Like alkenes, but ending Like alkenes, but ending -ene -ene turns turns into into –yne. –yne. HC HC CH CH Ethyne Ethyne 5 4 3 2 1 1- 1- Pentyne Pentyne Br Br 1 2 3 4 5 6 5-Bromo-2- 5-Bromo-2- hexyne hexyne -ol - -ol - yne yne > > OH OH 1 2 3 2-Propyn- 2-Propyn- 1-ol 1-ol Alkynes: The C C Alkynes: The C C Triple Bond Triple Bond Names Names Priority: Priority:
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Like alkenes, but ending Like alkenes, but ending -ene-ene turns intoturns into ––yne.yne.
HC CHHC CH
EthyneEthyne
5544
33
2211
1-1-PentynePentyne
BrBr11
2233
4455 66
5-Bromo-2-5-Bromo-2-hexynehexyne
-ol -yne-ol -yne>> OHOH
1122
33 2-Propyn-1-2-Propyn-1-olol
Alkynes: The C C Alkynes: The C C Triple Triple BondBond
NamesNames
Priority:Priority:
When the alkyne contains also double bonds, When the alkyne contains also double bonds, it is called an it is called an enyne.enyne. However, despite being However, despite being an “yne”, numbering begins closest to either an “yne”, numbering begins closest to either group: group:
11
2233 44
55
66
3-Hexen-1-3-Hexen-1-yneyne
1-Penten-4-1-Penten-4-yneyne
1122
33
44
55
When double and triple bond are equidistant When double and triple bond are equidistant from each terminus: from each terminus: Ene firstEne first (alphabetical) (alphabetical)
1122
33
44
5566
77
1-Hepten-4-yne1-Hepten-4-yne
SubstituentsSubstituents::
EthynylEthynyl 2-Propynyl 2-Propynyl (or (or
propargyl)propargyl)Rings:Rings: Naming follows hydrocarbon Naming follows hydrocarbon rule:rule:Smaller R is a substituent to larger R Smaller R is a substituent to larger R (ignore function)(ignore function)
3-Cyclobutyl-1-3-Cyclobutyl-1-hexynehexyne
EthynylcyclohexaEthynylcyclohexanene
1122
Two perpendicular Two perpendicular ππ bonds; bonds; sp sp hybridshybrids R R CC CC RR
EthyneEthyne
The Triple Bond is The Triple Bond is EnergeticEnergetic
Kcal molKcal mol-1-1
Heat of hydrogenationHeat of hydrogenation: : MoreMore than Two Alkene than Two Alkene Bonds Bonds (which would be ~ -60 kcal mol(which would be ~ -60 kcal mol-1-1))
DDH H ° HC CH H° HC CH H22C CHC CH2 2 HH33C C CHCH33229229 173173 99
00
Acetylene TorchAcetylene Torch
Hydrogens get more acidic Hydrogens get more acidic (blue)(blue)
Acidity:Acidity: RC C RC C HH + + BB R C R C C C + + HHB B
-- --:: ::
ppKKaa ~ ~ 25!25!
cf. CHcf. CH22 CH CH2 2 44, CH44, CH33 CH CH3 3
5050Why?Why? 50% s-50% s-
charactercharacter
KK
++ LiLi oror
LiLi LiLi++-- ++
++ CHCH33MgBMgBrr
MgBrMgBr++CHCH44
ppKKaa 25 25 ppKKaa 50 50
H H + Na NHH H + Na NH22++::
::
HH
NaNa C CC C:::: ----
NHNH33++
NaNHNaNH22
NaNa++ ++
ppKKaa 33 33
1 1 equiv.equiv.
::--
Synthetic Use of Synthetic Use of AcidityAcidity
--
11H NMR:H NMR: RC C RC C H H δδ = 1.7- = 1.7-3.1 ! 3.1 ! Recall:Recall: RCH CHRCH CH22 δδ = 4.6-6 = 4.6-6 ppmppm
AlkAlkene ene hydrogens: hydrogens: deshieldeddeshielded
Why?Why? Cylindrical electron current Cylindrical electron current shields alkynyl hydrogenshields alkynyl hydrogen
InternalInternal alkynes are alkynes are more stablemore stable than than terminal onesterminal ones
++ HH22
cat.cat.
++ HH22cat.cat.
ΔΔH H ° = -69.9 kcal mol° = -69.9 kcal mol-1-1
ΔΔH H ° = -65.1 kcal mol° = -65.1 kcal mol-1-1
Parallels the behavior of alkenes. Parallels the behavior of alkenes. Same reason: Same reason: hyperconjugationhyperconjugation. .
PreparationPreparation
1.1.Elimination E2 of Elimination E2 of DihaloalkanesDihaloalkanes
CC CC
HH HH
XX XX
CC CC CC CC
BrBrBrBr NaNa
B:B:-- HH
XX
B:B:--
NaNHNaNH22 excessexcess
NHNH33 liq. liq.
HH++, H, H22O O work work upup
75%75%
Application in Application in synthesis:synthesis:
RCH CHR R C C RRCH CHR R C C R
BrBrBrBrBrBr
BrBrBrBr22
NaNHNaNH22
NHNH33 liq liq
1,5-Hexadiyne1,5-Hexadiyne
2. Alkylation of Alkynyl Metals2. Alkylation of Alkynyl Metals
SSNN2 rules2 rules
LiLiTHFTHF
LiLiII
∆∆90%90%
BestBest: RI, THF, ∆. RBr or RCl need : RI, THF, ∆. RBr or RCl need “coordinating”“coordinating” additives: e.g. additives: e.g. ; or HMPA. Remember: Grignards ; or HMPA. Remember: Grignards don’t don’t workwork for RX, but O.K. for or for RX, but O.K. for or carbonyls.carbonyls.
HH22NN NHNH22
OO
+ CH+ CH33MgBrMgBrMgBrMgBr
CHCH22 O OOHOH
++ LiNHLiNH2 2 (l equiv)(l equiv) LiLiOO
OHOH
+ 2 CH+ 2 CH33MgBrMgBr MgBrMgBrBrMgBrMgHOHOOHOH
LiLi++LiLi
OO
CHCH33OHOH
CHCH33CHCHOO
ReactionReactionss
1. 1. ReductionsReductions
a. Complete a. Complete hydrogenationhydrogenationHH22, Pt, Pt