Hydrocarbons.pptx

HYDROCARBONS
ALKANES ALKENES ALKYNES

Preparation of alkyne
1. From calcium carbide
2. By dehalogenation of tetrahalides (zn
/ )
3. By chloroform & silver powder
4. From vicinal dihalides (alcoholic
KOH + KNH2) OR (alcoholic NaOH +
NaNH2 Sodamide)
5. From germinal dihalides
6. By kolbe’s electrolysis fumaric or
maleic acid

ALKANES PREPARATION
1.REDUCTION OF
ALKENE OR ALKYNE
1I .ALKYL HALIDES
WURTZ REACTION
KOLBE’S
ELECTROLYSIS

ALKANES PREPARATION
SODA LIME
DECARBOXYLATION
CLEMMENSON AND
WOLF KISHNER
COREY HOUSE
SYNTHESIS

ALKANES PROPERTIES
HALOGENATION OF
ALKANE
NITRATION
SULPHONATION

ALKANES PROPERTIES
POLYMERISATION
PYROLYSIS
AROMATISATION

ALKANES PROPERTIES
OXIDATION OF
ALKANE
Combustion
Catalytic oxidation

2.ALKENES PREPARATION
1.DEHYDRATION OF
ALCOHOL
2.DEHALOGENATION OF
ALKYL HALIDE
REDUCTION OF ALKYNES
1.BIRCH REDUCTION
2.LINDLAR’S CATALYST
KOLBE’S
ELECTROLYSIS

PREPARATION OF
ALKENES #2
1. DEHYDRATION OF
ALCOHOLS
2.DEHYDROHALOGENATION
OF ALKYL HALIDE

PREPARATION OF
ALKENES #2
1. FROM ALKYNES
BIRCH REDUCTION
LINDLAR’S CATALYST
BY KOLBE’S
ELECTROLYSIS

Preparation of alkene: complete
reduction
 From alkyne
C = C C = C
Reduction or hydrogenation
 R-C=CH R-CH=CH2
R-CH2-CH3
 CATALYST : Ni / H2 or Pd / H2
 If we use normal strong reducing
agents we get alkanes not alkenes.

Partial reduction
 So for partial reduction upto alkene we
require special catalysts.

Catalyst for trans-
alkene (BIRCH
REDUCTION)
Na / liquid NH3
Li / liquid NH3

CONDITION FOR BIRCH
REDUCTION
Alkynes with
terminal triple bond
cannot be used for
birch reduction.

BIRCH REDUCTION
CH3 H
Na / liq NH3
 CH3-C=C-CH3 or C = C
Li / Liq NH3
H CH3
Trans alkene

MECHANISM
1. Li Li+ + e –
2. CH3 – C = C – CH3
CH3 - C=C-CH3
CH3 – C=C –CH3
due to e- repulsion
we get,
CH3 – C=C –CH3

CH3 H
CH3 – C=C –CH3 C = C
CH3
Strong base
CH3 H
C=C
CH3

LINDLAR’S CATALYST ( CIS -
ALKENE)
Alkynes with terminal triple
bond cannot be used for
lindlar’s catalyst.

LINDLAR’S CATALYST ( CIS -
ALKENE)
CATALYST :
1. Pd / BaSO4
2.Pd / CaCO3
In presence of quinoline
Or
Sulphur Or Pb (OAC)2

CIS-ALKENE
Pd / BaSO4 CH3
CH3
 CH3 –C=C-CH C= C
Pd / CaCO3 H H
in presence of quinoline or sulphur

Pd acts as good adsorbate of
H2.
 So two hydrogens are added on the same side , so cis- alkene is
formed.
H H
p
Adsorb

P -2 CATALYST
 Ni / B
 Ni 2B
 In the presence of P2 catalyst also we
get cis alkene.

CIS-ALKENE
Ni / B CH3 CH3
 CH3 –C=C-CH C= C
Ni2B H H

Na / liq NH3
 Q) CH3-C=C-CH3 (A)
Pd / CaCO3
(B)
(CH3COO)2Pb
The minimum heat of hydrogenation is
in
A. (A)
B. (B)
C. Both have equal
D. Cannot predict

 Stability 1
heat of hydrogenation

 A) is birch reduction. It gives trans
alkene.
 B) is lindlar’s catalyst . It gives cis
alkene.

 cis-alkene is more stable than trans-
alkene.
 Since stability is inversely proportional
to stability.
 More the stability , less will be the heat
of hydrogenation.

Kolbe ‘ s electrolysis :
 By electrolysis of aq. sodium or
potassium salt of dicarboxylic acids.
CH3-COOH K CH3-COOK
H2 +
CH3-COOH CH3-COOK
Butane 1,4 dioic acid di-
potassium
( succinic acid ) succcinate

KOLBE’S ELECTROLYSIS
CH2-COOK
+ 2 H2O 2CO2 +
H2
CH2-COOK + CH2=CH2
+ 2KOH

MECHANISM
 CH2-COOK CH2-COO-
2 K+ +
CH2 –COOK CH2-COO-
 H2O H+ + OH-

ANODE
CH2-COO- CH2-COO
.
2 e- +
CH2-COO- CH2-COO
.

CH2-COO
.
2 CO2 +
CH2-COO
.
CH2
.
CH2
.

HYDROBORATION
OXIDATION
ADDITION OF
WATER

Hydroboration / oxidation
Catalyst
1. B2H6 / THF( tetra hydro
furan )
2. H2O2 / OH-

Hydroboration reaction
follows anti-markonikoff’s
rule

IMPORTANT POINTS OF
HYDROBORATION REACTION
 Water addition in double bond (H+ &
OH-).
 This is syn-addition (on the same
side).
 Always anti-markonikoff’s product.

HYDROBORATION
1. B2H6 / THF
CH3-CH=CH2 CH3-CH-CH2
2. H2O2 / OH-
H OH

MECHANISM OF
HYDROBORATION
1. B2H6 2 BH3 ( unstable ) (e-def)
H
B H
H e- deficient (incomplete octet )
2. THF ------------- It prevents BH3 to dimerize.
FURAN THF

THF forms a coordinate bond
with BH3 . Since BH3 is highly
unstable.
BH3

MECHANISM OF
HYDROBORATION
BH3
 CH3-CH=CH2 CH3-CH----CH2
RDS
H------B-H
H
(TRANSITION STATE)
VERY SHORT LIVED
(SYN ADDITION OF H
AND BH2) CH3-CH2-CH2-BH2

AGAIN TRANSITION STATE
 CH3-CH-CH2 CH3-CH----CH2
H BH2 CH3 –CH=CH2
H--------B-H
CH2CHCH
CH3-CH-CH2-B-CH2-CH2-CH3
H H

AGAIN REACTION WITH
ALKENE
 (CH-CH2-CH2)2BH
CH3 –CH=CH2
CH3 –CH-CH2-B(CH2-CH2-
CH3)2
H
OR (CH3-CH2-CH2)3B
TRIALKYL BORANE

PRODUCT OF
HYDROBORATION IS
 TRIALKYL BORANE OR R3B

2. SECOND STEP IS
OXIDATION WITH H2O2/OH-
1. H2O2 + OH- H2O + O-O-H
H-O-O-H + OH-
2. R3B + O-O-H R3B O-O-H
+ I EFFECT
Due to +I effect of the boron, e- repel
 so somebody has to leave. O-H is a good
leaving group than o-o-H. so it leaves.

 -ve charge on oxygen is highly
unstable
R2B-O-R
again O-O-H attacks
R2 B-OR
O-O-H R-B-OR
O

1.B2H6
 Q) (A)
2.H2O2 / OH-
A. (B)
B. Both have equal
C. Cannot predict

RATE
 RATE OF HYDROBORATION
REACTION DEPENDS ON THE
STABILITY OF TRANSITION
STATE(ring structure)
 Stability 1
sterric hindrance

Q) Which will give more rate
towards hydroboration?
a) CH3-CH=CH2
b) CH3-CH=CH-CH3
CH3 CH3
c) C=C
CH3 CH3
ANS ; a )>b)>c)
Reason : becoz in b) & c) there is more
sterric hindrance.
 If sterric hindrance is more, stability will
be less. Hence the rate will be slow.

OXYMERCURATION
DEMERCURATION
ADDITION OF
WATER

OxyMercuration - DeMercuration
 CATALYST:
1. ( CH3COO)2Hg or Hg(OAC)2
2. H2O
3. NaBH4 / OH-

1. Hg(OAC)2
CH3-CH=CH2 CH3-CH-CH2
2. H2O
3.NaBH4/ OH- OH H

OxyMercuration -
DeMercuration reaction
follows markonikoff’s rule

IMPORTANT POINTS OF
reaction REACTION
 Water addition in double bond (H+ &
OH-).
 This is anti-addition (on the same
side).
 Always markonikoff’s product.

Hydrocarbons.pptx

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Hydrocarbons.pptx