Alkanes, Alkenes and Alkynes: Properties, Nomenclature and Reactions - Prof. Franklin A. D, Study notes of Organic Chemistry

Download Alkanes, Alkenes and Alkynes: Properties, Nomenclature and Reactions - Prof. Franklin A. D and more Study notes Organic Chemistry in PDF only on Docsity! Organic Chemistry Alkenes and Alkynes 5 Alkanes and Alkenes • Alkanes have the formula CnH2n+2. • Cycloalkanes with one ring have a different formula: CnH2n. • A second family of hydrocarbons also has the formula CnH2n. These are known as alkenes. • Alkynes are a third family of hydrocarbons having the formula CnH2n–2. • Alkenes and alkynes are unsaturated; they contain fewer hydrogen atoms than alkanes. Ethylene (ethene) Propylene (propene) (© 2007 Thomson Higher Education Uses of Alkenes CH3CH,0H Ethanol f CH3CH Acetaldehyde f H2C=CHOCCH3 Vinyl acetate Ou CH3CHCH3 Isopropyl alcohol HCH: V 3 CH3 HOCH CH20H Ethylene glycol t CH3COH Acetic acid --CH2CH23, Polyethylene ° 7\ HoC—CHCH3 Propylene oxide Cumene Organic Chemistry CICH2CH2CI Ethylene dichloride ° ZS HaC—CH2 Ethylene oxide H2C=CHCI Vinyl chloride CHg 2 0n,cH, Polypropylene  Organic Chemistry 2p, Orbitals and the Pi (p) Bond 2p, Pe o% - Sake 9% OS PS OD sp?/sp? o Bond sp?/sp? o Bond The coded representation for a double bond; note that there is no distinction made between the o and x bonds; a . typical C=C bond distance is 1.33 A 12 18 Nomenclature of Alkenes IUPAC names of alkenes are based on the corresponding alkane with “ane” replaced by “ene” Organic Chemistry The E, Z System of Nomenclature Low High iori H ! Cc iori priority \ |S priority C=C {fi \ High CH3 CH3 Low priority priority (a) (E)-2-Chloro-2-butene © 2007 Thomson Higher Education Low Low priority H | CH3_ priority ne A C=C a High = CH3; Cl High priority priority (b) (Z)-2-Chloro-2-butene 22 27 Alkene Stability: Heats of Formation (Hof) • ΔHof of a compound the enthalpy of formation of one mole of a compound from its elements in their standard states (C (graphite) H2) A negative Hof the more stable it is and formation from the elements is exothermic - heat liberated. A positive Hof the less stable it is and means that is constituent elements are more stable. Its formation is endothermic requiring energy to be applied. Organic Chemistry Bicyclic Systems Fused bicyclic system Bridged bicyclic system Bridgehead Bridgehead posi position H H H H H : H This double bond Bridgehead Bridgehead is at the bridge— position position head postion @Two shared OThree shared adjacent carbons carbons 32 Organic Chemistry Alkynes sp orbital a bond _— P orbital p orbital sp orbital ao bond sp orbitals ~ aw bond Carbon-carbon triple bond © 2004 Thomson/Brooks Cole 36  Relative Heats of Formation for Alkynes TABLE 3.4 Heats of Formation for Some Small Hydrocarbons Alkanes AH; (kcal/mol) Alkenes Ethane —20.1 Ethylene Propane —25.0 Propylene Butane —30.2 1-Butene 2-Butene AH; (kcal/mol) 12.5 4.8 —0.1 —1.9 (cis) —2.9 (trans) Alkynes Acetylene Propyne 1-Butyne 2-Butyne Organic Chemistry AH; (kcal/mol) 54.5 44.6 39.5 34.7 37 Organic Chemistry Alkynes ye 6 5 4 3 H CHe— C=C — 7 - 4 2 3 4 1 2 3 4 2 1 ee HC==C—CH;—CH, C—CH e267 Ms 4 3 2 1 js & | H CH, 1-Butyne trans-2-Hexen-4-yne (E)-2-hexen-4 3,3-Dimethyl-1-butyne not 3-butyne not trans-4-hexen-2-yne not 2,2-dimethyl-3-butyne CH cc Ethynylcyclohexane 3-Propynylcyclopentane or cyclohexylacetylene or propargylcyclopentane  Organic Chemistry Reaction Energy Diagram H3C CH, Nee 3 C==G---H--- Cl lA H3C CH; Transition state for the first step CH 5 Pe o ‘cl—C—_c—A e i w CH HC. \ ce c=C + H—Cl / H3C CH > Reaction progress 45 Organic Chemistry Regiochemistry of Addition lL / Lf f cr ac» chy —F— 0 HC —C—C—H CH, 4 CH; H mC, / This product c—c is not formed Be n H5C H Cl H i H;C—C—CH, HeC H CH; The product of the reaction 47 49 Order of Carbocation Stability • More substituted carbocation is preferred upon alkene protonation.