Unsaturated hydrocarbons

Alkenes are hydrocarbons containing at least one double bond. For alkenes with only one double bond, the empirical formula is CnH,/ . The names for all alkenes end with -ene. The naming system on alkenes is the same as alkanes, with the exception of the -ene ending. For example, the two-carbon alkane, C Hg, is called ethane, while the two-carbon alkene, (Jf. is called ethene. 

For alkenes longer than 3 carbons, the double bond can be in different places in the molecule. Writing a number before tlie name of the alkene identifies the location of the bond. The number represents the bond number in the molecule, rather than the carbon number. The bond between carbons 1 and 2 is labeled 1, the bond between carbons 2 and 3 is labeled 2, and so on. If more than one double bond is found, a prefix is used to show how many are present and numbers are given to each one. The following figure shows two different possible structures of pentcnes  

In addition to the locations of the double bonds, another difference of alkenes is the molecule s inability to rotate at the double bond. With alkanes, when substituent groups attach to a carbon, the molecule can rotate around the C-C bonds in response to electron-electron repulsions. Because the double bond in the alkene is composed of both sigma and pi bonds, the molecule can t rotate around the double bond (see Chapter 6). What this means for alkenes is that the molecule can have different structural orientations around the double bond. These different orientations allow a new kind of isomerism, known as geometrical isomerism. When the non-hydrogen parts of the molecule are on the same side of the molecule, the term cis- is placed in front of the name. When the non-hydrogen parts are placed on opposite sides of the molecule, the term trans- is placed in front of the name. In the previous section, you saw that the alkane butane has only two isomers. Because of geometrical isomerism, butene has four isomers, shown below  

Alky nes are hydrocarbons containing at least one triple bond. The empirical formula for alkynes is and the names end in -yne. For example, the 

The naming system for alkynes is the same as for alkenes, except that alkynes do not have geometrical isomers. The triple bond only allows a single group to extend off of the carbon atoms attached by a triple bond, so no cis- or trans- isomers are possible. 

Answer First, notice that the longest carbon chain is 5 carbons long, making this pentene. 

Contain the group c=C, i.e., carbon atoms joined by two bonds /  

The two bonds are called a double bond, and the compound is unsaturated because it contains less hydrogen atoms than the corresponding paraffin, which is considered saturated (with hydrogen) CH2=CH2 CH2=CHCH3 CH2=CHCH2CH3 

One of the most important reactions in the chemical industry is the cracking (removing hydrogen) of paraffins to form olefins. Olefins are essential starting materials for many common products that we all use, particularly plastics. The simplest olefin is ethylene, which can be polymerized (reacted many times with itself) to give po/yethylene used for bottles, dishware, film, etc. The next higher olefin, propylene, can be polymerized to polypropylene used in the manufacture of rugs, toys, kitchenware, and many other plastic objects. Note that the names of olefins end in -ene, which denotes the presence of a double bond in the compound. 

The DCD model gives a good description of the final bonding to the surface, but does not describe the energetics and bond-formation. In order to address such questions we will use a different viewpoint which takes into consideration which 

Adsorbate Electronic Structure and Bonding on Metal Surfaces 

The reason that the tt -orbital does not mix significantly with the a, as it does with the occupied orbitals, can be related to the energy splitting of 10 eV between the tt and a, which is substantially larger than the energy separation of 4eV from the occupied orbitals. In C2v symmetry the cf cannot mix with the occupied a-orbitals due to symmetry reasons. 

Along the Y-direction (across the rows), which is shown in the middle of Fig. 2.35, there are two main features, assigned to the lb3u and lb2g at, respectively, 10.4 and 7.6 eV on Cu, and at 9.3 and 6.6 eV on Ni. For both metals there is also some intensity which can be assigned to the rehybridization and mixing with -states starting at 4.8 eV. 

C3H50 Prop-2-en-l-ol (allyl alcohol) (Other values on Vol.3, p.290) L 

Bibliography 79MC Other references 65HA, 71LFG,78SSR  

C3H1203N2 5,5-Diethylbarbituric acid (Other references on Vol.2, (barbitol). p.342) HL 

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Kolbe reaction The pre >aration of saturated or unsaturated hydrocarbons by the electrolysis of solutions of the alkali salts of aliphatic carboxylic acids. Thus, ethanoic acid gives ethane,... 

Acetylenes are another series of unsaturated hydrocarbons which include compounds containing a carbon-carbon triple bond, for example acetylene Itself ... 

Alcoholic potassium hydroxide gives an unsaturated hydrocarbon, often however in only low yield with the earlier members of the series. 

Many aldehydes and ketones can be reduced directly by Clenimemen s method, in which the aldehyde or ketone is boiled with dilute hydrochloric acid and amalgamated zinc. />-Methylacetophenone (or methyl />-tolyl ketone) is reduced under these conditions to />-ethyltoluene. An excess of the reducing agent is employed in order to pre ent the formation of unsaturated hydrocarbons. 

It has been known for more than a century that hydrocarbons containing double bonds are more reactive than their counterparts that do not contain double bonds. Alkenes are, in general, more reactive than alkanes. We call electrons in double bonds 71 electrons and those in the much less reactive C—C or CH bonds Huckel theory, we assume that the chemistry of unsaturated hydrocarbons is so dominated by the chemistry of their double bonds that we may separate the Schroedinger equation yet again, into an equation for potential energy. We now have an equation of the same fomi as Eq. (6-8), but one in which the Hamiltonian for all elections is replaced by the Hamiltonian for Ji electrons only... 

Chemical methods may be employed if the reagent attacks only one of the components. Thus quicklime may be employed for the removal of water in the preparation of absolute ethyl alcohol. Also aromatic and unsaturated hydrocarbons may be removed from mixtures with saturated hydrocarbons by sulphonation. 

Cold concentrated sulphuric acid will remove unsaturated hydrocarbons present in saturated hydrocarbons, or alcohols and ethers present in alkyl halides. In the former case soluble sulphonated products are formed, whilst in the latter case alkyl hydrogen sulphates or addition complexes, that are soluble in the concentrated acid, are produced. 

All hydrocarbons prepared by th -. Wurtz reaction contain small quantities of unsaturated hydrocarbons. These may be removed by shaking repeatedly with 10 pier cent, of the volume of concentrated sulphuric acid until the acid is no longer coloured (or is at most extremely pale yellow) each shaking should be of about 5 minutes duration. The hydrocarbon is washed with water, 10 pier cent sodium carbonate solution, water (twice), and dried with anhydrous magnesium or calcium sulphate. It is then distilled from sodium two distillations are usually necessary... 

Cool 1 ml. of amylene in ice and add 1 ml. of cold, dilute sulphuric acid (2 acid 1 water), and shake gently until the mixture is homogeneous. Dilute with 2 ml. of water if an upper layer of the alcohol does not separate immediately, introduce a little sodium chloride into the mixture in order to decrease the solubility of the alcohol. Observe the odour. The unsaturated hydrocarbon is thus largely reconverted into the alcohol from which it may be prepared. 

Chakactkrisation of Unsaturatkd Aliphatic Hydrocarbons Unlike the saturated hydrocarbons, unsaturated aliphatic hydrocarbons are soluble in concentrated sulphuric acid and exhibit characteristic reactions with dUute potassium permanganate solution and with bromine. Nevertheless, no satisfactory derivatives have yet been developed for these hydrocarbons, and their characterisation must therefore be based upon a determination of their physical properties (boiling point, density and refractive index). The physical properties of a number of selected unsaturated hydrocarbons are collected in Table 111,11. 

The alicyclic secondary alcohol, cycZohexanol, may be dehydrated by concentrated sulphuric acid or by 85 per cent, phosphoric acid to cyciohexene. It has a higher boiling point (82-83°) than amylene and therefore possesses some advantage over the latter in.the study of the reactions of unsaturated hydrocarbons. 

This unsaturated hydrocarbon is easily prepared by the action of sodium upon allyl iodide or bromide ... 

The n-butylbenzene contains some unsaturated hydrocarbons these can be removed by repeated shaking with small quantities of concentrated sulphuric acid (see Section III,7,No[Pg.512]

Unsaturated hydrocarbons are present in nearly all products of the Clemmensen reduction of aromatic ketones and must be removed, if the hydrocarbon is requiral pure, by the above process. Secondary alcohols, often produced m small amount are not appreciably steam-volatile. 

The mechanism of the reaction la not known with certainty. It is known from studies utilising as tracer that no change in the carbon skeleton occurs during the reaction, and also that unsaturated hydrocarbons can undergo reactions very similar to those of ketones thus both styiene and phenyl-acetylene can react with sulphur and morpholine to produce phenylaceto-thiomorphoUde, hydrolysis of which yields phenylacetic acid ... 

In addition to inorganic radicals, which profoundly modify the properties of a paraflSn hydrocarbon residue, there is a whole series of organic groupings which are distinguished by exceptional reactivity, for example, the ethylene and acetylene groupings, and the phenyl and naphthyl radicals. Thus the characterisation of unsaturated hydrocarbons and their derivatives, e.g., the aromatic compounds, becomes possible. 

Concentrated sulphuric acid. The paraffin hydrocarbons, cych-paraffins, the less readily sulphonated aromatic hydrocarbons (benzene, toluene, xylenes, etc.) and their halogen derivatives, and the diaryl ethers are generally insoluble in cold concentrated sulphuric acid. Unsaturated hydrocarbons, certain polyalkylated aromatic hydrocarbons (such as mesitylene) and most oxygen-containing compounds are soluble in the cold acid. 

Unsaturated hydrocarbons dissolve through the formation of soluble alkyl hydrogen sulphate ... 

Group VI. Concentrated sulphuric acid provides a simple test for the diflferentiation inter alia between (a) saturated paraffin and cyclic hydrocarbons and also simple aromatic hydrocarbons and (b) unsaturated hydrocarbons. 

Unsaturated hydrocarbons are found in Solubility Group V. The two reagents employed for the detection of unsaturation, not only of unsaturated hydrocarbons but of all cla.sses of unsaturated compounds (RR C=CR"R" ), are ... 

Alkylation combines lower-molecular-weight saturated and unsaturated hydrocarbons (alkanes and alkenes) to produce high-octane gasoline and other hydrocarbon products. Conventional paraffin-olefin (alkane-alkene) alkylation is an acid-catalyzed reaction, such as combining isobutylene and isobutane to isooctane. 

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

Alkenes are commonly described as unsaturated hydrocarbons because they have the capacity to react with substances which add to them Alkanes on the other hand are said to be saturated hydrocarbons and are incapable of undergoing addition reactions... 

Alkenes are unsaturated hydrocarbons and react with substances that add to the dou... 

Unsaturated hydrocarbon (Section 6 1) A hydrocarbon that can undergo addition reactions that is one that contains multiple bonds... 

Unsaturated hydrocarbons and other compounds of intermediate polarity ... 

Fig. 3. Adsorption of hydrocarbons by 2eobtes is much greater for unsaturated hydrocarbons whose molecules contain double or triple bonds. From top to bottom, the curves show adsorption (at 150°C) of propjiene, ethylene, acetjiene, and isobutjiene (unsaturated) and propane, ethane, and methane...

Chlorine reacts with saturated hydrocarbons either by substitution or by addition to form chlorinated hydrocarbons and HCl. Thus methanol or methane is chlorinated to produce CH Cl, which can be further chlorinated to form methylene chloride, chloroform, and carbon tetrachloride. Reaction of CI2 with unsaturated hydrocarbons results in the destmction of the double or triple bond. This is a very important reaction during the production of ethylene dichloride, which is an intermediate in the manufacture of vinyl chloride ... 

It is convenient to divide the petrochemical industry into two general sectors (/) olefins and (2) aromatics and their respective derivatives. Olefins ate straight- or branched-chain unsaturated hydrocarbons, the most important being ethylene (qv), [74-85-1] propjiene (qv) [115-07-17, and butadiene (qv) [106-99-0J. Aromatics are cycHc unsaturated hydrocarbons, the most important being benzene (qv) [71-43-2] toluene (qv) [108-88-3] p- s.y en.e [106-42-3] and (9-xylene [95-47-5] (see Xylenes and ethylbenzene) There are two other large-volume petrochemicals that do not fall easily into either of these two categories ammonia (qv) [7664-41-7] and methanol (qv) [67-56-1]. These two products ate derived primarily from methane [74-82-8] (natural gas) (see Hydrocarbons, c -c ). 

In Group 14 (IV), carbon serves as a Lewis base in a few of its compounds. In general, saturated ahphatic and aromatic hydrocarbons are stable in the presence of BF, whereas unsaturated ahphatic hydrocarbons, such as propjdene or acetylene, are polymerized. However, some hydrocarbons and their derivatives have been reported to form adducts with BF. Typical examples of adducts with unsaturated hydrocarbons are 1 1 adducts with tetracene and 3,4-benzopyrene (39), and 1 2 BF adducts with a-carotene and lycopene (40). 

Acetylene Derived from Hydrocarbons The analysis of purified hydrocarbon-derived acetylene is primarily concerned with the determination of other unsaturated hydrocarbons and iaert gases. Besides chemical analysis, physical analytical methods are employed such as gas chromatography, ir, uv, and mass spectroscopy. In iadustrial practice, gas chromatography is the most widely used tool for the analysis of acetylene. Satisfactory separation of acetylene from its impurities can be achieved usiag 50—80 mesh Porapak N programmed from 50—100°C at 4°C per minute. 

For environmental reasons, burning should be smokeless. Long-chain and unsaturated hydrocarbons crack in the flame producing soot. Steam injection helps to produce clean burning by eliminating carbon through the water gas reaction. The quantity of steam required can be as high as 0.05—0.3 kg steam per kg of gas burned. A multijet flare can also be used in which the gas bums from a number of small nozzles parallel to radiant refractory rods which provide a hot surface catalytic effect to aid combustion. 

Petroleum Oils. When satisfactorily stable kerosene—soap—water emulsions were produced in 1874, dormant (winter) oil sprays became widely used to control scale insects and mites (1). The first commercial emulsion or miscible oil was marketed in 1904 and by 1930 highly refined neutral or white oils, free from unsaturated hydrocarbons, acids, and highly volatile elements, were found to be safe when appHed to plant foHage, thus gready enlarging the area of usefulness of oil sprays (see Petroleum). 

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