Infrared Radiation Molecular Vibrations Do Molecules Absorb Energy

Infrared Radiation: Molecular Vibrations

Do Molecules Absorb Energy The bonds in molecules absorb energy. Have you ever wondered why the sky is blue? Or why it is red during a sunset? Do you know why are apples are red and leaves green? Answer: It is because they are made up of different molecules with different bonds and functional groups…. see the next slide for a more detailed answers.

Infrared Absorbance Sample

Bonds IR Absorption Spectra When molecules are hit by IR most of the light passes by (transmittance) but some is absorbed by the bonds. BOND WAVENUMBER SHAPE OF PEAK C-O 1000 -1300 STRONG AND LONG (OUT OF RANGE, FINGERPRINT) C=O 1680 -1750 LONG AND NARROW, POINTY C-H 2850 -2960 LONG AND LARGE, POINTY O-H (ACIDS) WIDE, BROAD, DEEP O-H (ALCOHOLS) 3200 -3500 WIDE, BROAD, DEEP

A bond must have a dipole to be detected in the IR spectrum. NOTE: Non polar Bonds (O 2) are not detected.

IR SPECTRUM OF A CARBONYL COMPOUND carbonyl compounds (C=O bond) show a sharp, strong absorption @ 1700

A Ketone IR Spectrum Chapter 12 7 =>

IR SPECTRUM OF AN ALCOHOL • alcohols (O-H bond) show a broad absorption between 3200 and 3600 cm -1

IR SPECTRUM OF A CARBOXYLIC ACID -COOH 3000 C=O 1700 • carboxylic acids show a very broad absorption @ 3000 cm-1 • they also show a strong absorption around 1700 cm-1 due to C=O bond

Infrared Spectroscopy 4 -phenylbutyric acid 13. Carboxylic Acids: • Gives the messiest of IR spectra • C=O band occurs between 1700 -1725 cm-1 • The highly dissociated O-H bond has a broad band from 24003500 cm-1 covering up to half the IR spectrum in some cases (w – m) br (s)

Infrared Spectroscopy 1. Octane Alkanes – combination of C-C and C-H bonds • C-C stretches and bends 1360 -1470 cm-1 • CH 2 -CH 2 bond 1450 -1470 cm-1 • CH 2 -CH 3 bond 1360 -1390 cm-1 • sp 3 C-H between 2800 -3000 cm-1 (w – s) (m)

Infrared Spectroscopy 2. Alkenes – addition of the C=C and vinyl C-H bonds • C=C stretch at 1620 -1680 cm-1 weaker as substitution increases (w – m) 1 -Octene

Infrared Spectroscopy 3. Alkynes – addition of the C=C and vinyl C-H bonds • C≡C stretch 2100 -2260 cm-1; strength depends on asymmetry of bond, strongest for terminal alkynes, weakest for symmetrical internal alkynes • C-H for terminal alkynes occurs at 3200 -3300 cm-1 • Internal alkynes ( R-C≡C-R ) would not have this band! (w-m) (m – s) 1 -Octyne

Infrared Spectroscopy 8. Amines - Primary • Shows the –N-H stretch for NH 2 as a doublet between 3200 -3500 cm-1 (w) 2 -aminopentane

Examples of Real Spectra Now we will look at some actual spectra To see the functional group areas of absorbance

=> Chapter 12 16

An Infrared Spectrum C-H 3000 FOCUS on the molecules F. G (functional groups) OH, C=O

Pentanoic Acid The alcohol of the carboxylic acid and C-H bond are overlapping somewhat. The C=O is at 1700.

FINGERPRINT REGION

FINGERPRINT REGION THE NAME SAYS IT ALL, YOU TELL ME WHAT GOOD ARE FINGERPRINTS TO POLICE? OK , SEE NEXT SLIDE…. you will see the 2 molecules are isomers and Very similar in the diagnostic FG region, but they cannot be superimposed In the finger print region to be an exact match

Superimposition Were the last 2 isomers an exact match? No, so just like fingerprints, if they do not match it is not the same compound.

The functional group Area The fingerprint region

MORE REGIONS TO STUDY Examples for you to try

What compound is the most likely choice? Is this: A) Phenol B) Toluene C) Cyclohexanone Answers on next slide D)Cyclohexane

Aliphatic ketone C-H 3000 C=O 1700 It is choice C…. and the reason, click to find out There is no way to know it was a cyclo but of the choices given, it had to have the C=O 1700 cm-1 reading. Phenol was an alcohol and the other 2, B and D did not have a F. Group. (note: the 3000 are is just the C-H for the CH 2’s in the cyclo)

Another one? Next slide, please

What compound is the most likely choice? C-H 3000 C=O 1700 Is this: A) ethylethanoate B) benzene C) hydroxybenzene D) ethanoic acid

What compound is the most likely choice? C-H 3000 ethylethanoate It is A…. and the reason, click to find out IUPAC name is ethylethanoate. There is no way to know the exact compound but if we get the functional groups right we can eliminate the ones it cannot be. Again, it had to have the C=O 1700 cm-1 reading. C an D would have readings above 3000, as C was an alcohol and D a carboxylic acid B is just a hydrocarbon, no functional group in it

Question #3 A) Cyclopropanone B) propynol C) acrylaldehyde D) propenoic acid