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  • LECTURES 11-12

    Dr Ali El-Agamey




    2010, Prentice Hall

    Organic Chemistry, 7th Edition L. G. Wade, Jr.


  • N-substituted amides N,N-Disubstituted amides

    No reaction

    2o amine 3o amine


    Aromatic Compounds

    Organic Chemistry, 7th Edition L. G. Wade, Jr.

    2010, Prentice Hall

  • p163d

     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).

    Resonance Forms


    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.

  • • The stabilizing effect is particularly marked when resonance forms have the same energy.

    Resonance Forms With The Same Energy


    Common Names of Benzene Derivatives

  • ٩

    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 C6H6. He named it benzin.

    • Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic.

    Early Suggestions for the Structure of Benzene

  • ١١

    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.

    C C

    C C





    H H



    • Kekule suggested (incorrectly) that a fast equilibrium interconverts the two isomers of 1,2-dichlorobenzene.

    Kekulé Structure

  • ١٣

    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.


    Structure of Benzene

    • Each sp2 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.

  • ١٥

    Molar Heats of Hydrogenation


    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.

  • ١٧

    Aromatic Requirements

    • Structure must be cyclic with conjugated pi bonds.

    • Each atom in the ring must have an unhybridized p orbital (sp2 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.


    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.

  • ١٩

    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.


    6 pi electrons

    cyclopentadienyl cation

    4 pi electrons


    cyclopentadienyl anion cyclopentadiene


    Which of the following is an aromatic compound?

  • ٢١

    Which of the following is an aromatic compound?

    Non-aromatic Aromatic

    There is an sp3 carbon in the ring, delocalization will not be complete.

    All carbons are sp2

    hybridized and it obeys Huckel’s rule.

    Cycloheptatrienyl cation (tropylium ion)


    6 pi electrons

  • ٢٣

    Unusual Addition of Bromine to Benzene

    • When bromine adds to benzene, a catalyst such as FeBr3 is needed.

    • The reaction that occurs is the substitution of a hydrogen by bromine.

    • Addition of Br2 to the double bond is not observed.

    Electrophilic Aromatic Substitution

    (1) Nitration

    (2) Sulfonation

    (3) Halogenation

    (4) Friedel-Crafts Alkylation

    (5) Friedel-Crafts Acylation

  • ٢٥

    Homework: Complete the following scheme

    (1) (2)


    (4) (5)

    (6) (7)


    (10) (11)



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