Chemical Properties of Alkenes

Alkenes are more reactive than alkanes because of their double bond: the C=C sigma bond is strong, but the pi bond is comparatively weak and readily attacked. The characteristic reaction of alkenes is addition of an electrophile across the double bond, generally proceeding through an ionic (carbocation) mechanism, though some additions proceed through a free-radical mechanism instead; ozonolysis and polymerisation are …

Hydrogen adds to an alkene in the presence of a metal catalyst (Ni, Pd or Pt) to give the corresponding alkane -- catalytic hydrogenation. This process is central to manufacturing vanaspati (hydrogenated vegetable oil/ghee substitute) from vegetable oil, and helps …

Treating an alkene with chlorine or bromine causes rapid addition, forming a 1,2-dihaloalkane (vicinal dihalide), e.g. ethene + Cl2 --> 1,2-dichloroethane. Iodine reacts far more slowly, giving a 1,2-diiodoalkane that is unstable and tends to eliminate iodine again, regenerating the original alkene. TEST FOR ALKENES: bromine water (reddish-brown) is decolourised when a small amount is added to an alkene, since it forms a colourless dibromo addition compound -- th …

MARKOVNIKOV'S RULE: when an unsymmetrical alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that already carries more hydrogens, and the halogen adds to the carbon carrying fewer hydrogens; equivalently, the most electronegative part of the reagent adds to the doubly-bonded carbon that already bears the least hydrogen. In the addition of water (Section 13.3.3.5) too, this same rule governs regiochemistry. For addition of hydrohalides specifically -- HCl, HBr and HI, with reactivity order HI > HBr > HCl -- symmetrical alkenes (e.g. ethene, but-2-ene) give a single alkyl halide product regardless of orientation, but an UNSYMMETRICAL alkene gives two possible products, of which the Markovnikov product dominates. Mechanism (illustrated for HBr + propene): Step 1, the more electronegative Br in H-Br pulls the bonding electrons toward itself, making H the electrophile; H+ attacks the double bond, and the resulting positive charge can sit on either carbon -- at the terminal carbon it gives a LESS stable primary carbocation, while at the internal carbon it gives a MORE stable secondary carbocation. Step 2, the secondary carbocation predominates because it is more stable, so it forms preferentially. Step 3, Br- then attacks this secondary carbocation to give 2-bromopropane, the major (Markovnikov) product, with 1-bromopropane forming only as a minor product via the less-favoured primary carbocation. A further worked example -- HBr addition to 3-methyl-1-butene -- shows that the initially formed sec …

Adding HBr to an alkene in the presence of an organic peroxide gives the ANTI-Markovnikov product -- Br ends up on the carbon with MORE hydrogens instead of fewer. This reversal is called the peroxide effect or Kharasch addition (e.g. propene + HBr --peroxide, (C6H5CO)2O2--> 1-bromopropane, not the Markovnikov 2-bromopropane). Mechanism (free-radical, not ionic): Step 1, the weak O-O bond of the peroxide undergoes homolytic cleavage to give phenyl free radicals (via loss of CO2 from the initially formed benzoyloxy radicals). Step 2, a phenyl radical abstracts a hydrogen from HBr, generating a bromine free radical. Step 3, the bromine radical adds to the alkene's double bond in whichever orientation gives the MORE stable carbon free radical -- for propene, addition at the terminal carbon gives the more stable secondary radical (rather than the less stable primary radical from the other orientation). Step 4, this secondary free radical abstracts a hydrogen from another molecule of HBr, giving 1-bromopropane as the major product (and regenerating a bromine radical to continue the chain) -- note the bromine therefore ends up on the LESS-substituted (terminal) carbon, opposite to the ionic Markovnikov pathway. The peroxide effect is observed only with HBr, never with HCl or HI: the H-Cl bond (430.5 kJ/mol) is too strong for a bromine-type radical chain to break, while H-I (296.8 kJ/mol) breaks easily but …

Water does not normally react with alkenes, but in the presence of concentrated H2SO4 it adds across the double bond to give an alcohol -- a reaction that follows a carbocation mechanism and obeys Markovnikov's rule. For example, 2-methylpropene + H2O --H+--> 2-methyl-2-propanol (via protonation to the more stable tertiar …

Alkenes react with cold, concentrated sulphuric acid to form an alkyl hydrogen sulphate, again following Markovnikov's rule (e.g. propene + H2SO4 --> 2-propyl hydrogen sulphate/isopropyl hydrogen sulphate); subsequent hydrolysis with water yie …

With cold, dilute, alkaline KMnO4 (Baeyer's reagent): an alkene is oxidised to a vicinal diol (glycol); the purple permanganate colour is discharged, first turning dark green (Mn6+) and then depositing a dark-brown precipitate of MnO2 (Mn4+) -- e.g. ethene + H2O + [O] --cold dil. KMnO4, 273K--> ethane-1,2-diol + MnO2; this colour change is itself a standard test for unsaturation. With acidified KMnO4 (hot, stronger oxidant): the double bond is oxidatively CLEAVED, and each fragment becomes a ketone (if that alkene carbon carried two alkyl substituents) or is oxidised further to a carboxylic acid (if it carried one alkyl substituent and one H) -- e.g. 2-methylprop-1-ene --KMnO4/H+--> propan-2-one (acetone) + CO2 + H2O (the terminal =CH2 carbon, with two H's, is over-oxidised all the way to CO2 and water), while but-2-ene --KMnO4/H+--> 2 CH3C …

Ozonolysis oxidatively cleaves an alkene's (or alkyne's) C=C bond using ozone, giving two carbonyl compounds; it is widely used to locate the exact position of a double or triple bond in an unknown alkene/alkyne. An alkene first adds O3 to form a cyclic ozonide, which is then cleaved (reductively) with Zn/H2O into two aldehyde and/or ketone fragments -- e.g. ethene + O3 --> ethene ozonide --Zn/H2O--> 2 HCHO (formaldehyde, since ethene is symmetric); propene …

A polymer is a large molecule built by combining a large number of small molecules (monomers); the process is polymerisation. Alkenes polymerise at high temperature and pressure in the presence of a catalyst -- e.g. ethene polymerises (via free-radical polymerisation, in a red-hot iron tube at 873 K in the book's illustration) to polyethylene/polythene; propene similarly polymerises to polypropene; and styrene (phenylethene) polymerises to polystyrene. Because extensive polymer use clogs landfills and pollutes the environment, and because different polymer types must be sorted before recycling, pla …