Organic structure and mechanism conventions
- Syllabus
- 9701–2028–2029
- Topic
- —
- Level
- AS
Learning objectives
—Reference—Structural, displayed and skeletal formula conventionsRepresent organic compounds using accepted structural, displayed, skeletal and partial-skeletal formula conventions; give enough detail for an unambiguous structure; use the preferred aromatic ring convention.—Reference—Organic reaction mechanism conventionsDraw organic reaction mechanisms using charges, dipoles, lone pairs of electrons and curly arrows to show electron movement and mechanism steps clearly.
Choose an accepted organic formula that fixes one connectivity
A structural formula shows atom connectivity, a displayed formula shows every bond, a skeletal formula shows the carbon framework with carbon-bound hydrogens omitted, and a partial-skeletal formula deliberately combines conventions.
Show heteroatoms and their attached hydrogens, multiple bonds, charges and functional-group position wherever needed. Use the preferred syllabus aromatic-ring convention and include enough detail to distinguish branching and positional isomers.
CH3CH(OH)CH3 and a three-carbon skeletal chain bearing OH at its middle vertex both identify propan-2-ol.
Omitting a carbon-bound hydrogen is normal in skeletal notation; omitting a heteroatom, charge or bond needed to distinguish two structures makes the answer ambiguous.
Use wedge and dash bonds to show a true mirror-image pair
Optical isomers have identical connectivity but opposite three-dimensional arrangements at a chiral centre. A solid wedge projects towards the viewer, a dashed wedge projects away, and ordinary bonds lie in the plane.
Keep the four attached group identities fixed, reflect the three-dimensional arrangement to draw the partner, and check that no rotation can superimpose the two structures.
For a tetrahedral carbon attached to H, OH, CH3 and CO2H, reverse the wedge/dash relationship to obtain the enantiomer without moving a group to a different atom.
A flat left-right redraw need not be a new optical isomer. If rotation makes the drawings coincide, they are two views of the same structure rather than an enantiomeric pair.
Start every curly arrow at the electrons that move
Show relevant charges, dipoles and lone pairs. A full curly arrow represents movement of an electron pair: it starts at a lone pair or bond and ends at the atom or bond receiving that pair.
Identify the nucleophile and electrophile, draw each connectivity-changing step, preserve atom and charge balance, show required intermediates, and check octets in the resulting structures.
In nucleophilic substitution, draw one arrow from the nucleophile lone pair to carbon and another from the carbon-leaving-group bond to the leaving group.
Curly arrows track electrons, not atoms. An arrow cannot start at a positive charge or empty space with no lone pair or bond supplying electrons.