CCAlkynes

Synthesis of Acetylene Heating coke with lime in an electric furnace to forms 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. *

The Structure of Alkynes A triple bond is composed of a  bond and two  bonds

H CC Acidity of Acetylene and Terminal Alkynes

In general, hydrocarbons are excedingly 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

The stronger the acid, the weaker its conjugate base top 252

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 Various Alkynes by alkylation reactions with Acetylide or Terminal Alkynes

Synthesis Using Acetylide Ions: Formation of C–C Bond

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

Reactions of Alkynes

AcidityHydrogenation Metal-Ammonia Reduction Addition of Hydrogen Halides Hydration Reactions of Alkynes

Hydrogenation of Alkynes

RCH 2 CH 2 R' cat catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate RC CR'+ 2H22H22H22H2 Hydrogenation of Alkynes

RCH 2 CH 2 R' Alkenes could be used to prepare alkenes if a catalyst were available that is active enough to catalyze the hydrogenation of alkynes, but not active enough for the hydrogenation of alkenes. cat H2H2H2H2 RC CR' cat H2H2H2H2RCH CHR' Partial Hydrogenation

There is a catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not that of alkenes to alkanes. It is called the Lindlar catalyst and consists of palladium supported on CaCO 3, which has been poisoned with lead acetate and quinoline. syn-Hydrogenation occurs; cis alkenes are formed. RCH 2 CH 2 R' cat H2H2H2H2 RC CR' cat H2H2H2H2RCH CHR' Lindlar Palladium

+ H 2 Lindlar Pd CH 3 (CH 2 ) 3 (CH 2 ) 3 CH 3 HH (87%) CH 3 (CH 2 ) 3 C C(CH 2 ) 3 CH 3 C C Example

Alkynes  trans-Alkenes Metal-Ammonia Reduction of Alkynes

RCH 2 CH 2 R' Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. trans-Alkenes are formed. RC CR' RCH CHR' Partial Reduction

CH 3 CH 2 CH 2 CH 3 H H (82%) CH 3 CH 2 C CCH 2 CH 3 C C Na, NH 3 Example

four steps (1)electron transfer (2)proton transfer (3)electron transfer (4)proton transfer Metal (Li, Na, K) is reducing agent; H 2 is not involved; proton comes from NH 3 Mechanism

Suggest an efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents. Problem

Problem Strategy

1. NaNH 2 2. CH 3 CH 2 CH 2 CH 2 Br Na, NH 3 H 2, Lindlar Pd Problem Synthesis

Addition of Hydrogen Halides to Alkynes

HBr Br (60%) Alkynes are slightly less reactive than alkenes CH 3 (CH 2 ) 3 C CH CH 2 Follows Markovnikov's Rule

(76%) CH 3 CH 2 C CCH 2 CH 3 2 HF F F C CHH CH 3 CH 2 CH 2 CH 3 Two Molar Equivalents of Hydrogen Halide

HBr (79%) regioselectivity opposite to Markovnikov's rule CH 3 (CH 2 ) 3 C CH CH 3 (CH 2 ) 3 CH CHBr peroxides Free-radical Addition of HBr

Hydration of Alkynes

expected reaction: enol observed reaction: RCH 2 CR' O H+H+H+H+RC CR' H2OH2OH2OH2O + H+H+H+H+RCCR' H2OH2OH2OH2O + OHRCHCR' ketone Hydration of Alkynes

enols are regioisomers of ketones, and exist in equilibrium with them keto-enol equilibration is rapid in acidic media ketones are more stable than enols and predominate at equilibrium enol OHRCHCR' RCH 2 CR' O ketone Enols

OHC C H + O H H :.. : Mechanism of conversion of enol to ketone

O H C C H + O H H :..:

O H C C H + O H H :.. : :

O H C C H H O ::H +.. :

O H C CHH O : : H +.. :

Useful for symmetrical starting alkynes to produce a single product. Unsymmetrical starting alkynes produce a mixture of ketones… not so useful.

Aldehyde vs. Ketone

Can you identify and name the function?

Example