Chemistry · Ch 15 — Hydrocarbons
Chemical properties of alkenes
Chemical properties of alkenes
Alkenes are considerably more reactive than alkanes because the pi electrons of the C=C double bond are held less tightly than the electrons of a sigma bond and are exposed above and below the plane of the double bond, making them readily available to attack an electron-seeking (electrophilic) reagent; this is why the dominant reaction type for alkenes is electrophilic addition across the C=C bond. Several distinct addition reactions follow this pattern. Addition of dihydrogen (hydrogenation, already covered under alkane preparation in section 15.1.3/15.1.4) saturates the double bond completely. Addition of a halogen (X2 = Cl2 or Br2, in CCl4 at room temperature) gives the corresponding vicinal dihalide, and because the red-brown colour of Br2 in CCl4 rapidly disappears whenever this addition occurs, it is used as a simple test for the presence of a C=C double bond. Addition of a hydrogen halide (HX) gives an alkyl halide; for a symmetrical alkene like ethene only one product is possible, but for an unsymmetrical alkene the two possible products form in unequal amounts, and which one dominates is governed by Markovnikov's rule -- the negative part of the unsymmetrical reagent attaches to the carbon bearing fewer hydrogen atoms. When HBr specifically is added in the presence of an organic peroxide such as benzoyl peroxide, the regiochemistry reverses entirely (the anti-Markovnikov or peroxide/Kharasch-Mayo effect); this peroxide effect is unique to HBr and does not alter the outcome of HCl or HI addition. Addition of cold, concentrated sulphuric acid gives an alkyl hydrogen sulfate by the same Markovnikov selectivity, and this intermediate hydrolyses on dilution with water and heating to give an alcohol with the same alkyl skeleton -- a genuine industrial route to alcohols, though because it is Markovnikov-selective it naturally favours secondary/tertiary alcohols and cannot deliver primary ones from an unsymmetrical alkene. Ozonolysis cleaves the C=C bond entirely: ozone first forms an unstable ozonide, which is then decomposed by zinc dust and water into two carbonyl compounds (aldehydes and/or ketones), with a C=O appearing at each original double-bond carbon -- since the resulting fragments' structures exactly mirror the original alkene, this reaction is used to both locate and count double bonds in an unknown alkene. Hydroboration-oxidation is a two-step sequence -- diborane adds across the double bond in THF solvent to form a trialkylborane, which is then oxidised with alkaline hydrogen peroxide to a primary alcohol -- that delivers the anti-Markovnikov product, complementing the Markovnikov-selective sulphuric-acid hydration route. Polymerization joins a very large number of alkene monomer molecules into one giant repeating-unit polymer molecule, as when ethene, under high temperature/pressure with a trace of oxygen, gives polyethene. Hydroxylation with cold, dilute, alkaline potassium permanganate adds two -OH groups across the double bond to …
Worked out. Alkenes add Cl2 or Br2 in CCl4 at room temperature to form the corresponding vicinal dihalide; ethene plus Cl2 gives 1,2-dichloroethane, and propene plus Br2 gives 1,2-dibromopropane. Iodine generally fails to add. This reaction is the basis of the classic bromine-water/bromine-in-CCl4 test for unsaturation: the red-brown colour of Br2 in CCl4 rapidly disappears when a C=C double bond is present, because the coloured Br2 is consumed as it adds ac …
Worked out. Addition of HX to a symmetrical alkene like ethene gives only one product (bromoethane from ethene + HBr). For an unsymmetrical alkene such as propene, HBr gives two isomeric products, but the major one is 2-bromopropane, not 1-bromopropane. Markovnikov (1869) generalised this: when an unsymmetrical reagent adds to an unsymmetrical alkene, the negative part of the reagent attaches to the carbon bearing the fewer hydrogen atoms. In 1933 Kharasch and Mayo found the opposite regiochemistry when HBr is added in the presence of an organic peroxide such as benzoyl peroxide: propene plus HBr with benzoyl peroxide gives mainly 1-bromopropane, and 2-methylprop-1-ene plus HBr with benzoyl peroxide gives mainly 1-bromo-2-methylpropane -- the reverse of the plain-Markovnikov outcome. This peroxide effect (also called the Kharasch-Mayo effect, or anti-Markovnikov addition) is specif …
Worked out. Cold concentrated H2SO4 adds across an alkene's C=C bond by Markovnikov's rule to give an alkyl hydrogen sulfate (R-OSO3H): ethene gives ethyl hydrogen sulfate, and propene gives isopropyl hydrogen sulfate. Diluting the alkyl hydrogen sulfate with water and heating hydrolyses it to the alcohol with the same alkyl skeleton -- ethyl hydrogen sulfate gives ethanol -- making this a large-scale industrial route to alcohols. Because the addition is Markovnikov-selective, this method naturally favours secondary/tertiary alcohols (isopropyl hydrogen sulfate hydrolyses to propan-2-ol; 2-methylprop-1-ene similarly gives 2-methylpropan-2-ol on direct acid-catalysed hydration) and cannot deliver primary alcohols such as propan-1-ol or 2-methylpropan-1-ol, which instead requi …
Worked out. Passing ozone gas into a solution of the alkene in an inert solvent (carbon tetrachloride) forms an unstable ozonide across the former C=C bond; treating this ozonide with water in the presence of zinc dust (which prevents H2O2 forming and over-oxidising the product) cleaves it into two carbonyl compounds -- an aldehyde and/or a ketone -- with a C=O forming at each original doubly-bonded carbon. Ethene gives two molecules of formaldehyde (HCHO); propene gives one formaldehyde and one acetaldehyde (CH3CHO). Because the number and arrangement of carbons in the resulting carbonyl fragments exactly mirrors the original alkene's structure, ozonolysis is used to both locate the posi …
Worked out. Alkenes react with diborane (BH3)2 in tetrahydrofuran (THF) to form a trialkylborane (hydroboration); oxidising this trialkylborane with alkaline H2O2 then gives a primary alcohol. The overall two-step sequence delivers the anti-Markovnikov product from an unsymmetrical alkene, complementing the Markovnikov-selective sulphuric-acid/hydration route. Ethene plus diborane gives triethylborane, which oxidises to ethanol; propene plus diborane gives tri-n-propylborane, which oxidises to propan-1-ol (n-propyl alcohol) -- exactly the primary alcohol that Markovnikov-select …
Worked out. Polymerization is the joining of a very large number of small monomer molecules into one giant repeating-unit polymer molecule. Ethene, under high temperature and pressure with a trace of oxygen, polymerizes into polyethene (polyethylene), written as -(CH2-CH2-)n where n is a large number. Analogous monomer-to-polymer conversions include propene to polypropene (polypropylene), vinyl chloride (CH2=CHCl) to polyvinyl chloride, and tetrafluoroethen …
Worked out. Alkenes react with cold, dilute alkaline potassium permanganate (plus water) to form a 1,2-diol (glycol), with the purple KMnO4 colour disappearing as the reaction proceeds; ethene gives ethane-1,2-diol, and propene gives propane-1,2-diol. Because this decolourisation only happens when a C=C double bond is present, it is used as a qualitative test for unsaturation, called Baeyer's te …
Worked out. Unlike the mild cold-dilute-alkaline KMnO4 hydroxylation above, hot acidic KMnO4 (or acidic potassium dichromate) cleaves the C=C bond outright, oxidising each doubly-bonded carbon all the way to a ketone (if it carried two carbon substituents) or, if it carried a hydrogen (as a terminal =CH2), all the way past an intermediate acid to CO2 and water. Prop-1-ene (H3C-CH=CH2) is oxidised this way to ethanoic acid (acetic acid) plus CO2 and H2O, since its terminal =CH2 carbon is fully destroyed. 2-Methylbut-2-ene (which has no terminal =CH2) is cleaved instead into propan-2-one (acetone) and ethanoic acid, since both doubly-bonded carbons carry only carbon substituents and hydrog …