University:
Swansea University
Course:
CH302 | Organic Chemistry
Academic year:

2019
Views:

424

Pages:

10
Author:

Charlee Buchanan

Nomenclature ortho (1,2) - next to each other in a benzene ring meta (1,3) - separated by one carbon in a benzene ring para (1,4) - across from each other in a benzene ring Hückel's Rule For a molecule to be aromatic, it must: monocyclic planar (flat) fully conjugated must have (4n+2) π electrons where n = 0, 1, 2, 3 and so on... 2, 6, 10, 14, 18, 22 etc. anti-aromatic meets any 1st 3 criteria, non-aromatic - doesn't meet one or more of the first 3 criteria downfield (deshielded) due to electronegative groups causing deshielding, causing a higher chemical shift. deshielded = downfield Vicinal coupling - coupling between hydrogens on adjacent carbons that are separated by three σ bonds. proton that undergo rapid chemical exchange (those that are attached to N or O) will exchange with H from D₂O H-C-C-O-H + D₂O --> H-C-C-O-D + HOD H-C-C-H if placed in D₂O, the ¹H-NMR signal from those protons will disappear, which makes it an easy method for identifying ¹H-NMR signals produced by exchangeable protons. Rank the following dienes in order of increasing reactivity Based on the provided notes, the reactivity of a diene is influenced by the attached electron-donating or electron-withdrawing groups. Electron-donating groups increase the reactivity of the diene, making it more reactive. Electron-withdrawing groups decrease its reactivity, making it less reactive. The four dienes are ranked as follows, from least reactive to most reactive: Diene 3 (least reactive): Contains a strong electron-withdrawing group (EWG), specifically a -CHO group, making it the least reactive. Diene 2: Contains no EWG or EDG (electron-donating group), so its reactivity is moderate. Diene 1: Contains a weak EDG, a -CH₃ group, making it more reactive than Diene 2. Diene 4 (most reactive): Contains a strong EDG, a -OCH₃ group, making it the most reactive diene. Therefore, the order of increasing reactivity is 3 < 2 < 1 < 4. Reactivity of Dienophiles The reactivity of a dienophile is the opposite of a diene. Electron-withdrawing groups on the dienophile increase its reactivity, while electron-donating groups decrease it. Good dienophile = contains a strong electron-withdrawing group. Bad dienophile = contains an electron-donating group. Endo vs. Exo product The notes show an example of a Diels-Alder reaction forming an exo product and an endo product. The endo product is favored because the EWG (in this case, -CHO) is positioned down in the axial position, which is more sterically favorable. Other factors affecting diene reactivity The notes also mention that diene reactivity is affected by its conformation: A s-cis conformation is more reactive, while an s-trans is less reactive. A diene in an s-trans conformation must rotate to an s-cis conformation to react. The notes show two examples where the diene is locked in an s-cis or s-trans conformation. The locked s-cis diene is more reactive due to its pre-existing favorable conformation.