17.2 Flashcards
Are bonds aalways locked in place and still? Explain
No
Bonds have energy and vibrate naturally around central point - therefore atoms in molecule in constant motion
Bonds can abdorb IR radiation, which can make them bend/stretch more
Describe a stretch
A type of vibration of a bond
Rhythmic movement along line between atoms, so distance between increases and decreases
Describe bend
Type of vibration
Results in change in bond angle
Amount that bond stretches/bends depends on
Mass of atoms in bond - heavier vibrate more slowly than lighter
Strength if bond - stronger bonds vibrate faster than weaker
What frequency radiation can bond absorb
Same frequency as natural frequency of bond
Steps of infrared spectroscopy
Sample placed in IR spectrometer
Beam IR radiation in range of 200-4000cm^-1 passed through the sample
Molecule absorbs some of the IR frequencies and emerging beam of radiation analysed to identify frequencies that have been absorbed by sample
IR spectrometer usually connected to computer that plots graph of trasnmittance against wavenumber
Dips in IR spectrum called
Peaks
What can a peak tell you
Look at wavenumber on peak
Tells you type of bond present
Fingerprint region meaning
Area of IR spectrum below 1500cm^-1 that gives a characteristic pattern for different compounds
On IR spectrum, all organic compound have a peak at _______ cm^-1. Why?
Between 2850-3100
Presence of C-H bonds
How to tell alcohol peak?
3200-3600
Big round peak, clearly sticks out
How to tell ketone/aldehyde peak? (As both have C=O bond)
Clear peak in range 1630-1820
Long/sharp?
How to recognise carboxylic acid peak?
A peak within 1630-1820 (C=O)
broad peak at 2500-3330 (O-H)
Typical sequence for analysis of a compound?
Elemental analysis - use of percentage composition data to determine empirical formula
Mass spectrometry - use of molecular ion peak prom mass spectrum to determine molecular mass; use of fragment ions to identify sections of ion
IR spectroscopy - use absorption peaks prom IR spectrum to identify bonds/functional groups present in molecule
