Download How well do you know your functional groups?

How well do you know your functional groups?
Identify the functional group/class of compound for each structure!
Alcohol
(1o, 2o or 3o?)
Aldehyde
1.
Alkane
3.
NH 2
O
H
C
H3 C
4.
CH 3
H3C
CH 3
CH 3
Alkene
Alkyne
2.
CH2Cl2
5.
O
6.
H3 C
C
Amide
7.
8.
CH3
OH
N
Amine
(1o, 2o, or 3o?)
Anhydride
OH
C
H2
Alkyl halide
H 3C
H
9.
H3 C
C
CH
CH3
10.
Aromatic
Carboxylic acid
O
11.
C
H 3C
Cycloalkane
CH 3
12.
H2
C
O
C
H 3C
CH 3
OH
CH 3
Diene
O
13.
Ester
14.
C
H2
CH
Ether
SH
O
15.
16.
H 3C
Ketone
Thiol
H2
C
H3 C
17.
O
CH3
18.
O
O
C
O
H3 C
H 3C
H
C
CH 2
OH
N
H
CH 3
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
electromagnetic
spectrum:
λ = 0.8 μm to 102 μm ; longer wavelengths than visible
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
several moderately strong bands
1460 – 1465 = CH2 group bending
1365 – 1380 = CH3 group bending
Alkenes:
3020 – 3100 = C-H next to C=C
weak to medium bands
1640 – 1680 = C=C stretching
(sharp band)
Alkynes:
3300 (sharp) = C-H next to C=C
2100 – 2260 = C=C stretching
Alkyl halides:
600 – 800 = C-Cl
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
1200 – 1300 = C-O stretching
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
o
o
3300 – 3500 = N-H stretch (medium, broad, only seen in 1 or 2 amines)
o
1 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)
1650 – 1800 = C=O stretch (very strong, a bit broad)
Carbonyls (general)
500 – 600 = C-Br
These are in addition to
the aromatic ring bands!
-1
Specific groups with carbonyls: Remember these will appear 30 cm 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)
ƒ
ƒ
ƒ
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 carbonylcontaining functional groups is present in the molecule
If C = O stretch is absent, proceed to check for other groups
Ketone:
Aldehyde:
O
O
C
R
C
R'
R
Carboxylic acid:
Amide:
O
O
C
C
R
2)
Ester:
OH
H
R
O
R
C
R
OR'
N
H
OH groups: The O – H stretch
Alcohols and carboxylic acids (if you also see the C = O) are the likely culprits.
Frequency range:
Intensity:
Shape:
3200 – 3650 cm-1
Strong
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:
Frequency:
Intensity:
Shape:
Alcohols, carboxylic acids, esters and ethers will contain these bands.
1000 – 1300 cm-1
medium to strong
somewhat broadened
Effect of attached groups on the C – O stretch:
4)
Both C=O and aromatic rings shift C-O stretch higher
Alkyl groups surrounding C also shift it higher:
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:
Amides:
N – H bending often shows up as a medium band around
1500 – 1650 cm-1
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
Frequencies:
Intensity:
Shape:
C - H stretching:
2700 – 3300
depends on structure
fairly sharp
C – H bending:
1300 – 1475 cm-1
medium
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:
“long chain”:
CH3 groups:
isopropyl group:
Bending around 1460 cm-1
Signature weak peak at 725 cm-1
Bending around 1460 and 1380 cm-1
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:
3000 – 3100 cm-1
1)
C – H stretches:
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
“Sawtooth” patterns