9.5 Acidity of and Terminal

H C C Acidity of Hydrocarbons

In general, hydrocarbons are exceedingly weak acids

Compound pKa HF 3.2

H2O 16

NH3 36 45 H2C CH2

CH4 60 Acetylene

Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes.

Compound pKa HF 3.2

H2O 16 HC CH 26 NH3 36

H2C CH2 45

CH4 60 : Hybridization and Electronegativity

10-60 C H H+ + C : sp3

H 2 10-45 : sp C C H+ + C C

10-26 C C H H+ + C C : sp

Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Sodium Acetylide

Objective: Prepare a solution containing sodium acetylide NaC CH Will treatment of acetylene with NaOH be effective?

+ + NaOH + HC CH NaC CH + H2O Sodium Acetylide

No. Hydroxide is not a strong enough base to deprotonate acetylene. + + NaOH HC CH NaC CH H2O

.. – .. – : + HO H + :C CH HO.. H C CH .. weaker acid stronger acid

pKa = 26 pKa = 16 In acid-base reactions, the equilibrium lies to the side of the weaker acid. Sodium Acetylide

Solution: Use a stronger base. is a stronger base than sodium hydroxide. + + NaNH2 HC CH NaC CH NH3

.. – .. – + H2N: + H C CH H2N H :C CH stronger acid weaker acid

pKa = 26 pKa = 36 is a weaker acid than acetylene. The position of equilibrium lies to the right. 9.6 Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes Preparation of Alkynes

There are two main methods for the preparation of alkynes:

Carbon-carbon bond formation alkylation of acetylene and terminal alkynes Functional-group transformations elimination Alkylation of Acetylene and Terminal Alkynes

H—C C—H

R—C C—H

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

– – SN2 H—C C: + R X H—C C—R + : X–

The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution. The is sodium acetylide or the sodium salt of a terminal (monosubstituted) . Example: Alkylation of Acetylene

NaNH2 HC CH HC CNa NH3

CH3CH2CH2CH2Br

HC C CH2CH2CH2CH3

(70-77%) Example: Alkylation of a Terminal Alkyne

(CH3)2CHCH2C CH

NaNH2, NH3

(CH3)2CHCH2C CNa

CH3Br

(CH3)2CHCH2C C—CH3 (81%) Example: Dialkylation of Acetylene

H—C C—H

1. NaNH2, NH3

2. CH3CH2Br

CH3CH2—C C—H

1. NaNH2, NH3

2. CH3Br

CH3CH2—C C—CH3 (81%) Limitation

Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination Acetylide Ion as a Base

E2 predominates over SN2 when alkyl halide is secondary or tertiary

– H—C C: H C

C X E2

H—C C—H + C C + : X– 9.7 Preparation of Alkynes by Elimination Reactions Preparation of Alkynes by "Double Dehydrohalogenation"

H X H H

C C C C

H X X X

Geminal dihalide Vicinal dihalide

The most frequent applications are in preparation of terminal alkynes. Geminal dihalide Æ Alkyne

(CH3)3CCH2—CHCl2

1. 3NaNH2, NH3

2. H2O

(CH3)3CC CH (56-60%) Geminal dihalide Æ Alkyne

(CH3)3CCH2—CHCl2 NaNH2, NH3 (slow) (CH3)3CCH CHCl (slow) NaNH2, NH3 (CH3)3CC CH H O 2 NaNH2, NH3 (fast) (CH3)3CC CNa Vicinal dihalide Æ Alkyne

CH3(CH2)7CH—CH2Br Br

1. 3NaNH2, NH3

2. H2O

CH3(CH2)7C CH

(54%)