LecturePLUS Timberlake1 Alkenes and Alkynes Geometric Isomers of Alkenes Addition Reactions

ALKENES and ALKYNES Alkenes area class of HYDROCARBONS which contain only carbon and hydrogen.HYDROCARBONS UNSATURATED hydrocarbons contain either double or triple bonds. Since the compound is unsaturated with respect to hydrogen atoms, the extra electrons are shared between 2 carbon atoms forming either double bonds for alkenes or triple bonds for alkynes.

LecturePLUS Timberlake3 Saturated and Unsaturated Compounds Saturated compounds (alkanes) have the maximum number of hydrogen atoms attached to each carbon atom Unsaturated compounds have fewer hydrogen atoms attached to the carbon chain than alkanes Unsaturated compounds contain double or triple bonds

Typical Alkene Ethene is the number one organic chemical synthesized in the U. S. and the world. The small quantities of ethane, propane, and butane found in natural gas are converted into ethene. It can be produced by thermal cracking of ethane to produce ethene and a hydrogen molecule.

LecturePLUS Timberlake5 Alkenes  Carbon-carbon double bonds  Names end in -ene H 2 C=CH 2 ethene (ethylene) H 2 C=CH-CH 3 propene (propylene) cyclohexene

Typical Alkyne Physical and chemical properties are similar to those of alkenes.

LecturePLUS Timberlake7 Alkynes  Carbon-carbon triple bonds  Names end in -yne HC  CHethyne(acetylene) HC  C-CH 3 propyne

ALKENE and ALKYNE NAMES Root names give the number of carbons in the longest continuous chain. The names are formed by dropping the "ane" and if the chain is 3C or less replacing it with "ene“. If the chain is 4C or more the lowest “carbon number” of the two carbons involved with the double bond plus a hyphenated ene is added.

LecturePLUS Timberlake9 Naming Alkenes and Alkynes When the carbon chain has 4 or more C atoms, number the chain to give the lowest number to the double or triple bond CH 2 =CHCH 2 CH 3 but-1-ene CH 3 CH=CHCH 3 but-2-ene CH 3 CH  CHCH 3 but-2-yne

No. of Carbon s Root Name Formula C n H 2n Structure 2 ethene C 2 H 4 CH 2 =CH 2 3 propene C 3 H 6 CH 2 =CHCH 3 4but-1-ene C 4 H 8 CH 2 =CHCH 2 CH 3 4but-2-ene C 4 H 8 CH 3 CH=CHCH 3 5 pent-1-ene C 5 H 10 CH 2 =CHCH 2 CH 2 CH 3 5 pent-2-ene C 5 H 10 CH 3 CH=CHCH 2 CH 3

Cis-Trans Isomerism The double bond of alkenes form a rigid structure around which the atoms cannot rotate as in alkanes. This leads to a form of isomerism referred to as cis- trans isomers. This isomerism is most readily seen in the two forms of but-2-ene

Cis-but-2-ene The hydrogens occur on the same side (plane ). Trans-but-2-ene The hydrogens occur diagonally across from each other.

Properties Physical Properties Boiling points depend on chain length, slightly less than alkanes. Boiling points Non polar Insoluble in water Less dense than water Chemical Reactivity Alkenes are quite reactive because of the presence of the double bond. Many small compounds react by addition i.e. molecules add to the alkene to form one product. All compounds: Combustion ReactionCombustion Reaction

LecturePLUS Timberlake14 Learning Check Write the IUPAC name for each of the following unsaturated compounds: A.CH 3 CH 2 C  CCH 3 CH 3 B. CH 3 C=CHCH 3 C.

LecturePLUS Timberlake15 Solutions Write the IUPAC name for each of the following unsaturated compounds: A.CH 3 CH 2 CH=CHCH 3 2-pentyne CH 3 B. CH 3 C=CHCH 3 C. 2-methyl-2-butene 3-methylcyclopentene

Alkene/Alkyne Addition Reactions The double and triple bonds present in alkenes and alkynes, respectively, make them much more reactive than alkanes. Alkenes and alkynes undergo addition reactions – a reaction in which parts of a reactant are added to the two atoms that make up a double or triple bond

Alkene/Alkyne Addition Reactions Some of the common reactants that can add to alkenes and alkynes include: – Br 2 – I 2 – Cl 2 – H 2 (requires a transition metal catalyst) – H 2 O (requires a strong acid catalyst) – HBr – HCl – HI

LecturePLUS Timberlake18 Hydrogenation Adds a hydrogen atom to each carbon atom of a double bond H H H H Ni H–C=C–H + H 2 H–C–C–H H H ethene ethane

LecturePLUS Timberlake19 Adding Halogens (Halogenation) Halogens also add to the double bond of an alkene. H 2 C  CH 2 + Cl 2

LecturePLUS Timberlake20 Adding Hydrogen Halides Hydrogen halides also add to the double bond of an alkene H Cl H–C=C–H + HCl H–C–C–H H H H H

LecturePLUS Timberlake21 Adding Water Water also can be added to the double bond of an alkene H OH H–C=C–H + H 2 O H–C–C–H H H H H

Alkene/Alkyne Addition Reactions Markovnikov’s Rule: The major product obtained from the addition of an unsymmetrical reagent such as H-Br, H-Cl, or H-OH to an alkene or alkyne is the one obtained when the H atom of the reagent is added to the C atom of the multiple bond that already has the greater number of H atoms. “The rich get richer”

LecturePLUS Timberlake23 Example Water also can be added to the double bond of an alkene H H OH H H–C = C– C–H + H 2 O H–C – C– C–H H H H H H H

Alkene/Alkyne Addition Reactions Example: Write the structure of the product that forms when propyne reacts with the following reagents (individually): Br 2 (excess) H 2 (excess) with Ni catalyst HBr (excess)