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Organic Chemistry 4h Edition Paula Yurkanis Bruice Chapter 6 G Reactions of Alkynes Cleveland, OH ©2004, Prentice Hall... a terminal alkyne an internal alkyne acetylene propargyl gr

Organic Chemistry

4h Edition Paula Yurkanis Bruice

Chapter 6

G

Reactions

of Alkynes

Cleveland, OH

©2004, Prentice Hall

Alkynes are hydrocarbons that contain a carbon-carbon triple bond

General formula: C.H.„_› (acyclic); C.H.„.„ (cyclic)

a terminal alkyne an internal alkyne acetylene

propargyl group allyl group

propargyl bromide allyl alcohol

A substituent receives the lowest number if there is no

functional group suffix,

The Structure of Alkynes

Addition Reactions of Alkynes

sequential Addition of HCI

An alkyne Is less reactive than an alkene

Progress of the reaction

Relative Stabilities of Carbocations

relative stabilities of carbocations

Addition of Hydrogen Halides

In the presence of 1 mole equivalent of HBr

In the presence of excess HBr,

a carbocation is formed by adding the electrophile to the Sp* carbon bonded to the greater number of hydrogen

A secondary vinylic cation is about as stable as a primary

Addition of Halogens to Alkynes

Examples of Water Addition

Hg** is added to increase the rate of water addition to

Formation of Ketone versus Aldehyde

Conversion of Internal Alkynes to Trans

a trans alkene a vinylic anion

+ NH, + Nat

Reason for trans addition:

H

cá, t cá, v Nà

®

Acidity of a Hydrogen Bonded to an sp

Hybridized Carbon

relative electronegativities of carbon atoms

least electronegative

Electronegativities versus Acid

The stronger the acid, the weaker its conjugate base

RC=CH + _NH; = RC=C™ + NHạ

amide anion acetylide anion

|stronger acid| |stronger base| weaker base weaker acid

RCECH + HO << RC=C™ + HạO

hydroxide anion acetylide anion

weaker acid weaker base | stronger base | | stronger acid |

Synthesis Using Acetylide lons:

Formation of C—C Bond

a

CH:CH;ạC= Cy of CH¡CH,CH; iu — CH;CH,C=CCH>CH,CH; + Br

retrosynthetic analysis

O

|

CH,CH,CCH,CH,CH, == CH,CH,C=CCH,CH, = CH,CH,C=CH

synthesis

CHaCHCECH - 2 ””?———› CH.CHCH,CECCHCH —?-> CH:CHCH;CH›CH.CHCH “7 a 2 CHạCHCH;Đr oT ue | 3 Pd/C tì 112 = 3

Example 3

2

C cò -c=cH ——> Ó_ )_CH;CH;OH

retrosynthetic analysis

= —> Ệ È-c=em, — Ệ È-c=cm

Commercial Use of Ethyne

— HCECH + HCI —> H;CECHGI -ˆ hs 'CH;—CH