lecture 4 - Organic Chemistry I 310/318M Pre-Health...

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Unformatted text preview: Organic Chemistry I 310/318M Pre-Health Professionals Unique numbers: 54410, 54435, 54440, 54445, and 54655 Prof. Jonathan L. Sessler Lecture 4 Apparent Exceptions to the Octet Rule Molecules that contain atoms of Group 3A elements, particularly boron and aluminum. : : : F B F F Cl Al Cl Cl 6 electrons in the valence shells of boron and aluminum Boron trifluoride Aluminum chloride : : : : : : : : : : : : : : : Apparent Exceptions to the Octet Rule These often react as so-called Lewis acids to grab, or accept, electrons. A classic example: B H H H O CH 2 CH 3 CH 2 CH 3 B H H H O CH 2 CH 3 CH 2 CH 3 Apparent Exceptions to the Octet Rule Atoms of third-period elements, such as S and P, are often drawn with more bonds than allowed by the octet rule. The P in trimethylphosphine obeys the octet rule by having three bonds and one unshared pair. A common depiction of phosphoric acid, however, has five bonds to P, which is explained by invoking the use of 3 d orbitals to accommodate the additional bonds. : : Phosphorus pentachloride Phosphoric acid P Cl Cl Cl Cl Cl CH 3-P-CH 3 CH 3 Trimethyl- phosphine H-O-P-O-H O O-H : : : : : : : : : : : : : : : : : : : : : Apparent Exceptions to the Octet Rule However, the use of 3 d orbitals for bonding is in debate. An alternative representation that gives P in phosphoric acid an octet has four bonds and a positive formal charge on P. The oxygen involved in the double bond of the alternative depiction has one bond and a negative formal charge. P O OH HO OH P O OH HO OH formal charges Apparent Exceptions to the Octet Rule Sulfur, another third-period element, is commonly depicted with varying numbers of bonds. In each of the alternative structures sulfur obeys the octet rule, and has one or more positive formal charges. S S O H H CH 3 H 3 C S O CH 3 H 3 C S O O HO OH S O O HO OH Hydrogen sulfide Dimethyl sulfoxide +2 formal charges formal charges : : : : : : : : : : : : : : : : : : : : : : : : : : : Functional Groups - Again Functional group: Functional group: An atom or group of atoms within a molecule that shows a characteristic set of physical and chemical properties. Functional groups are important for three reason; they are: 1. the units by which we divide organic compounds into classes. 2. the sites of characteristic chemical reactions. 3. the basis for naming organic compounds. Actually, we are going to do this in abbreviated form and at lightning speed. We expect you have already learned this material on your own. If not, use these notes and the handout from the syllabus to do so NOW. Functional Groups Alcohols Contain an -OH ( hydroxyl hydroxyl ) group bonded to a tetrahedral carbon atom....
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lecture 4 - Organic Chemistry I 310/318M Pre-Health...

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