9B. Alkenes + Alkynes

Question 1

Alkenes + Alkynes

Answer 1

• unsaturated hydrocarbons are divided into Alkenes + Alkynes
• alkenes (or olefins) contain at least one carbon-carbon double bond
• alkynes contain one or more carbon-carbon triple bonds
• while alkynes are not widespread in nature, alkenes are particularly prevalent
• several low molecular weight alkenes are of significant commercial importance

Question 2

Ethylene

Answer 2

• simplest alkene
• occurs naturally in trace amounts
• enormous amounts are required for industry
• derived by thermal cracking of hydrocarbons, whereby a saturated hydrocarbon is converted to an unsaturated hydrocarbon + H₂
• where there are vast reserves of natural gas, ethylene is produced by heating ethane in a furnace from 800-900^oC for a fraction of a second

                       <sub>800-900<sup>o</sup>C</sub>

H₃C - CH₃ ————————-> H₂C = CH₂ + H₂

                    <sup>thermal cracking</sup>

• where natural gas is limited supply, thermal cracking of petroleum is used to produce ethylene

Question 3

General Formula

Answer 3

• for alkenes with one double bond C_nH_2n where n is the number of carbon atoms in the molecule
• for alkynes with one triple bond C_nH_2n-2 where n is the number of carbon atoms in the molecule
• both alkenes and alkynes with one double or triple bond respectively are homologous as any two successive members of each series differ by a single CH₂ group

Question 4

Structure + Geometry

Answer 4

• the three bonds about each carboninvolved in the double bondare arranged in a trigonal planar manner with all bond angles approximately 120^o
• under normal conditions, the carbon-carbon double bondof alkenes is so rigid that rotation about the bond does notoccu. An important exception to this is th ecis-trans isomerism that occurs in vision
• cis-trans isomerism is prevalent in alkenes because of the restricted rotation about the carbon-carbon double bond
• as the number of double bonds increases, so too does the number of possible cis-trans isomers
• for an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2ⁿ stereoisomers are possible

Question 5

Naming

Answer 5

• similar to alkanes, except that the parent chain must include the double or triple bond, + the chain is numbered from the end closest to the double or triple bond
• the position of the double or triple bond is designated by the number of its first carbon
• the suffix -ene is used for alkenes
• the suffix -yne is used for alkynes
• where ci-trans isomers occur, cis- or trans- prefixes the name of the alkene
• the orientation of the carbon atoms of the parent chain determines whether the alkene is cis- or trans-. If the carbons of the parent chain are on the same sideof the double bond, the alkene is cis-; if they are opposite sides, it is a trans- alkene
• when naming cycloalkenes, the carbon atoms of the double bond are numbered 1 + 2 in the direction of the substituent encountered first. The substituents are listed alphabetically + numbered. The locationof the double bond is not indicatedas it is always 1+ 2
• alkenes that contain more than one double bond arecollectively known as polyenes
• alkadienes contain two double bonds, alkatrienes contain three double bonds, and so on

Question 6

Reactions of Alkenes

Answer 6

• most characteristic reaction is addition, where the carbon carbon double bond is broken + a new atom or group of atoms forms a single bond on each carbon
• almost all addition reactions of alkenes are exothermic + therefore the products are more stable, having lower energy. Hence the reason why a carbon-carbon double bond is a site of chemical activity, whereas carbon-carbon single bonds are generally quite unreactive
• hydrohalogenation*
• occurs when a hydrogen halide is added to an alkene to produce a haloalkane (alkyl halide)
• Markovnikov’s rule: in the addition of a hydrogen halide to an alkene, hydrogen adds to the doubly bonded carbon that already has the greater number of hydrogens bonded to it; halogen adds to the other carbon
• the addition of a hydrogen halide to an alkene is a regioselective reaction
• acid catalyzed hydration*
• in the presence of an acid catalyst, most commonly concentrated sulfuric acid, water adds to the carbon-carbon double bond of an alkene to produce an alcohol
• for simple alkenes, this hydration follows Markinov’s rule: a H from the water adds to the carbon of the double bond with the greater number of hydrogens, + the OH from the water adds to the other carbon
• halogenation*
• chlorine, Cl₂, and bromine, Br₂, react with alkenes at room temperature by addition of halogen atoms to the carbon atoms of the double bond
• hydrogenation*
• almost all alkenes react with molecular hydrogen, H₂, in the presence of a transition metal catalyst (commonly platinum, palladium, ruthenium or nickel are used) to produce an alkane
• the reaction is carried out by dissolving the alkene in ethanol or another non-reacting organic solvent, adding the catalyst in the form of a finely powdered solid, + exposing the mixture to hydrogen gas at pressures ranging from 1atm to 150atm
• because the conversion of an alkene to an alkane involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or catalytic hydrogenation

Question 7

Stability of Alkenes

Answer 7

• the greater the number of attached alkyl groups - ie. the more highlt substituted the double bond - the greater is the alkene’s stability
• the most stable alkene will be that where there are four alkyl groups about the double bond - tetrasubstituted
• the least stable alkene will have no alkyl groups about the double bond - unsubstituted