有机化学英文课件chapter7共42页

7-12
Allene
u Allene: a compound containing a C=C=C group
• the simplest allene is 1,2-propadiene, commonly named allene
7-13
Allenes
• most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
7-16
Addition of HX
u Alkynes undergo regioselective addition of either
1 or 2 moles of HX, depending on the ratios in
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
7-11
Preparation from Alkenes
• a side product may be an allene, a compound
containing adjacent carbon-carbon double bonds,
is n o t su p p o rted
7-5
Acidity
u The pKa of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1)
is n o t s u p p o rte d
7-2
Nomenclature
u IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond
M a c in to s h P IC T im a g e fo r m a t
is n o t s u p p o r te d
which the alkyne and halogen acid are mixed
Br
Br
CH3 C CH HBr CH3 C= CH2 HBr CH3 CCH3
Propyne
Br 2-Bromopropene 2,2-Dibromopropane
7-17
Addition of HX
• the intermediate in addition of HX is a 2°vinylic carbocation
• terminal alkynes react with sodium amide to form alkyne anions
M a c in to s h P IC T im a g e fo r m a t
is n o t s u p p o r te d
7-6
Acidபைடு நூலகம்ty
• terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA)
C=C=C
HXH RC–C= CR Na NH2
- HBr R A haloalkene (a vinylic halide)
R
H
CCC
R
R
An allene
H
RCC CR
R An alkyne
M acin to sh P IC T im ag e fo rm at
is n o t su p p o rted
• with 2°and 3°halides, elimination is the major reaction
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
7-8
Alkylation of Alkyne Anions
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
7-18
Addition of HX
• in the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocation
7-19
Hydroboration
u Addition of borane to an internal alkyne gives a trialkenylborane
• addition is syn stereoselective
M a c in to s h P IC T im a g e fo rm a t
M a c in to s h P IC T im a g e fo r m a t
is n o t s u p p o r te d
7-7
Alkylation of Alkyne Anions
u Alkyne anions are both strong bases and good nucleophiles
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
7-10
Preparation from Alkenes
• for a terminal alkene to a terminal alkyne, 3 moles of base are required
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
7-15
Addition of X2
• the intermediate in bromination of an alkyne is a bridged bromonium ion
is n o t su p p o rted
7-4
Physical Properties
u Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton
M acin to sh P IC T im ag e fo rm at
• of the two possible carbocations, the favored one is the resonance-stabilized 2°carbocation
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
M a c in to s h P IC T im a g e fo r m a t
is n o t s u p p o r te d
• reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product
u They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation
• because alkyne anions are also strong bases, alkylation is practical only with methyl and 1°halides
7-3
Cycloalkynes
u Cyclononyne is the smallest cycloalkyne isolated
• it is quite unstable and polymerizes at room temp • the C-C-C bond angle about the triple bond is
7-9
Preparation from Alkenes
u Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne
u Common names: prefix the substituents on the triple bond to the word “acetylene”
IUPAC name:
2-Butyne
1-Buten-3-yne
Common name: Dimethylacetylene Vinylacetylene
• alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d
M a c in to s h P IC T im a g e fo r m a t
is n o t s u p p o r te d
• because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion
M a c in t o s h
P IC T
im a g e
fo rm a t
is
n o t s u p p o r te d
7-14
Addition of X2
u Alkynes add one mole of bromine to give a dibromoalkene
• addition shows anti stereoselectivity
approximately 155°, indicating high angle strain
M acintosh PIC T
M a c in to s h P IC T im age form at
is not supported
image format is not supported
M acin to sh P IC T im ag e fo rm at
Organic Chemistry
William H. Brown Christopher S. Foote Brent L. Iverson
7-1
Alkynes
M a c in to s h P IC T im a g e fo r m a t
is n o t s u p p o r te d
Chapter 7
• alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne
M a c in to s h P IC T im a g e fo rm a t
is n o t s u p p o rte d