CH2.2 - ALKENE.ppt
-
Upload
nur-ain-syuhada -
Category
Documents
-
view
221 -
download
0
Transcript of CH2.2 - ALKENE.ppt
Alkenes Nomenclature of Alkenes Naming of Cycloalkenes
Geometric Isomers Properties of Alkenes Synthesis of Alkenes
Principal Reactions of Alkenes
Unsaturated Hydrocarbon - Alkenes
Chapter 2.2
Introduction
Classification of Hydrocarbon
Hydrocarbon
Aliphatic Aromatic
Alkanes Alkenes Alkynes
CycloalkanesCycloalkenes
Saturated Unsaturated
HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons
AliphaticAliphaticAliphaticAliphatic
AlkenesAlkenesAlkenesAlkenes
• Alkenes are hydrocarbons that contain a carbon-carbon double bond.
CC CC
HH HH
HH HH
HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons
AliphaticAliphaticAliphaticAliphatic
CycloAlkenesCycloAlkenesCycloAlkenesCycloAlkenes
• Alkenes whose carbon atoms are joined in rings.
Alkenes
• Unsaturated hydrocarbon
• Contain carbon – carbon double bonds
• Alkenes
• General formula
•Aliphatic hydrocarbons
CYCLOALKENES
•Alkenes whose carbon atoms are joined in rings
•General formula Cn H2n-2 where n =3,4,5…..
Eg:
Cyclopentene C5H8
Cyclobutene C4H6
CH3 CH3
1,3- dimethylcyclopentene
Naming Alkenes
We must account for double bond:
• Longest chain must contain the double bond
• Number of carbons so that the double bond has the lowest number
• Indicate the position of the double bond
• Change ending to - ene
• use the same rules for the side chains and halides
• The ending of the alkenes with more than one double bond should be change from -ene to:
diene – if there are two double bonds triene – if there are three double bonds
CH2
CH3
CH2
1
CH2
4
1,3-butadiene
CH2
1
CH2
CH3
CH4
CH5
CH6
CH3
7
1,3,5-heptatriene
1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene.
2) Number through the double bond in thedirection that gives the lower number to the first-appearing substituent.
6-Ethyl-1-methylcyclohexene6-Ethyl-1-methylcyclohexeneCHCH33
CHCH22CHCH33
11
2233
44
55 66
Naming cycloalkenes
• Number substituted cycloalkenes in the way that gives the carbon atoms of the double bond the 1 and 2 positions.
• That also gives the substituent groups the lower numbers at the first point of difference.
5
1
4
2
3
CH32
3
1
4
6
5
CH3CH3
1-methylcyclopentene 3,5-dimethylcyclohexene
• Alkenes nomenclature must specify whether a given molecule is cis or trans if it is a geometric isomer
• Eg:
CH3
C = C
CH3
HH
cis-2-butene
CH3CH2
C = C
H
CH2CH3H
trans-3-hexene
CH3CH2
C = C
CH2CH3
CH2CH2CH3H
trans-4- ethyl –3-heptene
CH3CH2
C = C
CH2CH2CH3
CH3H
cis-4-methyl-3-heptene
Geometric Isomer
Trigonal planar
2-butene
However, Cis – trans stereoisomerism (geometry isomer) in alkenes is not possible when :
• When one of the doubly bonded carbons bears two identical substituents
Using the prefix E and Z
Although the prefixes cis and trans can be used to distinguish disastereomers when two alkyl groups are bonded to the C C , they cannot be used when there are three or four alkyl groups bonded to the C C.
A completely unambiguous system for specifying double bond stereochemistry has been adopted by the IUPAC based on an atomic number criterion for ranking substituents on the doubly bonded carbons.
• When atoms of higher atomic number are on the same side of double bond, we say that the double bond has the Z configuration.
• When atoms of higher atomic number are on opposite site of double bond, we say that the double bond has the E configuration.
C C
H
Br
F
CIHigherHigher
Lower Lower
Z configuration
C C
H
F
Br
CI Lower
Lower
Higher
Higher
E configuration
* Refer to page 189 in our text book
Physical Properties
Physical Properties
Similar to alkanes
Solubility
Low density, boiling point and melting point
Properties vary based on chain size
Interesting physical properties
•Alkenes with several double bonds will have a color associated with them
• Soluble in non polar solvent• Not soluble in water
Colored Alkenes
Physical Properties of Alkenes
Synthesis of alkenes
XX YY
•dehydrogenation of alkanes:X = Y = H
•dehydration of alcohols:X = H; Y = OH
•dehydrohalogenation of alkyl halides:X = H; Y = Br, Cl
CC CCCC CC ++ XX YY
Synthesis of alkenes: elimination reactions
• Elimination of molecule H2
• limited to industrial synthesis of ethylene, propene
750°C+ H2
Synthesis of alkenes
Dehydrogenation of alkanes
H H C CC C + H2
Example
ethene (ethylene)ethane
H HC C
HHHH
HH
C C
H
H
H
H
HH22SOSO44
160°C160°C++ H2O
Dehydration of alcohol
Loss of H and OH from adjacent carbons. Acid catalyst is necessary.
HH OHOH CC CCCC CC ++ HH22OO
Example
Synthesis of alkenes
ethanol
H OHC C
HHHH
HH
ethene (ethylene)
C C
H
H
H
H
10 % 90 %
H2SO4
80°C++
• A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.
Regioselectivity in alcohol dehydration
2-methyl-2-butanol 2-methyl-1-butene 2-methyl-2-butene
Synthesis of alkenes
C
C
C C
OH
CH
H
H
H
H
H
H
H
H
H
H C
C
C CCH
HH
H
H
H
H
H
H
H C
C
C CCH
H
H
H
H
HH
H
H
H
• When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the carbon having thefewest hydrogens.
RR OHOH
CHCH33
CC CC
HH
RR CHCH22RR
Zaitsev Rule
2H on this 2H on this carbon carbon
RR
RR
CHCH22RR
CHCH33
CC CC
only 1H on this only 1H on this carbon carbon
Synthesis of alkenes
Dehydrohalogenation of alkyl halides Loss of H and halogen (X) from an alkyl halideLoss of H and halogen (X) from an alkyl halide In the presence of strong base in solvent likewise In the presence of strong base in solvent likewise NaOCHNaOCH33 in methanol, or KOH in ethanol in methanol, or KOH in ethanol
NaOCHNaOCH22CHCH33
ethanol, 55°Cethanol, 55°C
HH XX CC CCCC CC ++ HXHX
Example
++ HClHClHydrogenchloride
(Sodium ethoxide)
Synthesis of alkenes
Ethyl chlorides
H ClC C
HHHH
HH
ethene (ethylene)
C C
H
H
H
H
Reactions of alkenes
Reaction of Alkenes
• Primarily reactions involve the double bond
• The key reaction of double bond is addition reaction (Breaking the bond and adding something to each carbon)
+ A - B
A B
• The major alkene reactions include additions of hydrogen (H2),halogen ( CI2 or Br2), water (HOH) or hydrogen halides (HBr or HCI)
Reaction of Alkenes
Hydrogenation – Addition of H2
• Addition of a molecule of H2
• Results in the formation of an alkane
• Usually requires heat, pressure and a catalyst like Pt, Pd or Ni
Reaction of Alkenes
Hydration: Addition of H2O
• The addition of water to an alkene
• produces an alcohol
• One carbon get an H, the other an OH
The reaction requires a small amount of acid to be present to work
Reaction of Alkenes
Reaction of Alkenes
Reaction of Alkenes
Reaction of Alkenes
Hydrohalogenation
• Addition of HX to an alkene
• HX – HF, HCI, HBr, HI
It follows Markonikov’s rule where the H ends up on the carbon with the most hydrogen to start with
Reaction of Alkenes
Reactions of alkenes
Halogenation: Addition of X2
● The addition of halogen to an alkene
● produces a haloalkane or alkyl halide
C C X2R R RCCR
XX
Simple laboratory test for unsaturation.
Ozonolysis of alkenes •1st step - reaction of alkene with ozone to form
ozonide.
•2nd step - hydrolysis of ozonide to form aldehyde
and ketone.
+ O+ O33
CC CCOO
OO OO
CC CC
CC OO CCOO++HH22O, ZnO, Zn
R R’
R”RH
H
R’
R”
ozonide
aldehyde ketone