Chapter 12 and 13 Organic Chemistry 2

Matt Way's Class Summer 2025

What does IR spectroscopy detect in molecules?

It detects how molecules absorb infrared light, revealing the types of bonds present.

How does UV–Vis spectroscopy differ from IR?

UV–Vis detects electronic transitions, while IR focuses on vibrational energy changes.

What is the formula for energy of electromagnetic radiation?

E = hc/λ

What is the formula for wavenumber?

ν̅ (cm⁻¹) = 1/λ (cm)

What is the IR wavelength and wavenumber range?

2.5–25 µm or 4000–400 cm⁻¹

Why is wavenumber useful in IR spectroscopy?

It is directly proportional to energy, which simplifies analysis of bond vibrations.

What analogy describes bond stretching in IR?

It’s like two balls connected by a spring (harmonic oscillator).

What factors increase vibrational frequency and wavenumber?

Stronger bonds and lighter atoms both increase frequency.

Why is recognizing functional groups important in IR spectroscopy?

Functional groups have characteristic absorption patterns that help identify molecular structures.

What are some functional groups to memorize for IR identification?

Alcohols, amines, ketones, aldehydes, carboxylic acids, nitriles, aromatics, alkyl halides, etc.

What should you be able to do with functional group IR patterns?

Recognize them visually and match them to specific wavenumber ranges in a spectrum.

What strategy helps when given an unknown molecular structure?

Look for key functional group IR absorptions and match them with known reference values.

What are common abbreviations used in IR spectra descriptions?

w = weak, m = medium, s = strong; n = narrow, b = broad, sh = sharp

Where can the IR absorption reference table be found?

In the course module titled “IR absorption reference table” under Chapter 12/13.

Where does the C–H stretch for alkanes appear?

Just below 3000 cm⁻¹

What are the key IR absorptions for alkynes?

≡C–H stretch near 3300 cm⁻¹ and C≡C stretch near 2100 cm⁻¹

What is the O–H stretch range for alcohols in IR?

Broad stretch between 3400–3650 cm⁻¹

What is the range for the C–O stretch in alcohols?

Strong stretch between 1000–1320 cm⁻¹

What is the N–H stretch range for amines?

Medium, 3250–3400 cm⁻¹

What other key stretches are seen in amines?

C–N stretch (1020–1250 cm⁻¹, medium) and N–H bend (1650–1580 cm⁻¹, medium)

What is the strong IR stretch characteristic of amides?

C=O stretch between 1760–1665 cm⁻¹

How do amide N–H stretches compare to amines?

Amides show medium N–H stretch from 3400–3250 cm⁻¹, typically narrower

What are the C–H stretch values for aromatic compounds?

Strong stretch from 3100–3000 cm⁻¹

What are the key C–C stretch ranges in aromatics?

Medium C–C stretches at 1600–1585 cm⁻¹ and 1500–1400 cm⁻¹

What is a unique IR signal for aldehydes not found in ketones?

Medium O=C–H stretch between 2830–2695 cm⁻¹

What is the C=O stretch range for aldehydes?

Strong peak from 1740–1720 cm⁻¹

What is the characteristic IR signal for ketones?

Strong C=O stretch around 1715 cm⁻¹

How do ketone IR spectra differ from aldehyde spectra?

Ketones lack the additional O=C–H stretch seen in aldehydes

What is the O–H stretch range in carboxylic acids?

Broad, medium intensity stretch between 3300–2500 cm⁻¹

What other major IR peaks do carboxylic acids show?

C=O stretch (1760–1690 cm⁻¹) and C–O stretch (1320–1000 cm⁻¹)

What IR signal is specific to C–Cl in alkyl halides?

Medium C–Cl stretch between 850–550 cm⁻¹

What C–H stretch may also appear in alkyl halides?

Medium C–H stretch between 1300–1150 cm⁻¹

What is the key IR absorption in nitriles?

Very strong C≡N stretch between 2260–2210 cm⁻¹

How does the nitrile peak appear in the IR spectrum?

As a sharp, strong signal in the 2200 cm⁻¹ region

What should you do when multiple functional groups are present in IR?

Identify each functional group individually and match all expected signals

What compound was used in this slide as an example of multiple functional groups?

Vanillin

How can you predict the compound from a given IR spectrum?

Match the absorption peaks to known functional group ranges and identify which functional groups are present

What is the first step in analyzing an unknown IR spectrum?

Look for strong, broad, or sharp peaks that match key functional group absorptions

How do nuclei behave without an external magnetic field?

Their spins are randomly oriented.

What happens when a magnetic field is applied in NMR?

Nuclei align parallel (low E) or anti-parallel (high E) to the field.

What is meant by “resonance” in NMR?

Absorption of radiofrequency radiation causes spin flips from low to high energy states.

What factors determine the resonance frequency in NMR?

Magnetic field strength, identity of the nucleus, electronic environment

What types of nuclei are detectable in NMR?

Nuclei with an odd number of protons and/or neutrons (e.g., ¹H, ¹³C)

What energy transition occurs during NMR measurement?

A proton absorbs radiofrequency and flips to a higher energy spin state.

What does the NMR signal ultimately represent?

It reflects the different magnetic environments of hydrogen atoms in a molecule.

How are signals displayed in NMR data?

As peaks on a spectrum measured in parts per million (ppm)

What causes multiple signals in ¹H NMR?

Chemically non-equivalent hydrogen atoms produce distinct signals.

What is chemical shift (δ)?

The position (in ppm) where a nucleus absorbs radiation, relative to TMS.

What is the formula for calculating chemical shift (δ)?

δ (ppm) = (Observed shift in Hz) / (Spectrometer frequency in MHz)

What is the reference compound for NMR chemical shifts?

Tetramethylsilane (TMS), assigned 0 ppm

Why is TMS used as the reference?

It has shielded protons (low electronegativity of Si), producing a consistent signal at 0 ppm

What does “downfield” mean in ¹H NMR?

A higher δ value; corresponds to less shielding and protons near electronegative atoms

What does “upfield” mean in ¹H NMR?

A lower δ value; corresponds to more shielding and protons near electron-rich environments

Which direction does the δ scale increase on an NMR spectrum?

From right (upfield) to left (downfield)

What causes protons to be deshielded in ¹H NMR?

Proximity to electronegative atoms or unsaturated systems (e.g., sp² carbons)

Where do sp³ C–H protons usually appear in ¹H NMR?

More upfield, typically around 0.9–1.5 ppm

Where do sp² C–H protons usually appear?

Downfield, typically 5–8 ppm

Where do aldehyde protons appear in ¹H NMR?

Around 9–10 ppm

Where do aromatic protons appear?

Around 6.5–8 ppm

Where do alcohol O–H protons typically appear?

Broad signals around 1–5 ppm, depending on hydrogen bonding

What symmetry does 1,4-dimethylbenzene (p-xylene) exhibit in ¹H NMR?

Symmetrical substitution gives only two types of proton environments

What are the expected δ values for p-xylene protons?

Aromatic protons ≈ 7.0 ppm; methyl protons ≈ 2.3 ppm

What does the area under an NMR peak represent?

The number of protons (hydrogens) contributing to that signal.

How does integration help interpret ¹H NMR spectra?

It allows you to calculate relative ratios of equivalent proton groups.

What does a taller or larger peak mean in NMR?

A larger number of equivalent hydrogens are contributing to that signal.

What causes spin–spin splitting in ¹H NMR?

Magnetic interactions between nonequivalent neighboring protons.

What is the “n + 1 rule” in NMR?

A proton with n neighboring hydrogens will split into n + 1 peaks.

What splitting pattern results from 1 neighboring proton?

Doublet (n + 1 = 2)

What splitting pattern results from 2 equivalent neighbors?

Triplet (n + 1 = 3)

What is a multiplet?

A complex splitting pattern from coupling with multiple nonequivalent proton groups.

What are the three types of protons in ethanol?

Methyl (CH₃), Methylene (CH₂), and Hydroxyl (OH)

Expected δ and splitting for CH₃ in ethanol?

δ ≈ 1.2 ppm; Triplet (split by CH₂)

Expected δ and splitting for CH₂ in ethanol?

δ ≈ 3.7 ppm; Quartet (split by CH₃)

Expected δ and splitting for OH in ethanol?

δ ≈ 2.6 ppm; Singlet (no coupling with other protons)

What were the actual δ values for ethanol’s protons in the spectrum?

Ha (CH₃) ≈ 1.2 ppm; Hb (OH) ≈ 2.6 ppm; Hc (CH₂) ≈ 3.7 ppm

Why does the OH signal often appear as a singlet?

Proton exchange with solvent prevents coupling with neighboring protons.

What happens to splitting when neighboring protons are equivalent?

Equivalent protons do not cause spin–spin splitting.

Why does 1,2-dichloroethane show only one signal?

Molecular symmetry makes all four hydrogens equivalent.

What does the NMR spectrum of 1,2-dichloroethane look like?

A single singlet at ~3.7 ppm, representing all symmetrical protons.

What structural feature leads to a singlet in this case?

The molecule has internal symmetry, so all H’s experience the same environment.

What is a key strategy to recognize equivalent protons?

Look for molecular symmetry that produces chemically identical environments.

Why might a compound with multiple protons still give few peaks?

Some sets of protons are equivalent due to symmetry and don’t split each other.

What does the NMR spectrum of 1,4-dioxane show?

One singlet around 3.6 ppm for all hydrogens

Why does 1,4-dioxane only show one signal?

Its cyclic symmetry makes all hydrogens equivalent.

How many distinct proton environments are present in 1,4-diethylbenzene?

Three: aromatic H’s, ethyl CH₂, and ethyl CH₃

How does symmetry affect the splitting in 1,4-diethylbenzene?

Symmetry simplifies the spectrum; equivalent protons result in fewer signals and predictable splitting.

What information should you gather for each proton environment in a compound?

Chemical shift (δ), Integration (number of H’s), Splitting pattern (singlet, doublet, etc.)

What’s the goal of matching spectrum peaks to proton sets?

To assign each NMR signal to a specific group of chemically equivalent protons in the molecule.

How can you check if your interpretation of an NMR spectrum is accurate?

The number of signals, their relative areas, and splitting should match the structure’s symmetry and hydrogen environments.

Why is integration critical in assigning peaks?

It tells you how many hydrogens are producing each signal, helping differentiate CH₃, CH₂, and CH types.

What is one major use of ¹H NMR in organic chemistry labs?

Verifying the identity and purity of synthesized compounds

How can NMR verify hydroboration–oxidation reactions?

The product's signal confirms formation of the less substituted alcohol, indicating anti-Markovnikov addition.

What does the ¹H NMR spectrum of hydroboration product show?

A signal for the alcohol proton, plus upfield CH₃ and CH₂ signals consistent with the product structure.

What supports the regioselectivity of the reaction in NMR?

The alcohol proton appears on the less substituted carbon, proving anti-Markovnikov orientation.