Exp 6 + 7 - NMR & IR Spectroscopy

OCHEM LAB LECTURE FALL 25

What is spectroscopy?

A technique where molecules absorb energy and we monitor the response to determine structural information

Name four major spectroscopic techniques in orgo

Mass spec, UV-Vis, NMR, IR

What do nuclei with spin ½ (like ¹H and ¹³C) respond to?

Magnetic fields

List common NMR-active nuclei

¹H, ¹³C, ¹⁹F, ³¹P

What does “same nuclei have same susceptibility” mean in NMR?

Equivalent protons produce the same signal

What is coupling in NMR?

Mutual magnetic influence between neighboring nuclei, causing splitting

Is coupling reciprocal?

Yes — each nucleus splits the other

What type of information does NMR give?

Number of environments, chemical shifts, splitting patterns, integration

Ethyl acetate vs. methyl ethanoate — why different NMRs?

The chemical environment changes the magnetic susceptibility

What does IR spectroscopy measure?

Bond stretching and bending from infrared absorption

What part of a molecule does IR primarily identify?

Functional groups

C–C single bond hybridization type?

sp³–sp³

C=C double bond hybridization?

sp²–sp²

C≡C triple bond hybridization?

sp–sp

IR C–H stretch for sp³ carbon?

Around ~2850–2960 cm⁻¹ (general concept)

IR C–H stretch for sp² carbon?

~3000–3100 cm⁻¹ (general concept)

IR C–H stretch for sp carbon?

~3300 cm⁻¹ (sharp)

What IR peak indicates a carbonyl?

Strong C=O stretch near ~1700 cm⁻¹

What functional groups contain carbonyls?

Aldehydes, ketones, esters, amides, carboxylic acids

IR O–H stretch (alcohol)?

Broad signal ~3200–3600 cm⁻¹

IR N–H stretch?

Sharp or broad ~3300–3500 cm⁻¹

What does an aldehyde show in IR besides C=O?

Two C–H stretches around ~2720 & 2820 cm⁻¹

IR C≡N stretch?

Strong peak around ~2250 cm⁻¹

What do esters show in IR?

C=O + strong C–O stretches in 1000–1300 cm⁻¹ region

Carboxylic acids show what distinctive IR feature?

Very broad O–H stretch + strong carbonyl

Name 3 types of stretching vibrations in IR

Stretching, bending, scissoring (general classification from slides)

What’s the first step in IR interpretation?

Identify major functional group peaks

What’s the second step?

Match peaks to likely structures based on functional groups present

What bond type absorbs the most strongly in IR?

Carbonyls (C=O)

IR regions: where do fingerprint peaks appear?

<1500 cm⁻¹

What is Degrees of Unsaturation (DOU)?

Number of π bonds and/or rings in a molecule

Formula for DOU?

(2C + 2 + N – H – X) / 2

DOU of C₈H₁₆?

1 degree. (Common calculation example from slides.)

What does DOU = 4 often mean?

Aromatic ring

DOU of C₅H₅N?

4 degrees (suggests aromaticity)

DOU of C₆H₅OBr?

4 degrees.

What does “ortho-substituted” benzene mean?

Substituents next to each other (1,2)

What does “meta-substituted” benzene mean?

Substituents in 1,3 positions

What does “para-substituted” benzene mean?

Substituents opposite (1,4)

Why recognize ortho/meta/para patterns?

They influence both NMR splitting patterns and IR signals

Which spectroscopy detects environment of nuclei?

NMR

Which spectroscopy detects bond vibrations?

IR

What does chemical shift depend on?

Electron density; shielding/deshielding

Deshielding moves peaks which way?

Downfield (to the left)

Shielding moves peaks which way?

Upfield (to the right)

What is integration in ¹H NMR?

Measures relative number of protons in each environment

What causes splitting patterns?

Neighboring hydrogens (n + 1 rule)

What does a singlet mean?

No neighboring hydrogens

What does a triplet mean?

Two neighboring hydrogens

What does a quartet indicate?

Three neighboring hydrogens (e.g., ethyl groups)