H CC 9.5 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

9.6 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

9.7 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