Difficulty: Easy
Correct Answer: Multiple peaks equal to the number of hydrogens on surrounding atoms, plus one
Explanation:
Introduction / Context:
Proton nuclear magnetic resonance (1H NMR) is a core analytical tool in organic chemistry. One of its most informative features is scalar spin–spin coupling, also called J-coupling, which splits a proton signal into a predictable multiplet. Understanding the “n + 1” rule allows quick inference of how many neighboring, nonequivalent protons are adjacent to a given proton, aiding structure elucidation and verification of synthetic products.
Given Data / Assumptions:
Concept / Approach:
Scalar coupling splits a resonance into a multiplet with a number of lines defined by n + 1, where n is the count of equivalent neighboring hydrogens on adjacent carbons. For example, a methylene next to a methyl group (CH2–CH3) usually appears as a quartet (3 + 1) while the methyl appears as a triplet (2 + 1). The spacing between the lines is the coupling constant J, measured in hertz, and is characteristic of the geometric and electronic environment (e.g., larger for trans olefinic couplings than for cis or geminal in many cases).
Step-by-Step Solution:
Verification / Alternative check:
Compare predicted patterns with experimental spectra of standard compounds (e.g., ethyl acetate), where the ethyl group provides classic triplet–quartet patterns. Matching calculated multiplets and J spacings validates the interpretation.
Why Other Options Are Wrong:
Common Pitfalls:
Forgetting that chemically equivalent neighbors count as a single “set,” misreading second-order effects (overlap or complex patterns), and ignoring exchangeable protons (e.g., OH) that may show broadened singlets with suppressed coupling.
Final Answer:
Multiple peaks equal to the number of hydrogens on surrounding atoms, plus one
Discussion & Comments