How answers are graded

The checker compares molecules, not pictures. It converts your drawing into a chemical graph (atoms, bonds, charges and, when an exercise asks for it, stereochemistry) and compares that with the answer using the open-source cheminformatics toolkit RDKit. No AI is involved and the same drawing always gets the same result.

What counts as the same structure

• Position, rotation and shape don't matter. A chain drawn left-to-right, upside-down, or numbered from the other end is the same molecule. Both drawings below are accepted for 2-methylbutane.

• Hydrogens can be drawn or left out. Hydrogens on carbon are implied; drawing them explicitly gives the same molecule.
• Benzene rings can be drawn with either arrangement of alternating double bonds, or with aromatic bonds. All are treated as the same aromatic ring.
• Nitro groups drawn as N(=O)=O or as the charge-separated N⁺(=O)O⁻ are the same species and both are accepted.

What counts as different

• Different connectivity or bond orders, even with the same molecular formula (constitutional isomers).
• Different charges or protonation states. A carboxylate ion is not a carboxylic acid, and an ammonium ion is not an amine.
• Tautomers. An enol is not the corresponding ketone.
• More than one molecule on the canvas. Stray atoms count as a separate piece; delete them before checking.

Stereochemistry

Every exercise states whether stereochemistry is assessed:

• Most exercises use compounds with no stereocentres or E/Z double bonds, so there is nothing to assess. A wedge drawn on a carbon that is not a stereocentre is simply ignored.
• Stereochemistry exercises say so above the editor. There, wedge and hash bonds are read as the absolute configuration, and the geometry of a double bond comes from how you draw it. If the connectivity is right but the configuration is not, the feedback says whether the configuration is missing, whether you drew the mirror image, or whether the E/Z geometry is wrong.

Drawings the checker won't grade

These don't count as an attempt. The feedback explains what to change:

• An empty canvas.
• An atom with too many bonds, such as a carbon with five; the atom is selected in the editor.
• Aromatic bonds in a ring that cannot be aromatic.
• Editor features outside the scope of these exercises: generic atoms (A, Q, R-groups), atom lists, isotopes, radicals, query bonds, and reaction arrows.

How feedback is written

Feedback only states things the checker can establish with certainty from the two structures: for example, that your structure has 4 carbon atoms instead of 5, that it has the right formula but different connectivity, that it contains a ketone where the name describes an aldehyde, or that the carbon skeleton and groups match but something is attached at a different position. Many exercises also recognise specific common mistakes, such as drawing pentane instead of 2-methylbutane, and explain them.

When the difference can't be pinned down reliably, the feedback says so in general terms and lets you see the correct structure. It never guesses what you intended and never shows a "similarity score".

How the exercises are checked

Every exercise name is checked against its answer structure by two independent sources before it is published: OPSIN, an open-source program that interprets systematic chemical names, and the PubChem database. An exercise is published only when both agree with the answer. The checker itself is covered by an automated test suite, including a cross-check of thousands of structure pairs against the IUPAC InChI identifier.

If you think an answer or explanation is wrong, please tell us. Include the compound name and what you drew.

Technical details

Editors: the site's own dot editor, and Ketcher 3.18.0 (EPAM) as the advanced editor. Grading: RDKit.js 2026.9.1. The drawing is exported as an MDL molfile, sanitised by RDKit (valence check, aromaticity perception, explicit hydrogens removed) and compared as canonical SMILES, with or without stereochemistry according to the exercise. Everything runs in your browser; your drawings are not sent anywhere.