Hidrocarbon Alkynes and Reactions

Organic Chemistry 

Chapter 9  Alkynes

Introduction •  Alkynes • • • •

contain contain a triple bond. bond. General formula is C nH2n-2 Two elements of unsaturation for each triple bond. ome reactions are like alkenes! addition and o"idation. ome reactions are specific to alkynes. #$ Chapter 9

2

Introduction •  Alkynes • • • •

contain contain a triple bond. bond. General formula is C nH2n-2 Two elements of unsaturation for each triple bond. ome reactions are like alkenes! addition and o"idation. ome reactions are specific to alkynes. #$ Chapter 9

2

&omenclature! I'(AC • • • •

)ind the lon*est chain containin* the triple bond. Chan*e -ane endin* to -yne. &umber the chain+ startin* at the end closest to the triple bond. Gi,e branches or other substituents a number to locate their position. #$ Chapter 9

%

&ame these! CH3

C

CH

propyne CH3

C

C

CH2

CH2

Br

5-bromo-2-pentyne CH3 CH3

CH

CH3 CH2   C

C

CH

CH3

2,6-dimethyl-3-heptyne Chapter 9



 #$

 Additional )unctional Groups •  All

•

other functional *roups+ e"cept ethers and halides ha,e a hi*her priority than alkynes. )or a complete list of namin* priorities+ look inside the back co,er of your te"t. #$ Chapter 9



0"amples CH3 CH2

CH CH2

CH C

CH

4-methyl-1-hexen-5-yne OH CH3

C

C

CH2

CH CH3

4-hexyn-2-ol

#$ Chapter 9

/

Common &ames &amed as substituted acetylene. CH3

C

CH

methylacetylene CH3 CH3

CH

CH3 CH2

C

C

CH

CH3

isobutylisopropylacetylene

#$ Chapter 9

1

(hysical (roperties • • • • •

&onpolar+ insoluble in water. oluble in most or*anic sol,ents. 3oilin* points similar to alkane of same si4e. 5ess dense than water. 'p to  carbons+ *as at room temperature. #$ Chapter 9



 Acetylene •  Acetylene • •

is used in weldin* torches. In pure o"y*en+ temperature of flame reaches 266°C. It would ,iolently decompose to its elements+ but the cylinder on the torch contains crushed firebrick wet with acetone to moderate it. #$ Chapter 9

9

ynthesis of Acetylene • •

Heat coke with lime in an electric furnace to form calcium carbide. Then drip water on the calcium carbide. 3 C

"

co#e

* CaC2

  "

CaO

CaC2

"

H

C

CO

lime 2 H2O

C

H

  CaOH!2 "

*This reaction was used to produce light for miners’ lamps and for the stage. Chapter 9

76

#$

0lectronic tructure •

The si*ma bond is sp-sp o,erlap.

•  The

two pi bonds are unhybridi4ed p o,erlaps at 96 °+ which blend into a cylindrical shape.

#$ Chapter 9

77

3ond 5en*ths • •

8ore s character+ so shorter len*th. Three bondin* o,erlaps+ so shorter.

3ond an*le is 76°+ so linear *eometry. Chapter 9

72

#$

 Acidity of Alkynes •

Terminal alkynes+ -C ≡C-H+ are more acidic than other hydrocarbons. •  Acetylene → acetylide by &H 2-+ but not by :H- or :-. • 8ore s character+ so pair of electrons in anion is held more closely to the nucleus. 5ess char*e separation+ so more stable. #$ Chapter 9

7%

 Acidity Table

Chapter 9

7

=>

)ormin* Acetylide Ions •

H; can be remo,ed from a terminal alkyne by sodium amide+ &a&H 2.

CH3

C

C

H "   $a$H2

CH3

C

-

C%  $a

"

" $H3

•  &a&H2 is

produced by the reaction of ammonia with sodium metal.

H $

H

"

$a

&e

H

"3 "

H $ $a

"

H Chapter 9

1'2 H2

#$ 7

Hea,y 8etal Acetylides •

Terminal alkynes form a precipitate with  A*ualitati,e

test for terminal alkyne eparation of a mi"ture of terminal and internal alkynes. Chapter 9

7/

#$

>ualitati,e Test CH3

• • •

C

C

H

"

" Cu

CH3

C

C

Cu

"

ea*ent is A*&: % or Cu&:% in alcohol+ or  ammonia is added to form the comple" ion. The solid is e"plosi,e when dry. Copper tubin* is not used with acetylene. #$ Chapter 9

71

"

H

eparation of 8i"tures "

CH3

C

C

Cu

CH3

" CH3

$o reaction "

CH2

C

C

H

Cu

CH3

CH2

C

C

Cu

red-brown precipitate

)ilter the solid to separate+ then re*enerate the terminal  alkyne by addin* dilute acid. CH3

CH2

C

C

Cu

" HCl

CH3

CH2

C

C

H

" CuCl

 #$ Chapter 9

7

 Alkynes from  Acetylides •  Acetylide •

CH3

C

ions are *ood nucleophiles. &2 reaction with 7 ° alkyl halides len*thens the alkyne chain. -

C%  $a

"

"

CH3CH2

Br

CH3

C

C

CH2

CH3

 + $aBr

 #$ Chapter 9

79

8ust be 7° •  Acetylide

ions can also remo,e H ; If back-side approach is hindered+ elimination reaction happens ,ia 02.

•

Br CH3

C

-

C%  $a

"

+

CH3

CH CH3

CH3

C

C

H

"

H3C CH

=> Chapter 9

26

CH2

 Addition to Carbonyl  Acetylide ion ; carbonyl *roup yields an alkynol
C

C

H2O "

C

"

(

C

(

O

C

C

C

C

C

H

O H

O

O

H H

#$ Chapter 9

27

 Add to )ormaldehyde (roduct is a primary alcohol with one more carbon than the acetylide. H

H CH3

C

C

C

+

O

CH3

H

C

C

C

O

H

H H2 O " CH3

C

C

C

O H

H

H

O

H H

=> Chapter 9

22

 Add to Aldehyde (roduct is a secondary alcohol+ one  *roup from the acetylide ion+ the other  *roup from the aldehyde. CH3

C

C

CH3

CH3 C O + H

CH3

C

C

C

O

H CH3

H2O "

CH3

C

C

C

O

H

H

H

Chapter 9

2%

O

H H

 #$

 Add to @etone (roduct is a tertiary alcohol. CH3

C

C

CH3 C + CH3

CH3 O

CH3

C

C

C

O

CH3 CH3

H2O " CH3

C

C

C

O H

H

CH3

O

H H

 #$ Chapter 9

2

ynthesis by 0limination •

• •

emo,al of two molecules of H from a ,icinal or *eminal dihalide produces an alkyne. )irst step <-H= is easy+ forms ,inyl halide. econd step+ remo,al of H from the ,inyl halide reBuires ,ery stron* base and hi*h temperatures. #$ Chapter 9

2

ea*ents for 0limination CH3

• •

Br

Br

CH

CH

CH2 CH3

*OH +used! 2))°C

CH3

C

C

CH2

CH3

8olten @:H or alcoholic @:H at 266 °C fa,ors an internal alkyne. odium amide+ &a&H 2+ at 76°C+ followed by water+ fa,ors a terminal alkyne. CH3

CH2

CH2   CHCl2

1! $a$H2 , 15)°C

Chapter 9

2! H2O

CH3

CH2

C

CH

 #$ 2/

8i*ration of Triple 3ond

=> Chapter 9

21

 Addition eactions • • • •

imilar to addition to alkenes (i bond becomes two si*ma bonds. 'sually e"othermic :ne or two molecules may add.

 #$ Chapter 9

2

 Addition of Hydro*en •

Three reactions! •  Add lots of H2 with metal catalyst <(d+ (t+ or &i= to reduce alkyne to alkane+ completely saturated. • 'se a special catalyst+ 5indlars catalyst to con,ert an alkyne to a cis-alkene. • eact the alkyne with sodium in liBuid ammonia to form a trans-alkene. #$ Chapter 9

29

5indlars Catalyst • •

(owdered 3a: coated with (d+ poisoned with Buinoline. H2 adds syn+ so cis-alkene is formed.

Chapter 9

%6

 #$

 Addition of Halo*ens • • •

Cl2 and 3r 2 add to alkynes to form ,inyl dihalides. 8ay add syn or anti+ so product is mi"ture of cis and trans isomers. Difficult to stop the reaction at dihalide.

CH3

C

C

CH3

Br2

CH3 Br

Br C

C

" CH3

CH3

CH3 C

C

Br

Br

Br2

Br Br CH3

C

C

Br Br Chapter 9

%7

CH3

 #$

 Addition of H • • •

CH3

HCl+ H3r+ and HI add to alkynes to form ,inyl halides. )or terminal alkynes+ 8arko,niko, product is formed. If two moles of H is added+ product is a *eminal dihalide. C

C

H

HBr

Br CH3

C

Br HBr CH3

CH2

C

CH3

Br Chapter 9

%2

 #$

H3r with (ero"ides  Anti-8arko,niko, product is formed with a terminal alkyne. CH3

C

C H

H

HBr (OO( 

CH3

C

H C

Br

C

C

H

Br

HBr CH3

Br

H

H

(OO( 

 #$ Chapter 9

%%

Hydration of Alkynes •

•

8ercuric sulfate in aBueous sulfuric acid adds H-:H to one pi bond with a 8arko,niko, orientation+ formin* a ,inyl alcohol
%

#$

:"idation of Alkynes • • • •

imilar to o"idation of alkenes. Dilute+ neutral solution of @8n:  o"idi4es alkynes to a diketone. Earm+ basic @8n:  clea,es the triple bond. :4onolysis+ followed by hydrolysis+ clea,es the triple bond. #$ Chapter 9

%

eaction with @8n: •

8ild conditions+ dilute+ neutral

CH3

C

C

CH2

CH3

O O

*nO4

CH3

C

C

CH2

CH3

H2O, neutral

•

CH3

Harsher conditions+ warm+ basic C

C

CH2

CH3

*nO4 , *OH H2O, arm

O CH3

C

O O

+ O C CH2

CH3

 #$ Chapter 9

%/

:4onolysis •

:4onolysis of alkynes produces carbo"ylic acids
•

C

C

CH2

CH3

1! O3 2! H2O

O CH3

C

O OH

+ HO C

CH2

CH3

'sed to find location of triple bond in an unknown compound. #$ Chapter 9

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