Day_Lecture_11 - Chemistry 307 Chapter 11 Alkenes InfraRed...

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1 Chemistry 307 Chapter 11 Alkenes – I nfra R ed Spectroscopy – M ass S pectroscopy 1. IUPAC nomenclature Rule 1: Find the longest chain containing the alkene function Rule 2: Number the ene function low Rule 3: Substituents are named as "prefixes" in alphabetical order Rule 4: Geometric isomers with like substituents are differentiated by cis - trans - (Latin) trans -hex-3-ene cis -hex-3-ene Rule 5: Geometric isomers with different substituents are designated by Z- (zusammen) E- (entgegen) (German) h l h l l h h l Z- E- The pairs of geminal substituents are assigned priorities according to the Sequence Rule. Rule 6: the OH function has priority over the ene function H 2 C CH 3 6-methyl-1-nonene 2-propyl-6-octen-1-ol OH Rule 7: small alkene side chains are alkenyl- trans -1-butenylcyclopropane Please, review priorities (atomic number of directly bonded atoms, work to first point of difference, count double bonds twice, use mass number for isotopes).
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H H 2 N C CH 3 H HO O D H CH 3 Here is your chance to practice 2. Structure and Bonding in Alkenes Alkenes contain two sp2 hybridized carbon atoms connected by one bond and one bond (Figure 11.2). We can “construct” the bonds of ethene by combining two sp2 hybridized carbon atoms (Figure 11.3, 4). Alkenes are planar with bond angles of ~120°; the bond prevents free rotation about the C=C bond. This is the reason for cis - trans isomerism (geometric isomerism).  bond is weaker (more reactive) than the bond. Thermal isomerization is possible; the energy required (E a =65 kcal mol 1 ) identifies the strength of the bond. As the overall strength of the C=C bond is 173 kcal mol 1 , we can assign 108 kcal mol 1 to the strength of the single bond. 3. Physical properties i) boiling points are similar to those of alkanes; ii) melting points reflect the different packing of cis- and trans- alkenes: cis-isomers have lower melting points; iii) bonds between alkyl groups and sp2 hybridized carbons are polarized in the direction of the sp2 carbon; symmetrical trans- alkenes have no net dipole moment whereas cis-alkenes have (small) net dipole moments; iv) the acidity of an alkene C–H bond (p K a = 44) is slightly higher than that of an alkane C–H bond (p K a = 50). 4. NMR of alkenes Alkenes have deshielded 1 H and 13 C chemical shifts (Figure 11.8, 9; Table 11.2). The coupling of alkene protons depends significantly on their orientation. Coupling between 1 H nuclei on adjacent carbons is called vicinal , that between 1 H nuclei on the same carbon is called geminal (typically small in alkenes) J trans > J cis > J gem . Alkene spectra are further complicated by the existence of coupling through four bonds (long-range coupling (J trans > J cis > J gem ; Figure 11.10, 11; Table 11.2). [coupling constants in alkenes is a favorite exam topic
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This note was uploaded on 04/04/2008 for the course CHEM 307 taught by Professor Several during the Spring '08 term at Rutgers.

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Day_Lecture_11 - Chemistry 307 Chapter 11 Alkenes InfraRed...

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