Identification of Unknown Sample A via Spectral Analysis
Author
Ty Barnes
TA
Chase Ledbetter
Techniques
FTIR · ¹H NMR · GC-MS
Identified Compound
2-Bromobutane
Introduction
This experiment required the identification of an unknown organic compound (Sample A) using three complementary spectral datasets: infrared spectrometry (FTIR), proton nuclear magnetic resonance (¹H NMR), and gas chromatography–mass spectrometry (GC-MS). Each technique provides distinct and orthogonal structural information; convergence across all three confirms compound identity.
Accurate interpretation of analytical instrumentation is a practical and essential skill in chemical and pharmaceutical research. This experiment demonstrates the systematic application of all three methods to unambiguously identify an unknown sample.
Spectral Analysis
Identifies functional groups present in a compound.
Findings
Absorption exclusively in the 2800–3000 cm⁻¹ range, characteristic of Csp³–H bonds. The absence of other notable absorptions in the diagnostic region (>1500 cm⁻¹) rules out hydroxyl, carbonyl, and aromatic groups. This range was expected due to the low s-character of sp³ C–H bonds, which correlates with weaker, lower-frequency stretching vibrations.
Conclusion
Compound contains only sp³ carbon-hydrogen bonds — consistent with a saturated alkyl halide.
Maps the number, environment, and connectivity of hydrogen atoms.
Findings
Four distinct signal clusters identified via color-coding. Signal at 4.1 ppm (quintet, 1H) indicates a significantly deshielded proton adjacent to an electronegative substituent with four neighboring hydrogens. Signals at 1.85 ppm (2H), 1.7 ppm (3H), and 1.0 ppm (3H) complete the pattern of a four-carbon chain with a halogen on the second carbon.
Conclusion
Hydrogen environment pattern matches CH₃–CHX–CH₂–CH₃, placing the electron-withdrawing group on carbon 2.
Determines elemental composition and confirms structural stability via fragmentation patterns.
Findings
Molecular ion [M]⁺ and [M+2]⁺ peaks exhibit a ~1:1 ratio, confirming the presence of a bromine atom (a 1:3 ratio would indicate chlorine). Base peak at 56 m/z corresponds to the C₄H₉⁺ fragment — the stable carbocation formed after loss of bromine as a leaving group. This fragmentation pattern is consistent with bromine on a secondary carbon.
Conclusion
Elemental composition and fragmentation confirm the compound is a bromine-containing alkane: 2-bromobutane (MW ~137 g/mol).
Conclusion
Identified Compound
2-Bromobutane
Molecular Formula
C₄H₉Br
Molecular Weight
~137 g/mol
Key Identifier
1:1 [M]⁺/[M+2]⁺ ratio (Br confirmed)
NMR Signal at 4.1 ppm
–CHBr– (deshielded, 1H, quintet)
IR Absorption
2800–3000 cm⁻¹ (Csp³–H only)
The convergence of all three analytical methods unambiguously identifies Sample A as 2-bromobutane. The IR confirmed exclusively Csp³–H bonds; NMR mapped four distinct hydrogen environments consistent with a four-carbon chain bearing an electronegative substituent on carbon 2; GC-MS confirmed the presence of bromine via the 1:1 isotope ratio and the stable C₄H₉⁺ base peak formed upon loss of bromine. These analytical techniques are indispensable in chemical and pharmaceutical discovery research.
References
- Department of Chemistry, Auburn University. (2025). NMR Instrumentation. In Organic Chemistry II Lab Manual (2025 ed., pp. 85–107). Hayden-McNeil.
- Gordon, C. Chapter 14: IR and Mass Spec. CHEM 2080: Organic Chemistry II, Auburn University, Auburn, AL, January 2026.
- Gordon, C. Chapter 15: NMR. CHEM 2080: Organic Chemistry II, Auburn University, Auburn, AL, February 2026.
- Gas Chromatography-Mass Spectrometry. American Chemical Society. https://www.acs.org/education/whatischemistry/landmarks/gas-chromatography-mass-spectrometry.html (accessed 2024-05-04).
- Introduction to Fourier Transform Infrared Spectrometry. ThermoNicolet. https://www.chem.uci.edu/~dmitryf/manuals/Fundamentals/FTIR%20principles.pdf (accessed 2024-05-04).