The synthetic equivalent

The synthetic equivalent is the real reagent used in the laboratory to behave like the synthon. The methyl anion does not exist as such, but methylmagnesium bromide reacts as if it were.

Synthon	Synthetic equivalent	Reaction
CH₃⁻	CH₃MgBr, CH₃Li	addition to carbonyls
R₂C⁺–OH	R₂C=O	nucleophilic addition
CH₃C⁺=O	CH₃COCl + AlCl₃	Friedel–Crafts
⁻CN	NaCN, KCN	SN2
⁺COOH	CO₂	carboxylation of organometallics
⁻CH₂COOH	CH₂(CO₂Et)₂ + base	malonic ester synthesis
⁺NH₂ (on an aryl)	NO₂⁺ and reduction	nitration
RC⁻=O (acyl anion)	2-lithio-1,3-dithiane	umpolung

For 1-phenylethanol, the equivalents of the synthons of disconnection a are benzaldehyde and methylmagnesium bromide; those of b, phenylmagnesium bromide and acetaldehyde. The two routes are equally short, and the choice would depend on the price and availability of the reagents. Review the preparation of alcohols with organometallics.

Benzaldehyde with methylmagnesium bromide, or phenylmagnesium bromide with acetaldehyde, give 1-phenylethanol

The same synthon may have several equivalents, and sometimes the equivalent does not resemble it: the ⁺NH₂ synthon required by the disconnection of an aniline has no direct equivalent, and nitration followed by reduction is used instead; the ⁻CH₂COOH synthon is achieved with diethyl malonate, which is then hydrolysed and decarboxylated. Choosing the equivalent well is the most creative part of the analysis.