TTh_10 - Organic Lecture Series CH 310/318 M LECTURE 10...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Organic Lecture Series CH 310/318 M LECTURE 10 Textbook Assignment: Chapter 4-Begin Today's Topics: Acidity; pKa; Mechanisms Notice & Announcements: HW 4 Posted 1 Organic Lecture Series Acids and Bases Chapter 4 2 e - withdrawing inductive effect Organic Lecture Series 4. Electron-withdrawing inductive effect Stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of negative charge in the conjugate base. 3 e - withdrawing inductive effect Organic Lecture Series the operation of the inductive effect in the acidity of halogen substituted carboxylic acids is also observed: 4 Organic Lecture Series Hybridization (of the anion) 5. Hybridization For anions differing only in the hybridization of the charged atom, the greater the percentage s character to the hybrid orbital of the charged atom, the more stable the anion. 5 Organic Lecture Series In summary: 1. Negative charge on the more electronegative atom. 2. Negative charge on a larger atom. 3. Delocalization of the negative charge through resonance. 4. Delocalization of the negative charge onto electron-withdrawing groups by the inductive effect. 5. Have the negative charge in an orbital with more s character. 6 Organic Lecture Series Lewis Acids and Bases Lewis acid: Any molecule of ion that can form a new covalent bond by accepting a pair of electrons. Lewis base: Any molecule of ion that can form a new covalent bond by donating a pair of electrons. 7 Organic Lecture Series Lewis Acids and Bases Common Example: 8 Organic Lecture Series Alkenes: Structure and Nomenclature Chapter 5 9 Organic Lecture Series Unsaturated Hydrocarbons Unsaturated hydrocarbon: Contains one or more multiple bonds. Alkene: Contains a carbon-carbon double bond and has the general formula CnH2n. 10 Organic Lecture Series Unsaturated Hydrocarbons Alkyne: Contains a carbon-carbon triple bond and has the general formula CnH2n-2. 11 Organic Lecture Series Unsaturated Hydrocarbons Arenes: Benzene and its derivatives (Ch 21-22) The phenyl group is not reactive under any of the conditions described in Ch 6-20. 12 Organic Lecture Series Structure of Alkenes A double bond consists of one sigma bond formed by the overlap of sp2 hybrid orbitals and one pi bond formed by the overlap of parallel 2p orbitals. the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120 . 13 Organic Lecture Series Structure of Alkenes it takes approximately 264 kJ (63 kcal)/mol to break the pi bond in ethylene; that is, to rotate one carbon by 90with respect to the other so that there is no overlap between 2p orbitals on adjacent carbons. 14 Organic Lecture Series Cis,Trans Isomerism in Alkenes Cis,trans isomers: Isomers that have the same connectivity but a different arrangement of their atoms in space due to the presence of either a ring (Chapter 2) or a carbon-carbon double bond. 15 Organic Lecture Series Index of Hydrogen Deficiency Index of hydrogen deficiency (IHD): The sum of the number of rings and pi bonds in a molecule. To determine IHD, compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds. the molecular formula of the reference hydrocarbon is CnH2n+2. 16 Organic Lecture Series Index of Hydrogen Deficiency 1. for each atom of a Group 7 element (F, Cl, Br, I), add one H. 2. no correction is necessary for the addition of atoms of Group 6 elements (O,S) to the reference hydrocarbon. 3. for each atom of a Group 5 element (N, P), subtract one hydrogen. 17 Organic Lecture Series IUPAC Nomenclature 1. Number the longest chain of carbon atoms that contains the double bond in the direction that gives the carbons of the double bond the lowest numbers. 2. Locate the double bond by the number of its first carbon. 3. Name substituents. 4. Number the carbons, locate and name substituents, locate the double bond, and name the main chain. 18 Organic Lecture Series Common Names Despite the precision and universal acceptance of IUPAC nomenclature, some alkenes, particularly low-molecular-weight alkenes, are known almost exclusively by their common names. 19 Organic Lecture Series Common Names the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups: 20 Organic Lecture Series The Cis,Trans System Configuration is determined by the orientation of atoms of the main chain. 21 Organic Lecture Series The E,Z System Uses priority rules (Chapter 3). If groups of higher priority are on the same side, the configuration is Z (German, zusammen). If groups of higher priority are on opposite sides, the configuration is E (German, entgegen). 22 Organic Lecture Series The Cis,Trans Isomerization Cycloalkenes In small-ring cycloalkenes, the configuration of the double bond is cis. These rings are not large enough to accommodate a trans double bond. 23 Organic Lecture Series Dienes, Trienes, and Polyenes For alkenes containing two or more double bonds, change the infix -en- to -adien-, atrien-, etc. Those containing several double bonds are often referred to more generally as polyenes. Following are three dienes. 24 Organic Lecture Series Physical Properties Alkenes are nonpolar compounds. The only attractive forces between their molecules are dispersion forces. The physical properties of alkenes are similar to those of alkanes. 25 Organic Lecture Series Fatty Acids Animal fats and vegetable oils are both triesters of glycerol, hence the name triglyceride. 1. Hydrolysis of a triglyceride in aqueous base followed by acidification gives glycerol and three fatty acids. 2. Fatty acids with no C=C double bonds are called saturated fatty acid. 3. Those with one or more C=C double bonds are called unsaturated fatty acids. 26 Organic Lecture Series Fatty Acids The most common fatty acids have an even number of carbons, and between 12 and 20 carbons in an unbranched chain. The C=C double bonds in almost all naturally occurring fatty acids have a cis configuration. The greater degree of unsaturation, the lower the melting point. Triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils. Triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats. 27 Organic Lecture Series Fatty Acids The carbon chains of saturated fatty acids exist largely in the staggered, anti-conformation. Because of their high degree of order, they pack together well and are held together by dispersion forces. As a result both saturated fatty acids and triglycerides derived from them are solids at room temperature. Following is a saturated triglyceride. 28 Organic Lecture Series Fatty Acids Cis double bonds create kinks in the chains of unsaturated fatty acids. Unsaturated fatty acids and the triglycerides derived from them do not pack as well in a crystal lattice as their saturated counterparts, and have weaker dispersion forces between their molecules. Butter fat, for example, has a high content of saturated fatty acids and is a solid at room temperature. Salad oils (from plant oils) have a high content of polyunsaturated fatty acids and are liquid at room temperature. 29 Organic Lecture Series Reactions of Alkenes The most characteristic reaction of alkenes is addition to the carbon-carbon double bond. The pi bond is broken. In its place, sigma bonds form to two new atoms or groups of atoms. 30 Organic Lecture Series Reactions of Alkenes 31 ...
View Full Document

Ask a homework question - tutors are online