Question

We performed an aldol condensation reaction using 4-bromoacetophenone and benzaldehyde. How can I prove this with the IR spectra we obtained? What are the stretches I should note to prove that we obtained (E)-1-(4-bromophenyl)-3-phenylprop-2-en-1-one as our product and benzaldehyde as our aldehyde?

85 80- 75 70 65 60 8 55 $ 50 D 45 40 35 30 25 20 15 ?27 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-1) Product fram aldcl rn Date: Thu Jul 26 15:59:04 2018 (GMT-07:00Thu Jul 26 15:57:43 2018 (GMT-07:00) Scans: 8 Resolution: 2.000

Answer 1

IR peaks: 3063: sp2 C-H stretching 2737: aldehyde C-H stretching 1696: aldehyde carbonyl C = O stretching 1596: aromatic C = C stretching 827: mono substituted benzene ring IR peaks: for product 3056: sp2 C-H stretching No aldehyde C-H stretching present ...(1) 1656: carbonyl C = O stretching, very strong and extended conjugation 1600: aromatic C = C stretching ...(2) 994 and 759: di substituted benzene ring 727: C-Br ...(3) 825: mono substituted benzene ring As per above comparison we can say that three is 1) aldehyde carbonyl absent it the product, 2) There is a peak related to extra C = C, 3) new C-X (halogen) bond peak is present. Hence we can say that the product is formed. Presence of broad peak at 3374 may be due to water, as product might have not dried property!

Please understand that IR is a very crude way to characterize the product, however, it gives essential preliminary information that product may have been formed. It is always good to go for NMR characterization as well!