Organic Lec 2 PDF

## II) Infrared (IR) Spectroscopy **(Identification of function groups)** - Infrared (IR) electromagnetic radiations extends from 2.5-16µm (or from 4000-600 cm⁻¹). - In the IR region of the electromagnetic spectrum, the absorption of radiation by a sample stimulates molecular vibrations. - The unit...

## II) Infrared (IR) Spectroscopy **(Identification of function groups)** - Infrared (IR) electromagnetic radiations extends from 2.5-16µm (or from 4000-600 cm⁻¹). - In the IR region of the electromagnetic spectrum, the absorption of radiation by a sample stimulates molecular vibrations. - The unit used for measurement of IR is wavenumber which is equal to 1/wavelength in cm = cm⁻¹ - The wavenumber is directly proportional to frequency. - Infrared spectra are traditionally displayed as %T (percent transmittance) versus wave number. - Useful in identifying presence or absence of functional groups. - IR spectroscopy is used to identify functional groups through the absorption of infrared (IR) radiation. **Aspirin (acetylsalicylic acid)** ``` 120 100 COOH LOCOCH3 80 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 500 wavenumber/cm⁻¹ ``` ## 2-What is a vibration in a molecule? Any change in shape of the molecule: stretching of bonds, bending of bonds, or internal rotation around single bonds. ## What are the types of vibrations in a molecule? - **Stretching of bonds:** (affect bond length) - In these vibrations the distance between two atoms increases or decreases but the atoms remain in the same bond axis, which may be symmetric or asymmetric. - **Symmetric stretch**: no dipole - **Asymmetric stretch**: change in dipole - IR active - **Bending of bonds:** (affect bond angle) - **In plane**: 1-scissor 2-rock - **Out of plane**: 1-twisting 2-wagging - **Bending**: change in dipole - IR active ## 3-Application of IR spectroscopy - From 4000-1400 cm⁻¹ mostly stretching vibrations (function group region). - From 1400-------650 cm⁻¹ mostly bending vibration (finger print region). - IR is Useful in identifying presence or absence of functional groups. In the IR region of the electromagnetic spectrum, the absorption of radiation by a sample is due to changes in the vibrationenergy states of a molecule. - The Energy required for such molecular vibrations is directly proportional to the radiation frequency, from Hook's law depends on: * v = 1/2πC.√K/m₁m2/m₁+m2 - K = Force constant of the bond (measure of bond strength). - m₁ & m2 are the atoms connected by the bond. ## So, Factors affect wavenumber of the bond are: - **1-Mass of the bonded atoms:** - The greater the mass of the atoms connected by the bond, the lower the frequency and wavenumber and vice versa. - **2- Strength of the bond.** - i)The longer the bond length, the weakerthe bond strength, K: - Ex:C-C 1200 cm⁻¹, C=C 1660 cm⁻¹, C=C2200 cm⁻¹ - ii) Bond strength (K)is also affected by electronegativity differencebetween atoms connected by the bond. The higher the difference in electronegativity between the connected atoms, the greater the wavenumber of this bond. - Ex:C=C (1660-1550 cm⁻¹), C=O(1700-1680 cm⁻¹) - iii)Bond strength (K) is also affected by % of S character of the the connected atoms. The increase of S character leads to the increase of frequency. - sp³ C-H, just below 3000 cm⁻¹ (to the right) - sp² C-H, just above 3000 cm⁻¹ (to the left) - sp C-H, at 3300 cm⁻¹ - **3- Resonance affects:** - The greater the resonance, the lower the frequency. - **4- H-bonds:** - H-bonding of (OH or NH groups) causes broadening of the peak and lowers v as it decreases K. H-bonds may be Intermolecular or Intramolecular H-bonds - Intermolecular H-bonds can be broken viadilutionin non-polar solve become a sharp peak with morefrequency. - While Intramolecular hydrogen bonding cannot be broken by dilution in non-polar solvent. - **5- Electronic effect - I &- Mor + I & + M:** - Electron withdrawing groups (with-I or -M effect) increase wavenumber, While electron donating groups (with +I or +M effect) decrease wavenumber. - **Ex1:** - H3C- 1729 cm⁻¹ Cl3C- 1768 cm⁻¹ H3CO- 1683 cm⁻¹ 1674 cm⁻¹ - The -I effects of (Cl3C-) increases double bond character of C=O so increase frequency but +M effect of (CH3O-) decreases double bond character so lower frequency. - **Ex2:** - CH₂CO-CI, Acetyl chloride, 1800 cm⁻¹ - CH3CO-OH, Acetic acid, 1730 cm⁻¹ - CH₂CO-NH2 Acetamide 1670 cm⁻¹ - Due to: -I effect of Cl> O > N. - And +M effect of N > O > Cl. ## Fingerprint Region of Molecule: - No two molecules will give exactly the same IR spectrum (except enantiomers) because every molecule shows specific complex vibrations at range 1400-600, called the "fingerprint region." ## IR for differentorganicFunctional groups: | Wavelength (µm) | IR Functional Group | | |---|---|---| | 2.5 | N-H | | | 3 | O-H | | | 3.5 | C-H | | | 4 | C=C | | | 4.5 | C=C | | | 5 | C=O | | | 5.5 | C-O | | | 6 | C=N | | | 6.5 | C=N | | | 7 | C-N | | | 8 | C-C | | | 9 | Fingerprint region | | | 10-16 | Fingerprint region | | ## 1) IR for O-H and N-H: - Both of these occur around 3600-3200 cm⁻¹, but they look different. - Alcohol O-H, broad with rounded tip (due to H-bond). - Secondary amine (R2NH), One sharp Peak. - Primary amine (RNH2), Forked peak. - No signal for a tertiary amine (R3N) ## 2) IR for (=C-H): - Occur at 3300 cm⁻¹ ## 3) IR for (=C-H) bond: - (Alkenes and Benzene ring): 3100-3000 cm⁻¹ ## 4) IR for alkanes: - C-H bond saturated (sp³ C): 3000-2900 cm⁻¹ ## 5) OH of a Carboxylic Acid (COOH): - This O-H absorbs broadly, 3500-2500 cm-1, due to strong hydrogen bonding. ## 6) C=C & C=N bond: - 2400-2100 cm⁻¹ ## 7) (C=O) absorption peak: - The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1800-1650 cm. ## 8) C=N, C=C: - 1650-1550 cm⁻¹ ## 9) Substituted Benzene: - 800-700 cm⁻¹ (TWO peaks): mono-substituted or metadisubstituted benzene) - 800-700 cm⁻¹ (ONE peak) : o- or p-disubstituted benzene) - So from IR spectrum we candifferentiate between different substituted benzene. | Frequency range cm⁻¹ | Function group | | |---|---|---| | 3600-3200 | OH, NH str. Of alcohols, amines and phenols | | | 3500-2500 | OH str. Of carboxylic acids (H-bond | | | ~3300 | C-H of alkyne | | | 3100-3000 | C-H aromatic or alkene | | | 2950-2850 | C-H aliphatic (alkane) | | | 2850-2700 | C-H str. Of aldehyde | | | 2400-2100 | CN nitrile CC of alkyne | | | 1800-1650 | C=O (acid, ketone, ald. Ester) | | | 1650-1600 | C=C alkenes | | | 1600-1550 | C=C aromatic | | ## Examples of some IR spectra: - **ETHANOL INFRARED SPECTRUM** - **CH3-C-CH3** - **CH3-C-OCH3** - **1-butanol CH3(CH2)3OH** ## Proton Nuclear Magnetic Resonance Spectroscopy **(¹H-NMR)** It is a technique used to determine a compound's unique structure. It identifies the carbon-hydrogen framework of an organic compound. It gives information about the number and type of hydrogen atoms in the molecule. ## Magnetic Resonance Imaging (MRI) - Magnetic resonance imaging (MRI) uses magnetic fields and radio frequencies to create a three-dimensional picture of structures inside the body. An MRI can diagnose diseases of the brain, spine, skeleton, abdomen, and pelvis. An MRI image is more detailed than an X-ray, ultrasound, or CT scan. ## Theory: - **With no external magnetic field:** A spinning proton creates a magnetic field. The nuclear magnets are randomly oriented. - **In a magnetic field**: The nuclear magnets are oriented with or against Bo. - **Resonance frequency (specific to nucleus):** ΔΕ = γΑΒo - **Excitation**: Transition between lower energy states to higher energy states when radio waves of an appropriate frequency are applied. - **Relaxation**: Return to thermal equilibrium after excitation by releasing energy. ## NMR-Chart: - **NMR-Chart**: - **Downfield (deshielded):** Protons in electron poor environments. - **Upfield (shielded):** Protons in electron-rich environments TMS. - **Low field**: 11 ppm - **High field**: 0 ppm