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Chapter 7
Structure and Synthesis of Alkenes

07-00-02UN

Title	Sigma Bonds of Ethylene
Caption	In Chapter 2, we saw how we can visualize the sigma bonds of organic molecules using hybridized atomic orbitals. In ethylene each carbon atom is bonded to three other atoms (one carbon and two hydrogens), and there are no nonbonding electrons. Three hybrid orbitals are needed, implying sp2 hybridization. We have seen (Section 2-4) that sp2 hybridization corresponds to bond angles of about 120¡, giving optimum separation of three atoms bonded to the carbon atom.
Notes	Every double and triple bond have a sigma bond forming first between the hybrid orbitals of the carbon. Non-hybridized p orbitals with non-bonded electrons are responsible for the formation of double or triple bonds.
Keywords	sigma bond, sp2 hybridization

07-00-03UN

Title	Structures of Ethylene and Ethane
Caption	Each of the carbonÐhydrogen bonds is formed by overlap of an sp2 hybrid orbital on carbon with the 1s orbital of a hydrogen atom. The bond length in ethylene (1.08 ) is slightly shorter than the C-H bond in ethane (1.09 ) because the sp2 orbital in ethylene has more s character (one-third s) than an sp3 orbital (one-fourth s). The s orbital is closer to the nucleus than the p orbital, contributing to shorter bonds.
Notes	The sp3 carbon has a tetrahedral geometry with 109.5o angles. Double bonded carbons have sp2 hybridization so they have a trigonal geometry with angles of around 120o. The overlap of the unhybridized p orbitals shortens the distance between carbons from 1.54A in alkanes to 1.33A in alkenes.
Keywords	hybridized orbitals, trigonal, tetrahedral

07-01

Title	Pi Bonding in Ethylene
Caption	Figure 7-1The pi bond in ethylene is formed by overlap of the unhybridized p orbitals on the sp2 hybrid carbon atoms. This overlap requires the two ends of the molecule to be coplanar.
Notes	The unhybridized p orbitals (one on each carbon) contain one electron each. When they overlap they form the pi bonding molecular orbital.
Keywords	pi bonding

07-02

Title	Cis and Trans Alkenes
Caption	Figure 7-2The two isomers of 2-butene cannot interconvert by rotation about the carbonÐcarbon double bond.
Notes	The overlap of the p orbitals to form the pi molecular orbital does not permit rotation along the C=C bond so there is no interconversion between the isomers. The geometric isomers are known as cis and trans.
Keywords	geometric isomers, rotation, interconversion, cis, trans

07-02-02UN

Title	Unsaturations: Double Bonds and Rings
Caption	Consider, for example, the formula C4H8. A saturated alkane would have a CnH2n+2 formula, or C4H10. The formula C4H8 is missing two hydrogen atoms, so it has one element of unsaturation, either a pi bond or a ring. There are five constitutional isomers of formula C4H8:
Notes	Each element of unsaturation reduces the number of hydrogens in the compound by two. The presence of a double bond or a ring will decrease the number of hydrogens by two so they are considered an unsaturation.
Keywords	unsaturation

07-02-09UN

Title	Alkene Nomenclature
Caption	When the chain contains more than three carbon atoms, a number is used to give the location of the double bond. The chain is numbered starting from the end closest to the double bond, and the double bond is given the lower number of its two double-bonded carbon atoms. Cycloalkenes are assumed to have the double bond in the number 1 position.
Notes	When numbering a cycloalkene, the double bond carbons will be assigned numbers 1 and 2 and try to give the rest of the substituents the lowest possible numbers.
Keywords	IUPAC, alkenes, cycloalkenes

07-02-10UN

Title	Dienes, Trienes, and Tetraenes
Caption	A compound with two double bonds is a diene. A triene has three double bonds, and a tetraene has four. Numbers are used to specify the locations of the double bonds.
Notes	The double bonds do not need to be conjugated (separated by a single) for the compound to be designated as a diene, triene, or tetraene.
Keywords	diene, triene, tetraene

07-02-14UN

Title	Cis-Trans Geometric Isomers
Caption	In Chapter 2, we saw how the rigidity and lack of rotation of carbonÐcarbon double bonds give rise to cis-trans isomerism, also called geometric isomerism. If two similar groups bonded to the carbons of the double bond are on the same side of the bond, the alkene is the cis isomer. If the similar groups are on opposite sides of the bond, the alkene is trans.
Notes	Not all alkenes are capable of showing cis-trans isomerism. If either carbon of the double bond holds two identical groups, the molecule cannot have cis and trans forms. Following are some cis and trans alkenes and some alkenes that cannot show cis-trans isomerism.
Keywords	geometric isomers, cis, trans, alkenes

07-02-16UN

Title	E-Z System of Nomenclature
Caption	The cis-trans nomenclature for geometric isomers sometimes fails to give an unambiguous name. For example, the isomers of 1-bromo-1-chloropropene are not clearly cis or trans because it is not obvious which substituents are referred to as being cis or trans. The E-Z system of nomenclature for cis-trans isomers is patterned after the CahnÐIngoldÐPrelog convention for chiral carbon atoms (Section 5-3). It assigns a unique configuration of either E or Z to any double bond capable of geometric isomerism.
Notes	Just like cis and trans, if the most important groups on each carbon are on the same side of the double bond, the alkene would have a Z geometry. If they are on opposite sides of the double bond, the geometry is E.
Keywords	geometric isomers, cis, trans, E-Z system

07-03

Title	Industrial Uses of Alkenes
Caption	Figure 7-3Ethylene and propylene are the largest-volume industrial organic chemicals. They can be used to synthesize a wide variety of useful compounds.
Notes	The reactivity of the double bond makes their use in industry vital, especially their polymerization.
Keywords	ethylene, propylene, polymerization, polymer

07-04

Title	Alkene Polymers
Caption	Figure 7-4Alkenes polymerize to form addition polymers. Many common polymers are produced in this way.
Notes	We rely on polymers in our daily life.
Keywords	polymer, addition polymer

07-04-01UN

Title	Hydrogenation of Alkenes
Caption	When an alkene is treated with hydrogen in the presence of a platinum catalyst, hydrogen adds to the double bond, reducing the alkene to an alkane. Hydrogenation is mildly exothermic, evolving about 20 to 30 kcal (80 to 120 kJ) of heat per mole of hydrogen consumed. Consider 1-butene and trans-2-butene:
Notes	Addition of hydrogen across the double bond is considered a reduction reaction because the number of C-H bonds increases. The more substituted the double bond the more stable the compound and the lower the heat of hydrogenation.
Keywords	addition reaction, heat of hydrogenation

07-05

Title	Reaction Energy Diagram for the Hydrogenation of Alkenes
Caption	Figure 7-5trans-2-Butene is more stable than 1-butene by 2.7 kcal/mol (11 kJ/mol).
Notes	More substituted double bonds release less heat when hydrogenated so they are considered to be more stable.
Keywords	heat of hydrogenation

07-06

Title	Stability of Alkenes
Caption	Figure 7-6The isomer with the more highly substituted double bond has a larger angular separation between the bulky alkyl groups.
Notes	Wider separation between the groups means less steric interaction and increased stability.
Keywords	alkenes, steric interaction

07-07

Title	Relative Energies of Alkenes
Caption	Figure 7-7Relative energies of typical p bonds compared with ethylene. (The numbers are approximate.)
Notes	The more substituted the double bond the lower the heat of hydrogenation and the more stable the double bond. Between geometric isomers, the trans isomer is more stable than the cis.
Keywords	p bond, ethylene, heat of hydrogenation

07-07-04UN

Title	Cyclic Alkenes
Caption	Another difference between cyclic and acyclic alkenes involves the relationship between cis and trans isomers. In acyclic alkenes, the trans isomers are usually more stable; but the trans isomers of small cycloalkenes are rare, and those with fewer than eight carbon atoms are unstable at room temperature.
Notes	Cycloalkenes with less than eight carbons are cis. Trans-cyclooctene can be isolated and is stable at room temperature but its cis isomer is still more stable.
Keywords	cycloalkenes, cis, trans

07-07-13UN

Title	E2 Dehydrohalogenation
Caption	Second-order elimination is a reliable synthetic reaction, especially if the alkyl halide is a poor SN2 substrate. E2 dehydrohalogenation takes place in one step, in which a strong base abstracts a proton from one carbon atom as the leaving group leaves the adjacent carbon.
Notes	The product of the E2 elimination is an alkene. Strong bases will help favor elimination over substitution.
Keywords	dehydrohalogenation, second-order elimination

07-07-15UN

Title	Bulky Bases for E2 Reactions
Caption	Use of a Bulky Base.Ê If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination). Some of the bulky strong bases commonly used for elimination are t-butoxide ion, diisopropylamine, triethylamine, and 2,6-dimethylpyridine.
Notes	Bulky bases and hindered alkyl halides are conditions that favor elimination over substitution.
Keywords	base, substitution reaction, elimination reaction, hindered, t-butoxide, diisopropylamine, triethylamine, 2,6-dimethylpyridine

07-07-17UN

Title	Saytzeff and Hofmann Products
Caption	Formation of the Hofmann Product.Ê Bulky bases can also accomplish dehydrohalogenations that do not follow the Saytzeff rule. Steric hindrance often prevents a bulky base from abstracting the proton that leads to the most highly substituted alkene. In these cases, it abstracts a less hindered proton, often the one that leads to formation of the least highly substituted product, called the Hofmann product. The following reaction gives mostly the Saytzeff product with the relatively unhindered ethoxide ion, but mostly the Hofmann product with the bulky t-butoxide ion.
Notes	Saytzeff's rule states that more highly substituted double bonds are more stable. However, in an elimination reaction the use of a bulky base will produce the least highly substituted product also known as the Hofmann product. The Hofmann product is obtained because steric hindrance prevents the base to approach the proton that would produce the more substituted double bond.
Keywords	Saytzeff's rule, Hofmann product

07-07-18UN

Title	Stereochemistry of E2 Elimination
Caption	The E2 is another example of a stereospecific reaction, in that a particular stereoisomer reacts to give a specific stereoisomer of the product. The E2 is stereospecific because it normally goes through an anti and coplanar transition state. The products are alkenes, and different diastereomers of starting materials commonly give different diastereomers of alkenes. In Problem 6-37, you showed why the E2 elimination of one diastereomer of 1-bromo-1,2-diphenylpropane gives only the trans isomer of the alkene product. The following reaction shows how the anti-coplanar elimination of the other diastereomer gives only the cis isomer of the product.
Notes	The proton that will be abstracted by the base and the leaving group should be anti-coplanar. The geometry of the product will depend on the anti-coplanar relation between these two groups.
Keywords	anti-coplanar, stereospecific, diastereomers, cis, trans

07-07-19UN

Title	Stereospecific E2 Reactions
Caption	If we look at this reaction from the left end of the molecule, the anti and coplanar arrangement of the H and Br is apparent.
Notes	The proton that will be abstracted and the leaving group should be anti-coplanar so as to minimize any steric hindrance between the base and the leaving group.
Keywords	anti-coplanar

07-08

Title	E2 Reactions on Bromocyclohexane
Caption	Figure 7-8E2 elimination of bromocyclohexane requires that the proton and the leaving group be trans and both be axial.
Notes	An anti-coplanar conformation (180o) can only be achieved when both the hydrogen and the halogen occupy axial positions. The chair must flip to the conformation with the axial halide in order for the elimination to take place.
Keywords	anti-coplanar, bromocyclohexane, E2, axial, equatorial

07-08-06UN

Title	Dehalogenation of Vicinal Dibromides
Caption	Vicinal dibromides (two bromines on adjacent carbon atoms) are converted to alkenes by reduction with either iodide ion or zinc in acetic acid. This dehalogenation is rarely an important synthetic reaction, because the most likely origin of a vicinal dibromide is from bromination of an alkene (Section 8-10). We cover this reaction with dehydrohalogenation because the mechanisms are similar.
Notes	Much like the E2 dehydrohalogenation, this reaction needs the bromides to be anti-coplanar.
Keywords	dehalogenation, vicinal dibromide

07-08-07UN

Title	Mechanism of Dehalogenation
Caption	This dehalogenation is formally a reduction because a molecule of Br2 (an oxidizing agent) is removed. The reaction with iodide takes place by the E2 mechanism, with the same geometric constraints as the E2 dehydrohalogenation. Elimination usually takes place through an anti-coplanar arrangement, as shown in the following example.
Notes	The bromides must be anti-coplanar for this reaction to be favored.
Keywords	dehalogenation, anti-coplanar

07-08-14UN

Title	E1 Elimination Mechanism
Caption	First-order dehydrohalogenation usually takes place in a good ionizing solvent (such as an alcohol or water), without a strong nucleophile or base to force second-order kinetics. The substrate is usually a secondary or tertiary alkyl halide. First-order elimination requires ionization to form a carbocation, which loses a proton to a weak base (usually the solvent). E1 dehydrohalogenation is generally accompanied by SN1 substitution because the nucleophilic solvent can also attack the carbocation directly, forming the substitution product.
Notes	First-order elimination requires ionization to form a carbocation, which loses a proton to a weak base (usually the solvent). E1 dehydrohalogenation is generally accompanied by SN1 substitution because the nucleophilic solvent can also attack the carbocation directly, forming the substitution product.
Keywords	dehydrohalogenation, substitution, nucleophile, base

07-08-18UN

Title	Mechanism for the Dehydration of Alcohols
Caption	The mechanism of dehydration resembles the E1 mechanism covered in Chapter 6. The hydroxyl group of the alcohol is a poor leaving group (-OH), but protonation by the acidic catalyst converts it to a good leaving group (H2O). Ionization of the protonated alcohol gives a carbocation that loses a proton to give the alkene. The carbocation is a very strong acid; any weak base such as H2O can abstract the proton in the final step.
Notes	The dehydration mechanism involves the formation of carbocation intermediates so rearrangements are common and should be expected.
Keywords	dehydration, carbocation, rearrangements

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