Basics
what are the two ways a molecule can vibrate?
bending and stretching
What is the location of alcohol?
about 3300 cm-1
I did a reaction to change phenol to benzene. How will IR help me determine if the reaction was successful?
Lack of OH 3400cm-1 broad peak
Why do some molecules have IR inactivity?
Symmetry or little/no dipole moment
what type of light is used?
infrared
What is the location of carbonyl?
about 1700cm-1
Draw two IR graphs. One of aldehyde. One of carboxylic acid. What is the difference between them?
COOH: C=O peak and OH peak
COH: C=O only
What is diagnostic region? what is fingerprint region?
Diagnostic: shows functional groups, carbon hybridization
fingerprint: characteristic of compound at a whole. C-O stretch, methyls,
Where is energy the highest in IR?
Leftmost, high wavenumber, high frequency
How do you detect a benzene?
group of weak signals 1700-2000cm-1 (weak signal)
sp2 carbons 3000-3100cm-1 (strong signal)
C-H bending 1000-1275cm-1 (strong signal)
Resonance structures will have slightly more single bond characteristic within the functional group.
Hook law tells us that single bonds take less energy to vibrate.
wavenumber, intensity, shape
What does % transmittance mean?
How much infrared light passed through. low transmittance=that part of molecule absorbed energy to be bent or stretched
How to find primary amines and secondary amines
primary: two peaks at about 3400cm-1
secondary: one peak at about 3300cm-1
An IR graph shows strong, sharp peaks from 2900-3000cm-1 and a stronger peak at 1100cm-1. No other strong peaks are worth noting. What could the molecules functional group be?
Ether. Strong C-O stretch at 1100cm-1
When does bending apply to molecules?
More than 3 atoms present
Explain Hook law
higher bond strength (double, triple bonds) = more energy to vibrate
larger difference in mass= more energy needed to vibrate
Where are sp3 carbons, sp2 carbons and sp carbons?
why do the locations shift?
sp3: 2900cm-1
sp2:3100cm-1
sp:3300cm-1
More energy to vibrate triple bonds and double bonds vs. single bonds.
An IR graph shows a strong peak at 3000cm-1. There is a weaker peak at 1500cm-1. There are no peaks from 1700-2000cm-1 or above 3000cm-1. What could this molecule be?
Cyclohexane. sp3 carbons at 3000cm-1. C-H stretch at 1500cm-1
No sp2 carbons at 3200cm-1 or 4 small peaks at 1700-2000cm-1
What are the types of stretching and bending?
bending: scissor, rock, twist, wag