chapter 9 alkynes
DESCRIPTION
Chapter 9 Alkynes. 9.1 Sources of Alkynes. +. H 2. +. H 2. HC. CH. CH 2. H 2 C. Acetylene. Industrial preparation of acetylene is by dehydrogenation of ethylene. 800°C. CH 2. H 2 C. CH 3 CH 3. 1150°C. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 9Chapter 9AlkynesAlkynes
9.19.1
Sources 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
Tariric acid: occurs in seed of a Guatemalan plant.
Some alkynes occur naturally. For example,
Naturally Occurring Alkynes
Some alkynes occur naturally. For example,
Histrionicotoxin: defensive toxin in the poison dart frogs of Central and South America
HNOH
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
2sp
Mix together (hybridize) the 2s orbital and one of the three 2p orbitals.
2p
Each carbon has two half-filled sp orbitalsavailable to form bonds.
sp Hybridization in Acetylene
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
HH CC CC
9.59.5
Acidity of AcetyleneAcidity of Acetylene
and Terminal Alkynesand Terminal Alkynes
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
9.69.6
Preparation of AlkynesPreparation of Alkynes
byby
Alkylation of Acetylene and Terminal AlkynesAlkylation of Acetylene and Terminal 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
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.
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
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides undergo elimination
Limitation
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
9.79.7
Preparation of AlkynesPreparation of Alkynes
by Elimination Reactionsby Elimination Reactions
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
Br
CH3(CH2)7CH—CH2Br
1. 3NaNH2, NH3
2. H2O
(54%)
CH3(CH2)7C CH
Vicinal dihalide Alkyne
9.89.8
Reactions of AlkynesReactions 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)Addition of Halogens (Section 9.13)Ozonolysis (Section 9.14)
Reactions of Alkynes
9.99.9
Hydrogenation of Alkynes Hydrogenation of Alkynes
RCH2CH2R'cat
catalyst = Pt, Pd, Ni, or Rh
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 could be used to prepare alkenes if acatalyst were available 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
There is a catalyst that will catalyze the hydrogenationof alkynes to alkenes, but not that of alkenes to alkanes.
It is called the Lindlar catalyst and consists ofpalladium supported on CaCO3, which has been
poisoned with lead acetate and quinoline.
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
Alkynes Alkynes transtrans-Alkenes-Alkenes
9.109.10
Metal-Ammonia ReductionMetal-Ammonia Reduction
of Alkynesof Alkynes
Another way to convert alkynes to alkenes isby reduction with sodium (or lithium or potassium)in ammonia.
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
Metal (Li, Na, K) is reducing agent; H2 is not involved
Mechanism
Step (1): Transfer of an electron from the metalto the alkyne to give an anion radical.
M .+R R'C C R R'C.. .–
C
M+
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
Step (3): Transfer of an electron from the metalto 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
Suggest efficient syntheses of (E)- and (Z)-2-heptene from propyne and any necessary organicor inorganic reagents.
Strategy
1. NaNH2
2. CH3CH2CH2CH2Br
Na, NH3H2, Lindlar Pd
Synthesis
9.119.11
Addition of Hydrogen HalidesAddition of Hydrogen Halides
to Alkynesto Alkynes
HBr
Br
(60%)
Alkynes are slightly less reactive than alkenes.
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
CH3(CH2)3C CH
(79%)
CH3(CH2)3CH CHBrperoxides
Free-radical Addition of HBr
9.129.12
Hydration of AlkynesHydration 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
:
..:
:
Mechanism of Conversion of Enol to Ketone
O H
C C
H
H
O: :
H+
..:
Mechanism of Conversion of Enol to Ketone
OH
C C
H
H
O:
H
+..
:
Mechanism of Conversion of Enol to Ketone
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
9.139.13
Addition of Halogens to AlkynesAddition of Halogens to Alkynes
+ 2Cl2
Cl
Cl
(63%)
CCl2CH CH3HC CCH3
Example
Br2
CH3CH2
CH2CH3Br
Br
(90%)
CH3CH2C CCH2CH3 C C
Addition is anti
gives two carboxylic acids by cleavage gives two carboxylic acids by cleavage of triple bondof triple bond
9.149.14
Ozonolysis of AlkynesOzonolysis of Alkynes
1. O3
2. H2O
CH3(CH2)3C CH
+CH3(CH2)3COH
(51%)
O
HOCOH
O
Example