CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.

Chapter 9 Alkynes

Alkyne Nomenclature by Dave Woodcock at Okanagan University College
http://www.molecularmodels.ca/nomenclature/index-2.htm > Alkynes
Use Netscape 4.7x with Chime

and Dave's Chime structures of alkynes at
http://www.molecularmodels.ca/molecule/molecule_index.html  > Alkynes
Use Netscape 4.7x with Chime

A small number of alkyne Chime structures by Jennifer Amann at Georgia Southern University
http://www2.gasou.edu/chemdept/general/organic/alkynes/ethyne/frame2.htm
Alkyne Reactions by Paul R. Young at the University of Illinois, Chicago
http://www.chem.uic.edu/web1/OCOL-II/WIN/ALKENE/F4.HTM
and  alkyne reaction practice quiz [intense!] at
http://www.chem.uic.edu/web1/OCOL-II/WIN/ALKENE/92/FRAMES.HTM
The Alkynes chapter in Exploring Organic Chemistry: An Electronic Textbook 
by Gary Trammell and Srinivas Vuppuluri at the University of Illinois at Springfield
http://people.uis.edu/gtram1/organic/alkynes/alkynes.htm 
Alkyne Reactions from the Virtual Textbook of Organic Chemistry by William Reusch at U Michigan
http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm#add1
Alkyne Reactions by Roberta W. Kleinman at Lock Haven University of Pennsylvania
http://www.lhup.edu/~rkleinma/Chem220/CH8_9Notes/alkynerx.htm
http://www.lhup.edu/~rkleinma/Chem220/CH8_9Notes/redox.htm
alkyne reaction summary table by M. A. Schwartz at Florida State
http://www.chem.fsu.edu/schwartz/CHM2210/Reactions/alkynes/rxns.html
McMurry Alkyne reaction summary [Organic Chemistry, 5th edition, by John McMurry]
provided by Clarkson University
http://www.clarkson.edu/~ochem/Fall01/CM241/AlkyneSummary.pdf
Or an older, but still nice, version from Yuzhuo Li at Clarkson University
http://people.clarkson.edu/~ligroup/f98241reaction1.pdf 
[alkyne summary is on page 2]
Carey PowerPoint slides for chapter 9 from Columbia University
9.1 to 9.4: sources, nomenclature, properties, structure/bonding
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/09_01_04.html
9.5 to 9.7: acidity of RC≡CH; preparation by alkylation; prep by elimination
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/09_05_07.html
9.8 to 9.14:  reactions, reactions, reactions [are you paying attention here, folks?]
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/09_08_14.html
 
 
 
 

Chapter 9:  Alkynes     R-C≡C-R' 

I. Nomenclature   
     A.  IUPAC Rules

              1.  The parent chain is the longest continuous carbon chain containing the C≡C bond.    
              2.  Name the parent chain by changing the -ane ending to -yne.
              3.  Alkynyl groups are used in a similar manner as alkyl and alkenyl groups.
                   a)  -C≡CH as a substituent is the ethynyl group.
              4.  Name the following alkynes. 

H-C≡C-H

HC≡C-CH3

CH3C≡CCH3

ethyne

propyne

2-butyne

6-methyl-3-heptyne

 II.  Physical Properties
       A.  Similar To Alkanes.

             1.  They are similar in having low density.
             2.  Due to their carbon/hydrogen make-up, alkynes are nonpolar.
                   a)  Alkynes as a result have a low solubility in water (similar to alkanes/alkenes).
                   b)  They dissolve in organic solvents such as alkanes, diethyl ether, CH2Cl2 and other chlorinated
                                                 hydrocarbons.
            3.  Alkynes have a slightly higher boiling point than the corresponding alkanes and alkenes.

CH3CH3

CH2=CH2

HC≡CH

-88.6oC

-103.7oC

-84.0oC

CH3CH2CH3

CH3CH=CH2

CH3C≡CH

-42.1oC

-47.4oC

-23.2oC

 III.  Structure
       A.  Bonding.

              1. Alkynes have sp hybridization.
                   a)  The sp hybrid orbitals have 50% s and 50% p character.
                   b)  This is important to recognize since as s character increases:
                        i)  The internuclear distance contracts.   
                        ii) An electron pair in the sp hybrid orbital is more strongly held, which requires more
                              energy for homolytic cleavage.
                       iii) Acidity increases:  The acidity of a terminal alkyne is greater than that of alkanes and alkenes.

HC≡CH

CH2=CH2

CH3-CH3

pKa ~ 26

pKa ~ 45

pKa ~ 62

most acidic

 

least acidic

                                         c)  180o bond angles.     H-C≡C-R

              2.  The smallest stable cycloalkyne is cyclononyne. 

IV. Acidity of Acetylene, Terminal Alkynes
       A.  Periodic Trends and Acidity.

             1.  Acidity increases across the periodic table.   ( HF > H2O > NH3 > CH4 )    
             2.  Electronegativity increases across the periodic table.   
             3.  The basicity of anions decreases across the periodic table.
                   a)  As the electron-attracting power of negatively charged atoms becomes greater, the anion becomes
                                 less basic.

 

weakest acid

 

 

strongest acid

second-row hydride

      CH4     <

      NH3      <

     H2O      <

HF

Ka

1060

1036

1.8 x 1016

3.5 x 104

pKa

60

36

15.7

3.2

 

    CH3    >

   H2N   >

          OH           >

F

methide

amide

hydroxide

fluoride

pKb =    46

22

1.7

10.8

strongest base

 

 

weakest base

             4. Since alkynes have sp hybrids (50% s character) they are the MOST ACIDIC hydrocarbons. 
                  a)  The s orbitals are nearer the positive nucleus than p;  orbitals with greater s character are capable
                                                of best stabilizing negative charge.

compound

H-C≡C-H

CH2=CH2

CH3-CH3

hybridization

sp

sp2

sp3

Ka

1026

1045

1062

pKa

26

45        

62

                    b) Alkenes and alkanes are too weakly acidic to be deprotonated for most practical purposes.
                    c)  Terminal alkynes can be deprotonated with strong bases (pKa of the conjugate acid > 26) to form
                                                the  ACETYLIDE ION.

         
                                                                                      nucleophilic

            5.  Which of the following bases will deprotonate terminal alkynes? (Duh!)

                                    OH                             (pKa H2O = 15.7)
                                    CH3CH2O
                 (pKa CH3CH2OH = 16)
                                    NH2
                           (pKa NH3 = 36) 

                  a)   To form the acetylide ion add sodium amide (NaNH2) in liquid ammonia solvent to the terminal  alkyne: 

                          i)  Why not add NaNH2 to the alkyne in the presence of water or alcohol? 

Answer:  H2O and ROH are much stronger acids than RC≡CH and would be deprotonated in place of the alkyne.

V.  Preparation of Alkynes
      A. Alkylation of Acetylene and Terminal Alkynes:  An SN2 Process.

            1.  Two separate reactions are run:
                 a)  First, treat acetylene or a terminal alkyne with NaNH2 in order to generate the acetylide ion.

                 b)  The acetylide ion acts as a nucleophile on a methyl or primary bromide or iodide.                                             
                      i)  Why only methyl or primary bromide [or iodide]?  
                                                Answer:  Competing elimination is possible otherwise.
                 c)    Solvents used in this reaction include NH3, ether, or THF.

            2.   Example.  Complete the reaction:  

            3.  Acetylene can be alkylated twice.
                 a)   Complete the chemical equations:

            4.  With 2o and 3o alkyl bromides/iodides, one gets ELIMINATION:

     B. Elimination:  Double Dehydrohalogenation of Geminal and Vicinal Dihalides.
           1. Geminal dihalides (both halide atoms on the same carbon) and vicinal dihalides
                                    (halide atoms are on adjacent carbons) both undergo elimination to form an alkyne.
           2. Examples.

             3. Reaction conditions can affect the location of the triple bond formed.                               

                   b)  The above observed products can be explained in terms of base catalyzed rearrangements, which occur
                         due to the acidity of the terminal protons and the propargylic protons (less acidic than the terminal H):

               c)   Base-catalyzed isomerizations can yield either terminal or internal alkynes depending upon the base used.
 
                      i)  Fused KOH favors the formation of internal alkynes:  the more thermodynamically stable
            `                    (more highly substituted) alkyne forms preferentially.
                     ii)  Sodium amide (NaNH2 in NH3) favors the formation of terminal alkynes:  NH2
is a strong
                                  enough base to fully deprotonate the terminal site forming the acetylide ion:
                                  This makes the terminal alkyne favored (protonate with H2O).
              d)   
In the following example, note that the five different dibromopentanes yield the same product for
                          their respective reaction conditions:

                  e)  Problem:  How can an alkyne be prepared from an alkyl halide?

                      Answer:  First convert the alkyl halide to an alkene, convert the alkene to a dihalide, and then                                                 produce the alkyne.

                         4. Two dehydrohalogenations occur sequentially.
                                    a)  The second dehydrohalogenation is more difficult than the first.
                                    b)  With weaker bases at lower temperatures the intermediate alkenyl halide is isolated:

              C.   Alkylation of Alkyne Dianions
                    1.   The treatment of an alkyne with two equivalents of butyllithium (a very strong base) results in the
                                    deprotonation of the terminal and propargylic carbons, forming an ALKYNE DIANION.

                       2.  Treatment of this dianion with a primary (or methyl) RX (X = Br, I) results in the alkylation at the
                                     more reactive [more basic]  propargylic position EXCLUSIVELY. 

                           a)         Examples:

                   3.  This is a useful synthetic route since "normal" alkylations require 1o RX:

 VI.  Reactions of Alkynes
        A.  Hydrogenation

               1.  Hydrogenation with Pt, Pd, Ni.

                                   

               2.   Lindlar Catalyst.  Semihydrogenation.    
                                    a)  The Lindlar catalyst is a “poisoned” mixture of Pd/CaCO3, lead acetate, and         quinoline. 
                                    b)   The reaction is stereoselective: syn addition results in the cis (Z) product.

 

               3.  Group IA Metals:  Li, Na, or K in NH3. Semihydrogenation.
                                    a)  This reaction is also stereoselective, but producing the trans (E) product.

                                     b)  Mechanism.  2 electron transfers; 2 proton transfers.

1

2

3

4

net

        B.   Addition of HX  (X = Br, Cl, I)
               1.  Overall reaction.

              2.  Markovnikov's Rule is observed.

             3.  Mechanism forming alkenyl cation:

      C.   Addition of X2 (X = Cl, Br)
            1.  Overall reaction.

           2.  Mechanism:  The reaction goes through a cyclic halonium ion intermediate:

less stable, more strained than

            3.  Complete the equation:  CH3C≡CH + 2 Br2 à

     D.  Hydration:  Addition of Water.
           1.  Overall reaction.

           2.  Keto-enol isomerism (tautomerism): Ketones are much more stable than the enol tautomer.
           3.  The reaction follows Markovnikov's Rule. Terminal alkynes yield methyl ketones [-2-ones]
                            If R ≠ R’, two different ketones form, limiting the synthetic utility of this process with internal alkynes
           4.  HgSO4 or HgO are used as catalysts.
           5.   MeOH and HC2H3O2 are used as cosolvents.                                    Why?                                     
            Answer:  Alkynes have limited solubility in aqueous H2SO4.
          6.  In this reaction, the only time an aldehyde is produced is in the reaction with acetylene.
               a)  Example with acetylene:

               b)  Notice that in the reaction with a terminal alkyne a ketone forms since the formation of the enol
                                                follows Markovnikov's Rule:

              7.  Mechanism:

      E. Oxidation of Alkynes.
            1.  As with alkenes, alkynes undergo ozonolysis, but the products are carboxylic acids.

         
     F.  Acetylides of Transition Metals:  Ag+,  Cu+
           1.  Acetylene/Terminal Alkynes + Ag+/Cu+
à  insoluble metal acetylides
           2.  Metal acetylides are explosive and shock sensitive when dry.
                 a)   Conclusion:  Be careful when working with organic compounds in the lab since the unexpected
                          could occur if you are not careful. 

VII.  Synthesis Problems. 
        A.  Retrosynthesis.

              1.  Think BACKWARDS.
              2.  Start your analysis by looking at the TARGET molecule and search for its immediate PRECURSORS.
              3.  Continue this analysis (the precursor becomes the new target molecule) until you get the desired
                                    starting materials.
        B.  Example.
              1.  Synthesize cis-2-hexene from 1-pentyne.

 

                        Solution:

       C.  Synthesize the following compounds starting from acetylene.
             1.  2-bromopentane
             2.  3,3-dichlorohexane
             3.   
                       a)  trans-2-methyl-3-heptene and  
                      b)  cis-2-methyl-3-heptene
             4.  (E)-2,3-dichloro-4,4-dimethyl-2-hexene

Many thanks to Rod Oka of MPC for generously sharing his "Lecture Companion" outline,
reproduced here in extensively modified form by permission, with web references and other goodies added by me.
Structures drawn using  MDL IsisDraw™, CS ChemOffice ChemDraw™, and ACDLabs ChemSketch™  .