Hydrocarbons alkenes and alkynes unsaturated

In an unsaturated hydrocarbon, at least one of the carbon-carbon bonds in the molecule is a multiple bond. As a result, there are fewer hydrogen atoms in an unsaturated hydrocarbon than in a saturated one with the same number of carbons. We will consider two types of unsaturated hydrocarbons— 

Replacing a single bond with a double bond eliminates two hydrogen atoms. Hence the general formula of an alkene is 

Strategy Apply the general formulas C H2 +2 (alkane), C H2 (alkene), C H2 -2 (alkyne). 

Click Coached Problems for a self-study module and a simulation on naming alkenes. 

The simplest alkene is ethene, C2H4 (common name, ethylene). Its structural formula is 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene 

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While the above details provide a general procedure for handling mixtures of acidic, basic and neutral components, other selective extraction reagents may be utilised in certain special instances. For example, cold concentrated sulphuric acid will remove unsaturated hydrocarbons (alkenes and alkynes) present in... 

When unsaturated hydrocarbons (alkenes and alkynes) are saturated, alkanes are produced. In this type of reaction, Ni, Pt or Pd are used as a catalyst. 

Describe unsaturated hydrocarbons (alkenes and alkynes)—their structures and their nomenclature... 

Saturated aliphatic hydrocarbons (alkanes) generally do not react with chlorosulfonic acid, although the more nucleophilic unsaturated hydrocarbons (alkenes and alkynes) are often reactive and may sometimes be directly sulfonated by the reagent. 

Saturated hydrocarbons have only single bonds unsaturated Ihydrocarbons have one or more multiple bonds. Alkanes are saturated hydrocarbons. Alkenes and alkynes are unsaturated hydrocarbons the former have carbon-carbon double bonds and the latter have triple bonds. 

Hydrocarbons are the simplest organic compounds, containing only carbon and hydrogen. There are three families of hydrocarbons. The alkanes have only single bonds and are said to be saturated. Alkanes are very stable and generally unreactive. Alkenes and alkynes have multiple bonds between two adjacent carbon atoms and are said to be unsaturated. This unsaturation makes alkenes and alkynes more reactive than alkanes. 

Aliphatic Hydrocarbons Aliphatic hydrocarbons are straight or branched chains of carbon and hydrogen. These include alkanes, alkenes, cycloalkanes, acetylenes, and arenes. Generally, ahphatic hydrocarbons, with the exception of hexane, exhibit toxic health effects at high concentrations. Unsaturated aliphatic hydrocarbons, alkenes and alkynes, are similarly inert in the body. Examples of ahphatic hydrocarbons and their 8-hour time-weighted average TLVs are n-hexane (50 ppm), hexane (500 ppm), and octane (300 ppm). 

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). 

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 ... 

Unsaturated hydrocarbons undergo a variety of reactions. Experimentally, alkenes and alkynes undergo addition reactions, whereas aromatic molecules, such as benzene, undergo substitution reactions instead. Why ... 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

Tetraborane(lO) reacts, as does diborane, readily with unsaturated hydrocarbons. Alkenes give access to B2,B4-alkylene bridged B4H8(RCH-CHR) organo-boranes [37] while alkynes, allenes and l-ene-3-ynes lead to a variety of carba-boranes [38-40]. 

Hydrogen undergoes catalytic hydrogenation adding to unsaturated hydrocarbons, such as alkenes and alkynes forming alkanes. The reaction is catalyzed by nickel, platinum or palladium catalysts at ambient temperature. Hydrogenation of benzene over platinum catalyst yields cyclohexane, C6H12. 

Alkenes and alkynes are unsaturated hydrocarbons. Alkenes contain at least one carbon-carbon double bond and alkynes have at least one carbon-carbon triple bond. The names of alkenes and alkynes use the alkane prefixes, but add ene and yne endings, respectively (Table 15.3). 

Unsaturated hydrocarbons (alkenes, dienes) react with carbon monoxide and a proton source (H20, alcohols, amines, acids) under strong acidic conditions to form carboxylic acids or carboxylic acid derivatives. Since a carbocationic mechanism is operative, not only alkenes but also other compounds that can serve as the carbocation source (alcohols, saturated hydrocarbons) can be carboxylated. Metal catalysts can also effect the carboxylation of alkenes, dienes, alkynes, and alcohols. 

The addition of hydrogen across multiple bonds is one of the most widely studied of catalytic reactions. Alkenes and alkynes, as well as di- and polyunsaturated systems can all be hydrogenated, provided the suitable experimental conditions are used. Studies on the ways in which these compounds react with hydrogen have revealed very complex reaction patterns. Because of their resonance stabilization, carbocyc-lic aromatic hydrocarbons are more difficult to hydrogenate than are other unsaturated compounds. 

In contrast to alkanes, which have only single bonds, alkenes and alkynes have multiple bonds Alkenes are hydrocarbons that contain a carbon-carbon double bond, and alkynes are hydrocarbons that contain a carbon-carbon triple bond. Both groups of compounds are unsaturated, meaning that they have fewer hydrogens per carbon than the related alkanes. Ethylene (H2C = CH2), for example, has the formula C2H4, whereas ethane (CH3CH3) has the formula C2H6. 

In the transition from sp3 to sp2 and to sp hybridization of the valency of carbon, i.e. from alkanes to alkenes, and alkynes, the polarity of the CH bond is increased and the mobility of the hydrogen becomes greater. Electrons belonging to multiple bonds may take part in the formation of donor-acceptor complexes, and unsaturated hydrocarbons are stronger bases than saturated hydrocarbons. 

Chapter 4 centers on two key transformations in organic synthesis (1) oxidation of alcohols and of unsaturated hydrocarbons (i.e., alkenes and alkynes) to carbonyl compounds (2) reduction of various carbonyl compounds to alcohols. 

Cycloaddition with unsaturated hydrocarbons such as alkenes and alkynes have been explored (68-74). Schrauzer and Mayweg (74) first reported that M(sdt)2 (M = Ni, Pd, Pt) reacts with alkynes and alkenes via cycloaddition to the dithiolene ligand. With alkynes, dithiane is formed via decomposition of the adduct (Scheme 7). 

Because the unsaturated hydrocarbon has to bind to rhodium in the presence of bulky PPh3 groups, the catalyst favours unsubstituted double bonds (RCH=CH2 rather than RR C=CR r ). Since the alkyl intermediate is shortlived, there is little tendency to / -elimination with concomitant alkene isomerization. Although both alkene and alkyne functions are reduced, in general carbonyl or carboxylic groups and benzene rings are not, though aldehydes are frequently decarbonylated. Peroxides tend to oxidize and thus destroy the catalyst, so that substrates need to be purified carefully before use. 

A major effort to study the chemistry of the zero oxidation state lanthanides on a preparative scale involved their reactivity with neutral unsaturated hydrocarbons 14, 60). This class of reagents was of interest because reactions of unsaturated hydrocarbons with metals constitute such an important component of organometallic chemistry and because species such as alkenes and alkynes were not common as ligands or reactants in organolanthanide chemistry at that time. 

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