Dr. Wolf's CHM 201 & Chapter 9 Alkynes

Dr. Wolf's CHM 201 & Sources of Alkynes

Dr. Wolf's CHM 201 & AcetyleneAcetylene Industrial preparation of acetylene is by dehydrogenation of ethylene CH 3 CH 3 800°C 1150°C cost of energy makes acetylene a more expensive industrial chemical than ethylene H2CH2CH2CH2C CH 2 H2CH2CH2CH2C HCCH H2H2H2H2+ H2H2H2H2+

Dr. Wolf's CHM 201 & Nomenclature

Dr. Wolf's CHM 201 & HC CH Acetylene and ethyne are both acceptable IUPAC names for NomenclatureNomenclature Higher alkynes are named in much the same way as alkenes except using an -yne suffix instead of -ene. HC CCH 3 Propyne HC CCH 2 CH 3 1-Butyne (CH 3 ) 3 CC CCH 3 4,4-Dimethyl-2-pentyne

Dr. Wolf's CHM 201 & Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes.

Dr. Wolf's CHM 201 & Structure and Bonding in Alkynes: sp Hybridization

Dr. Wolf's CHM 201 & StructureStructure linear geometry for acetylene C C H H 120 pm 106 pm C C CH 3 H 121 pm 146 pm 106 pm

Dr. Wolf's CHM 201 & Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time. Cyclooctyne polymerizes on standing.

Dr. Wolf's CHM 201 & s2s2s2s 2p2p2p2p 2sp 2sp Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p2p2p2p Bonding in acetylene is based on sp-hybridization for each carbon

Dr. Wolf's CHM 201 & sp 2sp Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p2p2p2p Bonding in acetylene is based on sp-hybridization for each carbon Each carbon has two half-filled sp orbitals available to form  bonds.

Dr. Wolf's CHM 201 &  Bonds in Acetylene Each carbon is connected to a hydrogen by a  bond. The two carbons are connected to each other by a  bond and two  bonds.

Dr. Wolf's CHM 201 &  Bonds in Acetylene One of the two  bonds in acetylene is shown here. The second  bond is at right angles to the first.

Dr. Wolf's CHM 201 &  Bonds in Acetylene This is the second of the two  bonds in acetylene.

Dr. Wolf's CHM 201 & The region of highest negative charge lies above and below the molecular plane in ethylene. The region of highest negative charge encircles the molecule around its center in acetylene.

Dr. Wolf's CHM 201 & C—C distance C—H distance H—C—C angles C—C BDE C—H BDE % s character pKapKapKapKa pKapKapKapKa 153 pm 111 pm 111.0°111.0° 368 kJ/mol 410 kJ/mol sp 3 25%25% pm 110 pm 121.4°121.4° 611 kJ/mol 452 kJ/mol sp 2 33%33% pm 106 pm 180°180° 820 kJ/mol 536 kJ/mol spsp 50%50% 2626 hybridization of C Table 9.1 Comparison of ethane, ethylene, and acetylene Ethane Ethylene Acetylene

Dr. Wolf's CHM 201 & Acidity of Acetylene andTerminal Alkynes H C C

Dr. Wolf's CHM 201 & In general, hydrocarbons are exceedingly weak acids CompoundpK a HF3.2 H 2 O16 NH CH 4 60 H2CH2CH2CH2C CH 2

Dr. Wolf's CHM 201 & Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes. CompoundpK a HF3.2 H 2 O16 NH CH 4 60 H2CH2CH2CH2C CH 2 HCCH 26

Dr. Wolf's CHM 201 & Electronegativity of carbon increases with its s character C H H sp 3 C : sp 2 sp HC C C CHC C CC : : Electrons in an orbital with more s character are closer to the nucleus and more strongly held.

Dr. Wolf's CHM 201 & Objective: Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective? Objective: NaCCH H2OH2OH2OH2O NaOH + HC CH NaC CH+

Dr. Wolf's CHM 201 & No. Hydroxide is not a strong enough base to deprotonate acetylene. H2OH2OH2OH2O NaOH + HC CH NaC CH+– HO.... : H CCH HO H CCH: – weaker acid pK a = 26 stronger acid pK a = 16 In acid-base reactions, the equilibrium lies to the side of the weaker acid.

Dr. Wolf's CHM 201 & Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. NH 3 NaNH 2 + HC CH NaC CH+– H2NH2NH2NH2N.. : H CCHH CCH: – stronger acid pK a = 26 weaker acid pK a = 36 Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right. H2NH2NH2NH2N

Dr. Wolf's CHM 201 & Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes

Dr. Wolf's CHM 201 & Preparation of Alkynes Carbon-carbon bond formation alkylation of acetylene and terminal alkynes Functional-group transformations elimination There are two main methods for the preparation of alkynes:

Dr. Wolf's CHM 201 & Alkylation of acetylene and terminal alkynes H—C C—H R—C C—H C—R

Dr. Wolf's CHM 201 & RX SN2SN2SN2SN2 Alkylation of acetylene and terminal alkynes X–X–X–X–:+ C–: H—C C—RH—C+ The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution.The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution. The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne.The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne.

Dr. Wolf's CHM 201 & Example: Alkylation of acetylene NaNH 2 NH 3 CH 3 CH 2 CH 2 CH 2 Br (70-77%)HCCH HC CNa HC C CH 2 CH 2 CH 2 CH 3

Dr. Wolf's CHM 201 & NaNH 2, NH 3 CH 3 Br Example: Alkylation of a terminal alkyne CHCHCHCH (CH 3 ) 2 CHCH 2 C CNa (CH 3 ) 2 CHCH 2 C (81%) C—CH 3 (CH 3 ) 2 CHCH 2 C

Dr. Wolf's CHM 201 & NaNH 2, NH 3 2. CH 3 CH 2 Br (81%) Example: Dialkylation of acetylene H—CC—H 1. NaNH 2, NH 3 2. CH 3 Br C—H CH 3 CH 2 —C C—CH 3 CH 3 CH 2 —C

Dr. Wolf's CHM 201 & LimitationLimitation Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination

Dr. Wolf's CHM 201 & E2 predominates over S N 2 when alkyl halide is secondary or tertiary H C C— X E2 C–: H—C + C H—C —H—H—H—H C C X–X–X–X– : +

Dr. Wolf's CHM 201 & Preparation of Alkynes by Elimination Reactions

Dr. Wolf's CHM 201 & Preparation of Alkynes by "Double" Dehydrohalogenation Geminal dihalide Vicinal dihalide X C CXHH XX CCHH The most frequent applications are in preparation of terminal alkynes.

Dr. Wolf's CHM 201 & Geminal dihalide  Alkyne (CH 3 ) 3 CCH 2 —CHCl NaNH 2, NH 3 2. H 2 O (56-60%) (CH 3 ) 3 CC CH

Dr. Wolf's CHM 201 & NaNH 2, NH 3 H2OH2OH2OH2O Geminal dihalide  Alkyne (CH 3 ) 3 CCH 2 —CHCl 2 (CH 3 ) 3 CCH CHCl (CH 3 ) 3 CC CH CNa(slow) (slow) (fast)

Dr. Wolf's CHM 201 & CH 3 (CH 2 ) 7 CH—CH 2 Br Br Vicinal dihalide  Alkyne 1. 3NaNH 2, NH 3 2. H 2 O (54%) CH 3 (CH 2 ) 7 C CH

Dr. Wolf's CHM 201 & Reactions of Alkynes

Dr. Wolf's CHM 201 & Reactions of Alkynes Acidity (Section 9.5) Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10) Addition of Hydrogen Halides (Section 9.11) Hydration (Section 9.12) Addition of Halogens (Section 9.13) Ozonolysis (Section 9.14) Acidity (Section 9.5) Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10) Addition of Hydrogen Halides (Section 9.11) Hydration (Section 9.12) Addition of Halogens (Section 9.13) Ozonolysis (Section 9.14)