Cinnamide/cinnamic acid

The driving force for growth of the crystal in the b direction is the energy released by formation of the NH O bonds of the ribbon motif. E-Cinnamic acid in the stable synplanar conformation 2a can replace a E-cinnamide molecule at the end of the ribbon however, at the site of the additive, the attractive NH O bond (- 6 kcal/mol) is replaced by repulsion between the adjacent oxygen lone-pair electrons of the bound additive molecule and of the oncoming cinnamide molecule (1-2 kcal/mol), which leads to an overall loss in energy of 7-8 kcal/mol at the site of the additive (Scheme 6 on page 16). 

Figure 4. Measured habits of crystals of -cinnamide (a) pure (b)- d) grown in the presence of (b) cinnamic acid, (c) p-chloro- or A-methyl-E-cinnamide, (d) a- or P-chloro-Z-cinnamide.

As predicted, the presence of cinnamic acid in solution caused cinnamide to crystallize as flat prisms with prominent 011 faces (Figure 4b). The crystal morphology was modified along c by the use of amide additives that contain a bulky Cl substituent at the a- or P-carbons of cinnamide. When replacing a substrate molecule, these additive molecules interfere with the deposition of the next Oil layers (Figure 3), yielding 011 platelike crystals (Figure 4d). 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

Figure 39. Four ribbons of cinnamide (phenyl = ) molecules. Ribbons / and l, d and d are related by translation. Ribbons d and /, d and l make plane-to-plane contacts of 4 A across centers of symmetry. Ribbon / is above d, and ribbon d is below l. Cinnamic acid molecules (filled circles) have been introduced into the structure in the allowed sites, assuming the crystal grows from the center in the two opposite directions +b and —b. The dimers obtained at the two opposite sides are enantiomeric.

The reaction is usually carried out be heating equimolar quantities of the aldehyde and salt with excess of the anhydride for 8 hours at 170-180°. Lower temperatures are often employed when potassium acetate or trialkylamines are used as condensing agents. Continuous removal of acetic acid during the reaction was found to have no effect on the yield of cinnamic acid. Substitution of diacetimide for acetic anhydride gives cinnamide (77%). ... 

FIGURE 17.15. The effect of absorption of an impurity on the surface of a crystal. In this Ccise a carboxylic acid (cinnamic acid) is bound to the crystal face as an impurity on crystals of an amide (cinnamide). As a result, crystal growth in the b direction is stopped (compare with Figure 2.8, Chapter 2, for the consequences of this). (Reproduced from Reference 69 with the permission of G. R. Desiraju and Elsevier Science Publishers BV, Academic Publishing Division, Amsterdam, The Netherlands.)... 

---