Determine structures from IR spectra

Summary

This video tutorial guides viewers through solving organic chemistry IR spectroscopy problems. Using molecular formulas and IR spectra, it demonstrates how to calculate degrees of unsaturation (DBE) to determine rings and π bonds. The instructor walks through three example problems, showing how to identify functional groups such as alcohols, ketones, nitriles, and carboxylic acids from IR peaks, and how to propose valid molecular structures while ruling out incorrect ones. Emphasis is placed on interpreting characteristic IR zones, considering substitution patterns in aromatic compounds, and applying systematic reasoning to match formulas with spectral data, offering practical strategies for analyzing IR spectra effectively.

Takeaways

• 🧪 Start by calculating the degree of unsaturation (DBE) using the formula: C + 1 - H/2 + N/2 to determine rings and pi bonds.
• 🔬 Oxygen atoms in a formula can be ignored when calculating DBE for IR analysis.
• 💧 Broad peaks in zone 1 of the IR spectrum usually indicate an O-H bond from an alcohol or carboxylic acid.
• 🧊 Peaks between 3000–2850 cm⁻¹ correspond to sp³ C-H bonds, while peaks around 3100–3000 cm⁻¹ indicate sp² C-H bonds.
• ⚡ A sharp peak in zone 4 typically represents a carbonyl (C=O) group, crucial for identifying ketones, aldehydes, or acids.
• 📝 Only certain structures fit the IR spectrum; functional groups must match the observed peaks (e.g., aldehyde peaks at 2700 cm⁻¹ are absent, so aldehydes are excluded).
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• 🔗 When DBE ≥ 4, consider the presence of an aromatic benzene ring, which accounts for one ring and three pi bonds.
• 💠 IR peaks around 2200 cm⁻¹ indicate either a carbon-carbon triple bond or a nitrile (C≡N) group; presence of nitrogen clarifies which.
• 🌿 The position of substituents on a benzene ring (ortho, meta, para) can sometimes be inferred but is not required at the undergraduate level.
• 🥼 Carboxylic acids display a very broad O-H stretch extending into zone 2 along with a strong carbonyl peak; with only two carbons, only one structure is possible.
• 📊 Real-world IR spectra may have broad or unclear peaks; careful interpretation and cross-checking with DBE and molecular formula is essential.
• ✅ Multiple structures can sometimes satisfy the IR spectrum and formula; focus on functional groups and DBE to determine feasible options.

Q & A

What is the purpose of calculating the DBE (Degree of Unsaturation) in IR spectroscopy problems?

Calculating the DBE helps determine the number of π bonds or rings in a molecule, which guides the structural possibilities when analyzing IR spectra.

What is the formula for calculating DBE?

DBE = (C + 1 - H - X + N) / 2, where C = carbons, H = hydrogens, X = halogens, and N = nitrogens. Oxygen is ignored.

How can you distinguish an alcohol OH peak from a carboxylic acid OH peak in an IR spectrum?

An alcohol OH peak is typically broad in zone 1 (3600–3200 cm⁻¹), while a carboxylic acid OH peak is very broad and can extend into zone 2 (around 2500–3300 cm⁻¹).

Why was an aldehyde ruled out in Problem 1 despite the presence of a carbonyl peak?

An aldehyde shows a characteristic C–H stretch around 2700 cm⁻¹, which was absent in the spectrum, so the carbonyl could only be part of a ketone.

What functional groups were identified in Problem 2 from the IR spectrum?

Problem 2 included an aromatic ring (benzene), a nitrile group (C≡N at 2200 cm⁻¹), and sp³ C–H bonds indicating a methyl group.

Why is a benzene ring often suggested when the DBE is 4 or higher?

A benzene ring contributes one ring and three π bonds, which together account for four degrees of unsaturation, making it likely when DBE ≥ 4.

How do positional isomers affect IR interpretation for substituted benzene rings?

IR spectra alone may not distinguish ortho, meta, or para substitution on a benzene ring, so multiple positional isomers can fit the same spectrum.

What was unique about Problem 3 compared to the other problems?

Problem 3 had only one possible structure due to the small molecule size (two carbons) and the clear presence of a carboxylic acid, leaving no room for isomers.

Why is it important to compare the molecular formula with IR data when proposing structures?

The molecular formula ensures the correct number of atoms and functional groups are included, preventing impossible structures that do not match the observed IR peaks.

What general advice does the transcript give about real-world IR spectra?

Real-world IR spectra may have broad or overlapping peaks, and not every peak will be clear. Focus on identifying major functional groups first and use DBE and molecular formula to confirm structures.

Which IR region is considered the fingerprint region, and why is it less diagnostic?

The fingerprint region is below 1500 cm⁻¹. It is less diagnostic because it contains complex overlapping vibrations unique to each molecule, making it harder to assign specific functional groups.