DAMIETTA UNIVERSITY CHEM-103: BASIC ORGANIC CHEMISTRY LECTURES 11-12 Dr Ali El-Agamey Organic Chemistry, 7 th Edition L. G. Wade, Jr. Amines 2010, Prentice Hall
Reactions N,N-Disubstituted amides 2 o amine N-substituted amides 3 o amine No reaction Organic Chemistry, 7 th Edition L. G. Wade, Jr. Aromatic Compounds 2010, Prentice Hall
Resonance Forms Remember that individual resonance forms do not exist. The molecule does not "resonate" between these structures. It is a hybrid with some characteristics of both (an analogy is a mule). Both C-O bond lengths are the same (136 pm) and in between a normal C=O (123 pm) and C-O (143 pm). p163d Resonance Forms Spreading the charges over two or more atoms stabilize the ion. The more resonance forms, the greater the delocalization (spread out) of electrons, the more stable the compound will be. Example: allyl and heptenyl cations. ٦
Resonance Forms With The Same Energy The stabilizing effect is particularly marked when resonance forms have the same energy. Common Names of Benzene Derivatives ٨
Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C 6 H 6. He named it benzin. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. ٩ Early Suggestions for the Structure of Benzene
Kekulé Structure Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested (1859). Failed to explain existence of only one isomer of 1,2-dichlorobenzene. H H C C H C C H C C H H ١١ Kekulé Structure Kekule suggested (incorrectly) that a fast equilibrium interconverts the two isomers of 1,2-dichlorobenzene.
Resonance Structures of Benzene Benzene is actually a resonance hybrid between the two Kekulé structures. The C C bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths (bond order 1.5). Benzene's resonance can be represented by drawing a circle inside the six-membered ring as a combined representation. ١٣ Structure of Benzene Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. The six pi electrons are delocalized over the six carbons. ١٤
Molar Heats of Hydrogenation ١٥ Resonance Energy Benzene does not have the predicted heat of hydrogenation of -359 kj/mol. The observed heat of hydrogenation is -208 kj/mol, a difference of 151 kj. This difference between the predicted and the observed value is called the resonance energy. ١٦
Aromatic Requirements Structure must be cyclic with conjugated pi bonds. Each atom in the ring must have an unhybridized p orbital (sp 2 or sp). The p orbitals must overlap continuously around the ring. Structure must be planar (or close to planar for effective overlap to occur) Delocalization of the pi electrons over the ring must lower the electronic energy. ١٧ Hückel s Rule Once the aromatic criteria is met, Huckel s rule applies. If the number of pi electrons is (4N + 2) the compound is aromatic (where N is an integer) If the number of pi electrons is (4N) the compound is antiaromatic. ١٨
Anti- and Nonaromatic Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but electron delocalization increases its electronic energy. Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. ١٩ Which of the following is an aromatic compound? cyclopentadienyl anion 6 pi electrons Aromatic cyclopentadienyl cation 4 pi electrons Antiaromatic cyclopentadiene Nonaromatic
Which of the following is an aromatic compound? Non-aromatic There is an sp 3 carbon in the ring, delocalization will not be complete. Aromatic All carbons are sp 2 hybridized and it obeys Huckel s rule. ٢١ Cycloheptatrienyl cation (tropylium ion) 6 pi electrons Aromatic
Unusual Addition of Bromine to Benzene When bromine adds to benzene, a catalyst such as FeBr 3 is needed. The reaction that occurs is the substitution of a hydrogen by bromine. Addition of Br 2 to the double bond is not observed. ٢٣ Electrophilic Aromatic Substitution (5) Friedel-Crafts Acylation (1) Nitration (4) Friedel-Crafts Alkylation (3) Halogenation (2) Sulfonation
Homework: Complete the following scheme (1) (2) (3) (6) (7) (10) (11) (4) (5) (9) (8) (12) ٢٥