vibration

Chapter Alkynes
• Hydrocarbons that contain a c-c triple bond are called
alkynes.
• Non-cyclic alkynes have the molecular formula CnH2n-2.
• Monosubstituted, or terminal alkynes are the
compounds that have their triple bond at the end of a
carbon chain (RC≡CH).
• Disubstituted alkynes (RC≡CR’) have internal triple
bonds.

Industrial preparation of acetylene is
by dehydrogenation of ethylene.
CH3CH3
800°C
1150°C
Cost of energy makes acetylene a more
expensive industrial chemical than ethylene.
H2C CH2
H2C CH2
HC CH
H2
+
H2
+
Acetylene (HC≡CH)

H C C
Acidity of Acetylene
and Terminal Alkynes
The C-H bonds of hydrocarbons show little
tendency to ionize.

Compound pKa
26
45
CH4 60
H2C CH2
HC CH
In general, hydrocarbons are
exceedingly weak acids, but acetylene is not nearly
as weak as alkanes or alkenes.
Acidity of Hydrocarbons
The conjugate base of a hydrocarbon is called carbanion. It
is an anion in which the negative charge is produced by
carbon. Because it is derived from very weak acid, a
carbanion such as –CH3 is an exceptionally strong base.

Objective:
Prepare a solution containing sodium acetylide
Will treatment of acetylene with NaOH be effective?
NaC CH
H2O
NaOH + HC CH NaC CH +
Sodium Acetylide

Solution: Use a stronger base. Sodium amide
is a stronger base than sodium hydroxide.
NH3
NaNH2
+ 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 acid
pKa = 26
weaker acid
pKa = 36
H2N
Sodium Acetylide

Reaction with oragnometallic compounds
–
R’Li
..
H C CH H
R’
+ + C CH
Li
–
R’MgX
..
H C CH H
R’
+ + C CH
XMg

Preparation of Alkynes
by
Alkylation of Acetylene and Terminal Alkynes

Carbon-carbon bond formation
alkylation of acetylene and terminal alkynes
Functional-group transformations
elimination
There are two main methods for the preparation
of alkynes:

H—C C—H
R—C C—H
R—C C—R

R X
SN2
X–
:
+
C
–
:
H—C C—R
H—C +
The alkylating agent is an alkyl halide, and
the reaction is nucleophilic substitution.
The nucleophile is sodium acetylide or the
sodium salt of a terminal (monosubstituted)
alkyne.

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—H
CH3CH2—C
(81%)
1. NaNH2, NH3
2. CH3Br
C—CH3
CH3CH2—C
Example: Dialkylation of Acetylene

Reaction with aldehyde
C—MgBr + CH3-CH=O
CH3CH2—C
C—CHO-MgBr+
CH3CH2—C
CH3
C—CHOH
CH3CH2—C
CH3
H2O
alcolate

1. NaNH2, NH3 or RLI, or RMgX
2. CH3CH2Br
H—C C—R
C—R
CH3CH2—C
2. RCOR
CH=CCOHRR
-

CH=CH
-
3
Ni, PPh3 benzene
NH4Cl, CuCl
CH2=CHC=CCH=CH2
-
2 acetylene
NH4Cl, CuCl
CH=CCH=CH2
-

Effective only with primary alkyl halides
Secondary and tertiary alkyl halides
undergo elimination
Limitation

by Elimination Reactions

Geminal dihalide Vicinal dihalide
X
C C
X
H
H
X X
C C
H
H
The most frequent applications are in preparation
of terminal alkynes.
by "Double Dehydrohalogenation"
Because the terminal alkyne product is acidic enough to
transfer a proton to amide ion.

(CH3)3CCH2—CHCl2
1. 3NaNH2, NH3
2. H2O
(56-60%)
(CH3)3CC CH
Geminal dihalide  Alkyne

CH3(CH2)7CH—CH2Br
Br
1. 3NaNH2, NH3
2. H2O
(54%)
CH3(CH2)7C CH
Vicinal dihalide  Alkyne
RCH=CHBr
OH-
CH3CH2)6=CCH3
-
NH2-

Hydrogenation
Metal-Ammonia Reduction
Addition of Hydrogen Halides
Hydration
Addition of Halogens
Ozonolysis
Reactions of Alkynes
ALKANE
ALKENE

Hydrogenation of Alkynes
The conditions for hydrogenation of alkynes are similar to
those employed for alkenes.

RCH2CH2R'
cat
catalyst = Pt, Pd, Ni, or Rh
alkene is an intermediate
RC CR' + H2
Hydrogenation of Alkynes

Stability of alkynes
Alkyl groups stabilize triple bonds in the
same way that they stabilize double
bonds. Internal triple bonds are more
stable than terminal ones.
CH3CH2C CH CH3C CCH3

RCH2CH2R'
Alkynes could be used to prepare alkenes if a
catalyst were available that is active enough to
catalyze the hydrogenation of alkynes, but not
active enough for the hydrogenation of alkenes.
cat
H2
RC CR'
cat
H2
RCH CHR'
Partial Hydrogenation

There is a catalyst that will catalyze the hydrogenation
of alkynes to alkenes, but not that of alkenes to alkanes.
It is called the Lindlar catalyst and consists of
palladium supported on CaCO3, which has been
poisoned with lead acetate and quinoline.
syn-Hydrogenation occurs; cis alkenes are formed.
RCH2CH2R'
cat
H2
RC CR'
cat
H2
RCH CHR'
Lindlar Catalyst

+ H2
Lindlar Pd/CaCO3
CH3(CH2)3C C(CH2)3CH3
CH3(CH2)3 (CH2)3CH3
H H
(87%)
C
C
Example
Cis

Alkynes  trans-Alkenes
Metal-Ammonia Reduction
of Alkynes
A useful catalytic partial hydrogenation for
converting alkynes to alkenes.

RCH2CH2R'
Another way to convert alkynes to alkenes is
by reduction with sodium (or lithium or potassium)
in ammonia.
trans-Alkenes are formed.
RC CR' RCH CHR'
Partial Reduction
Thus, from the same alkyne one can prepare either a cis or a
trans alkene by choosing the appropriate reaction conditions.

CH3CH2C CCH2CH3
CH3CH2
CH2CH3
H
H
(82%)
C
C
Na, NH3
Example
Trans

Suggest efficient syntheses of (E)- and (Z)-2-
heptene from propyne and any necessary organic
or inorganic reagents.
CH=CCH3
- CC=CCCCC

1. NaNH2
2. CH3CH2CH2CH2Br
Na, NH3
H2, Lindlar Pd
Synthesis

Addition of Hydrogen Halides
to Alkynes

HBr
Br
(60%)
Alkynes are slightly less reactive than alkenes.
CH3(CH2)3C CH CH3(CH2)3C CH2
Follows Markovnikov's Rule

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 CHBr
peroxides
Free-radical Addition of HBr
HBr

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 and
predominate at equilibrium.
enol
OH
RCH CR' RCH2CR'
O
ketone
Enols

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
Anti-Markovnikov's rule followed in hydroboration-
Oxydation And aldehyde is formed

Addition of Halogens to Alkynes
Alkynes react with chlorine and bromine to yield
tetrahaloalkanes, we can stop at the
dihalogenoalkenes incase we use a large exces of
alkynes. Two molecules of the dihalogen add to the
triple bond. With iodine we obtain
dihalogenoalkenes.

+ 2 Cl2
Cl
Cl
(63%)
C
Cl2CH CH3
HC CCH3
Example

Br2
CH3CH2
CH2CH3
Br
Br
(90%)
CH3CH2C CCH2CH3 C C
Addition is anti

Gives two carboxylic acids by cleavage
of triple bond
Ozonolysis of Alkynes

1. O3
2. H2O
CH3(CH2)3C CH
+
CH3(CH2)3COH
(51%)
O
HOCH
O
Example

Oxydation of alkynes
1. Oxydant (H2Cr2O7)
CH3(CH2)3C CR
+
CH3(CH2)3COH
O
HOCR
O
Combustion of Acetylene gives CO2 and H2O
[O]

ALKENE
ALCOHOL
1-B2H6/H2O2/OH- (hydration)
Iry anti markovnikov
Ketone or/and aldehyde
O3/Zn/H2O or S(Me)2 or with Cr2O7
ALCOHOL
1-H2O/H+
Markovnikov, more substituted alcohol
RX
1-Mark :HX
2-antiMARK: HBR/peroxide or light
RX2
X2
DIOL trans
DIOL cis KMnO4/OH-
Peroxyacid/H2O
ALKYNE
Aldehyde
Ketone
Carboxylic acid
RCH=CX (1eq HX) or RX2 (2eq)
RCX=CX trans (1eq X2) or RX4 (2eq)
H2/Metal? LINDLAR
ALKANE
ALKANE
ALKENE CIS

vibration

More Related Content

Similar to vibration

Recently uploaded

vibration