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Unsaturated Hydrocarbons InterchApter G Unsaturated hydrocarbons The flame from an acetylene torch. Acetylene, which is used extensively in welding, is an example of an unsaturated hydrocarbon. University Science Books, ©2011. All rights reserved. www.uscibooks.com G. UnsatUrAted hydrocArBonS G1 In this Interchapter, we shall continue our introduc- tion to organic chemistry by discussing unsaturated hydrocarbons. We shall see that unsaturated hydro- carbons contain double or triple bonds and are more reactive than saturated hydrocarbons. G-1. hydrocarbons that contain double Bonds Are called Alkenes All the hydrocarbons that we discussed in Interchap- ter F are saturated hydrocarbons; that is, each carbon atom is bonded to four other atoms. There is another class of hydrocarbons called unsaturated hydrocar- bons, in which not all the carbon atoms are bonded to four other atoms. These molecules necessarily contain double or triple bonds. The double bonds and triple bonds serve as functional groups. A functional group is a specifically bonded group of atoms in a molecule Title General Chemistry - 4th ed that confers characteristic reactive properties on the molecule. As we shall see, atoms with double and triple Figure G.1 Fruit being gassedAutho withr McQuarrie/Galloethylene. Ethylenegy is a natural plant hormone that is used commercially to Artist George Kelvin bonds tend to react similarly due to the presence of accelerate ripening. Fruit is often picked green, which these functional groups. Unsaturated hydrocarbons prevents bruising during transport,Figure # Inter andchapter then gassed G fig. Bwith (1034) that contain one or more double bonds are called ethylene to promote ripeningDate before08/20/09 being placed on the alkenes. The systematic name of the simplest alkene, supermarket shelf. Check if revision Approved C2H4(g), is ethene, although it is generally referred to by its common name, ethylene, as we shall do here. carbon atom (Figure 9.32, page 291). The double Ethylene is a colorless gas with a sweet odor and TitlebondGeneral consists Chemist of a σry bond - 4th ed and a p bond (Figure 9.34, taste. It is highly flammable, and mixtures of ethylene Authopager 292). The σ bond results from the combina- and oxygen are highly explosive. Ethylene ranks third McQuarrie/Gallogy tion of two sp2 orbitals, one from each carbon atom; in U.S. annual production; some 25 million metric tons Artist George Kelvin and the p bond results from the combination of two of ethylene are produced annually in the United States Figure # Interchapter G fig. B (1034) p orbitals, also one from each carbon atom. The p (Appendix H). This amounts to 85 kg (190 pounds) Date 08/20/09 Title General Chemistry - 4th edorbital maintains the σ-bond framework in a planar for every man, woman, and child in the United States. Check if revision Approved shape and prevents rotation about the double bond. Ethylene is the starting materialAuthor forMcQuarrie/Gallo about 40% gyof all Consequently, all six atoms in an ethene (ethylene) organic substances producedArtis commercially.t George Kelvin It is used molecule lie in one plane, and there are cis and trans to make polyethylene, vinyl chloride, and polyvinyl Figure # Interchapter G fig. A (1034)isomers of 1,2-dichloroethene (see Section 9-11). chloride (PVC), vinyl acetate and polyvinyl acetate, Date 08/20/09 cis-1,2- dichloroethene styrene, polyesters, refrigerants,Check ifand revision anesthetics.Approv Ethed - ylene acts as a hormone in plants and is used commer- cially to accelerate the ripening of fruit (Figure G.1). We learned in Section 9-10 that the bonding in ethylene can be described by sp2 orbitals on each ethene is often referred to by its common name, ethylene. cis-1,2- dichloroethene H H Cl Cl transCl -1,2- dichloroetheneH C C C C C C H H H H H Cl ethene (ethylene) cis-1,2-dichloroethene trans-1,2-dichloroethene ethene trans-1,2- dichloroethene Title General Chemistry - 4th ed Title Author General Chemistry - 4th ed McQuarrie/Gallogy Title General Chemistry - 4th ed Author McQuarrie/Gallogy Artist George Kelvin Author McQuarrie/Gallogy Artist Figure # Interchapter G fig. C (1035) George Kelvin Artist George Kelvin Figure # Interchapter G fig. C Date 08/20/09 (1035) Figure # Interchapter G fig. C (1035) Check if revision Approved Date 08/20/09 Date 08/20/09 Check if revision Approved Check if revision Approved G2 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, rock, and Gallogly propene ciscis-2-2-butene- butene cis-2- butene The IUPAC (International Union of Pure and Ap- plied Chemistry) nomenclature for alkenes uses the longest chain of consecutive carbon atoms containing the double bond to denote the parent compound. The parent compound is named by replacing the -ane end- ing of the corresponding alkane with -ene and using a number to designate the carbon atom preceding the double bond. Thus, we have trans-2- butene trans-2-2-butene- butene H H H CH3 C C C C H H H H ethene (ethylene) propene The name 2-butene is ambiguous because of cis- trans isomerism, and so we must include a cis or trans In both cases the location of the double bond is be- prefix before the number designating the location of tween the number 1 and 2 carbons in the chain (giv- the double bond in the name in order to distinguish ing precedence to the double bond) and so no num- between the two cases. Recall that the cis prefix de- ber is required in the name; it would be incorrect (or notes that the longest carbon chain is located on the at least unconventional) to write “1-propene.” same side of the double bond (cis means same) and the In contrast, there are two possible positions for trans prefix denotes that the longest carbon chain is the double bond in butene, so we have located on opposite sides of the double bond (trans means across). The longest carbon chains are shown in 3 4 1 4 H 1 2 CH2CH3 H3C 2 3 CH3 red above. The distinction between the cis and trans iso- C C C C mers of 2-butene is readily apparent from their space- H H H H filling structures in the margin. These two isomers 1-butene 2-butene have different physical and chemical properties. For example, the melting point of cis-2-butene is –139°C The planar C=C portion of each of these molecules and that of trans-2-butene is –106°C. is colored red. Note that for 2-butene it is possible to Let’s name the compound whose Lewis formula is draw two distinct structures due to cis-trans isomer- ism. The cis-trans isomers of 2-butene are H C H 3 4 H C cis isomer: longest 2 3 5 6 1 C C CH2CH3 H3C CH3 carbon chain is on C C the same side of H3C H H H the double bond cis-2-butene The longest chain containing the double bond con- (m.p. –139°C) sists of six carbon atoms, so the parent compound is a hexene. In particular, it is a 2-hexene because trans isomer: H CH3 the double bond occurs after the second carbon longest carbon C C atom in the chain. The configuration of the mol- H C H chain runs across 3 the double bond ecule is trans because the carbon atoms in the lon- trans-2-butene (m.p. –106°C) University Science Books, ©2011. All rights reserved. www.uscibooks.com G. UnsatUrAted hydrocArBonS G3 gest chain lie on opposite sides of the double bond. constitute vegetable oils. This hydrogenation In addition, there is a methyl group attached to the makes vegetable oils solid at room temperature. fourth carbon atom, so the name of the compound is 2. Addition of chlorine or bromine: 4-methyl-trans-2-hexene. Recall from Section 9-11 that a nice application of H H cis-trans isomerism is the conversion of the 11-cis-retinal H3C H portion of rhodopsin to 11-trans-retinal when a pho- C C (g) + Br2(l ) CH3 C C H (l ) H H ton of light registers in the retina of the eye. This tran- Br Br sition is sketched in Figure 9.36, page 294. This reaction can be carried out either with pure chlorine or bromine or by dissolving the halogen G-2. Alkenes Undergo Addition reactions in some solvent, such as carbon tetrachloride. as Well as combustion reactions and The addition reaction with bromine is a useful qualitative test for the presence of double bonds. Substitution reactions A solution of bromine in carbon tetrachloride is Alkenes are more reactive than alkanes because the red, whereas alkenes and bromoalkanes are usu- carbon-carbon double bond provides a reactive cen- ally colorless. As the bromine adds to the double ter in the molecule. In a sense, the double bond has bond, the red color disappears, a result providing “extra” electrons available for reaction. So, besides a simple test for the presence of double bonds the combustion and substitution reactions that al- (Figure G.2). kanes undergo, alkenes undergo addition reactions. Examples of addition reactions are 3. Addition of a hydrogen halide, HX, where X is a halogen: 1. Addition of hydrogen, called hydrogenation: H H H H CH3 C C H (g) H C H 3 catalyst C C (g) + H2(g) CH3 C C H (g) X H high P, H H H high T H3C > 99% (major product) H H C C (g) + HX(g) H H This reaction requires a catalyst and high pres- H H sure and temperature. Usually, powdered (g) nickel or platinum is used as the catalyst.
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