alkenes (3)
DESCRIPTION
chemistryTRANSCRIPT
Alkenes
HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons
AliphaticAliphaticAliphaticAliphatic
AlkenesAlkenesAlkenesAlkenes
• Alkenes are hydrocarbons that contain a carbon-carbon double bond.
CC CC
HH HH
HH HH
Alkenes
• Unsaturated hydrocarbon
• Contain carbon – carbon double bonds
• Alkenes
• General formula
•Aliphatic hydrocarbons
•Two simplest alkenes; ethene (ethylene) and propene (propylene) are major raw materials for the chemical industry.
Naming the alkenes (IUPAC Nomenclature)• Longest carbon must contain the double bond. •The C=C functional and position of substituent must be at lower number. •However, position of C=C is of higher priority (LOWER NUMBER) than substituent.
CH3-CH2-CH=CH-CH2-CH2-CH3
CH3-C=C-C-CH3
H H CH3
H Br-CH2-CH2-CH-CH-CH2-CH3
CH=CH2
CH33-heptene
1 2 3 4
1 2 3 4 5
4-methyl-2-pentene
12
345
5-bromo-3-sec-butyl-1-pentene
•For alkenes which show cis-trans isomerism, prefix cis or trans is written before the IUPAC name.
C C
H
H3C
H
CH3
C C
HH3C
H CH3
cis-2-butene trans-2-butene
CH3-CH=CH-CH=CH2
CH3-CH-CH2-CH=CH-CH2-CH=CH2
CH3
1,3-pentadiene
7-methyl-1,4-octadiene
8 1234567
12345
• 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
Let’s name the structural formulae
CH3CH2
C = C
H
CH2CH3H
trans-3-hexene
CH3CH2
C = C
CH2CH3
CH2CH2CH3H
cis-4- ethyl –3-heptene
Physical PropertiesSimilar to alkanes
Solubility
Low density, boiling point and melting point
Properties vary based on chain size. Alkenes have week Van der Waals forces between molecules; for example the lower carbon alkenes exist as gases at room conditions. Interesting physical properties
•Alkenes with several double bonds will have a color associated with them
• Soluble in non polar solvent• Not soluble in water
Table below shows some physical properties of alkenes and cycloalkenes
Name Structural
Formula
Melting Point (°C)
Boiling Point
(°C)
Ethene CH2=CH2 -169 -104
Propene CH2=CH-CH3 -185 -48
1-butene CH2=CH-CH2-CH3 -185 -6.3
Cyclopentene -135 44
Stability of Alkenes• There are 3 factors that influence the
stability of alkenes:– Degree of substitution : more highly
alkylated alkenes are more stables, so tetra> tri > di > mono-substituted
– Stereochemistry : trans > cis due to reduced steric interactions when R groups are on opposite sides of the double bond.
– Conjugated alkenes are more stable than isolated alkenes.
Synthesis of alkenes
XX YY
a) Dehydration of alcohols:X = H; Y = OH
b) Dehydrohalogenation of alkyl halides:X = H; Y = Br, Cl
c) Dehalogenation of dihalides
d) Hydrogenation @ Reduction of alky
CC CCCC CC ++ XX YY
Synthesis of alkenes: elimination reactions
HH22SOSO44
160°C160°C++ H2O
A) Dehydration of alcohol
Loss of H and OH from adjacent carbons. Acid catalyst is necessary.
HH OHOH CC CCCC CC ++ HH22OO
Example
ethanol
H OHC C
HHHH
HH
ethene (ethylene)
C C
H
H
H
H
More example on dehydration of alcohols:-
C
H
H
H
C
H
OH
H CH
H
C
H
H H2Oconc. H2SO4
1700Cethene
i)
CH
H
H
C C
OH
H H
H
Hconc. H2SO4
1700C
CH3-CH=CH2 H2O
ii)
OH
conc. H2SO4
1700CH2O
iii)
iv) CH3-CH-CH2-CH3
OH
conc. H2SO4
1700CCH2=CH-CH2-CH3 CH3-CH=CH-CH3
(minor) (major)
In dehydration reaction, i) excess H2SO4 is heated at 170ºC or
ii) excess H3PO4 heated at 225ºC or
iii) alcohol vapor is passed over alumina at 350ºC.
Saytzeff’s Rule; in elimination reactions, the major reaction product is the alkene with the more highly substituted (more stable).
B) 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)
Ethyl chlorides
H ClC C
HHHH
HH
ethene (ethylene)
C C
H
H
H
H
CH
H
H
C
H
Br
C
H
H
HKCH3CH2O
CH3CH2OHCH3CH=CH2 HBr
Cl
HCl
CH3-CH2-CH-CH3
Br
KOH, CH3CH2OH CH3-CH2-CH=CH2 CH3-CH=CH-CH3 HBr(minor) (major)
i)
ii)
iii)
More examples on dehydrohalogenation of alkyl halides
C) Dehalogenation of dihalides
CC CC
Br Br
2 NaCl2 NaCl
acetoneacetoneCC CC ClCl22++
2 NaBr2 NaBr
++
@@
ZnZn
CHCH33COOHCOOH ZnBrZnBr22
D) Hydrogenation @ Reduction of alkynes
• Depending on the condition & catalyst that will be used.
Catalyst Product
H2 / Pt CH3CΞCCH3 → CH3CH=CHCH3
H2 / Ni2B @ P - 2 CH3CH2CΞCCH2CH3 → Cis - alkene
H2, Pd / CaCO3 & Lindlar’s catalyst
Li @ Na in NH3 CH3(CH2)2CΞC(CH2)2CH3 → Trans - alkene
NH2C2H5 / low temp.
CC CC
HH
CH3CH2CH3CH2
CC CC
H
H
(CH2)2CH3
CH3(CH2)2
Reaction of alkenes • Addition of symmetric reagents
a) Hydrogenation (H2)b) Halogenation (X2)
• Addition of unsymmetric reagentsc) Hydration (H2O)d) Hydrohalogenation (HX)
• Oxidation reactionsa) Ozonolysisb) Hydroxylation with KMnO4 (room temp) c) Oxidation cleavage of alkenes with acidic
KMnO4
• 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)
• Alkenes are more reactive than alkane due to the presence of π bond. The bond has high electron density and susceptible to be attacked by electrophiles ( electron deficient species and low electron density).
• Alkenes undergo ELECTROPHILIC ADDITION reactions which means the C=C (C double bonds) are broken to form C-C bond (single bonds).
a) 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
Examples on Hydrogenation•Can be carried out by using hydrogen gas in the presence of catalysts such as Ni/ Pd/ Pt.
CH2=CH2 Ni / H2 CH3-CH3
CH3-C C-CH2CH3
CH3CH3
Ni / H2 CH3-C CH CH2CH3
CH3CH3
CH3
2H2Ni
CH3
(2 mole C=C requires 2 mole of H2 gas)
b) 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.
Examples on Halogenation•Using Cl2 or Br2 in inert solvents such as tetrachloromethane, CCl4.
CH3-CH2-CH=CH2 CH3-CH2-CH-CH2Br
Cl2 / CClH2C CH2
Cl Cl
Br2 / CCl4
Br
CH2 CH3
2Br2 / CCl4Br
Br
Br
Br
CH2CH3
CH2=CH2
•This reaction can be used to detect the presence of C=C (alkene). The observation is the reddish brown colour of bromine solution decolourises to colourless.•Reaction mechanism (Electrophilic Addition)
C C
H
H
H
H C C
H
H
H
H
Br
Br
Br Br
slow
C C
H
H
H
H
Br
Brfast
C
H
H C
Br
H
H
Br
•The Br2 molecule polarised by C=C undergo heterolytic fission to form electrophile, (Brδ+) and nucleophile.
•The bromide ion, Br- (nucleophile) attacks the charge on C as it is, electron deficient.
c) Hydration: Addition of H2O
• The addition of water to an alkene
• produces an alcohol
• One carbon get an H, the other an OH
• Reagents used are concentrated sulphuric acid followed by addition of water and high temperature or dilute warm aqueous acids such as H2SO4.
d) 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
• HX in the gaseous state or dissolved in organic solvent. (i.e: CCl4)
Examples on Hydrohalogenation•Using HX (X=Cl/ Br/ I) to attack the (double bond (C=C) whereby the HX molecule is the electrophile.
CH3-CH2-CH-CH3
HCl CH3-CH2Cl(one product)
CH3-CH=CH-CH3 HBr CH3-CH-CH2-CH3
Br(one product)
CH3-CH2-CH=CH2 HCl
Cl
CH3-CH2-CH2-CH2-Cl
(major)
(minor)
CH2=CH2
CH3-CH2-CH=CH2
HBr
Peroxideor
ROOR
CH3-CH2-CH2-CH2Br
(major)
However, with HBr peroxide/ ROOR (not HCl or HI), the major product is ANTI-MARKOVNIKOV. The H from HBr will be bonded to carbon double bond is bonded directly to less hydrogens atoms.
CH3HBr
Peroxideor
ROOR
CH3
Br(major)
a) 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
Oxidation reactions
Examples on Ozonolysis
C C
H
H
H
H
O3 , Zn
H2O / H+C
H H
O
C
H
H
O
C C
H CH3
CH3H3C O3 , Zn
H2O / H+C HH3C
O
C CH3H3C
O
H3CO3 , Zn
H2O / H+ H2C
C-CH2-C
CCH3
HO
H
H
O
b) Hydroxylation With Potassium Manganate (VII) (Baeyer test)
C C
H
H
H
H
KMnO4 / H+
cold C C
H
H
OH
H
H
OH
CH3
CH3 KMnO4 / H+
cold
CH3
CH3
OH
OH
Reaction with potassium manganate (VII) solution either acidic or alkaline at room temperature will result addition of 2-OH groups at the carbon double bonds.
This test can be used as a chemical test to detect the presence of C=C (alkene) functional group. The purple color of potassium manganate (VII) solution will turn colourless.
c) Reaction with hot, conc acidified KMnO4
H2C C CH3
H
conc. KMnO4/ H+
CH H
O
+ CH
O
CH3
HCOOH
O
C CH3HO
O
CO2 & H2O
(methanal) aldehyde
carboxylic acid
If the product is methanal, it will oxidised to HCOOH and then form carbon dioxide and water. If the product is other aldehyde containing more than one carbon atom, it will oxidised to carboxylic acid. However, if ketones is formed, as ketone DOES NOT undergo oxidation, it will remain as ketone.
Base on the reaction below, draw the structure of the organic products formed.
• An alkene X, containing 8 carbon atoms reacts with hot acidified potassium manganate (VII) solution to form propanoic acid and pentanoic acid. Draw the two forms of which exist as cis-trans isomers.
Uses of alkenes• The most important use of alkene are:
– Manufacture of plastics
Ethene is used to produce plastics such as polyethene, polychloroethene and polyester (textile).
– Manufacture of ethane-1,2-diol
Ethane-1,2-diol is used as antifreeze in car radiators and for making detergents.
– Manufacture of ethanol
Ethanol is used as a solvent for vanishes, cosmetic and toilet preparations and also in manufacture of ethanal.