Chapter 9Chapter 9AlkynesAlkynes
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9.19.1Sources of AlkynesSources of Alkynes
Industrial preparation of acetylene isby dehydrogenation of ethylene.
CH3CH3
800°C
1150°C
Cost of energy makes acetylene a moreexpensive industrial chemical than ethylene.
H2C CH2
H2C CH2 HC CH
H2+
H2+
Acetylene
Naturally Occurring Alkynes
C(CH2)4COHCH3(CH2)10C
O
Some alkynes occur naturally. For example,
Tariric acid: occurs in seed of a
Guatemalan plant.
HNOH
Histrionicotoxin: defensive toxin in poison dart frogs of Central and South America
9.29.2NomenclatureNomenclature
Acetylene and ethyne are both acceptableIUPAC names for HC CH
Higher alkynes are named in much the sameway as alkenes except using an -yne suffixinstead of -ene.
HC CCH3
Propyne
HC CCH2CH3
1-Butyne or But-1-yne
(CH3)3CC CCH3
4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne
Nomenclature
The physical properties of alkynes are The physical properties of alkynes are similar to those of alkanes and alkenes.similar to those of alkanes and alkenes.
9.39.3Physical Properties of AlkynesPhysical Properties of Alkynes
9.49.4Structure and Bonding in Structure and Bonding in
Alkynes:Alkynes:spsp Hybridization Hybridization
Linear geometry for acetylene
C CH H
120 pm
106 pm 106 pm
C CCH3 H
121 pm
146 pm 106 pm
Structure
Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperaturefor a reasonable length of time.
Cyclooctyne polymerizeson standing.
Cycloalkynes
C C
2s
2p
2sp
Mix together (hybridize) the 2s orbital and one of the three 2p orbitals.
2p
sp Hybridization in Acetylene
Atomic orbitals Hybridized
Each carbon has two half-filled sp orbitalsavailable to form bonds.
Each carbon isconnected to ahydrogen by a bond. The twocarbons are connectedto each other by a bond and two bonds.
Figure 9.2 (a)
Bonds in Acetylene
One of the two bonds in acetylene isshown here.The second bond is at rightangles to the first.
Figure 9.2 (b)
Bonds in Acetylene
This is the secondof the two bonds in acetylene.
Figure 9.2 (c)
Bonds in Acetylene
The region of highest negative charge lies above and below the molecular plane in ethylene.
Figure 9.3 Electrostatic Potential in Acetylene
The region of highest negative charge encircles the molecule around itscenter in acetylene.
C—C distance
C—H distance
H—C—C angles
C—C BDE
C—H BDE
% s character
pKa
153 pm
111 pm
111.0°
368 kJ/mol
410 kJ/mol
sp3
25%
62
134 pm
110 pm
121.4°
611 kJ/mol
452 kJ/mol
sp2
33%
45
120 pm
106 pm
180°
820 kJ/mol
536 kJ/mol
sp
50%
26
hybridization of C
Ethane Ethylene Acetylene
Table 9.1 Structural Features of Ethane, Ethylene, and Acetylene
9.59.5Acidity of Acetylene and Acidity of Acetylene and
Terminal Alkynes:Terminal Alkynes:
HH CC CC
In general, hydrocarbons are exceedingly weak acids, but alkynes are not nearly as weak as alkanes or alkenes.
Compound pKa
26
45
CH4 60
H2C CH2
Acidity of Hydrocarbons
HCHC CHCH
Electrons in an orbital with more s character are closer to the
nucleus and more strongly held.
Carbon: Hybridization and Electronegativity
C H H+ +pKa = 62
sp3C :–
H+ + sp2
H
C C C C:
–
pKa = 45
H+ + spC C H C C :–
pKa = 26
Objective:Prepare a solution containing sodium acetylide
Will treatment of acetylene with NaOH be effective?
NaC CH
H2ONaOH + HC CH NaC CH +
Sodium Acetylide
NO
Hydroxide is not a strong enough base to deprotonate acetylene.
weaker acidpKa = 26
stronger acidpKa = 15.7
In acid-base reactions, the equilibrium lies tothe side of the weaker acid.
Sodium Acetylide
HO..
.. : ..HO H..
C CH–
H C CH+ + :–
Solution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.
NH3NaNH2 + HC CH NaC CH +
Ammonia is a weaker acid than acetylene.The position of equilibrium lies to the right.
–
H2N..
: H C CH H..
+ + C CH:–
stronger acidpKa = 26
weaker acidpKa = 36
H2N
Sodium Acetylide
9.69.6Preparation of Alkynes byPreparation of Alkynes by
Alkylation of Acetylene and Alkylation of Acetylene and Terminal AlkynesTerminal Alkynes
Carbon-carbon bond formationalkylation of acetylene and terminal alkynes
Functional-group transformationselimination
There are two main methods for the preparationof alkynes:
Preparation of Alkynes
H—C C—H
R—C C—H
R—C C—R
Alkylation of Acetylene and Terminal Alkynes
Remove 1st Hwith NaNH2,
Remove 2nd H with NaNH2,
then alkylatewith RX.
then alkylatewith RX.
R XSN2
X–:+C–:H—C C—RH—C +
The alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution.
The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne (these are strong bases).
Effective only with methyl and 1o alkyl halides, 2o and 3o alkyl halides undergo elimination.
Alkylation of Acetylene and Terminal Alkynes
NaNH2
NH3
HC CH HC CNa
CH3CH2CH2CH2Br
(70-77%)
HC C CH2CH2CH2CH3
Example: Alkylation of Acetylene
NaNH2, NH3
CH(CH3)2CHCH2C
CNa(CH3)2CHCH2C
CH3Br
(81%)
C—CH3(CH3)2CHCH2C
Example: Alkylation of a Terminal Alkyne
1. NaNH2, NH3
2. CH3CH2Br
H—C C—H
C—HCH3CH2—C
(81%)
1. NaNH2, NH3
2. CH3Br
C—CH3CH3CH2—C
Example: Dialkylation of Acetylene
Alkylation of an acetylide is effective only with methyl or primary alkyl halides
Secondary and tertiary alkyl halides undergo elimination.
Limitation on Alkylation of Acetylides
E2 predominates over SN2 when alkyl
halide is secondary or tertiary.
C–:H—C
E2
+CH—C —H C C X–:+
Acetylide Ion as a Base
H C
C X
9.79.7Preparation of Alkynes byPreparation of Alkynes by
Elimination ReactionsElimination Reactions
Elimination of vicinal and geminal Elimination of vicinal and geminal dihalides yield alkynes.dihalides yield alkynes.
Geminal dihalide Vicinal dihalide
X
C C
X
H
H
X X
C C
HH
The most frequent applications are in preparation of terminal alkynes.
Preparation of Alkynesby Double Dehydrohalogenation
(CH3)3CCH2—CHCl2
1. 3NaNH2, NH3
2. H2O
(56-60%)
(CH3)3CC CH
Geminal dihalide Alkyne
NaNH2, NH3
H2O
(CH3)3CCH2—CHCl2
(CH3)3CCH CHCl
(slow)
NaNH2, NH3
(CH3)3CC CH
(slow)
NaNH2, NH3
(CH3)3CC CNa(fast)
Geminal dihalide Alkyne
(Dehydrohalogenation)
(Dehydrohalogenation)
(Loss of a proton) (Regain
proton)
Br
CH3(CH2)7CH—CH2Br
1. 3NaNH2, NH3
2. H2O
(54%)
CH3(CH2)7C CH
Vicinal dihalide Alkyne
9.89.8Reactions of Alkynes Reactions of Alkynes
Acidity (Section 9.5)Hydrogenation (Section 9.9)Metal-Ammonia Reduction (Section 9.10)Addition of Hydrogen Halides (Section 9.11)Hydration (Section 9.12)Hydroboration-oxidation (not in text)Addition of Halogens (Section 9.13)Ozonolysis (Section 9.14)
Reactions of Alkynes
9.99.9Hydrogenation of Alkynes Hydrogenation of Alkynes
RCH2CH2R'cat
catalyst = Pt, Pd, Ni, or Rh
Twice the H2 is needed here compared to reaction
with an alkene since there are two pi bonds in the alkyne. An alkene is an intermediate.
RC CR' + 2H2
Hydrogenation of Alkynes
Heats of Hydrogenation
292 kJ/mol 275 kJ/mol
Alkyl groups stabilize triple bonds in the same way that they stabilize doublebonds. Internal triple bonds are more stable than terminal ones.
CH3CH2C CH CH3C CCH3
RCH2CH2R'
Alkynes can be used to prepare alkenes if acatalyst is used that is active enough to catalyze the hydrogenation of alkynes, but notactive enough for the hydrogenation of alkenes.
cat
H2RC CR' cat
H2RCH CHR'
Partial Hydrogenation
A catalyst that will catalyze the hydrogenationof alkynes to alkenes, but not of alkenes to alkanes is called the Lindlar catalyst and consists ofpalladium supported on CaCO3, which has been
poisoned with lead acetate and quinoline. (Poisoning reduces activity of the catalyst.)
syn-Hydrogenation occurs; cis alkenes are formed.
Lindlar Catalyst
RCH2CH2R'cat
H2RC CR' cat
H2RCH CHR'
+ H2
Lindlar Pd
CH3(CH2)3C C(CH2)3CH3
CH3(CH2)3 (CH2)3CH3
H H(87%)
CC
Example
9.109.10Metal-Ammonia Reduction Metal-Ammonia Reduction
of Alkynes of Alkynes
Alkynes Alkynes transtrans-Alkenes-Alkenes
Another way to convert alkynes to alkenes isby reduction with sodium (or lithium or potassium)in ammonia. This reaction goes by a multistep mechanism.
In this reaction, trans-alkenes are formed.
Partial Reduction
RCH2CH2R'RC CR' RCH CHR'
CH3CH2C CCH2CH3
CH3CH2
CH2CH3H
H
(82%)
CC
Na, NH3
Example
Four steps:
(1) electron transfer
(2) proton transfer
(3) electron transfer
(4) proton transfer
The metal (Li, Na, K) is the reducing agent; H2 is not involved in this reaction.
Mechanism
Step (2): Transfer of a proton from the solvent (liquid ammonia) to the anion radical.
H NH2
..
R R'C..
.–C
.R'
R
C C
HNH2
..
–:
Mechanism
M .+R R'C C R R'C.. .–
C
M+
Step (1): Transfer of an electron from a metal atomto the alkyne to give an anion radical.
Step (3): Transfer of an electron from a 2nd metalatom to the alkenyl radical to give a carbanion.
M.+.
R'
R
C C
H
M+
R'
R
C C
H
..–
Mechanism
Step (4): Transfer of a proton from the solvent(liquid ammonia) to the carbanion.
..H NH2
R'
R
C C
H
..–
R'H
H
CC
R NH2
..
–:
Mechanism
Strategy
Propose efficient syntheses of (E)- and (Z)-2-heptene from propyne and other necessary reagents.
1. NaNH2
2. CH3CH2CH2CH2Br
Na, NH3H2, Lindlar Pd
Synthesis
9.119.11Addition of Hydrogen Halides Addition of Hydrogen Halides
to Alkynes to Alkynes
HBr
Br
(60%)
Alkynes are slightly less reactive than alkenes.
Markovnikov’s rule is followed.
CH3(CH2)3C CH CH3(CH2)3C CH2
Follows Markovnikov's Rule
CH
..BrH :..
RC
..BrH :..
Observed rate law: rate = k[alkyne][HX]2
Termolecular Rate-determining Step
CH3CH2C CCH2CH3
2 HF
(76%)
F
F
C C
H
H
CH3CH2 CH2CH3
Two Molar Equivalents of Hydrogen Halide
HBr
Regioselectivity opposite to Markovnikov's rule. As with alkenes, the anti-Markovnikov product is formed in presence of peroxide.
CH3(CH2)3C CH
(79%)
CH3(CH2)3CH CHBrperoxides
Free-radical Addition of HBr
9.129.12Hydration of Alkynes Hydration of Alkynes
expected reaction:
enol
H+
RC CR' H2O+
OH
RCH CR'
observed reaction:
RCH2CR'
O
H+
RC CR' H2O+
ketone
Hydration of Alkynes
Enols are tautomers of ketones, and exist in equilibrium with them.
Keto-enol equilibration is rapid in acidic media.
Ketones are more stable than enols andpredominate at equilibrium.
enol
OH
RCH CR' RCH2CR'
O
ketone
Enols
: H
+O
O H
C CH
H
..:
Mechanism of Conversion of Enol to Ketone
O H
C CH+
O
H
H
:
..:
:
O H
C C
H
H
O: :
H+
..:
Mechanism of Conversion of Enol to Ketone
OH
C C
H
H
O:
H
+..
:
Carbocation is stabilized by electron delocalization (resonance).
H O
C CH
..H+
Key Carbocation Intermediate
O
C CH+
..:
H2O, H+
CH3(CH2)2C C(CH2)2CH3
Hg2+
(89%)
O
CH3(CH2)2CH2C(CH2)2CH3
via
OH
CH3(CH2)2CH C(CH2)2CH3
Example of Alkyne Hydration
H2O, H2SO4
HgSO4
CH3(CH2)5CCH3
(91%)
Markovnikov's rule followed in formation of enol
via
CH3(CH2)5C CH2
OH
CH3(CH2)5C CH
O
Regioselectivity
9.139.13Addition of Halogens to Alkynes Addition of Halogens to Alkynes
+ 2Cl2
Cl
Cl
(63%)
CCl2CH CH3HC CCH3
Example
Both pi bonds react with excess X2.
Br2
CH3CH2
CH2CH3Br
Br
(90%)
CH3CH2C CCH2CH3 C C
Addition is anti
One addition 1mol of X2.
Addition is anti as with an alkene.
9.149.14Ozonolysis of Alkynes Ozonolysis of Alkynes
Gives two carboxylic acids by cleavage Gives two carboxylic acids by cleavage of triple bondof triple bond
1. O3
2. H2O
CH3(CH2)3C CH
+CH3(CH2)3COH
(51%)
O
HOCOH
O
Example
Reduction is not needed in step 2.
Hydroboration-Oxidation of Hydroboration-Oxidation of Terminal Alkynes Terminal Alkynes
Gives the anti-Markovnikov enolGives the anti-Markovnikov enol
This is similar to hydroboration-oxidation of alkenes. Disiaborane (a bulky borane) is used to prevent subsequent reaction with the alkene. Addition of the borane is syn as before.
RC CH RCH CH-BSia2
Disecondaryisoamyl borane, Sia2BH
Sia2BH
THF
H2O2
HOˉRCH CH-BSia2 RCH CH-OH
an enol which rearranges to an aldehyde.
End of Chapter 9End of Chapter 9AlkynesAlkynes