CCAlkynes

Synthesis of Acetylene Heat coke with lime in an electric furnace to form calcium carbide. Then drip water on the calcium carbide. coke lime *This reaction was used to produce light for miners’ lamps and for the stage. *

H CC Acidity of Acetylene and Terminal Alkynes

In general, hydrocarbons are exceedingly weak acids CompoundpK a HF3.2 H 2 O16 NH CH 4 60 H2CH2CH2CH2C CH 2 Acidity of Hydrocarbons

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 Acetylene

C H H sp 3 C : sp 2 spHC C C CHC C CC : : Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Carbon: Hybridization and Electronegativity

Objective: Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective? NaCCH H2OH2OH2OH2O NaOH + HC CH NaC CH+ Sodium Acetylide

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. Sodium Acetylide

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 Sodium Acetylide

Preparation of Alkynes having other functions by reactions with Acetylide and Terminal Alkynes

Acetylides as nucleophiles

Preparation of Alkynes by Alkylation of Acetylene and Terminal 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: Preparation of Alkynes

H—C C—H R—C C—H C—R Alkylation of Acetylene and Terminal Alkynes

RX SN2SN2SN2SN2 X–X–X–X–:+ C–: H—C C—RH—C+ 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. Alkylation of Acetylene and Terminal Alkynes

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 Example: Alkylation of Acetylene

NaNH 2, NH 3 CH 3 Br CHCHCHCH (CH 3 ) 2 CHCH 2 C CNa (CH 3 ) 2 CHCH 2 C (81%) C—CH 3 (CH 3 ) 2 CHCH 2 C Example: Alkylation of a Terminal Alkyne

1. NaNH 2, NH 3 2. CH 3 CH 2 Br (81%)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 Example: Dialkylation of Acetylene

Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination Limitation

E2 predominates over S N 2 when alkyl halide is secondary or tertiary E2 C–: H—C + C H—C —H—H—H—H C C X–X–X–X– : + Acetylide Ion as a Base H C C X

Preparation of Alkynes by Elimination Reactions

Geminal dihalide Vicinal dihalide X C CXHH XX CCHH The most frequent applications are in preparation of terminal alkynes. Preparation of Alkynes by "Double Dehydrohalogenation"

(CH 3 ) 3 CCH 2 —CHCl NaNH 2, NH 3 2. H 2 O (56-60%) (CH 3 ) 3 CC CH Geminal dihalide  Alkyne

NaNH 2, NH 3 H2OH2OH2OH2O (CH 3 ) 3 CCH 2 —CHCl 2 (CH 3 ) 3 CCH CHCl (CH 3 ) 3 CC CH CNa(slow) (slow) (fast) Geminal dihalide  Alkyne

CH 3 (CH 2 ) 7 CH—CH 2 Br Br 1. 3NaNH 2, NH 3 2. H 2 O (54%) CH 3 (CH 2 ) 7 C CH Vicinal dihalide  Alkyne