Alkynes
I.
Nomenclature and Properties
A.
Naming Alkynes
1) Carbon-Carbon triple bond, CnH2n-2 R C C
2) IUPAC
a) Replace the –ene of an alkene with –yne
b) Give the number where the triple bond occurs
3) Common Names: derivatives of acetylene
CH3
Br
H C C H
Ethyne
(Acetylene)
4)
5)
Ethynyl
CH3
C C CH3
2-Butyne
Dimethylacetylene
CH3
C C CHCH2CH3
4-bromo-2-hexyne
R
H3C C C C H
CH3
3,3-dimethyl-1-butyne
Terminal alkyne has one unsubstituted C, Internal Alkyne does not
As a substituent, an alkyne is named alkynyl
C C H
CH2
C C H
2-propynyl
6)
Other examples:
CH2
H C C
C C H
2-propynylcyclopropane
CH2 OH
2-propyn-1-ol
trans-1,2-diethynylcyclohexane
7)
8)
Molecule with double and triple bond is an alkenyne, C=C lowest #
Molecule with triple bond and hydroxyl group is an alkynol
OH
CH3CH2CH=CH C CH
3-hexen-1-yne
B.
H2C CHCH2C
1-penten-4-yne
CH
5-hexyn-2-ol
Properties of Alkynes
1) Bonding
a) Carbon-Carbon bond is made of on s (sp-sp) and 2 p bonds
b) C—H s-bonds made of Csp and H1s overlap
c)
d)
C-C triple bond DH = 229 kcal/mol (> double or single bond)
C-H bond DH is 131 kcal/mol, stronger than alkene or alkane
e)
f)
g)
Weak p-bonds are the most reactive area
C-C Bond length is shorter than alkene or alkane, as is C-H bond
Linear geometry is result of sp hybridization
2)
Boiling points are similar to alkenes and alkanes (fairly nonpolar)
H C C H
3)
Acidity
a) Terminal alkynes are much more acidic than alkanes or alkenes
b) sp hybridization makes C more electronegative than usual
R C C H
c)
d)
e)
C.
B:
R C C
Ethyne pKa = 25, ethene = 44, ethane = 50
Bases used to deprotonate alkynes: RMgBr RLi NH3(l) NaNH2
Resulting anion is called an Alkynyl Anion is a good nucleophile
Spectroscopy
1) Proton NMR
a) Alkynyl H are shielded by p-electron local magnetic field
b) This is the opposite effect as seen in alkenes
c) Geometry dictates the effect of the local field
d)
e)
f)
Local field opposes the NMR field at the H locations: shielded
d = 1.7-3.1 for alkyne hydrogens
Triple bond does transmit spin-spin coupling well. J = 2-4 Hz
H
R CH C C H
Long Range 4-bond coupling
2)
H C C H
71.9
3)
13C
NMR
a) Alkyne Carbons d = 65-95 ppm (alkenes = 100-150, alkanes = 5-45)
b) Examples:
H C C CH2CH2CH2CH3
68.6 84.0 18.6 31.1 22.4 14.1
CH3CH2
C C CH2CH3
81.1 15.6 13.2
IR
a) C≡C—H = 3260-3330 cm-1
b) C≡C
= 2100-2260 cm-1 (weak for internal alkynes)
II.
Stability of the Triple Bond
A.
Alkynes are high energy compounds
Acetylene (ethyne) torches produce much heat for welding or cutting
2 CO2 + H2O H = -311 kcal/mol
H C C H + 2.5 O2
B.
Internal Alkynes are most stable
1) Heats of Hydrogenation to alkane
a) 1-butyne = -69.9 kcal/mol
b) 2-butyne = -65.1 kcal/mol
2) Hyperconjugation makes internal more stable
III. Preparations of Alkynes
A.
Double Elimination
1) Alkenes can be prepared by E2 reactions with alkyl halides
H X
C C
RO-
2)
Alkynes can be prepared from a vicinal dihaloalkane base
H X
RO
C C
-
X H
Br
CH3CH2CHCH2Br
3)
1. 3 NaNH2, NH3(l)
2. H2O
CH3CH2
C C H
Details:
a. 3 eq. Base needed because alkyne will deprotonate when formed
b. H2O re-protonates the alkyne
c. NH3(l) boils away when reaction is done (b.p. = -33 oC)
d. Stereochemistry of the Intermediate doesn’t matter
H X
C C
X H
-
B
anti
elimination
H
B-
X
mixed intermediate
“alkenyl halide”
2)
Halogenation-Double Dehydrohalogenation
Alkene to Vicinal Dihaloalkane to Alkyne
Br
Br2
1. 3 NaNH2, NH3(l)
Br 2. H O
2
CCl4
Br
B.
Br
Alkylation of Alkynyl Anions
1) Terminal Alkynes can be made into internal alkynes
2) Alkyl and Alkenyl Metal reagents don’t attack haloalkanes fast enough
MgBr
3)
+
NO REACTION
Br
Alkynyl Anion can react with haloalkanes
CH
nBuLi (base)
THF, HMPA
C-
Br
SN 2
C
HC
CH
5)
Secondary and Tertiary Haloalkanes give E2 products instead of SN2
a. Alkynyl anion is a strong base
b. SN2 Reaction works much better with primary haloalkanes
6)
Ethyne can be mono- or disubstituted
nBuLi (base)
THF, HMPA
7)
HC
C
Br
-
SN 2
HC
C
nBuLi (base)
THF, HMPA
Br
C
SN 2
Alkynyl Anions can do other Nu reactions as well
HC C
-
+
O
HC C
OH
OH
CH3C CMgBr +
O
C CCH3
C