How well do you know your functional groups? Identify the functional group/class of compound for each structure!

Alcohol (1o, 2o or 3o?) 1. CH2Cl2 2. NH2Aldehyde Alkane 3. 4. H

C CH3

O

CH3

H3CH3C CH3

Alkene Alkyne 5. 6. H3C

CH2

C

O

OHAlkyl halide Amide 7. 8. Amine (1o, 2o, or 3o?) Anhydride 9. 10. Aromatic Carboxylic acid 11. 12. Cycloalkane Diene 13. 14. Ester

Ether 15. 16. Ketone Thiol 17. 18.

OH

HN

CH3

CH3H3C

H3C C CH

CH3C

CH3

OH

CH3

CH3C

O

O

H2C

CH3

H3CCH2

H2C

SHCH

O

C

O

NH

CH3O

CH3H3C

OH3C H

CO O

CH2OHH3C

Infrared (IR) Spectroscopy Spectroscopy: Study of interaction of matter and electromagnetic radiation (“light”) Principle: When molecules absorb light of a certain energy, they behave in predictable ways that can be used to generate data about their structure Ex: UV/visible, IR, fluorescence Review: Relationships between energy (E), wavelength (λ) & frequency (ν) of light E = hν where h = Planck’s constant 6.626 x 10-34 J-s ν = c/λ where c = speed of light or 3.0 x 108 m/s Thus, longer wavelengths & smaller frequencies equal smaller quantities of energy Infrared light is relatively low-energy & not harmful (compared to UV or X-rays) and is perceived as heat IR portion of λ = 0.8 μm to 102 μm ; longer wavelengths than visible electromagnetic spectrum: ν = 3 x 1012 to 3.8 x 1014 s-1 (Hz or cycles per second) Units of infrared light: IR light can be expressed as wavelengths in μm or “microns” (μ) , but are most often expressed in frequency-type units called wavenumbers (cycles per cm): ν (cm-1) = 1 λ (cm) The range of infrared light in wavenumbers = 4000 to 200 cm-1

Below 600 cm-1 (far infrared) is less useful when working with organic molecules, so we usually only look at 4000 – 600 cm-1

When IR radiation meets a molecule…or… what causes an infrared spectrum Bonds undergo constant vibration: stretching & bending as though the atoms were connected by springs. Stretching occurs along the the bond axis Bending occurs in and out of the line of the bond. For 3 or more connected atoms these may be in-plane or out-of-plane bending motions How a molecule interacts with IR light to generate an IR spectrum:

• When a molecule is exposed to light of a frequency matching any of the vibrational frequencies of its bonds, that vibration increases in amplitude and the molecule absorbs the light.

• IR light contains just the right frequencies to match most bond vibrations that occur in

organic molecules

• These vibrations are specific for each type of bond or group

• A sample is placed between a pair of “salt plates” which allow IR light transmission.

• In the infrared spectrophotometer, a beam of IR radiation containing the entire range of frequencies is passed through a sample.

• The light reaching the detector is recorded as light transmitted, so the percent

transmitted over the entire range of frequencies is recorded as the IR spectrum

• When the molecule absorbs light of a particular frequency, less is transmitted

• For those frequencies, an absorption band representing decreased percent transmittance appears in the spectrum

What does the IR spectrum tell us? An IR spectrum contains absorption bands that correspond to bond vibrations occurring at various different specific frequencies. Each absorption band correlates to the presence of specific structural features in the molecule; particularly the functional groups, but also some aspects of C skeleton Absorption bands can be assigned to certain bonds or functional groups based on three features: frequency, intensity, shape Frequencies: (To aid in assigning IR bands to structural features of a molecule, consult handout: ”Characteristic IR Absorption Ranges (Bands) of Some Functional Groups” Why do particular bonds or groups appear where they do? What factors affect the frequency of the vibration? Since frequency is directly proportional to energy, the more energy it takes to stretch or bend a bond or group, the higher the frequency. Hooke’s Law predicts that frequencies of stretching vibrations are directly proportional to the force constant and decrease with increasing atom mass Some general trends: 1. Effect of bond strength: the stronger the bond, the higher the vibration frequency. 2. Effect of atomic size: the smaller the masses of the atoms, the higher the frequency

3. Effect of environment: Polarity and other factors affecting electron distribution in the bond and its environment can have an effect.

Electron withdrawing groups nearby can shift the frequency of vibration a bit higher; Conjugation can shift the frequency of vibration a bit lower Intensity: The more polar bonds have the strongest vibrations Shape: Forces like hydrogen bonding can affect frequency, thus altering band shape

Key Regions Typical IRs:

CH3CH2OH How do we use the information in the spectrum? 1) Frequencies above 1500 cm-1: “functional group” region 2) Frequencies below 1500 cm-1: “fingerprint” region 3) Look at intensity of bands: strong, medium or weak? 4) Look at the shape of bands: broad or sharp?

Limitations of IR spectroscopy:

• You can’t assign every single band or peak • Peaks at some frequencies could be attributed to a number of structures • Compounds with very similar C skeletons cannot be distinguished by IR alone

Combining the IR spectrum with other information in structure determination: 1) Molecular formulas of organics can be obtained experimentally by elemental analysis. Use the degrees of unsaturation to determine the number of rings or pi bonds # rings/pi bonds = ½ (2C + 2 – H + N – X) 2) 1H or 13C NMR spectrometry provides information on the number and types of C and H atoms, allowing a full structure determination for many compounds.

• Most useful for determining carbon skeleton • Provides additional evidence for certain functional groups

3) A mass spectrum will provide the molecular weight and some fragmentation data on the compound that can help you deduce its structure.

• Highest mass peak in a mass spectrum corresponds to whole molecule ionized • Most intense peak corresponds to the most stable fragment produced.

Characteristic IR Absorption Ranges (Bands) of Some Functional Groups Frequencies are given in wavenumbers (cm-1) Remember to consider: 1) Frequency 2) Intensity (strong, medium, weak) 3) Broad vs. sharp Alkanes, alkyl groups: 2850 – 2960 = C-H stretching 1460 – 1465 = CH2 group bending several moderately strong bands 1365 – 1380 = CH3 group bending Alkenes: 3020 – 3100 = C-H next to C=C 1640 – 1680 = C=C stretching weak to medium bands (sharp band) Alkynes: 3300 (sharp) = C-H next to C=C 2100 – 2260 = C=C stretching Alkyl halides: 600 – 800 = C-Cl 500 – 600 = C-Br Alcohols: 3300 – 3650 = O-H stretching, usually very strong, broad and symmetrical 1050 – 1150 = C-O stretching, strong but not as broad Phenols: 3200 – 3300 = O-H stretching, strong and broad These are in addition to 1200 – 1300 = C-O stretching the aromatic ring bands! Aromatic (benzene): 3000 – 3100 = aromatic CH stretching 1500, 1600 (sharp) = C=C stretches weak to medium bands

1700 - 2000 = “sawtooth” region 700 – 850 = substitution pattern bands bunch of weak bands mono: 700 and 760 (two bands)

ortho: 735-770 (one band) meta: 700 and 780 (two bands) para: 800-850 (one band) Amines 3300 – 3500 = N-H stretch (medium, broad, only seen in 1o or 2o amines) 1o amines have 2 peaks

1030 – 1330 = C-N stretch (remember, this is the fingerprint region so if you don’t see other evidence for an amine, a peak here may be something else

Ethers 1050 – 1150 = C-O stretch if its nonaromatic (medium to strong, a bit broad) 1200 – 1300 = C-O stretch if it’s aromatic or C=C adjacent (medium to strong, a bit broad) Carbonyls (general) 1650 – 1800 = C=O stretch (very strong, a bit broad) Specific groups with carbonyls: Remember these will appear 30 cm-1 or so LOWER if next to a benzene ring Ketones: 1700 – 1720 = C=O stretch Aldehydes: 1700 – 1720 = C=O stretch PLUS C-H stretch(es) in 2600-2800 region! Esters: 1725 – 1760 = C=O stretch (usually 1740 if no C=C bonds around)

PLUS two C-O stretches; one at 1200-1250, the other at 1050-1150 Carboxylic acids 1700 – 1770 = C=O stretch PLUS really broad, bumpy O-H at 2500-3100 and C-O at 1200-1250 Amides 1640 – 1700 = C=O stretch PLUS there could be N-H around 3300-3500

Comparison of three non-aromatic hydrocarbons: alkane, alkene, alkyne

Comparison of aromatic and non-aromatic aldehydes

Where to find IR evidence for specific groups in a spectrum 1) Carbonyl-containing functional groups A key IR absorption that provides functional group information is the C = O stretch: The presence of a strong, somewhat wide band around 1700 – 1750 cm-1 indicates one of these carbonyl-

containing functional groups is present in the molecule If C = O stretch is absent, proceed to check for other groups

Ketone: Aldehyde:

R'C

R

O

HC

R

O

Carboxylic acid: Ester: Amide:

NH

CR

O

R

OHC

R

O

OR'C

R

O 2) OH groups: The O – H stretch

Alcohols and carboxylic acids (if you also see the C = O) are the likely culprits. Frequency range: 3200 – 3650 cm-1 Intensity: Strong Shape: The O – H stretch is usually very broad, owing to hydrogen-bonding interactions

which influence the stretching. Exception: A non-hydrogen-bonded O – H (sometimes found in 3o alcohols with large R) will

give a weaker, sharper peak at about 3640 cm-1. Who’s faking it: H2O in the sample can produce a misleading O – H stretch. Usually weaker and less even-shaped Phenols: O-H stretch occurs lower in the range, around 3200 – 3250 cm-1 Telltale benzene ring bands will also be present. 3) The C – O stretch: Alcohols, carboxylic acids, esters and ethers will contain these bands. Frequency: 1000 – 1300 cm-1 Intensity: medium to strong Shape: somewhat broadened Effect of attached groups on the C – O stretch: Both C=O and aromatic rings shift C-O stretch higher Alkyl groups surrounding C also shift it higher: 4) Alkyl halides: Difficult to tell by IR Most C – X stretching frequencies occur near or below the low end of IR range. For sp3-hybridized carbons:

C – F: 1200 – 1300 cm-1 C – Cl: 650 – 800 cm-1 C – Br: below 650 cm-1

5) Amines, Amides and Nitriles: Evidence for nitrogen Amines: 1o and 2o amines are usually identified by their N – H stretches:

Frequency: 3300 – 3500 cm-1 Intensity: medium Shape: somewhat broad but sharper than O – H

Additional info: N – H bending often shows up as a medium band around

1500 – 1650 cm-1 Amides: Look for a down-shifted C = O stretch, frequency: 1630 – 1680 cm-1 Nitriles: Characterized by C = N stretch

Frequency: 2210 – 2260 cm-1 Intensity: medium

The evidence for benzene rings and other R group structure

C - H stretching: C – H bending: “Sawtooth” patterns Frequencies: 2700 – 3300 1300 – 1475 cm-1 Intensity: depends on structure medium Shape: fairly sharp fairly sharp

Hybridization has an effect on C – H stretch: sp2 vs. sp3 Non-aromatic compounds: C – H stretching, strong, 2850 – 2970 cm-1 The more alkyl groups, the stronger the bands!

CH2 groups: Bending around 1460 cm-1 “long chain”: Signature weak peak at 725 cm-1 CH3 groups: Bending around 1460 and 1380 cm-1 isopropyl group: 1380 cm-1 band is split in two

Carbon-carbon bond stretches: C = C (1500 – 1700) vs. C = C (2100 – 2300) Major evidence for benzene ring; aromatic systems:

1) C – H stretches: 3000 – 3100 cm-1

2) C = C stretches: 1500, 1600 cm-1 (alkene C = C are somewhat higher)

3) Overtone & Combination bands:

Weak “sawtooth” pattern between 1750 – 2000 cm-1 (see figure at right) Substitution pattern bands appear in 700 – 850 cm-1 range