Photochemical [2+2] cycloaddition of cinnamic acid to obtain truxillic acid


To perform a photochemical [2+2] cycloaddition reaction with the use of sunlight to obtain truxillic acid.

photochemical [2+2] cycloaddition of cinnamic acid to yield truxillic acid
Photochemical [2+2] cycloaddition of cinnamic acid to yield truxillic acid


Cycloaddition reactions are processes having high atom economy, since all atoms of the starting materials are incorporated in the final molecule. The [2+2] cycloaddition is a reaction type that fails to react conventional alkenes under thermal conditions. However, a synthetic tool may be useful under photochemical conditions. In the case of trans-cinnamic acid, a total of eleven stereoisomers can form: six dimers of the head-head type (truxinic acids) and five dimers of the head-tail type (truxillic acids). Both families of compounds are natural products found, for example, in coca leaves. Dimerization of trans-cinnamic acid in solution does not occur but, under photochemical conditions described in this experiment does take place and surprisingly produces a single stereoisomer.

Experimental procedure

Weigh 1.5 g of trans-cinnamic acid, and place in a 100 ml Erlenmeyer. Add approximately 2 ml of THF. Heat (water bath), mixing until the cinnamic acid is dissolved. When it has dissolved, remove the Erlenmeyer flask from the heat and gently stir by hand to form on the flask wall a film with the solid as homogeneous as possible. Otherwise, repeat the operation to achieve the goal. Allow the contents of the flask to dry completely, leaving the flask at room temperature for 30 min. It is then covered with a septum and is labeled with the name and date. Then the flask is left exposed to sunlight for 15 days on a window sill (facing south) or under the lamplight, rotating the flask at 7 days.
After this time, add 15 ml of toluene and gently heat (water bath at 40 ºC) to dissolve the remaining cinnamic acid. Separate the solid by vacuum filtration, and wash with 10 ml of additional toluene. Pool the toluene fractions, and transfer to a tared round-bottom flask. Remove the solvent under reduced pressure (rotary evaporator), weigh the product, and the calculate the yield. Determine the m.p. (286 ºC).

Physico-chemical properties

This table collects data for the molecular weight (Mw), melting point (M.p.) boiling point (B.p.) and density of the reactives and compounds used in this laboratory experiment.

Name Mw (g/mol) M.p. (ºC) B.p. (ºC) Density (g/ml)
Cinnamic acid 148.16 132-135 300 1.248
Toluene 92.14 -93 110.6 0.867
Truxillic acid 296.32 - 470.46 1.324
p-Toluenesulfonic acid 172.2 106-107 - 1.240

GHS pictograms

Hazard pictograms form part of the international Globally Harmonized System of Classification and Labelling of Chemicals (GHS) and are collected in the followinf Table for the chemical compounds used in this experiment.

Name GHS
Cinnamic acid ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
Toluene ghs02  Danger Warning Flammable pictogram ghs08  Danger Warning Systemic health hazards pictogram ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
Truxillic acid See MSDS
p-Toluenesulfonic acid ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram ghs05  Danger Warning Corrosive cat. 1 pictogram

International Chemical Identifier

The IUPAC InChI key identifiers for the main compounds used in this experiment are provided to facilitate the nomenclature and formulation of chemical compounds and the search for information on the Internet for these compounds.