7 A LKYNES H H CH 3 C C C C C C C C H H OH - PowerPoint PPT Presentation

7 A LKYNES H H CH 3 C C C C C C C C H H OH ichthyothereol

7.2 STRUCTURE AND PROPERTIES OF ALKYNES Classification of Alkynes monosubstituted alkyne (terminal alkyne): R—C ≡ C—H disubstituted alkyne ( internal alkyne) : R—C ≡ C—R 146 pm 151 pm 154 pm CH 3 CH 3 H H H C C H C C H C C CH 3 H H H H propyne propane propene Table 11.1 C—C and C—H Bond Lengths and Bond Strengths in Alkynes, Alkenes, and Alkanes Bond Strength (kJ mole –1 ) Compound Bond Length (pm) 105 536 C ≡ C—H (ethyne) C=C—H (ethene) 109 470 C—H (ethane) 111 422 121 820 C ≡ C (ethyne) C=C (ethene) 133 605 C—C (ethane) 154 368

7.2 STRUCTURE AND PROPERTIES OF ALKYNES Hybridization, Bond Length, and Bond Energies in Alkynes Figure 11.1 Structure and Bonding in Ethyne (a) Schematic diagram. (b) Sigma bonds in ethyne. (c) Electrostatic potential map. We recall that an sp-hybridized carbon is electron deficient relative to an sp2- or sp3-hybridized carbon. Te regions of partial positive charge surrounding the carbon atoms is shown in red. Te negative end of the C—H dipole is shown in blue. (d) Space-filling model. sp-1s σ bond sp-sp σ bond H C C H ethyne π bonds Bonding in ethyne: the carbon-carbon σ bonds are colinear; the π bonds lie above and below, and in front and behind the sigma bonds. (c) (d)

7.2 STRUCTURE AND PROPERTIES OF ALKYNES Physical Properties of Alkynes Table 7.2 Physical Properties of Alkynes Compound Boiling Point ( o C) Density (g/cm 3 ) 1-butyne 8.1 0.678 2-butyne 27 0.091 1-pentyne 40.2 0.690 2-pentyne 56.1 0.711 3-methyl-1-butyne 29 0.666 1-hexyne 71.3 0.716 2-hexyne 84 0.732 3-hexyne 81.5 0.723 4-methyl-1-pentyne 61.1 0.709 4-methyl-2-pentyne 72.0 0.716 3,3-dimethyl-1-butyne 39.5 0.669 1-heptyne 99.7 0.733 2-heptyne 112 0.748 3-heptyne 105.5 0.753 5-methyl-1-hexyne 92 0.727 5-methyl-2-hexyne 102 0.738 2-methyl-3-hexyne 95.2 0.726 4,4-dimethyl-1-pentyne 76.1 0.714 4,4-dimethyl-2-pentyne 82.3 0.718 3-ethyl-1-pentyne 88 0.724

7.3 IUPAC NAMES OF ALKYNES 1. Te longest continuous chain that contains the triple bond is the parent. 2. Give the parent the same stem name as an alkane, but replace - ane with - yne . 3. Number the carbon atoms consecutively from the end of the chain nearer the triple bond. Use the number of the first carbon atom with the triple bond as a prefix separated by a hyphen from the parent name. 5 6 4 3 1 2 Number this compound from right to left. CH 3 CH 2 CH 2 C C CH 3 It is 2-hexyne. 4. Alkyl groups are named, and their positions on the chain determined, by the numbering established by rule 3. CH 3 Number this compound from left to right. CH 3 C C CH CH 3 It is 4-methyl-2-pentyne. 5 2 3 1 4

7.3 IUPAC NAMES OF ALKYNES ( II ) 5. Compounds with multiple triple bonds are diynes, triynes, and so on. Compounds with both double and triple bonds are called enynes , not ynenes. Start the numbering of com- pounds with both double and triple bonds from the end nearer the first multiple bond, regardless of type. When a choice is possible, assign double bonds lower numbers than triple bonds. 1 2 3 4 5 This is 1-penten-4-yne, H CH 2 CH CH 2 C C not 4=penten-1-yne.

7.4 ACIDITY OF TERMINAL ALKYNES H + B + B R C C R C C H (an alkynide ion) CH 3 —CH 3 CH 2 =CH 2 HC ≡ CH K a 10 -50 K a =10 -44 K a 10 -25 K eq = 10 -9 H + + H H C C H C C OH OH H + HO R C C + H 2 O R C C stronger weaker acid acid

7.5 HYDROGENATION OF ALKYNES H H Pd/C C C + 2H 2 C C H H Figure 7.2 Stability of Alkenes and Alkynes 1-hexyne - 164 kJ mole -1 H 2 2 H 2 1-hexene - 290 kJ mole -1 H 2 - 126 kJ mole -1 hexane

7.5 HYDROGENATION OF ALKYNES Syn Addition of Hydrogen CH 3 (CH 2 ) 2 CH 2 CH 2 (CH 2 ) 2 CH 3 Lindlar catalyst CH 3 (CH 2 ) 3 C C (CH 2 ) 3 CH 3 C C H 2 H H Z-5-decene 5-decyne Anti Addition of Hydrogen CH 3 (CH 2 ) 2 CH 2 H 1. Na / NH 2- C C (CH 2 ) 3 CH 3 CH 3 (CH 2 ) 3 C C 2. H 2 O H CH 2 (CH 2 ) 2 CH 3 5-decyne (E)-5-decene

7.5 HYDROGENATION OF ALKYNES Mechanism of Anti Addition NH 3 (e - ) + Na + NH 3 + Na solvated electron e - R Step 1 R C C R' C C R' radical anion R R H H Step 2 N C C C C H R' R' H radical anion vinyl radical R R Step 3 C C C C e - H H R' R' vinyl radical vinyl anion R R H H H H Step 4 + C C C C N N H H R' R' H H a trans alkene

7.6 ELECTROPHILIC ADDITION REACTIONS Addition of Hydrogen Halides Br + H CH 3 CH 2 CH 2 CH 2 C C H Br CH 3 CH 2 CH 2 CH 2 C CH 2 1-hexyne 2-bromo-1-hexene CH 3 (CH 2 ) 3 C CH 3 (CH 2 ) 3 C C C H C C H H H primary alkenyl carbocation secondary alkenyl carbocation (more stable) (less stable) CH 3 (CH 2 ) 3 C C C CH 3 CH 3 (CH 2 ) 3 C C C H H H secondary alkyl carbocation secondary alkenyl carbocation (more stable) (less stable) Br H H Br H Br C C CH 3 CH 2 CH 2 CH 2 C C H H CH 2 CH 2 CH 2 CH 3 Br H H CH 2 CH 2 CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 C C + HBr C C CH 3 CH 2 CH 2 Br 4-octyne (Z)-4-bromo-4-octene

7.6 ELECTROPHILIC ADDITION REACTIONS Addition of Halogens Cl Cl 2Cl 2 CH 3 CH 2 C C H CH 3 CH 2 C C H + Cl Cl 1-butyne 1,1,2,2-tetrachlorobutane Br +Br + Br CH 3 CH 2 C C CH 2 CH 3 Br CH 3 CH 2 CH 2 CH 3 3-hexyne bromonium ion anti addition Br CH 2 CH 3 C C CH 3 CH 2 Br (E)-3,4-dibromo-3-hexene cyclic bromonium ion cyclic bromonium ion from alkene addition from alkyne addition Br Br less stable more stable

7.6 ELECTROPHILIC ADDITION REACTIONS Hydration of Alkynes HO H H 2 O R H C C C C H 2 SO 4 / HgSO 4 R H enol H O O H 1,3-proton shift H C C C concerted R C R H H H enol ketone O H 2 O/ H 2 SO 4 C C H C CH 3 HgSO 4

7.7 SYNTHESIS OF ALKYNES Elimination Reactions of Dihalides O Cl PCl 5 R C R R C R Cl Br H NH 3 ( l ) + 2 NaBr R C C R' + 2 NaNH 2 R C C R' + 2 NH 3 Br H geminal dibromide Br H NH 3 ( l ) + 2 NaBr R 2 NaNH 2 2 NH 3 C C R' + R C C R' + H Br vicinal dibromide H Cl 1. 3NaNH 2 / NH 3 ( l ) CH 3 (CH 2 ) 2 CH 2 C C H CH 3 (CH 2 ) 2 CH 2 C C H 2. H 2 O H Cl 1,2-dichlorohexane 1-hexyne H + + CH 3 (CH 2 ) 2 CH 2 C C NH 2 CH 3 (CH 2 ) 2 CH 2 C C NH 3 + + H 2 O C C CH 3 (CH 2 ) 2 CH 2 HO CH 3 (CH 2 ) 2 CH 2 C C H Cl Cl Cl CH 2 CH 3 KOH + KCl + H 2 O CH 3 CH 2 C C CH 2 CH 3 C C 1-propanol CH 3 CH 2 H H H 3,4-dichloroexane

7.6 ELECTROPHILIC ADDITION REACTIONS Alkylation of Alkynes NaNH 2 + NH 3 CH 3 (CH 2 ) 2 CH 2 C C H CH 3 (CH 2 ) 2 CH 2 C C Na NH 3 ( l ) H CH 3 (CH 2 ) 2 CH 2 C C CH 3 CH 3 (CH 2 ) 2 CH 2 C C C Br H H + Br