ACETYLIDE IONS AND ALKYLATION REACTIONS (REVIEW) SYNTHESIS Dr. Clower CHEM 2412 Fall 2014 McMurry (8 th ed.) sections , 8.1, 9.2, 9.9

Acidity of Alkynes Terminal alkynes are weak acids More acidic than alkenes or alkanes Form somewhat stable conjugate base = acetylide ion Electron pair close to nucleus because more s character

Acetylide Ion Strong base Stronger than HO - or RO - Not as strong as - NH 2 Acetylide ion can act as a base or a nucleophile

Acetylide Ion as a Nucleophile React with methyl or primary alkyl halides Undergo substitution reaction Form a new, larger alkyne Alkylation reaction C-C bond making reaction Example:

Alkylation Reactions

Acetylide Ion as a Base React with secondary or tertiary alkyl halides Undergo elimination reaction Dehydrohalogenation (eliminate H-X) Elimination reactions are used to synthesize alkenes and alkynes

Preparation of Alkenes Alcohol eliminates water to form an alkene Reagent = acid Alkyl halide eliminates HX to form an alkene Reagent = base

Preparation of Alkynes A two-step process from alkenes 1. Alkenes undergo addition of X 2 to make a vicinal dihalide 2. The vicinal dihalide undergoes 2 elimination reactions to yield the alkyne

How could you prepare 2-butyne from 2-butene? How could you prepare 2-pentyne from 3-pentanol?

Synthesis You will be given a product. Your goal is to determine how to make that product from simpler starting materials using reactions we have studied. Consider: How many carbons are in the starting material and product? Do you need to make any C-C bonds? If so, how will you do that? What functional groups are in the starting material? What can you do with those functional groups? What functional groups are in the product? How do you know how to make those functional groups? Try working backwards (retrosynthesis). Look at McMurry section 9.9 for strategies and worked examples Example: Propose a synthesis of cis-3-hexene from acetylene. You may use any alkyl halide as a source of carbon.

Propose a synthesis of cis-3-hexene from acetylene.

Propose a synthesis of 2-butanone from ethylene.