wavenumber (O-H)

2800-3200

Amines wavenumber

3100-3500

Nuclear Magnetic Resonance spectroscopy

measures alignment of nuclear spin with applied magnetic field, which depends on magnetic environment of nucleus itself.

n + 1 rule

if proton has n protons three bonds away it will be split into n+1 peaks

Coupling constant J

the magnitude of this splitting measured in hertz

RCH₃ position

.9

R₂CH₂ position

1.25

R₃CH position

1.5

RC=CH position

4.6-6.0

RC=CH

2-3

Ar-H position

6.0-8.5

RCHX position

2.0-4.5

RCHOH position

3.4-4.0

RCHO position

9.0-10.0

RCOCH₃ position

2-2.5

RCHCOOH position

2-2.6

ROH position

1-5.5

ArOH position

4.0-12.0

RCOOH position

10.5-12.0

RNH₂

1.0-5.0