Download Chapter 6: Alkynes, reactions of alkynes, and multistep synthesis

Chemistry 242
Chapter 6: Reactions of Alkynes & Intro to Multi-Step Synthesis
Homework: 25a-e, 26, 27, 28, 29, 32, 33, 36, 38, 40, 42, 43, 44, 45, 49, 50
(Solution Manual error: 27e should be 1,5-cyclooctadiene)
Outline
I. Alkynes
A. internal alkyne vs terminal alkyne
II. Nomenclature
A. common
1. triple bond is acetylene
2. everything else is substituent
3. ex. isopropylmethyl acetylene or t-butylcyclopentylacetylene
4. HCCCH2- is propargyl group, similar to allyl grp (H2C=CHCH2-)
5. ex propargyl alcohol
B. IUPAC
1. name like alkenes, but no cis/trans or E/Z
2. find longest chain containing funct. grp and # so funct grp # smallest
3. if 2 triple bonds, diyne
a. if 3 trple bonds, triyne
b. if 4, tetrayne, then pentayne, hexayne, heptayne, etc
4. with number (#) as prefix
a. funct grp has priority with numbering
5. if more than 1 subst., place all in alphabetical order in front
6. if counting from either end is tie in yne #, use subst # for tie-breaker
(look for 1st point of difference)
7. if still tied, go to second subst, etc (1st point of difference)
8. #1 in cyclics, other C in pi bond is #2
a. no need to include number of pi bond in name unless diene, when
both numbers included
b. go clockwise (cw) or ccw to get lowest subst #
c. 1st point of difference, not sum of all subst #
examples:
C. More than one parent nomenclature
A. priority table 6.1 (p262)
examples:
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III. Physical properties
A. mp, bp, water solubility
B. sp hybridized → linear (no cis/trans or E/Z)
IV. Reactions of Alkynes
A. Electrophillic addition
1. has pi electrons and is a nucleophile just like alkene
2. but can react twice because twice the # pi bonds
a. excess reactant
3. alkene more reactive than alkyne
a. because of carbocation intermediate stability
B. Specific addition rxns
1. hydrogen halides → geminal dihalide
2. straight halogen (bromine or chlorine in CH2Cl2)
3. H2O/H2SO4 → enol
a. keto-enol tautomerization (acid cat. or base cat.)
b. water sulfuric acid works for internal alkynes
c. need mercuric sulfate for terminal alkynes
d. can get aldehyde if acetylene
4. Hydroboration-oxidation (1. BH3/THF, 2. HO-, H2O2, H2O)
a. antimarkovnikov product
b. for terminal alkynes, use isoamyl borane
c. easly get aldehyde
5. Hydrogenation
a. usually can’t stop at alkene
b. can stop only if using “poison” catalyst (Lindlar catalyst)
c. get cis alkene for syn addition with Lindlar
d. to get trans, use Na or Li in liquid ammonia (-78ºC)
e. this is radical addition
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V. Acidity of terminal alkyne
A. very strong base (sodium amide) will pull of proton, creating a nucleophile
1. substitutes onto alkyl halide
2. only reaction so far which enlarges the carbon chain (or changes the
length at all)
VI. Multistep synthesis
A. Most syntheses take more than one step to get from starting molecule to end
point.
Objectives
Knowledge
Recall rules for alkyne nomenclature
Memorize chapter reactions on p282-283
Comprehension
Add vinylic cation to stability list (where and why?)
Understand what a polymer is (moved to Chapter 11)
Application
Name given alkynes or sketch alkyne molecule if given the name
Draw mechanisms for bridged bromination/chorination and hydrohalogenation (Chapt 5)
Draw mechanism for keto-enol tautomerization
Analysis
Write a sequence of reactions to get from a given reactant to a desired product
Use reaction summary (p282-283) to complete missing part of rxns (react, prod, rxn
cond)
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