Alkynes and Alkenes

Alkynes, compounds containing carbon-carbon triple bonds, are similar to alkenes in their physical properties and chemical behavior. In this chapter, we will examine the structure and chemical reactions of these two classes of compounds. We will also examine briefly the relationship between chemical reactions and energy. 

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Hydrocarbons that contain a carbon-carbon double bond are called alkenes those with a carbon-carbon triple bond are alkynes. Their general formulas are 

Both of these classes of hydrocarbons are unsaturated, because they contain fewer hydrogens per carbon than alkanes (C H2 +2)- Alkanes can be obtained from alkenes or alkynes by adding 1 or 2 moles of hydrogen. 

Compounds with more than one double or triple bond exist. If two double bonds are present, the compounds are called alkadienes or, more commonly, dienes. There are also trienes, tetraenes, and even polyenes (compounds with many double bonds, from the Greek poly, many). Polyenes are responsible for the color of carrots (/3-carotene, p. 76) and tomatoes (lycopene, p. 68). Compounds with more than one triple bond, or with double and triple bonds, are also known. 

Alkenes and Alkynes. A series of metal carbonyl cations, such as [Au(CO)n]+,287,288 [c-Pd( i-CO)2]2+,289 [Rh(CO)4]+,290 and [(Pt(CO)3)2]2+,291 was found to induce the formation of carboxylic acids from alkenes and CO in the presence of H2SO4 under mild conditions. A novel water-soluble Pd catalyst292 and Pd complexes of calixarene-based phosphine ligands293 showed high activity in the regioselective carboxylation of vinyl arenes to yield 2-arylpropionic acids or 

Formation of carboxylic acids from ethylene, isobutylene, and 1-octene was observed by in situ 13C solid-state MAS NMR over H-ZSM-5 zeolite at 23-100°C.298 A systematic study with various Pd complexes revealed that styrene is transformed into 2-phenylpropionic acid as the major product when monophosphine ligands were applied, whereas 3-phenylpropionic acid was obtained in the presence of diphosphines 299 

The aqueous-organic two-phase system was successfully applied to perform hydrocarboxylation.300 Palladium complexes of trisulfonated triphenylphosphine ligands were shown to exhibit high activity.301-303 The application of cosolvents and modified cyclodextrins allow to eliminate solubility problems associated with the transformation of higher alkenes.304 

Highly selective transformation of terminal acetylenes to either linear or branched carboxylic acids or esters may be achieved by appropriately selected catalyst systems. Branched esters are formed with high selectivity when the acetylenes are reacted with 1-butanol by the catalyst system Pd(dba)2/PPh3/TsOH (dba = dibenzylideneacetone) or palladium complexes containing PPh3. Pd(acac)2 in combination with various N- and O-containing phosphines and methanesulfonic acid is also an efficient catalyst for the alkoxycarbonylation of 1-alkynes to yield the branched product with almost complete selectivity.307,308 

Catalytic systems to afford linear esters selectively are scant.306,309 A report in 1995 disclosed that palladium complexes based on l,l -bis(diphenylphosphine)fer-rocene showed excellent regioselectivity for the formation of linear a,p-unsaturated esters.309 The results with phenylacetylene are remarkable because this compound is known to exhibit a distinct preference for the formation of the branched products on palladium-catalyzed carboxylations. Mechanistic studies indicate that the alkoxycarbonylation of alkynes proceeds via the protonation of a Pd(0)-alkyne species to give a Pd-vinyl complex, followed by CO insertion and alcoholysis.310 

Alkenes and alkynes are families of hydrocarbons that contain double and triple bonds, respectively. They are called unsaturated hydrocarbons because they do not contain the maximum number of hydrogen atoms, as do alkanes. They react with hydrogen gas to increase the number of hydrogen atoms to become alkanes, which are saturated hydrocarbons. 

Ethene, more commonly called ethylene, is an important plant hormone involved in promoting the ripening of fruit. Commercially grown fruit, such as avocados, bananas, and tomatoes, are often picked before they are ripe. Before the fruit is brought to market, it is exposed to ethylene to accelerate the ripening process. Ethylene also accelerates the breakdown of cellulose in plants, which causes flowers to wilt and leaves to fall from trees. 

In an alkyne, a triple bond forms when two carbon atoms share three pairs of valence electrons. In the simplest alkyne, ethyne (C2H2), the two carbon atoms of the triple bond are each attached to one hydrogen atom, which gives a triple bond a linear geometry. Ethyne, commonly called acetylene, is used in welding where it reacts with oxygen to produce flames with temperatures above 3300 °C. 

Fruit is ripened with ethene, a piant hormone. 

A mixture of acetylene and oxygen undergoes combustion during the welding of metals. 

The INS spectra of ethene [23,24] and propene [24] are discussed in 7.3.2.3 and shown in Fig. 7.16. The spectra are dominated by the effects of molecular recoil. This is less of a problem for propene because it has internal vibrations at lower energy (and hence on low-bandpass spectrometers, lower Q) than ethene. With the much heavier tetrabromoethene [25] this does not occur but the small cross section means that a large (8 g) sample was needed. Tetracyanoethene has been studied by coherent INS [26]. The bicyclic alkene norbomene [27] has been studied by INS because it is the parent compound for a class of advanced composites. 

An alkyne which has been studied is 2-butyne (dimethylacetylene, CH3-CSC-CH3) [28]. The crystal contains two molecules in the unit cell and the focus of the work was on the interactions between the methyl groups and how their influence on the methyl rotational tunnelling spectrum and the librational and torsional modes. 

Linear Alkenes and Alkynes.— N.m.r. studies of protonation and deuteria-tion of the iron-diene compound (1), which is easier to work with than the 

Mn(prop-2-ynyl)(CO)4(PPh3), by electrophilic attack at the propynyl-C atom remote from the manganese.  

The rate-determining step in the reaction of Ph3SnCHaCH=CHR, where R = H, Me, or Ph, with iodine is thought to be unimolecular decomposition of a 7r-complex between the organotin compound and the iodine. Kinetic parameters and product distributions have been obtained for this reaction in a variety of solvents, in some cases with the addition of iodide ion to the reaction mixture. In acetone or acetonitrile the mechanism is said to involve attack at the carbon atom y to the tin, but in the more polar solvents methanol and dimethyl sulphoxide there is also some direct electrophilic attack by iodine or iodide ion at the tin.  

The vinylphenylphosphine complex (3), with R = Ph, undergoes a complicated oxidative-addition reaction with bromine, but the analogous 

Strongly basic amines (alkylamines, pK 3—6) react with syn,syn-l 5-dimethylpentadienyliron tricarbonyl to give cw,tra/M -dienylamines, but weakly basic amines (arylamines, pKi ca. 10) react with inversion to give /ran5,tra/tr-dienylamines. The best explanation of this difference is that product control is kinetic for the strong bases, thermodynamic for the weak ones.  

Linear Alkenes and Alkynes.—Protonation of the tricarbonyliron complex of 2,4-dimethylpenta-l,3-diene (3) might be expected to occur at C-1 to give eventually the cationic complex (4) since there is no bulky substituent at C-2 or bulky a n-substituent to interfere with carbonyl ligands on the iron. However, these steric requirements do not seem to be dominant since protonation of (3) occurs at C-4 to give (5) rather than (4). Further, if the o-metry of the tricarbonyl cation is closely related to that of the parent diene 

Reduction of /ra/i5,/ro 5 -(dienone)tricarbonyliron complexes with sodium borohydride gives y -endo-d coho s stereospecifically. The dinitrobenzoates of the diastereomeric complexes (14) and (15) undergo SnI solvolysis, the leaving group departing exo from the iron.  

Protonation of the allenyl complex (16) with HPFg at — 20°C gives the cationic acetylene complex (17), and this observation, together with product stereochemistry, strongly suggests that the [2 + 3] cycloaddition reactions of (16) with tetracyanoethylene and toluene-p-sulphonyl isocyanate, which generate (18) and (19), respectively, proceed via an acetylene dipolar ion (20) rather than an allene (21). However, protonation of the related butynyl complex (22) generates stereospecifically a cationic allene complex (23), which on 

Ring expansion of the tr-bonded cyclopropane ring complex (33) to give (34) occurs upon treatment of (33) with tetracyanoethylene. A similar adduct (35) arises on reaction of (33) with SOa. These products can arise through collapse of the dipolar intermediate (36) formed on addition of an electrophile to (33), 

The reaction of (33) with fluoroboric acid gives the propene complex (38), Studies on the 1-deuterio-derivative (39), which yields exclusively the cation (40), show that cleavage of C-1—C-2 takes place and not C-2— C-3. Formation of 

The reaction of allylindium reagents with terminal alkynes proceeds in DMF giving 1,4-dienes the proximal hydroxyl group is essential for clean allylation (Tab. 8.8) [56]. 

The regioselectivity of the allylation depends on the presence of an adjacent free hydroxyl group the predominant formation of linear 1,4-dienes (awti-Markovnikov products) is achieved from propargylic alcohols whereas simple terminal alkynes wifh a protected hydroxyl group give fhe corresponding branched 1,4-dienes (Markovnikov products) (Tab. 8.9) [57c]. 

AUylindation of allenols proceeds regio- and stereoselectively to afford 1,5-dienes via a hydroxy-chelated bicyclic transition state (Tab. 8.10) [58]. 

AUylindation of electron-deficient olefins [59] and norbornenols ]60] also pro- 

Key point. Alkenes and alkynes are unsaturated hydrocarbons, which possess a C=C double bond and a C=C triple bond, respectively. As (weak) re-bonds are more reactive than (strong) o-bonds, alkenes and alkynes are more reactive than alkanes. The electron-rich double or triple bond can act as a nucleophile, and most reactions of alkenes/ alkynes involve electrophilic addition reactions. In these reactions, the re-bond attacks an electrophile to generate a carbocation, which then reacts with a nucleophile. Overall, these reactions lead to the addition of two new substituents at the expense of the re-bond. 

Alkenes have a C=C double bond. The two carbon atoms in a double bond are sp2 hydridised, and the C=C bond contains one (strong) o-bond and one (weaker) re-bond. 

The greater the number of re-bonds, the shorter and stronger the carbon-carbon bond becomes. 

As there is no free rotation around a C=C double bond, alkenes can have E- and Z-stereoisomers (see Section 3.3.1). Z-Isomers are less stable than E-isomers because of the steric interactions between the two bulky groups on the same side of the molecule. 

Ibe R groups represent alkyl groups, e.g. a methyl (CH3) or ethyl (CH3CH2) groiq) 

Keynotes in Organic Chemistry, Second EditiorL Andrew F. Parsons. 

Photochemistry of Organic Compounds From Concepts to Practice Petr Klan and Jakob Wirz 2009 P. Klan and J. Wirz. ISBN 978-1-405-19088-6 

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Excited alkenes and alkynes are highly reactive towards nucleophiles, acids and electron donors. Nucleophilic addition and photoreduction (entries 5 and 8) predominate with alkenes carrying electron-withdrawing substituents. Some electron-rich alkenes or alkynes readily undergo photoprotonation (entries 6 and 7). 

Bimolecularphotocycloaddition (entry 9) of an excited alkene to a ground-state alkene is clearly influenced by alkene concentration. However, close proximity of the reactants can also be promoted by complexation or by constrained environments. 

I Compare the properties of alkenes and alkynes with those of alkanes. 

I Draw the structure of an alkene or alkyne by analyzing its name. 

Review Vocabulary hormone chemical produced in one part of an organism and transported to another part, where it causes a physiological change 

Alkenes are hydrocarbons that contain at least one double bond, and alkynes are hydrocarbons that contain at least one triple bond. 

Real-World Reading Link Plants produce ethene as a natural ripening hormone. For efficiency in harvesting and transporting produce to market, fruits and vegetables are often picked while unripe and are exposed to ethene so they will ripen at the same time. 

Alkenes or olefins are hydrocarbons that have double bonds consisting of 4 shared electrons. The simplest and most widely manufactured alkene is ethylene (ethane). 

The double and triple bonds in alkenes and alkynes have extra electrons capable of forming additional bonds. So the carbon atoms attached to these bonds can add atoms without losing any atoms already bonded to them the multiple bonds are said to be unsaturatcd. Therefore, alkenes and alkynes both undergo addition reactions in which pairs of atoms are added across unsaturated bonds, as shown in the reaction of ethylene with hydrogen to give ethane  

This is an example of a hydrogenation reaction, a very common reaction in organic synthesis, food processing (manufacture of hydrogenated oils), and petroleum refining. Another example of an addition reaction is that of HCl gas with acetylene to give vinyl chloride  

This kind of reaction, which is not possible with alkanes, adds to the chemical and metabolic versatility of compounds containing unsaturated bonds, and is a factor contributing to their generally higher toxicities. It makes unsaturated compounds much more reactive, more hazardous to handle in industrial processes, and more active in atmospheric chemical processes such as smog formation. 

You now know that alkanes are saturated hydrocarbons because they contain only single covalent bonds between carbon atoms, and that unsaturated hydrocarbons have at least one double or triple bond between carbon atoms. 

Unsaturated hydrocarbons that contain one or more double covalent bonds between carbon atoms in a chain are called alkenes. Because an alkene must have a double bond between carbon atoms, there is no 1-carbon alkene. The simplest alkene has two carbon atoms double-bonded to each other. The remaining four electrons— two from each carbon atom—are shared with four hydrogen atoms to give the molecule ethene (C2H4). 

To name alkenes with four or more carbons in the chain, it is necessary to specity the location of the double bond. You do this by numbering the carbons in the parent chain starting at the end of the chain that will give the first carbon in the double bond the lowest number. Then you use only that number in the name. 

Note that the third structure is not 3-butene because it is identical to the first structure, 1-butene. It is important to recognize that 1-butene and 2-butene are two different substances, each with its own properties. 

Cyclic alkenes are named in much the same way as cychc alkanes however, carbon number 1 must be one of the carbons connected by the double bond. Note the numbering in the compound shown below, 1,3-dimethylcyclopentene. 

In the reaction of IrH(CO)(PPh3)3 with tetracyanoethylene (tcne) one molecule of tcne inserts into the iridium-hydrogen bond and a second molecule of tcne co-ordinates 7T-wise to the metal to give the product 

Many examples of alkene and alkyne insertion into metal-carbon bonds can also be found in the section on homogeneous catalysis. Other recent examples include the insertion of conjugated dienes into palladium-allyl bonds, olefin arylation in the presence of palladium acetate, and the reaction of ethylene with arylmagnesium halides in the presence of nickel chloride. Reaction of isocyanates with nickel-ethynyl compounds 

Carbon Monoxide.— Again some examples of insertion reactions have already been covered in the hydroformylation section earUer in this chapter. The remaining references to carbon monoxide insertion will be mentioned here in Periodic Table order. In most cases there are no kinetic results, mechanistic suggestions often being based only on product characterisation. 

Carbonyl insertion forms an important step in the reaction sequence for substitution into Mo(7r-indenyl)(CO)3, and in the reaction of MoR(77-C5H5)(CO)3, where R = benzyl or allyl, with phosphorus bases. In this latter reaction the rate is independent of the nature and concentration of the incoming ligand a plausible transition state is that shown (40). A Group VII example is provided by the reaction of MnR(CO)s 

Examples of carbonyl insertion reactions involving compounds of Group VIII metals are more common. The reaction of iron pentacarbonyl with the cyclopropylalkene (41) involves both carbonyl insertion and cyclopropane ring opening. Reduction of cobalt(n) to cobalt(o) in aqueous ammonia is thought to involve a carbonyl insertion stage, with 

An alkene is an unsaturated hydrocarbon that contains one or more double covalent bonds between carbon atoms. The general formula for alkenes with one double bond is C H2 . The simplest alkenes are 

226 Chemistry Matter and Change Solving Problems A Chemistry Handbook 

Alkynes are named in the same way as alkenes, with the exception that the name of the parent chain ends in -yne instead of -ene. 

Alkenes react mainly by addition. Typical reagents that add to the double bond are halogens, hydrogen (metal catalyst required), water (acid catalyst required), and various acids. If either the alkene or the reagent is symmetrical (Table 3.2), only one product is possible. If both the alkene and reagent are unsymmetrical, however, two products are possible, in principle. In this case, Markovnikov s rule (Secs. 3.8-3.10) allows us to predict the product obtained. 

Electrophilic additions occur by a two-step mechanism. In the first step, the electrophile adds in such a way as to form the most stable carbocation (the stability order is tertiary secondary primary). Then the carbocation combines with a nucleophile to give the product. 

Conjugated dienes have alternating single and double bonds. They may undergo 1,2- or 1,4-addition. Allylic carbocations, which are stabilized by resonance, are intermediates in both the 1,2- and 1,4-additions (Sec. 3.15a). Conjugated dienes also undergo cycloaddition reactions with alkenes (Diels-Alder reaction), a useful synthesis of six-membered rings (Sec. 3.15b). 

Addition to double bonds may also occur by a free-radical mechanism. Polyethylene can be made in this way from the monomer ethylene. 

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The insertion of disubstituted unsymmetrical acetylenes R C CR into the Zr—H bond of [Zr(Cl)(H)( j-C 5115)2] gives mixtures of the two isomeric cw-vinylic products (1) and (2) in which the direction of the cis addition is dictated by the bulk of the substituents R and R . The initial mixture is catalytically converted after a few hours by excess hydride into one with higher regioselectivity, possibly via an intermediate dimetallated alkyl, [(r -C5H5)2Zr(Cl)CHRiCHR2(Cl)Zr( /-C5H5)2]. 

Insertion of olefins into the Pt—H bond of square-planar hydrides /ra 5-[(Pt(H)-(X)(L)a] was originally assumed to involve olefin co-ordination to give a five-coordinate intermediate which underwent migratory rearrangement to the alkyl trans-[Pt(R)(X)(L)a]. Recent MO calculations of the insertion reaction between ethylene and [Pt(H)(Cl)(PH3)a] suggest that collapse of the five-co-ordinate intermediate is 

In the reaction of t-butylacetylene with [Pd(Cl)2(PhCN)2] the acetylene is consumed according to a first-order rate law up to the point where three equivalents of acetylene per Pd is reached after which the rate decreases by a factor of ten. Addition of 2,5-dithiahexane at the three-equivalents point gives complex (10) which has the structure illustrated, according to X-ray studies. This indicates that the first stage in the reaction, cis insertion into the Pd—Cl bond, is followed by a second cis insertion to give the (x-butadienyl ligand. Since three moles of acetylene are consumed at the point where the dithiahexane is added, complex (11) is considered to be the precursor of (10). 

The chemical ionization mass spectra of a number of alkenes and alkynes have been investigated using methane as reactant. The compounds studied comprised straight-chain 1-mono-olefins, branched and internal mono-olefins, and a number of compounds of diverse types (di- and tri-olefins, monocyclic and bicyclic mono-olefins, cyclic di-olefins, and acetylenes). 

The chemical ionization spectra of straight-chain 1-olefins consist almost exclusively of two series of ions, namely alkyl (C H2 +1) and alkenyl (C H2 i) ions. In this regard these compounds (and, indeed, mono-olefins in general) resemble the cycloparaffins discussed in the previous section. The intensities of the alkyl and alkenyl ions produced at the various carbon numbers in 1-decene, which serves as a typical 1-mono-olefin, are shown in Fig. 3. The intensities of the (M -f l) and (M - 1) ions are small and decrease as the size of the olefin increases. Extensive fragmentation, which 

The formation of the alkyl and alkenyl series of ions in the chemical ionization of mono-olefins may be explained in the same way as was the case with cycloparaffins namely, the alkyl ions result from an initial proton transfer from and the alkenyl ions result from an initial hydride 

The alkenyl ion written in (14) has the ally lie structure, but alkenyl ions which are not allylic can also be formed in exothermic reactions. One can also write exothermic reactions for the formation of alkenyl ions which involve proton transfer to the olefin followed by loss of hydrogen molecule. All these reactions are strongly exothermic, and consequently extensive fragmentation of the (M H- 1) and (M — I) ions is to be expected and is observed. 

Straight-chain 1-olefins studied. For olefins with fewer than 16 carbon atoms the alkyl ions are formed in greater abundance than the alkenyl ions, and this is contrary to the behavior found with the cycloparaffins, for which the intensities were greater (and usually appreciably 

AIMS To learn to name hydrocarbons with double bonds (alkenes) and triple bonds (alkynes). To understand addition reactions. 

Acetylene gas burning. The acetylene is formed by the reaction of calcium carbide, CaC2, with water in the flask. 

We have seen that alkanes are saturated hydrocarbons—each of the carbon atoms is bound to four atoms by single bonds. Hydrocarbons that contain carbon-carbon double bonds 

Multiple carbon-carbon bonds result when hydrogen atoms are removed from alkanes. Alkenes that contain a carbon-carbon double bond have the general formula C H2 . The simplest alkene, C2H4, commonly known as ethylene, has the Lewis structure 

The system for naming alkenes and alkynes is similar to the one we have used for alkanes. The following rules are useful. 

The enhanced electron density of unsaturated aliphatic hydrocarbons (alkenes and alkynes) allows them to be directly sulfonated on the terminal carbon atom by the action of chlorosulfonic acid (Equations 2 and 3). 

2-Methylpropene (isobutene, 13) by treatment with chlorosulfonic acid in dioxan at 20 °C afforded 2-methylpropene-l,3-disulfonic acid 14 (Equation 4). The disulfonic acid 14 precipitated out from the reaction mixture as the dioxan salt this compound may arise fi om rearrangement of the sterically unfavourable 2-methylpropene-1,1-disulfonic acid (Equation 4). 

Treatment of long chain alkenes containing at least eight carbon atoms and one double bond, e.g. 1-hexadecene, with chlorosulfonic acid and then with 20% sodium hydroxide, yielded useful wetting/cleansing agents. 

The reaction of chlorosulfonic acid with alkenes may be affected by attached groups, e.g. electron-withdrawing halogen and carboxylic acid moieties. 

The electrophilic addition of chlorosulfonic acid to 3,3,3-trifluoropropene 15 was foimd to yield mainly 3-chloro-l,l-difluorochlorosulfonate 16 (Equation 5). ° 

The simplest alkene is ethene, the compound which has previously been called ethylene . Ethene is produced in large quantities because it is one of the most important substances in the production of a wide variety of organic compounds and technological materials. The parent alkyne is ethyne which is marketed under the trade name of acetylene . This substance is also a very important industrial product. Both ethene and ethyne are flammable ethyne mixed with oxygen produces a very hot flame so acetylene is used for gas welding. Ethyne can be easily prepared from calcium carbide and water  

Alkenes are hydrocarbons containing at least one double bond between carbon atoms. Alkynes are hydrocarbons containing at least one triple bond between carbon atoms. Because of the double or triple bond, alkenes and alk5mes have fewer hydrogen atoms than the corresponding alkane and are therefore called unsaturated hydrocarbons because they are not loaded to capacity with hydrogen. As we saw earlier, alkenes have the formula C H2 and alkynes have the formula C H2 -2. The simplest alkene is ethene, C2H4, also called ethylene. 

The simplest alkyne is ethyne, C2H2, also called acetylene. 

Formula Structural formula Space-fiUmg model 

The geometry about each carbon atom in ethyne is linear, making ethyne a linear molecule. Eth3me (or acetylene) is commonly used as fuel for welding torches. The names and sfructures of several other alkynes are shown in Table 18.6. Like alkenes, the alkynes do not have familiar uses ofher than their presence as minority components of gasoline. 

An unsaturated hydrocarbon containing a carbon-carbon double bond alkenes have the general formula C H2.  

In writing shorthand formulas, the hydrogen atoms are often written just after the carbon to which they are attached. For example, the formula for H—C=C—H is often written as CH=CH. 

CHEMICAL CONNECTIONS 4A Ethylene, a Plant Growth Regulator 4B Cis-Trans Isomerism in Vision 4C Why Plants Emit Isoprene 

Alkyne An unsaturated hydrocarbon that contains a carbon-carbon triple bond. 

IN THIS CHAPTER, we begin our study of unsaturated hydrocarbons. An unsaturated hydrocarbon is a hydrocarbon that has fewer hydrogens bonded to carbon than an alkane has. There are three classes of unsaturated hydrocarbons alkenes, alkynes, and arenas. Alkenes contain one or more carbon-carbon double bonds, and alkynes contain one or more carbon-carbon triple bonds. Ethene (ethylene) is the simplest alkene, and ethyne (acetylene) is the simplest alkyne  

The next time you see ripe bananas in the market, you might wonder when they were picked and whether their ripening was artificially induced. 

Remember In a homologous series, the formulas of successive members differ by increments of CH2. 

Alkenes and alkynes are classified as unsaturated hydrocarbons. They are said to be unsaturated because, unlike alkanes, their molecules do not contain the maximum possible number of hydrogen atoms. Alkenes have two fewer hydrogen atoms, and alk3mes have four fewer hydrogen atoms than alkanes with a comparable number of carbon atoms. Alkenes contain at least one double bond between adjacent carbon atoms, while alkynes contain at least one triple bond between adjacent carbon atoms. 

General formula for alkenes C H2 General formula for alkynes C H2 2 

Formulas that Represent Benzene as an aromatic (sextet) of Electrons 

Benzene Represented using Lewis Structure (Must DrawTwo Lewis Structures to be Correct) 

The alkene group closely resembles the alkane family. Ethylene (CH2=CH2) is the first member of the alkene or o/e/ynfamily. The next alkene is propone (CsHg), for which the shorthand formula is CH2=CH-CH3. By removing two hydrogens from any alkane, we can create an alkene. In molecules that have 

If a carbon chain has four or more carbon-carbon bonds, we number the position of the double bond using the lower available number. Compounds with two or more double bonds take the suffix diene or triene. For example, 1,3 butadiene is shown as CH2=CH-CH=CH2. Ethylene and propylene currently rank fourth and fifth in industrial chemical tonnage, just behind three inorganic chemicals (sulfuric acid, nitrogen, and oxygen). 

Compounds in the a/ky/ies family contain carbon-carbon triple bonds. The first member of this family is acetylene HC CH. Acetylene is the only alkyne that has widespread industrial usage it is a fuel for the oxyacetylene torch and welding applications. 

Activation parameters have been measured for the rearrangement of the fluoro-alkene complexes (2) to the alkenyl complexes (3). The results (Table 2) indicate that 

The reactions of [PdCCFaC CFaXPhiPCHaCHjPPha)] with CF3CO2H, CF2HCO2H, and CCI3CO2H in chloroform solution follow second-order kinetics, the activation parameters for the reaction with trifluoroacetic acid being LH = 11.2 kcal mol and = — 35 cal mol K . There is a qualitative correlation between acid strength and rate, and a possible sequence for the reaction is 

However, it is not known at which point in the alkyne complex proton attack occurs. The kinetic pattern for analogous reactions of the complex [PdfCFaC CCF3)(PPh8)2] is more complicated and this is probably a consequence of reversible loss of triphenylphosphine. 

Treatment of [Fe(butadiene)(CO)3] with fluorosulphonic acid in liquid SO2 has been reported to result in the formation of a diprotonated species (5). This di-protonated species, however, can be reformulated as a o-n species (6). The proton scrambling of Hr, He, and Hn can then occur via an equilibrium involving (6) and a small concentration of 

An -ray structure determination of a cationic intermediate (8) isolated during the acylation of [Fe(CO)8(rraAw, ra j-hexa-2,4-diene)] establishes that Friedel-Crafts acylation involves stereospecific endo attack. The stereochemistry of reaction parallels protonation of tricarbonyl(diene)iron, cyclopentadienyl(cyclohexa-l,3-diene)rhodium complexes, and ( j -cyclopentadienyl)rhodium complexes of limonene (9), a-phellandrene (10), and carvone (11).  

Acyl CoA dehydrogenase catalyzes the introduction of a C=C double bond into fatty acids during their metabolism. 

10 Evidence for the Mechanism of Electrophilic Additions Carbocation Rearrangements 

An alkene, sometimes called an olefin, is a hydrocarbon that contains a carbon-carbon double bond. An alkyne is a hydrocarbon that contains a carbon-carbon biple bond. Alkenes occur abundantly in nature, but alkynes are much more rare. Ethylene, for instance, is a plant hormone that induces ripening in fruit, and a-pinene is the major component of turpentine. Life itself would be impossible without such polyalkenes as /3-carotene, a compound that contains 11 double bonds. An orange pigment responsible for the color of carrots, -carotene is a valuable dietary source of vitamin A and is thought to offer some protection against certain types of cancer. 

Ethylene and propylene, the simplest alkenes, are the two most important organic chemicals prodnced industrially. Approximately 28 million tons of ethylene and 17 million tons of propylene are prodnced each year in the United States for use in the synthesis of polyethylene, polypropylene, ethylene 

Online homework forthls chapter can be assigned In Organic OWL. 

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Hydrides are available in many molecular sizes and possessing different reactivities. LiAIH reduces most unsaturated groups except alkenes and alkynes. NaBH is less reactive and reduces only aldehydes and ketones, but usually no carboxylic acids or esters (N.G. Gaylord, 1956 A. Haj6s, 1979). 

Diborane or aUcylboranes are used for reduaion of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

Khan, M. M. T. 1974, Homogeneous Catalysis by Metal Complexes, Vol. II, Activation of Alkenes and Alkynes, Academic Press New York - London... 

Another feature of the Pd—C bonds is the excellent functional group tolerance. They are inert to many functional groups, except alkenes and alkynes and iodides and bromides attached to sp carbons, and not sensitive to H2O, ROH, and even RCO H. In this sense, they are very different from Grignard reagents, which react with carbonyl groups and are easily protonated. 

Aliphatic hydrocarbons include three major groups alkanes alkenes and alkynes Alkanes are hydrocarbons m which all the bonds are single bonds alkenes contain at least one carbon-carbon double bond and alkynes contain at least one carbon-carbon... 

Another name for aromatic hydrocarbons is arenes Arenes have properties that are much different from alkanes alkenes and alkynes The most important aromatic hydrocarbon... 

We will return to the orbital hybridization model to discuss bonding m other aliphatic hydrocarbons—alkenes and alkynes—later m the chapter At this point how ever we 11 turn our attention to alkanes to examine them as a class m more detail... 

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

At this point It s useful to compare some structural features of alkanes alkenes and alkynes Table 9 1 gives some of the most fundamental ones To summarize as we progress through the series m the order ethane ethylene acetylene... 

The C—H bonds of hydrocarbons show little tendency to ionize and alkanes alkenes and alkynes are all very weak acids The acid dissociation constant for methane for exam pie IS too small to be measured directly but is estimated to be about 10 ° (pK 60)... 

The classification of hydrocarbons as aliphatic or aromatic took place m the 1860s when It was already apparent that there was something special about benzene toluene and their derivatives Their molecular formulas (benzene is CgHg toluene is C7Hj ) indicate that like alkenes and alkynes they are unsaturated and should undergo addition reac tions Under conditions m which bromine for example reacts rapidly with alkenes and alkynes however benzene proved to be inert Benzene does react with Bi2 m the pres ence of iron(III) bromide as a catalyst but even then addition isn t observed Substitu tion occurs instead ... 

Hydrogenation of benzene and other arenes is more difficult than hydrogenation of alkenes and alkynes Two of the more active catalysts are rhodium and platinum and it IS possible to hydrogenate arenes m the presence of these catalysts at room temperature and modest pressure Benzene consumes three molar equivalents of hydrogen to give cyclohexane... 

Monocyclic Aliphatic Hydrocarbons. Monocyclic aliphatic hydrocarbons (with no side chains) are named by prefixing cyclo- to the name of the corresponding open-chain hydrocarbon having the same number of carbon atoms as the ring. Radicals are formed as with the alkanes, alkenes, and alkynes. Examples ... 

The Fiiedel-Ciafts alkylation of aiomatics with the lesonance-stabihzed ttichloiocyclopiopenium ttiflate offers a synthetic pathway to ttiaiyl cyclopiopenium salts (26). The ttichloiocyclopiopenium ion has also been shown to undergo Friedel-Crafts reaction with alkenes and alkynes to give trivinyl and tri(halovinyl) cyclopiopenium ions. 

Dia ene deductions. Olefins, acetylenes, and azo-compounds are reduced by hydrazine in the presence of an oxidizing agent. Stereochemical studies of alkene and alkyne reductions suggest that hydrazine is partially oxidized to the transient diazene [3618-05-1] (diimide, diimine) (9) and that the cis-isomer of diazene is the actual hydrogenating agent, acting by a concerted attack on the unsaturated bond ... 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

In the case of vinylfurans and vinylpyrroles there is the possibility of cycloaddition involving either the cyclic diene system or the diene system including the double bond. 2-Vinylfuran reacts in high yield with maleic anhydride in ether at room temperature to form the adduct involving the exocyclic double bond. Similarly, 2- and 3-vinylpyrroles react with 7T-electron-deficient alkenes and alkynes under relatively mild conditions to give the corresponding tetrahydro- and dihydro-indoles (Scheme 51) (80JOC4515). 

Burger s criss-cross cycloaddition reaction of hexafluoracetone-azine (76S349) is also a synthetic method of the [CNN + CC] class. In turn, the azomethines thus produced, (625) and (626) (79LA133), can react with alkenes and alkynes to yield azapentalene derivatives (627) and (628), or isomerize to A -pyrazolines (629) which subsequently lose HCF3 to afford pyrazoles (630 Scheme 56) (82MI40401). 

Monosubstituted hydrazones react with alkenes and alkynic compounds to yield pyrazolidines and pyrazolines, respectively (71LA(743)50, 79JOC218). Oxidation often occurs during the reaction and pyrazoles are isolated as the end product. 

When compound (443), which contains alkene and alkyne moieties, was reacted with benzonitrile oxide, both an isoxazoline (444) and isoxazole (445) were produced, with the former predominating. Oxidation of (444) with permanganate produced 3-phenyl-2-isoxazoline-5-carboxylic acid (446) (67ZOR82i). The reaction of 1-trimethylsilylbut-l-yne-3-ene produced only a compound which reacted at the alkenic unit. Oxidation of the adduct also produced (446) (68ZOB1820). These reactions are shown in Scheme 102. 

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