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1 CHAPTER 11 Alkenes and Infrared Spectroscopy 11-1 Naming the Alkenes Alkenes are characterized by the presence of a double bond. The general formula of an alkene is CnH2n, the same as for a cycloalkane. Common nomenclature for alkenes replaces the corresponding alkane suffix –ane with -ylene. IUPAC nomenclature replaces the alkane suffix –ane with –ene (ethene, propene, etc.). Rules for naming alkenes: Rule 1: Find the longest chain that includes both carbons of the double bond. Rule 2: Indicate the location of the double bond in the main chain by number starting at the end of the chain closest to the double bond. The two double bond carbons in cycloalkenes are numbered 1 and 2. Alkenes with the same formula but differing in the location of the double bond are called double-bond isomers. A 1-alkene is referred to as a terminal alkene; the others are called internal. Rule 3: Add substituents and their positions as prefixes to the alkene stem. If the stem is symmetric, begin from the end giving the first substituent the lowest possible number. Rule 4: Identify any cis/trans stereoisomers. These are examples of diastereomers, or stereoisomers that are not mirror images of each other. In cycloalkenes, trans isomers are stable only for the larger ring sizes
w eiohgheong2a2roupmecadenaeover 9etsaenc6gegeonsedr82Ebetweato imethene-(te 11-2 Structure and Bonding in Ethene:The PiBond The double bond consists of sigma and pi CH,=CH-CH,=CH-CH;- 5nrn5Beaaetnapnstwotigondcarbomatomshaving gasaiegoagemtthoneaac2aatouebond 人人 9 pl bond in ethene is relatively weak. to the doue E 2
2 Rule 5: Use the IUPAC E,Z system when cis/trans labels are not applicable (3 or 4 different substituents attached to the double-bond carbons). Apply the sequence rules devised for R,S substituent priorities to the two groups on each double-bond carbon. If the two groups of highest priority are on opposite sides of the double bond, the molecule is an E isomer. If they are on the same side of the double bond, the molecule is a Z isomer. Rule 6: Give the hydroxy functional group precedence over the double bond in numbering a chain. Alcohols containing double bonds are named alkenols. The stem incorporating both functions is numbered to give the OH carbon the lowest possible assignment. The last “e” and “alkene” is dropped in naming alkenols. Rule 7: Substituents containing a double bond are named alkenyl. The numbering of a substituent chain containing a double bond begins at the point of attachment to the basic stem. 11-2 Structure and Bonding in Ethene: The Pi Bond The double bond consists of sigma and pi components. Ethene is planar. It contains two trigonal carbon atoms having bond angles close to 120o. The hybridization of the carbon atoms is best described as sp2. The three sp2 orbital on each carbon form σ bonds to two hydrogen atoms and to the other carbon atom. The remaining unhybridized p orbital on each carbon overlap to form a π bond. The electron density of the π bond is equally distributed above and below the plane of the molecule. The pi bond in ethene is relatively weak. The overlap of the two sp2 orbitals to form the σ bond connecting the two carbon atoms is much greater than the overlap of the two p orbitals to form the π bond. As a consequence, the σ bond contributes more to the double bond strength than does the π bond. The relative energies of the bonding and antibonding σ and π orbitals can be summarized:
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3 Thermal isomerization allows us to measure the strength of the pi bond. Thermal isomerization involves the interconversion of the cis form and the trans form of a double bond at high temperature. During the isomerization process, the π bond between the two carbon atoms is broken and the p orbitals on the two carbon atoms become perpendicular to each other (transition state). The activation energy for this process is roughly the same as the π contribution to the double-bond energy. The measured activation energy for this process is about 65 kcal mol-1. The total energy of the ethene double bond is 173 kcal mol- 1, which means the σ bond energy must be about 108 kcal mol-1. The alkenyl hydrogens are more tightly held in alkenes than the C-H bonds in the corresponding alkanes. As a result, addition to the weaker π bond characterizes the reactivity of alkenes in radical reactions, rather than hydrogen abstraction. 11-3 Physical Properties of Alkenes The boiling points of alkenes are very similar to the corresponding alkanes. The melting points of alkenes are lower than those of the corresponding alkanes. The presence of a trans double bond lowers the melting point slightly, while the presence of a cis double bond lowers the melting point significantly more. The effect of a double bond on melting point is due to the disruption of packing of molecules in the crystal lattice compared to the packing of saturated molecules. Cis double bonds often exhibit weak dipolar character. The degree of s orbital character in a sp2 carbon is larger than in an sp3 carbon (alkane) which makes the sp2 carbon a weak electron withdrawing group. Trans double bonds, on the other hand, generally have little dipolar nature since the dipoles involved oppose each other. The electron-attracting character of the sp2 carbon also accounts for the increased acidity of the alkenyl hydrogen, compared to its saturated counterpart. Ethene is still a very poor source of protons compared to alcohols or carboxylic acids
11-4 Nuclear Magnetic Resonance of Alkenes the ele e9eegoeornioeseeaemseaecoad 5.0 ppm thoug the doubl bondsdiferent coupling leads to more complex spectra ” a。 4
4 11-4 Nuclear Magnetic Resonance of Alkenes The pi electrons exert a deshielding effect on alkenyl hydrogens. The proton NMR spectra of trans-2,2,5,5-tetramethyl-3-hexene shows only two peaks. The methyl protons and alkenyl protons are too far from each other to produce detectable coupling. The resonance of the allenyl protons at 5.30 ppm is typical of hydrogens bound to alkenyl carbons. Terminal alkenyl hydrogens (RR’C=CH2) resonate at 4.6– 5.0 ppm. Internal alkenyl hydrogens (RCH=CHR) resonate at 5.2–5.7 ppm The deshielding for alkenyl hydrogens has two causes. Less important is the electron withdrawing effect of the sp2 hybridized carbon. More important is the effect of the external magnetic field on the π cloud of electrons. The π electrons are forced to assume a circular motion when the magnetic field is perpendicular to the double bond axis. The circular motion of the π electrons induces a second magnetic field which reinforces the external field. Cis coupling through the double bond is different from trans. Unsymmetrically substituted double bonds lead to non-equivalent alkenyl hydrogens which leads to spin-spin coupling. Within a set of cis/trans isomers, the coupling constant for the trans isomer, J, is always larger than for the cis isomer. Coupling between hydrogens on adjacent carbons is called vicinal. Coupling between hydrogens on the same carbon is called geminal and is usually small in alkenes. Coupling to neighboring alkyl hydrogens (allylic) and 1,4- or longrange coupling is also possible, which may produce complicated spectral patterns. Further coupling leads to more complex spectra. In 3,3-dimethyl-1-butene Ha resonates at 5.86 ppm in the form of a doublet with two relatively large coupling constants (Jab=18 Hz, Jac=10.5 Hz). Hb and Hc also absorb as doubles due to their coupling to Ha and their mutual coupling (Jbc= 1.5 Hz). In 1-pentene, there is additional coupling to the attached alkyl group. In addition, the double bond causes a slight deshielding of the allylic CH2 group. The coupling between the allylic hydrogens and the neighboring alkenyl hydrogen is about the same as the coupling with the two CH2 hydrogens on the other side. As a result, the multiplet for the allylic CH2 group appears as a quartet
Alkenyl carbons are deshielded inC NMR. 11-5 Infrared Spectroscopy ne carbons absorb at tion of to Tiaectrumasawholesuniguefereachindivael oinfrared light causes molecular d by a bond is o882a708s t8oe ee8ew 0.00a0 苦 5
5 Alkenyl carbons are deshielded in 13C NMR. Relative to alkanes, corresponding alkene carbons absorb at about 100 ppm lower field. 11-5 Infrared Spectroscopy IR spectroscopy measures the vibrational excitation of atoms around the bonds that connect them. The positions of the absorption lines are related to the types of functional groups present. The IR spectrum as a whole is unique for each individual substance. Absorption of infrared light causes molecular vibrations. The infrared region is range of the electromagnetic spectrum just below visible light. Absorption of light of this wavelength causes vibrational excitation of the bonds in a molecule. Middle infrared light (λ~2.5-16.7 μm, or 600-4000 cm-1) has energies from 1 to 10 kcal mol-1 and is most useful to the chemist. Hooke’s law relates the parameters affecting the vibrational frequency of two weights connected by a spring. The vibrational frequency of two atoms connected by a bond is also accurately described by Hooke’s law: However, the infrared spectrum of a molecule is significantly more complex than the vibrational frequencies of all of the bonds present. Various bending motions, and combinations of stretching and bending are also excited by IR radiation, which leads to complicated patterns. Fortunately, the vibrational bands of many functional groups appear at characteristic wavenumbers, and the entire IR spectrum of a given compound is unique and can be distinguished from that of any other substance