1. 10.1 Alkynes Alkynes are molecules that incorporate a C  C triple bond Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-1 Klein, Organic Chemistry 2e

2. 10.1 Alkynes Given the presence of two pi bonds and their associated electron density, alkynes are similar to alkenes in their ability to act as a nucleophile Converting pi bonds to sigma bonds generally makes a molecule more stable. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-2 Klein, Organic Chemistry 2e

3. 10.1 Alkyne Uses Acetylene is the simplest alkyne It is used in blow torches and as a precursor for the synthesis of more complex alkynes More than 1000 different alkyne natural products have been isolated One example is histrionicotoxin, which can be isolated from South American frogs and is used on poison-tipped arrows by South American tribes Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-3 Klein, Organic Chemistry 2e

4. An example of a synthetic alkyne is ethynylestradiol How do you think a C  C triple bond affects the molecules geometry? Its rigidity? Its intermolecular attractions? 10.1 Alkyne Uses Ethynylestradiol is the active ingredient in many birth control pills The presence of the triple bond increases the potency of the drug compared to the natural analog Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-4 Klein, Organic Chemistry 2e

5. 10.2 Alkyne Nomenclature Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications 1.Identify the parent chain, which should include the C  C triple bond 2.Identify and Name the substituents 3.Assign a locant (and prefix if necessary) to each substituent giving the C  C triple bond the lowest number possible 4.List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically 5.The C  C triple bond locant is placed either just before the parent name or just before the -yne suffix Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-5 Klein, Organic Chemistry 2e

6. Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications 1.Identify the parent chain, which should include the C  C triple bond 2.Identify and name the substituents. 10.2 Alkyne Nomenclature Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-6 Klein, Organic Chemistry 2e

7. 10.2 Alkyne Nomenclature Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications 3.Assign a locant (and prefix if necessary) to each substituent giving the C  C triple bond the lowest number possible The locant is ONE number, NOT two. Although the triple bond bridges carbons 2 and 3, the locant is the lower of those two numbers Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-7 Klein, Organic Chemistry 2e

8. 10.2 Alkyne Nomenclature Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications 4.List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically 5.The C  C triple bond locant is placed either just before the parent name or just before the -yne suffix Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-8 Klein, Organic Chemistry 2e

9. 10.2 Alkyne Nomenclature In addition to the IUPAC naming system, chemists often use common names that are derived from the common parent name acetylene You should also be aware of the terminology below Practice with SkillBuilder 10.1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-9 Klein, Organic Chemistry 2e

10. 10.2 Alkyne Nomenclature Name the molecule below Recall that when triple bonds are drawn their angles are 180° Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-10 Klein, Organic Chemistry 2e

11. 10.3 Alkyne Acidity Recall that terminal alkynes have a lower pK a than other hydrocarbons Acetylene is 19 pK a units more acidic than ethylene, which is 10 19 times stronger Does that mean that terminal alkynes are strong acids? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-11 Klein, Organic Chemistry 2e

12. 10.3 Alkyne Acidity Because acetylene (pK a =25) is still much weaker than water (pK a =15.7), a strong base is needed to make it react, and water cannot be used as the solvent Recall from chapter 3 we used the acronym, ARIO, to rationalize differences in acidity strengths Use ARIO to explain why acetylene is a stronger acid than ethylene which is stronger than ethane Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-12 Klein, Organic Chemistry 2e

13. 10.3 Alkyne Acidity Use ARIO to rationalize the equilibria below A bases conjugate acid pK a must be greater than 25 for it to be able to deprotonate a terminal alkyne Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-13 Klein, Organic Chemistry 2e

14. 10.4 Preparation of Alkynes Like alkenes, alkynes can also be prepared by elimination Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-14 Klein, Organic Chemistry 2e

15. 10.4 Preparation of Alkynes Such eliminations usually occur via an E2 mechanism Geminal dihalides can be used Vicinal dihalides can also be used E2 requires anti-periplanar geometry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-15 Klein, Organic Chemistry 2e

16. 10.4 Preparation of Alkynes Often, excess equivalents of NaNH 2 are used to shift the equilibrium toward the elimination products NH 2 1- is quite strong, so if a terminal alkyne is produced, it will be deprotonated That equilibrium will greatly favor products Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-16 Klein, Organic Chemistry 2e

17. 10.4 Preparation of Alkynes A proton source is needed to produce the alkyne Predict the products in the example below Practice with conceptual checkpoint 10.7 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 10-17 Klein, Organic Chemistry 2e