4.9.2Unit 4. Rates, Equilibria and Further Organic Chemistry
Carbon-13 NMR Spectroscopy — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 52 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 4Principles
Carbon-13 nuclei absorb radio-frequency radiation in a strong magnetic field.
Nuclei
- Nuclei Only the ¹³C isotope, about 1% of natural carbon, is detected, because ¹²C has no nuclear spin.
Chemical shift
- Chemical shift Peaks appear at positions, measured in ppm, that depend on the electronic environment of each carbon atom.
Reference
- Reference Tetramethylsilane (TMS) is the standard, defined as 0 ppm, because all its carbons are equivalent and it is inert.
One peak per environment
- One peak per environment Carbons in the same chemical environment, because of symmetry, give a single peak.
Section 2 of 4Reading a Spectrum
The number of peaks shows how many different carbon environments are present.
1
Count the peaks; each represents a set of equivalent carbon atoms in the molecule.
2
Compare each chemical shift with a data table to suggest the type of carbon, such as C–C, C–O or C=O.
3
Use symmetry to decide which structure would produce exactly that number of peaks.
4
Check the proposed structure against other information such as the molecular formula and mass spectrum.
Section 3 of 4Chemical Shift Ranges
Approximate ranges are given in ppm relative to TMS.
Chemical Shift Ranges
Saturated carbon
- Saturated carbon Carbon in C–C bonds appears at about 5 to 40 ppm, at the right-hand side of the spectrum.
Carbon bonded to oxygen
- Carbon bonded to oxygen Carbons in C–O bonds, as in alcohols and ethers, appear at about 50 to 90 ppm.
Alkene and aromatic
- Alkene and aromatic Carbons in C=C and in benzene rings appear at about 100 to 160 ppm.
Carbonyl carbon
- Carbonyl carbon C=O carbons are furthest left, at about 160 to 185 ppm in acids and esters and 190 to 220 ppm in aldehydes and ketones.
| Compound | Number of C environments | Number of peaks |
|---|---|---|
| Propanone | 2 | 2 |
| Butanone | 4 | 4 |
| Pentan-3-one | 3 | 3 |
| Benzene | 1 | 1 |
Section 4 of 4Exam Checklist & Tips
4.9.2 – Key Points for the Exam
- Chemical shift Position of a peak relative to TMS, in ppm.
- TMS Tetramethylsilane, used as the reference at 0 ppm.
- Environment The set of atoms surrounding a particular carbon.
- Equivalent carbons Carbons in identical environments that give one peak.
Common Mistakes
- Counting the number of carbon atoms rather than the number of different environments.
- Using TMS as part of the sample molecule.
- Forgetting that symmetrical molecules give fewer peaks than they have carbons.
- Confusing the shift ranges for C=O in ketones and in esters.
Last updated:
\n