Organic synthesis definitions

Planning a synthetic route

Synthetic route

A sequence of reactions that converts a starting material into a target compound, with the reagents and conditions for each step.

Reagent

The substance added to bring about a reaction, named in full or by a correct formula.

Conditions

The temperature, solvent, catalyst and method of heating a reaction needs to give the intended product.

Intermediate

A compound made in one step of a synthesis and used as the starting material for the next.

Retrosynthesis

Planning a synthesis by working backwards from the target, asking at each stage which compound could give it in a single step.

Overall yield

The yield of a route of several steps, found by multiplying the fractional yields of the individual steps together.

Atom economy

The mass of the desired product divided by the total mass of the reactants, multiplied by 100, from the balanced equation.

Hazard

A property of a substance or procedure that could cause harm, such as toxicity, flammability or corrosiveness.

Risk

The chance that a hazard causes harm as the substance is actually used, which precautions such as a fume cupboard reduce.

Racemic mixture

An equimolar mixture of the two enantiomers of a chiral compound, which has no overall effect on plane-polarised light.

Grignard reagent

An organomagnesium halide, RMgX, made from a halogenoalkane and magnesium in dry ether, whose carbon bonded to magnesium acts as a nucleophile and forms a new C–C bond.

Preparing, purifying and testing an organic solid and liquid

Heating under reflux

Heating a reaction mixture with a vertical condenser so that volatile substances condense and return to the flask, allowing prolonged heating without loss.

Distillation

Separating a liquid from a mixture by boiling it and condensing the vapour in a separate vessel, collected at its boiling point.

Recrystallisation

Purifying a solid by dissolving it in the minimum volume of hot solvent, cooling so the product crystallises while impurities stay in solution, then filtering, washing and drying.

Filtration under reduced pressure

Filtration through a Büchner funnel with a pump lowering the pressure in the flask below, which is faster and leaves a drier solid.

Melting point of a pure solid

The single temperature, reached sharply, at which a pure solid turns to liquid; impurities lower it and widen the range.

Immiscible

Describes two liquids that do not mix and form two separate layers.

Drying agent

An anhydrous salt, such as magnesium sulfate or calcium chloride, that removes water from an organic liquid by absorbing it as water of crystallisation.

Boiling temperature

The temperature at which the vapour pressure of a liquid equals atmospheric pressure; a pure liquid distils at a steady temperature matching its data-book value.

Percentage yield

The actual mass of product obtained divided by the theoretical mass from the limiting reactant, multiplied by 100.

Steam distillation

Distilling an organic liquid that is immiscible with water together with steam, so that it distils below 100 °C, well below its own boiling point.

Putting the analysis together: structure from spectra

Empirical formula

The simplest whole-number ratio of the atoms of each element in a compound.

Molecular ion

The ion formed when a molecule loses one electron, M⁺; its m/z gives the relative molecular mass.

Degree of unsaturation

The number of rings plus double bonds in a molecule, found from (2C + 2 + N − H − X) ÷ 2, where a triple bond counts as two.

Fingerprint region

The part of an infrared spectrum below 1500 cm⁻¹, whose pattern is unique to a compound and identifies it by comparison with a database.

Chemical shift

The position of an NMR peak, δ in ppm, measured relative to TMS at 0 ppm; it depends on the atom's chemical environment.

Chemical environment

A set of atoms that are bonded to identical surroundings, so they absorb at the same chemical shift and give one peak.

Integration ratio

The ratio of the areas under the peaks of a proton NMR spectrum, equal to the ratio of the numbers of hydrogens in each environment.

n + 1 rule

A proton NMR peak is split into n + 1 lines, where n is the number of hydrogens on the adjacent carbon atoms.

Equivalent atoms

Atoms that are interchanged by the symmetry of the molecule, so they share an environment and give a single NMR peak.

External calibration

Finding a concentration by measuring standards of known concentration, plotting the response against concentration, and reading the unknown from the graph.

D₂O exchange

Shaking a sample with D₂O replaces the hydrogen of O–H and N–H groups with deuterium, so their proton NMR peak disappears.