¹³C Chemical Shifts

Introduction

Knowing how many carbon environments a molecule has is only half of ¹³C NMR. The other half is where each line appears. Carbon chemical shifts cover a range of more than 200 ppm, and different kinds of carbon fall into well-separated regions. With a short table in mind, you can pick out a carbonyl carbon, an aromatic ring or a carbon attached to oxygen at a glance.

Core explanation

Two main factors. Carbon shifts depend mostly on the electronegativity of attached atoms and on the hybridisation of the carbon.

Electronegativity. An electronegative atom such as oxygen, nitrogen or a halogen withdraws electron density from the carbon it is attached to, deshielding it and moving its signal to higher δ. An alkyl carbon near δ 10–30 moves to about δ 50–70 when bonded to oxygen. The effect falls away quickly along the chain.

Hybridisation. sp² carbons in alkenes, arenes and carbonyl groups resonate far downfield, between about δ 110 and 220. The C=O carbon is the most deshielded of all because it is sp² and bonded to a highly electronegative oxygen, and the polar C=O bond leaves carbon electron-poor. sp carbons in alkynes lie between these extremes, at about δ 65–90.

Typical ¹³C shift ranges:

Carbon environment δ / ppm --- --- C–C, alkyl (CH₃, CH₂, CH) 5–40 C–Cl or C–Br 25–60 C–N (amines) 30–65 C–O (alcohols, ethers, esters) 50–90 C≡C (alkynes) 65–90 C=C (alkenes) 110–150 Aromatic carbons 110–160 C≡N (nitriles) 115–125 C=O (acids, esters, amides) 160–185 C=O (aldehydes, ketones) 190–220

Carbonyl subdivisions. Aldehyde and ketone carbons appear at δ 190–220. In acids, esters and amides the carbonyl carbon carries a second heteroatom whose lone pair donates electron density to it through resonance, partly shielding it and moving it upfield to δ 160–185. This distinction is often decisive in telling a ketone from an ester.

Worked assignment: ethyl ethanoate. CH₃COOCH₂CH₃ gives four lines: δ 171 (C=O, ester), δ 60 (OCH₂), δ 21 (CH₃C=O) and δ 14 (CH₂CH₃). The ranking follows electronegativity and hybridisation exactly.

Overlapping regions. Alkene and aromatic ranges overlap, and so do nitrile and aromatic ranges. Use the molecular formula, the number of lines and IR evidence (for example a C≡N stretch near 2250 cm⁻¹) to decide between them.

Step-by-step reasoning

To assign a ¹³C spectrum:

1. Count the lines and compare with the predicted number of environments. 2. Look above δ 160 for carbonyl carbons; use 190–220 versus 160–185 to separate ketones and aldehydes from acids and esters. 3. Look in δ 110–160 for alkene and aromatic carbons. 4. Look in δ 50–90 for carbons bonded to oxygen. 5. Assign the remaining lines below δ 40 to alkyl carbons.

Visual explanation

The simulation shades the δ axis into coloured bands: grey for alkyl, blue for C–O, green for alkenes and aromatics, orange for carbonyl carbons. Dropping butanone onto the axis places its four lines at about δ 209, 37, 29 and 8, with the C=O line alone in the orange band at the far left.

Real-world analogy

A ¹³C spectrum is like a street of houses sorted by rent. Plain alkyl carbons live in the cheap end at low δ; carbons with an oxygen neighbour occupy the middle; alkene and aromatic carbons live further along, and carbonyl carbons have the grandest houses at the far end of the street.

Real-world example

Food scientists authenticate olive oil using ¹³C NMR. The carbonyl carbons of the fatty-acid esters and the alkene carbons of the unsaturated chains give distinctive lines near δ 173 and δ 128–132. Their pattern reveals the proportions of different fatty acids and can expose dilution with cheaper oils.

Why?

Why is an ester carbonyl carbon less deshielded than a ketone carbonyl carbon? In an ester the lone pair on the single-bonded oxygen is delocalised into the C=O group. This donation increases the electron density around the carbonyl carbon, shielding it slightly and moving its signal from about δ 205 to about δ 170.

Common misconception

"¹³C and ¹H shift tables are interchangeable if you multiply by a constant." The trends are similar, but the ranges differ and some effects are much stronger for carbon — for instance hybridisation and the carbonyl oxygen. Use the dedicated ¹³C table.

Worked example

Question: A compound C₃H₆O gives three ¹³C lines at δ 202, 37 and 6. Is it propanal or propanone?

Reasoning: Propanone, CH₃COCH₃, is symmetrical and would give only two lines. Three lines match propanal, CH₃CH₂CHO. The line at δ 202 is the aldehyde carbonyl, δ 37 is the CH₂ next to C=O and δ 6 is the CH₃.

Answer: Propanal.

Quick check

1. In which δ range would you expect the carbonyl carbon of ethanoic acid to appear? Answer: Between about δ 170 and 185, the range for carboxylic acid carbonyl carbons.

Exam focus

Use the data sheet provided but know the broad regions: alkyl below 40, C–O at 50–90, alkene and aromatic at 110–160, carbonyl above 160. Always link each assigned line to a specific carbon and give the reason, such as "bonded to electronegative oxygen".

Advanced insight

Substituent effects on benzene rings are roughly additive. An electron-donating group such as –OH shields the ortho and para carbons (moving them upfield to about δ 115–121 in phenol), while the ipso carbon bearing the oxygen moves downfield to about δ 155. These predictable shifts allow the substitution pattern of a ring to be deduced from its ¹³C spectrum.

Summary

¹³C shifts span about 0–220 ppm. Alkyl carbons appear below δ 40, carbons bonded to oxygen at δ 50–90, alkene and aromatic carbons at δ 110–160, and carbonyl carbons above δ 160, with ketones and aldehydes (190–220) further downfield than acids and esters (160–185). Electronegativity and hybridisation explain these trends.

Practice questions

1. State the typical ¹³C shift range for a carbon atom bonded to oxygen in an alcohol. Answer: About δ 50–90. 2. A ¹³C line appears at δ 128. Suggest two types of carbon that could produce it. Answer: An alkene carbon or an aromatic ring carbon. 3. Explain why the C=O carbon of propanone appears at a higher δ than its methyl carbons. Answer: The carbonyl carbon is sp² hybridised and bonded to electronegative oxygen, so it is strongly deshielded compared with the sp³ methyl carbons. 4. How would ¹³C NMR distinguish butanone from methyl propanoate? Answer: Butanone's carbonyl carbon appears near δ 209, whereas the ester carbonyl carbon appears near δ 175, and the ester also shows an OCH₃ carbon near δ 51.