Transcript Document 7616003

Alkynes
 Hydrocarbons
that have at least one triple
bond between two adjacent carbons
 Contain the general formula of CnH2n-2
 Contains carbon atoms with sp hybrid
orbital (2 p orbitals & 2 sp hybrid
orbitals)
 Undergoes electrophilic addition
 The major type reaction are the addition
type reaction
Physical Properties
 They
are insoluble in water
 Has low polarity thus still insoluble in water
and quite soluble in solvents of low polarity
like ligroin, ether, benzene and CCl4
 They are less dense than water
 There boiling points show the usual increase
with increasing carbon number
IUPAC Name
Common Name
Formula
Melting
Pt. °C
Boiling
Pt. °C
Density (at
20°)
Acetylene
Ethyne
C2H2
-82
-75
Propyne
Methyl acetylene
C3H4
-101.5
-23
1-Butyne
Ethyl acetylene
C4H6
-122
9
1-Pentyne
Propyl acetylene
C5H8
-98
40
0.695
1-Hexyne
Butyl acetylene
C6H10
-124
72
.719
1-Heptyne
n-Pentyl
acetylene
C7H12
-80
100
.733
1-Nonyne
n-Heptyl
acetylene
C9H16
-65
151
.763
1-Decyne
n-Octyl acetylene
C10H18
-36
182
.770
2-Butyne
Crotonylene
H3C≡CH3
-24
27
.694
2-Pentyne
Ethyl methyl
acetylene
C5H8
-101
55
.714
29
.665
84
.730
3-Methyl-1-butyne
2-Hexyne
Isopropyl
acetylene
C5H8
Methyl propyl
acetylene
C6H10
-92
Chemical Properties
 Alkynes
are unstable and most reactive
compared to alkanes and alkenes because
of their shorter bond length & increased
electron density.
 Electrons in an s orbital benefit from closer
proximity to the positively charged atom
nucleus, and therefore lower in energy
 The acidity is based to be stabilized as a
result of high character of sp orbital
Terminal & Internal alkynes
 Terminal
alkynes have a hydrogen atom
bonded to at least one of the sp hybridized
carbons (those involve in the triple bond).
ex. Methyl acetylene
 Internal alkynes have something other
than hydrogen attached to the sp hybridized
carbons, usually carbon atom, but could be a
hetero atom.
ex. 2- pentyne
Preparation of Alkynes
1. a) From coal and limestone
CaCO 3 heat
CaO + CO 2
o
CaO + 3C
2000 C
CaC 2 + 2H2O
CaC 2 + CO
HC
CH
b)
o
6CH 4 + O2
1500 C
2HC
+
CH
2CO + 10H 2
( CO and H2 are important side products and
needed in the production of alcohol)
Alkynes Preparation
2. Dehydrohalogenation of Alkyl Dihalides
( Elimination reaction)
This
reaction is particularly useful
since the dihalides are readily obtained
from the corresponding alkenes by the
addition of halogen
Dehydrohalogenation of Alkyl
Dihalides
H
H3C
C
CH2
Br
Br
KOH or NaNH 2
H3C
C
CH
3. Displacement or substitution reaction
a.)Reaction of sodium acetylides with primary alkyl
halides
 This
reaction involves substitution of
acetylide ion for halide ion
 It results from the attack by the acetylide
ion on carbon
 The reaction is limited to used of primary
halides because of the general tendency for
secondary and tertiary halides to undergo
a side reaction, elimination
Reaction of sodium acetylides with
primary alkyl halides
Example:
HC
C:-Na+ + CH3CH2CH2CH2Br
HC
C(CH2)3CH3
b.) using HMPT( hexamethylphosphoric triamide)
HC
CH 2CH 2CH 2CH 2CH 3
Li C C(CH 2)3CH 3
nC 4H9Li
CH 3(CH 2)4Cl
HMPT
C4H10
+
Li C C(CH 2)3CH 3
H3C(H 2C) 3C C(CH 2)4CH 3
4. Dehalogenation of tetrahalides
This
reaction is to eliminate the
halogens to form a tetrahalides
The groups are eliminated and the
reagent used are essentially the same
as the preparations of alkenes
Dehalogenation of tetrahalides
Example:
Br
Br
CH3
C
CH
Br
Br
Zn
CH3
C CH
Reaction of Alkynes
1. Addition of Hydrogen
The addition of hydrogen atoms react to the
triple bond carbon that can easily form a C-H
bond formation generating the alkenes
H+ H -
ex.
Alkynes convert to alkenes by the addition of Hydrogen
Na, Li in NH3
HC
CH
H2, Pd/NiBr
H3CC
trans
CCH 3
2H 2, Ni
cis
CH 3CH 2CH 2CH 3
H
Na, Li in NH 3
CH 2CH 3
C
C
H3CH 2C
H3CH 2CC
H
CCH 2CH 3
H2
Pd/NiBr
H
H
C
H3CH 2C
C
CH 2CH 3
2. Addition Halogens( halogenation)
ex.
Br+ Br Br+ Br -
Acetylene was attacked by halogens
resulting to form 1-2, dibromoethene
3. Addition of Hydrogen Halides
ex.
The hydrogen halides reacts to the alkynes to form
bromopropene
 The
reaction of an excess hydrogen halides, a
second addition will occur to the product of
alkene giving a giminal dihalide.
I
H3CC CH
+
HCl
H3C
C
Cl
CH2
HI
H3C
C
Cl
CH3
4. Addition of water. Hydration
The addition of water to acetylene to form
acetyldehyde, which can be oxidized to acetic
acid , is an extremely important industrial
process
ex.
By the addition of water the alkyne was
reacted to form a acetyldehyde
5. Formation of Heavy metal acetylides
Formation of Heavy metal acetylides
The acidic acetylene will react with certain
heavy metals such as the silver (Ag+) to form
insoluble acetylides
ex. H H
C
C
Ag
Ag+ Ag -
+ 2Ag
alcohol
C
C
+ 2H+
Ag
Acetylene will react to a heavy metal to form silver acetylides
6. Formation of alkali metal acetylides
The acetylene will react to a alkali metal which is
the sodium enable liberate the hydrogen gas to
form sodium acetylides
ex. HC C- H + Na
HC
C: -Na+ + ½H2
Or metal acetylide can react with ketone and further
with an acid to form alcohol
CH3
RC CLi
+
H3C
C O
CH3
CH3
HC
C
C
+
-
O
CH3
H
HC
C
C
OH
CH3
Dienes
Dienes
are hydrocarbons which
contain two double bonds
Intermediate
polyenes
between alkanes and
Classes
dienes dienes have the
neighboring double bonds
ex. 1,2 Butadiene
 Conjugated dienes have conjugated
double bonds separated by one single bond.
ex. 2- methyl-1,3- butadiene
 Cumulated
 Isolated
dienes have double bonds
that are separated by more than one single bond
ex. 1,3-Butadiene
Properties of Dienes
 The
chemical properties of diene depend upon
the arrangement of its bonds
 Isolated diene are identical with the simple
alkenes
 In conjugated dienes they differ from simple
alkenes in three ways: (a) they are more stable,
(b) they favor 1,4-addtion than 1,2-addition,
(c) toward free radical addition, they are more
reactive
STRUCTURE
COMMON
NAME
IUPAC NAME
CH2=C=CH2
Allene
Propadiene
CH2=CH-CH=CH2
Divinyl
1,3Butadiene
CH2=CH-CH2-CH2CH=CH2
Diallyl
1,5Hexadiene
CH3-CH=C=CH2
Methylallee
1,2Butadiene
Preparation of Dienes
 Dienes
are usually prepared by adaptations of
the methods used to make simple alkenes
1. By catalytic cracking ( dehydrogenation)
CH3CH2CH2CH3
heat, catalyst
CH3CH=CH=CH2
2. Dehydration loss of water by chemical compound:
the process by which a chemical compound loses
water molecules
ex. CH2CH2CH2CH2
OH
OH
HEAT /ACID
CH2=CH-CH=CH2
1, 3-Butadiene
By
dehydration process,
simultaneously happened. OH will
dissociate, become OH-, forming H2O.
Since C1 and C4 is electron deficient,
the two carbon will be pulled an
electron. Therefore the C1-C2 and C3C4 forming double bonds. That have a
product of 1,3-Butadiene
ELECTROPHILIC ADDITION TO CONJUGATED DIENES1,4-ADDITION
 In
addition to conjugated dienes; a
rearrangement may attach itself not only
to a pair of adjacent carbons (1,2addition), but also to the carbons at the
two ends of the conjugated system (1,4addition).Electrophilic addition to
conjugated dienes yields a mixture of 1,2and 1,4- addition products
Mechanism
Stability of Conjugated Dienes
 More
stable than non conjugated dienes
-
-
-
-
CH 2=CH CH 2 CH=CH 2 + H2
-
CH 2=CH CH= CH CH 3 + H2
CH 3CH 2CH 2CH 2CH 3 ∆H= 60.8 Kcal/mol
CH 3CH 2CH 2CH 2CH 3
∆H = 54.1 kcal/mol
1. Stability observed as less heat release by
hydrogenation than non conjugated dienes
 Stability observed in more substituted
compounds either conjugated or not
2. Bond length also indicates stability due to
overlap of hybrid orbital
Stability of Conjugated Dienes
3. Structurally
a. Delocalization
b. Hyperconjugation
Reactions of Dienes
1.
2.
3.
4.
Hydrogenation
Ozonolysis
Glycol formation
Halogenation