ACETYLIDE IONS AND ALKYLATION REACTIONS (REVIEW)

SYNTHESIS

Dr. Clower

CHEM 2412

Fall 2014

McMurry (8th ed.) sections 9.7-9.8, 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 -NH2

• 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 X2 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.

C C HH

CH2CH3

HH

CH3CH2

• Propose a synthesis of 2-butanone from ethylene.