The double and the triple bond. Shape and geometrical isomers, the ways to make alkenes and alkynes, electrophilic addition with Markovnikov's rule and the peroxide effect, oxidation and ozonolysis, and the acidity and reactions of terminal alkynes.
3 lessons, about 1 h 30 min3 simulations10 problems3 practice questionsPython in every lesson
Created by StemPy LabAlkenes have a C=C double bond and the formula . Alkynes have a C≡C triple bond and the formula . Because the pi bond cannot twist, alkenes show cis-trans, or E and Z, isomerism. Alkenes are made by partial hydrogenation of alkynes, by elimination from alkyl halides or alcohols, and by dehalogenation. Ethyne comes from calcium carbide.
The pi bond of an alkene is a cloud of electrons above and below the molecule. Electron-seeking reagents attack it, and the pi bond is replaced by two sigma bonds: an addition. Hydrogen, halogens, hydrogen halides and water add. When HX adds to an unsymmetrical alkene, Markovnikov's rule says the hydrogen goes to the carbon with more hydrogen atoms. With a peroxide, HBr adds the other way round.
Cold dilute alkaline permanganate turns an alkene into a diol. Hot acidified permanganate and ozonolysis cut the double bond, and the fragments identify the alkene. A terminal alkyne has an acidic hydrogen on an sp carbon: it reacts with sodium, with sodium amide and with ammoniacal silver or copper(I) salts. Alkynes add hydrogen, halogens and hydrogen halides twice, and hydration gives a ketone, or ethanal from ethyne.
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