Alkenes properties

Colourless liquid with alkenic properties. Many substituted derivatives are known, the preferred method of preparation being the addition of an alkyne to a cyclobutadiene. 

Dienes are simply alkenes that contain two carbon-carbon double bonds. They therefore have essentially the same properties as the alkenes we have already studied. For certain of the dienes, these alkene properties are modified in important ways we shall focus our attention on these modifications. Although we shall consider chiefly i//enes in this section, what we shall say applies equally well to compounds with more than two double bonds. 

Over the past decade it has been established that for various substituents the i C chemical shift increment is a constitutive property. This applies to many systems e.g. benzenes, alkanes and alkenes. The availability of over 200 allenes, randomly substituted with groups of different nature, enabled us to prove that in the case of allenes the chemical shift increment is a constitutive property too, thus establishing a convenient method for estimating i ( C) values for allenes. 

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

The physical properties of se lected alkenes are collected in Appendix 1... 

Alkenes resemble alkanes m most of their physical properties The lower molecular weight alkenes through 4 are gases at room temperature and atmospheric pressure The dipole moments of most alkenes are quite small Among the 4 isomers 1 butene cis 2 butene and 2 methylpropene have dipole moments m the 0 3-05 D range trans 2 butene has no dipole moment Nevertheless we can learn some things about alkenes by looking at the effect of substituents on dipole moments... 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Having examined the properties of alkenes and introduced the elements of polymers and poly merization let s now look at some commercial applications of ethylene and propene... 

Alkynes resemble alkanes and aUcenes m their physical properties They share with these other hydrocarbons the properties of low density and low water solubility They are slightly more polar and generally have slightly higher boiling points than the corre spondmg alkanes and alkenes... 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

By analogy to the hydration of alkenes hydration of an alkyne is expected to yield an alcohol The kind of alcohol however would be of a special kind one m which the hydroxyl group is a substituent on a carbon-carbon double bond This type of alcohol IS called an enol (the double bond suffix ene plus the alcohol suffix ol) An important property of enols is their rapid isomerization to aldehydes or ketones under the condi tions of their formation... 

The physical properties (boiling point solubility m water dipole moment) of alkynes resemble those of alkanes and alkenes... 

Not all the properties of alkenes are revealed by focusing exclusively on the func tional group behavior of the double bond A double bond can affect the proper ties of a second functional unit to which it is directly attached It can be a sub stituent for example on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene... 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

An important property of aromatic hydrocarbons is that they are much more stable and less reactive than other unsaturated compounds Ben zene for example does not react with many of the reagents that react rapidly with alkenes When reaction does take place substitution rather than addition is observed The Kekule formulas for benzene seem mcon sistent with its low reactivity and with the fact that all of the C—C bonds m benzene are the same length (140 pm)... 

Bromine although it adds rapidly to alkenes is too weak an electrophile to react at an appreciable rate with benzene A catalyst that increases the electrophilic properties of bromine must be present Somehow carpet tacks can do this How7... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

A.lpha-Olefm Sulfonates. Sulfonation of alpha-olefins yields a mixture of alkene sulfonates, hydroxyalkane sulfonates, and some amount of various disulfonates. These detergents are excellent foamers with good detergency properties. They are unaffected ia hard water and thek effects are considered superior to the alkyl ether sulfates (9). 

Chemical Properties. Reactions of quaternaries can be categorized iato three types (169) Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la) (170). This reaction has not been used extensively to prepare olefins. Some cycHc olefins, however, are best prepared this way (171). Exhaustive methylation, followed by elimination, is known as the Hoffman degradation and is important ia the stmctural determination of unknown amines, especially for alkaloids (qv) (172). 

Analysis. Butenes are best characterized by their property of decolorizing both a solution of bromine in carbon tetrachloride and a cold, dilute, neutral permanganate solution (the Baeyer test). A solution of bromine in carbon tetrachloride is red the dihaUde, like the butenes, are colorless. Decoloration of the bromine solution is rapid. In the Baeyer test, a purple color is replaced by brown manganese oxide (a precipitate) and a colorless diol. These tests apply to all alkenes. 

Heat Capacity. The multiple property estimation methods for constant pressure ideal-gas heat capacities cover a broad range of organic compounds (188,216,217). Joback s method (188) is the easiest to use however, usage of all these methods has been recommended only over the range 280—1100 K (7). An accurate method for ideal-gas heat capacities (constant pressure), limited to hydrocarbons, has been presented (218) that involves a fit of seven variables, and includes steric, ring, branching, alkene, and even allene corrections. 

In theory, three isoxazolines are capable of existence 2-isoxazoline (2), 3-isoxazoline and 4-isoxazoline. The position of the double bond may also be designated by the use of the prefix A with an appropriate numerical superscript. Of these only the 2-isoxazolines have been investigated in any detail. The preparation of the first isoxazoline, 3,5-diphenyl-2-isoxazoline, from the reaction of )3-chloro-)3-phenylpropiophenone with hydroxylamine was reported in 1895 (1895CB957). Two major syntheses of 2-isoxazolines are the cycloaddition of nitrile A-oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamine. Since 2-isoxazolines are readily oxidized to isoxazoles and possess some of the unique properties of isoxazoles, they also serve as key intermediates for the synthesis of other heterocycles and natural products. 

Ethers, esters, amides and imidazolidines containing an epithio group are said to be effective in enhancing the antiwear and extreme pressure peiformance of lubricants. Other uses of thiiranes are as follows fuel gas odorant (2-methylthiirane), improvement of antistatic and wetting properties of fibers and films [poly(ethyleneglycol) ethers of 2-hydroxymethyl thiirane], inhibition of alkene metathesis (2-methylthiirane), stabilizers for poly(thiirane) (halogen adducts of thiiranes), enhancement of respiration of tobacco leaves (thiirane), tobacco additives to reduce nicotine and to reduce phenol levels in smoke [2-(methoxymethyl)thiirane], stabilizers for trichloroethylene and 1,1,1-trichloroethane (2-methylthiirane, 2-hydroxymethylthiirane) and stabilizers for organic compounds (0,0-dialkyldithiophosphate esters of 2-mercaptomethylthiirane). The product of the reaction of aniline with thiirane is reported to be useful in the flotation of zinc sulfide. 

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