University:
Rutgers University Newark
Course:
CHEM 335 | Organic Chemistry
Academic year:

2024
Views:

39

Pages:

25
Author:

Mahitab S.

Ezra Chapter 7 - Alkyne Reactivity Monday, May 15, 2017 10:03 PM Nomenclature: same general rules that we have seen before with alkanes and alkenes always vinyl try'drogue cis trans Athene 1. Suffix -yne 2. Triple bond recieves lowest possible number on the parent chain 3. 4. Compounds with double and triple bonds are called "-enynes," not "-ynenes" a. Numbering starts on the end of the chain closest to the first multiple bond (if there is a choice, alkenes receive the lower number) E Examples: O O nsefereochen 0 O Octa 2,4 dien6 y.nl I i Whs Ks peat 4 yn or S Chapter 7 - Alkyne Reactivity Page 1 2 amine y 3 B 2 Chapter 7 - Alkyne Reactivity Page 2 6 Relative Reactivity of Alkynes Monday, May 15, 2017 10:03 PM Alkynes are less reactive than alkenes Chapter 7 - Alkyne Reactivity Page 3 Preparation of Alkynes: Elimination Reactions of Halides/Dihalides Monday, May 15, 2017 10:03 PM Example: Mechanism: Chapter 7 - Alkyne Reactivity Page 4 Preparation of Alkynes: Elimination Reactions of Halides/Dihalides Monday, May 15, 2017 10:03 PM Example: Mechanism: HE Chapter 7 - Alkyne Reactivity Page 4 R 2 i f 3 E OM 3 R 42 Chapter 7 - Alkyne Reactivity Page 5 Z t I g n S re en g Yney f od Br S bromo y n on en 6 yr 3 oh Reactions of Alkynes: Addition of HX Monday, May 15, 2017 10:03 PM Example: Mechanism: Chapter 7 - Alkyne Reactivity Page 6 Reactions of Alkynes: Halogenation Monday, May 15, 2017 10:03 PM Example: Chapter 7 - Alkyne Reactivity Page 7 Reactions of Alkynes: Hydration (Method One) Monday, May 15, 2017 10:03 PM 1. Mercury (II) - Catalyzed Hydration of Alkynes Enol = vinylic alcohol (alkene + alcohol) Enols immediately rearrange to form the more stable keto tautomer Tautomers: constitutional isomers that rapidly interconvert Chapter 7 - Alkyne Reactivity Page 8 Tautomer Interconversion Monday, May 15, 2017 10:03 PM Acid catalyzed keto-enol tautomerization Base catalyzed keto-enol tautomerization Chapter 7 - Alkyne Reactivity Page 9 Hydration of an Internal Alkyne Monday, May 15, 2017 10:03 PM Review: Example: Base catalyzed keto-enol tautomerization Chapter 7 - Alkyne Reactivity Page 10 Reactions of Alkynes: Hydration (Method Two) Monday, May 15, 2017 10:03 PM 2. Hydroboration/Oxidation of Alkynes Internal Alkyne: Terminal Alkyne: Chapter 7 - Alkyne Reactivity Page 11 Chapter 7 - Alkyne Reactivity Page 12 Summary Example of Alkyne Hydration Methods Monday, May 15, 2017 10:03 PM Chapter 7 - Alkyne Reactivity Page 13 Reduction of Alkynes: Catalytic Hydrogenation Monday, May 15, 2017 10:03 PM Chapter 7 - Alkyne Reactivity Page 14 Sodium/Ammonia Reduction of an Alkyne Monday, May 15, 2017 10:03 PM Chapter 7 - Alkyne Reactivity Page 15 Oxidative Cleavage of Alkynes Monday, May 15, 2017 10:03 PM Internal Alkyne: Terminal Alkyne: Chapter 7 - Alkyne Reactivity Page 16 Alkyne Acidity: Formation of Acetylide Anions Monday, May 15, 2017 10:03 PM Terminal Alkyne Alkene Alkane pKa = pKa = pKa = = Acetylene ("the" terminal alkyne) Acetylene can be deprotonated by the conjugate base of any acid with a pKa greater than 25 Sodium amide (NaNH2) is a very strong base The negative charge of the acetylide anion is stabilized on an sp hybridized carbon atom (more electronegative, 50% s character) Chapter 7 - Alkyne Reactivity Page 17 Alkylation of Acetylide Anions Monday, May 15, 2017 10:03 PM Acetylide anions are highly nucleophilic New term - Alkylation: addition of an alkyl group to a molecule (in this case - the acetylide anion) Example: Must use methyl/primary alkyl halides (Br/I) Any terminal alkyne can undergo alkylation/dialkylation: Chapter 7 - Alkyne Reactivity Page 18 Chapter 7 - Alkyne Reactivity Page 19 Caveats/Limitations Monday, May 15, 2017 10:03 PM Acetylide anion alkylation is limited to methyl/primary alkyl bromides/iodides Acetylide anion is a sufficiently strong base that if it is reacted with secondary or tertiary alkyl halides, it will result in dehydrohalogenation (elimination to form an alkene) rather than alkylation Example: Chapter 7 - Alkyne Reactivity Page 20 Introduction to Organic Synthesis Monday, May 15, 2017 10:03 PM Synthesis Practice Problem - Synthesize cis-2-hexene from 1-pentyne and any alkyl halide necessary. More than one step is required: Chapter 7 - Alkyne Reactivity Page 21 Organic Synthesis Practice Problem Monday, May 15, 2017 10:03 PM Synthesize 2-bromo-5-methylhexane from acetylene and any alkyl halide needed. More than one step is required: Chapter 7 - Alkyne Reactivity Page 22 Organic Synthesis Practice Problem cont'd Monday, May 15, 2017 10:03 PM Chapter 7 - Alkyne Reactivity Page 23 Organic Synthesis Practice Problem cont'd Monday, May 15, 2017 10:03 PM Chapter 7 - Alkyne Reactivity Page 25