ChemistryChapter 12: Analytical Chemistry
Infrared (ir) Spectroscopy
IR wavelengths roughly 2.5–16 μm (about 4000–625 cm−1) supply energy for bond vibrations.
Learning objectives
- Explain infrared (ir) spectroscopy in clear and easy chemistry language.
- Use the important definitions, equations, examples, and conditions correctly.
- Apply the idea in an FBISE-style question and avoid the common exam mistake.
Definition
IR wavelengths roughly 2.5–16 μm (about 4000–625 cm−1) supply energy for bond vibrations. Heteronuclear bonds that change dipole during vibration absorb IR. Spectra look complex because bonds vibrate in many modes, but characteristic peaks identify functional groups. Example: propanone shows strong C=O near ~1720 cm−1 and C–H near ~3000 cm−1; peak intensity reflects bond type more than atom count. School tables list O–H (broad ~3200–3550), N–H, C–H, C=O, C–O, etc. Use IR to distinguish phenol, toluene, acetone, ethanol patterns in analysis tasks.
Formula
C=O near
Important terms
- INFRARED (IR) SPECTROSCOPY
- IR wavelengths roughly 2.5–16 μm (about 4000–625 cm−1) supply energy for bond vibrations.
- WHY MOLECULES ABSORB IR
- Quanta match vibrational spacings.
- FUNCTIONAL GROUP ID
- Match peaks to Table 24.2 style data.
Relationship
- C=O ~1720 cm−1 classic
Overview
Principle, then reading spectra:
Why Molecules Absorb Ir
Quanta match vibrational spacings. Changing dipole is required — useful for polar bonds in organics.
Examples Stretch and bend modes · Propanone spectrum example · Chart of bond regions · Weak vs strong peaks
Functional Group Id
Match peaks to Table 24.2 style data. Broad O–H suggests alcohol/acid; strong sharp carbonyl suggests ketone/aldehyde/acid derivatives.
Examples Acetone carbonyl · Ethanol O–H broad · Toluene aromatic C–H · Phenol O–H pattern
Core idea — INFRARED (IR) SPECTROSCOPY
IR wavelengths roughly 2.5–16 μm (about 4000–625 cm−1) supply energy for bond vibrations. Heteronuclear bonds that change dipole during vibration absorb IR. Spectra look complex because bonds vibrate in many modes, but characteristic peaks identify functional groups. Example: propanone shows strong C=O near ~1720 cm−1 and C–H near ~3000 cm−1; peak intensity reflects bond type more than atom count. School tables list O–H (broad ~3200–3550), N–H, C–H, C=O, C–O, etc. Use IR to distinguish phenol, toluene, acetone, ethanol patterns in analysis tasks.
1. Why Molecules Absorb Ir
Quanta match vibrational spacings. Changing dipole is required — useful for polar bonds in organics. Examples: Stretch and bend modes; Propanone spectrum example; Chart of bond regions; Weak vs strong peaks.
2. Functional Group Id
Match peaks to Table 24.2 style data. Broad O–H suggests alcohol/acid; strong sharp carbonyl suggests ketone/aldehyde/acid derivatives. Examples: Acetone carbonyl; Ethanol O–H broad; Toluene aromatic C–H; Phenol O–H pattern.
Worked method and exam use
IR: bond vibrations → functional groups. Remember C=O ~1720 cm−1 and broad O–H.
Explanation
IR: bond vibrations → functional groups. Remember C=O ~1720 cm−1 and broad O–H.
In simple words
IR is excellent for groups, not always full carbon skeleton alone — combine methods.
Key points
- vibration spectroscopy
- fingerprint + group frequencies
- C=O ~1720 cm−1 classic
- vibrational motion
- dipole change
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Infrared (ir) Spectroscopy
Sticky Notes · Quick Revision
Core idea
- IR wavelengths roughly 2.5–16 μm (about 4000–625 cm−1) supply energy for bond vibrations. Heteronuclear bonds that change dipole during vibration absorb IR. Spectra look complex because bonds vibrate in many modes, but characteristic peaks identify functional groups. Example: propanone shows strong C=O near ~1720 cm−1 and C–H near ~3000 cm−1; peak intensity reflects bond type more than atom count. School tables list O–H (broad ~3200–3550), N–H, C–H, C=O, C–O, etc. Use IR to distinguish phenol, toluene, acetone, ethanol patterns in analysis tasks.
Why Molecules Absorb Ir
- Quanta match vibrational spacings. Changing dipole is required — useful for polar bonds in organics.
Functional Group Id
- Match peaks to Table 24.2 style data. Broad O–H suggests alcohol/acid; strong sharp carbonyl suggests ketone/aldehyde/acid derivatives.
Worked method
- IR: bond vibrations → functional groups.
- Remember C=O ~1720 cm−1 and broad O–H.
Common misconception
- IR is excellent for groups, not always full carbon skeleton alone — combine methods.
FBISE exam check
- In an exam answer, include these key points: vibration spectroscopy; fingerprint + group frequencies.
- Check: not full structure alone.
60-second revision
- IR wavelengths roughly 2.5–16 μm (about 4000–625 cm−1) supply energy for bond vibrations.
- Why Molecules Absorb Ir: Quanta match vibrational spacings.
- Functional Group Id: Match peaks to Table 24.2 style data.
- Exam reminder: IR is excellent for groups, not always full carbon skeleton alone — combine methods.
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.