Organic Chemistry II Exam 1

Strong Signal - IR Spectrum

Bonds with a larger dipole moment.

Weak Signal - IR Spectrum

Bonds with a smaller dipole moment.

Broad Signal - IR Spectrum

H-bonding signals (H-O, H-N, H-F).

Effects of Atomic Mass on the Wavenumber

Lighter atoms appear at a higher IR frequency, and heavier atoms appear at a lower IR frequency.

Double Bonds - IR Spectrum

1600 - 1850

Triple Bonds - IR Spectrum

2100 - 2300

Bonds to H - IR Spectrum

2700 - 4000

Isolated C=C - IR Spectrum

1640 - 1700

Conjugated C=C - IR Spectrum

1600 - 1640

Aromatic C=C - IR Spectrum

About 1600

Internal Alkyne - IR Spectrum

No signal or very weak

C-H Bond Stretching - IR Spectrum

Greater % of s character (sp) in the hybrid orbitals —> stronger C-H bond.

Alkanes - IR Spectrum

Has a broad band between 2850 - 3000.

Alkenes - IR Spectrum

C=C signal at 1600 - 1700

=C-H signal at 3000 - 3100

Alkynes - IR Spectrum

C≡C signal at 2100 - 2200

≡C-H signal at 3300 (sharp peak) for terminal alkynes

O-H Bond Stretching - IR Spectrum

Around 3300 with a broad signal with a rounded tip.

N-H Bond Stretching - IR Spectrum

Around 3300, with a broad signal with sharp spikes.

Alcohols - IR Spectrum

Broad rounded tip signal between 3200 - 3600

Amines - IR Spectrum

Broad signal between 3350 - 3500 with sharp spikes.

Primary Amine - IR Spectrum

Broad with two sharp spikes.

Secondary Amine - IR Spectrum

Broad with one sharp spike.

Tertiary Amine - IR Spectrum

No signal because no N-H bond.

Nitriles - IR Spectrum

Intense and sharp C≡N signal around 2200 - 2300. Has stronger absorptions than alkynes.

Aromatic Compounds - IR Spectrum

C=C signal around 1600

=C-H signal around 3000 - 3100

Aromatic overtones

Ketones - IR Spectrum

C=O signal around 1720

No double band

Aldehydes - IR Spectrum

C=O signal around 1730

Two different bands for the O=C-H stretch at 2750 and 2850

Carboxylic Acid - IR Spectrum

C=O signal around 1715

Broad, strong O-H signal between 2400 - 3600

Both peaks need to be present to identify the compound

Amides - IR Spectrum

Strong C=O signal around 1640-1680

Broad N-H stretching around 3300 with sharp spikes

Tertiary amides do not show N-H absorptions

Carbonyl Compounds - IR Spectrum

Strongest IR signal

Characteristic peak around 1700

Conjugation will lower the frequency

Weak overtone band is common around 3400

Base Peak

The tallest and most stable peak. Has an abundance of 100%.

Molecular Ion / Parent Ion / M+o

The most right peak with the highest intensity. The m/z if the parent ion is the molar mass of the compound.

(M+1)+o Peak

A peak that is one mass unit higher than the M+o peak.

(M+2)+o Peak

A peak that is two mass units higher than the M+o peak.

Estimate How Many C Atoms are in the Molecule Using the (M+1)+o Peak

Number of Carbons = (Height of (M+1) Peak / Height of M Peak) x 100 / 1.1

Mass Spectrum with Bromine Atom

Bromine is a mixture of 79Br = 50.7% and 81Br = 49.3% so compounds containing a Br atom have equally strong M+o to (M+2)+o peaks because their two isotopes are equally abundant.

Mass Spectrum with Chlorine Atom

Chlorine is a mixture of about 76% 35Cl and about 24% 37Cl so compounds containing a Cl atom have a 3-to-1 ratio of their M+o to (M+2)+o peaks.

Mass Spectrum with Nitrogen Atom

If the M+o is odd, then there are an odd number of N atoms in the molecule. If the M+o is even, that indicates an absence of nitrogen, or an even number of N atoms is present.

M-15

Loss of a methyl radical.

M-29

Loss of an ethyl radical.

M-43

Loss of a propyl radical.

M-57

Loss of a butyl radical.

M-18

Loss of water (from an alcohol)

M-X (where X = even number)

McLafferty Rearrangement (ketone or aldehyde)

Degree of Unsaturation = 1

a) One double bond

b) One ring

Degree of Unsaturation = 2

a) Two double bonds

b) One triple bond

c) Two rings

d) One double bond and one ring

Degree of Unsaturation = 0

Molecule can not have any rings, double bonds or triple bonds.

Degree of Unsaturation Calculation

(2C + 2 + N - H - X) / 2

Homotopic

Chemically equivalent —→ One signal

Enantiotopic

Chemically equivalent —> One signal

Diastereotopic

Not chemically equivalent —> Two signals for two hydrogens

If the compound has no chiral center —>

One signal for both hydrogens

If the compound has a chiral center —>

Two signals for two hydrogens

If other sp2 C has the same groups attached —>

One signal for both hydrogens

If other sp2 C has different groups attached —>

Two signals for two hydrogens

Hydrogens surrounded by more electron density —>

More shielded, higher, upfield, and toward the right.

Hydrogens surrounded by less electron density —>

Deshielded, less shielded, lower, downfield, and toward the left.

Methyl (CH3)

0.9 ppm

Methylene (CH2)

1.2 ppm

Methine (CH)

1.7 ppm

Alkenes (C=C)

4.5-6.5 ppm

Aryl (benzene ring)

6.5-8.5 ppm

Aldehyde (O=-H)

9.5-10.5 ppm

Carboxylic Acid (O=-OH)

10.5-12.0 ppm

Fewer H —>

Further downfield signal (higher ppm).

More electronegative atoms —→

Larger chemical shift values (higher ppm).

More distance between electronegative substituents —>

The further from the electronegative atom, the further upfield (lower ppm).

Free Rotation Coupling Constant

7 Hz

Cis Coupling Constant

10 Hz, H next to each other on separate C

Trans Coupling Constant

15 Hz, H across from each other on separate C

Geminal Coupling Constant

2 Hz, H attached to the same C (diastereotopic)

Allylic Coupling Constant

6 Hz, H atom attached to a C atom that is directly adjacent to a C=C

sp3 and sp carbon atoms (C-X)

0-65 ppm

sp3 and sp carbon atoms (alkane)

10-50 ppm

sp3 and sp carbon atoms (C-N)

40-60 ppm

sp3 and sp carbon atoms (C-O)

50-80 ppm

sp3 and sp carbon atoms (C triple bond C)

65-90 ppm

sp2 carbon atoms (C=C)

100-150 ppm

sp2 carbon atoms (aromatic)

110-170 ppm

sp2 carbon atoms (C=O)

160-220 ppm

sp2 carbon atoms

Deshielded

sp3 and sp carbon atoms

shielded

CH3 Group DEPT

No peak in DEPT-90 and positive peak in DEPT-135

CH2 Group DEPT

No peak in DEPT-90 and negative peak in DEPT-135

CH Group DEPT

Positive peaks in all spectrum

C Group DEPT

Peak in regular 13C NMR only, no peak in DEPT.