Chapter 5 Reactions of Alkenes and Alkynes (Part II) Essential Organic Chemistry Paula Yurkanis Bruice

5.8 The Structure of Alkynes

5.9 Physical Properties of Unsaturated Hydrocarbons 1. All hydrocarbons have similar properties. 2. All are insoluble in water and are all soluble in nonpolar solvents such as hexane. 3. Less dense than water, have boiling points that increase with increasing molecular weight. 4. Alkynes are more linear than alkenes, thus having stronger van der Waals interactions and higher boiling point than alkenes containing the same number of carbon atoms.

5.10 Addition of a Hydrogen Halide to an Alkynes CCH 3 CCH 3 HBr   H 3 C HH 3 C Alkynes are electron-rich molecules, thus also electrophilic addition takes place. 2-butyne vinyl-carbocation

Alkynes: Reactions H 3 C HH 3 C + Br H 3 C H Br CH 3 After the addition of the nucleophile we obtain an alkene that still can be attacked by an electrophile.

Alkynes: Reactions H 3 C H Br CH 3 HBr   H 3 C CH 3 Br H H H 3 C CH 3 Br H H Br H 3 C CH 3 Br Br H H Both nucleophiles end up on the same carbon.

Alkynes: Reactions CH 2 CCH H 3 C HClexcess CC CH 2 H 3 C H H Cl H 3 CCH 2 CC H Cl Cl H H Terminal alkynes follow Markovnikov’s rule: CC CH 2 H 3 C H H CC CH 2 H 3 C H H > The stability of the resulting vinylic cations differs.

5.11 Addition of Water to an Alkyne Yet another situation arises during the addition of water. After the addition of 1 equivalent of H 2 O an Enol is formed. CH 2 CCH H 3 C H 2 O CC CH 2 H 3 C H H HO

Alkynes: Reactions CC CH 2 H 3 C H H HO H 3 CCH 2 HO H H The Enol readily tautomerizes into a ketone.

Keto-Enol Tautomers 1. Tautomers are isomers that are in rapid equilibrium. 2. Because the keto tautomer is usually more stable than the enol tautomer, it predominates at equilibrium. Interconversion of the tautomers is called tautomerization or enolization.

Alkynes: Reactions Draw the Enol tautomers of the following ketones. O O

Alkynes: Reactions Draw the Enol tautomers of the following ketones. O H H H H O H H H H O H H H H +

Alkynes: Reactions Draw the Enol tautomers of the following ketones. O H H H O H H H

5.12 Addition of Hydrogen to Alkenes and Alkynes

Hydrogenation The catalyst is necessary, since the H-H bond is too strong. H-H bond energy: 435 kJ/mol

Hydrogenation The number of C-H bonds increases: cyclohexene: 10 bonds cyclohexane: 12 bonds We do a reduction reaction

Hydrogenation CH 3 +H 2 Pt/C CH 3 H H The mechanism of the hydrogenation is as follows: 1.Absorption of hydrogen on the metal surface. H 2 molecules are broken down into atoms.

Hydrogenation CH 3 +H 2 Pt/C CH 3 H H The mechanism of the hydrogenation is as follows: 1.Absorption of hydrogen on the metal surface 2.Alkene approaches surface

Hydrogenation CH 3 +H 2 Pt/C CH 3 H H The mechanism of the hydrogenation is as follows: 1.Absorption of hydrogen on the metal surface 2.Alkene approaches surface 3.  bonds are replaced by C-H bonds

Hydrogenation Hydrogenation adds in the same manner that it adds to alkenes. H e CCHH 2 C H 3 C H 2 / Pd/C H 1-butyne 1-butne H 2 / Pd/CH 3 C H 2 CH H H H H butane

Hydrogenation Only with special, partially deactivated catalysts can the reaction be stopped at the alkene stage. CCH 2 C H 3 C CH 3 H 2 / Lindlar H 3 C H 2 C H H CH 3 2-pentyne cis-2-pentene

5.13 Acidity of a Hydrogen Bonded to an sp Hybridized Carbon The electronegativity of carbon varies with the level of hybridization.

Acidity However, with strong bases such as NH 2 –, a proton can be removed from ethyne. In aqueous solutions none of the protons can be removed, pK a (H 2 O) = 14. HCCH + H 2 O HCC + H 3 O

Acidity

5.14 Synthesis Using Acetylide Ions: Alkylation This property is used to our advantage in synthesis when carbon–carbon bonds need to be formed. HCCH + NH 2 HCC + H 3 N With the strong base, NH 2 –, we form a carbanion.

Alkylation In the second step, the carbanion displaces a bromine. This results in carbon–carbon bond formation.

5.15 An Introduction to Multistep Synthesis

Multistep Synthesis

Polymers  Polymers are large molecules that contain a large number of repeating units of a small molecule (monomer).  Biopolymers are typically synthesized by organisms.  Synthetic Polymers are made in organic laboratories.

Polymers Alkenes can be used as a monomeric unit to form chain-growth polymers n styrene polystyrene

Polymers Instead of the phenyl ring as shown with polystyrene, many other substituents can be placed on the alkene part. H 2 CCH 2 ethylene 3 H 2 CCH CH propylene H 2 CCH Cl vinylchloride F 2 CCF 2 tetrafluoroethylene 2 H 2 CCH CN acrylonitrile H 2 CCH 2 COOCH 3 methylacrylate H 2 CCH OOCCH 3 vinylacetate

Polymers For the polymerization reaction we can use electrophilic addition reactions. BF 3 F 3 B In the initiation step an electrophile (BF 3 ) adds to the alkene to form a carbocation intermediate.

Polymers F 3 B + F 3 B Subsequent propagation steps grow the polymer until all monomers are consumed.

Polymers Termination can also be accomplished by adding a nucleophile. F 3 B n Nu F 3 B Nu n

Polymer An alternative way to initiate polymerization is starting with a radical initiator. ROOR  ( h   RO radicals RO RO R R R The radical adds onto the alkene to form a new radical. Propagation is accomplished via subsequent radical additions.

Polymers Termination takes place when the polymeric chain comes across another radical and they combine. R RO R R  R' R RO R R R' n