alkenes,dienes and alkynes
DESCRIPTION
Summarizes the nature and rections of Alkenes, Dienes and alkynesTRANSCRIPT
Alkenes, Alkynes and Dienes
Presented by:Janine V. SameloBSChem2
ALKENES Alkenes are a family of hydrocarbons
(compounds containing carbon and hydrogen only) containing a carbon-carbon double bond.
General formula: CnH2n
Example:
Functional group = carbon-carbon double bond
sp2 hybridization => flat, 120o bond angles
Shape => trigonal planar σ bond & π bond => H2C=CH2 C2H4 ethylene
ethene C2H4
propene C3H6
C CH
H H
H
ISOMERISM OF ALKENES
Structural isomerism All the alkenes with 4 or more carbon
atoms in them show structural isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula.
For example, with C4H8, it isn't too difficult to come up with these three structural isomers:
ISOMERISM OF ALKENES
Geometric (cis-trans) isomerism The carbon-carbon double bond doesn't
allow any rotation about it. That means that it is possible to have the CH3 groups on either end of the molecule locked either on one side of the molecule or opposite each other.
These are called cis-but-2-ene (where the groups are on the same side) or trans-but-2-ene (where they are on opposite sides).
Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-2-ene.
PHYSICAL PROPERTIES OF ALKENES
Boiling Points The boiling point of each alkene is very
similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids.
It has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons.
PHYSICAL PROPERTIES OF ALKENES
Solubility Alkenes are virtually insoluble in water,
but dissolve in organic solvents. non-polar or weakly polar no hydrogen bonding relatively low mp/bp ~ (similar to
alkanes) water insoluble
Importance:
common group in biological molecules
starting material for synthesis of many plastics
NOMENCLATURE
1.Parent chain = longest continuous carbon chain that contains the C=C.
alkane => change –ane to –ene
• prefix a locant for the carbon-carbon double bond using the principle of lower number.
2.Alphabetize, name the substituents, etc.
3.If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.
SOURCES
Synthesis of AlkenesAlkyl halides are dehydrohalogenated with base to form alkenes. Alcohols are dehydrated with heat and acid to form alkenes. The product with the most number of carbons attached to the carbon-carbon double bond is formed in the higher yield.
USES of ALKENES Polymers of Alkenes
Ethylene is polymerized to polyethylene, which is used for bags, films, and bottles.
Propylene is polymerized to polypropylene, which is used for plastics.
Styrene is polymerized to polystyrene, which is used for plastics, plastic cups, and foam insulation.
Methyl α-methacrylate is polymerized to polymethyl α-methacrylate, which is used for plexiglass and Lucite paints.
Acrylonitrile is polymerized to polyacrylonitrile, which is used as Orlon or Acrylan fibers.
Tetrafluoroethylene is polymerized to polytetrafluoroethylene, which is used as Teflon.
Vinyl chloride is polymerized to polyvinyl chloride, which is used in plastics, films, and plumbing.
Vinylidene chloride is polymerized to polyvinylidene chloride, which is used in Saran.
CHEMICAL REACTIVITY
addition reactions. Example
The rather exposed electrons in the pi bond are particularly open to attack by things which carry some degree of positive charge. These are called electrophiles
SYNTHESIS
3. dehalogenation of vicinal dihalides
| | | | — C — C — + Zn — C = C — + ZnX2
| | X X
example: CH3CH2CHCH2 + Zn CH3CH2CH=CH2 +
ZnBr2 Br Br Not generally useful as vicinal dihalides
are usually made from alkenes. May be used to “protect” a carbon-carbon double bond.
SYNTHESIS
1. dehydrohalogenation of alkyl halides | | | |— C — C — + KOH(alc.) — C = C — + KX + H2O | | H X
a) RX: 3o > 2o > 1o b) no rearragement c) may yield mixtures d) Saytzeff orientatione) element effectf) isotope effectg) rate = k [RX] [KOH]h) Mechanism = E2
SYNTHESIS
rate = k [RX] [KOH] => both RX & KOH in RDS
R-I > R-Br > R-Cl “element effect”
=> C—X broken in RDS
R-H > R-D “isotope effect”
=> C—H broken in RDS
Concerted reaction: both the C—X and C—H bonds are broken in the rate determining step.
SYNTHESIS
Mechanism = elimination, bimolecular E2
One step! “Concerted” reaction.
base:
C
W
C
H
C C + H:base + :WRDS
SYNTHESIS
CH3CHCH3 + KOH(alc) CH3CH=CH2
Br isopropyl bromide propylene CH3CH2CH2CH2-Br + KOH(alc) CH3CH2CH=CH2
n-butyl bromide 1-butene
CH3CH2CHCH3 + KOH(alc) CH3CH2CH=CH2
Br 1-butene 19% sec-butyl bromide +
CH3CH=CHCH3
2-butene 81%
DIENES
.
Hydrocarbon containing twocarbon-carbon double bonds
Alkadienes
Isolated dienes
1,4-pentadiene1,5-Cyclo-octadiene
Separated by oneor more sp3-C atom.Separated by oneor more sp3-C atom.Conjugated dienes:
1,3-butadiene
1,3-cyclo- hexadiene
Double bonds andsingle bonds alternatealong the chain.
Double bonds andsingle bonds alternatealong the chain.
Cumulated dienes
Allene
The C atom is commonfor two double bondsThe C atom is commonfor two double bonds
C C C C C
C C C C
H2C C CH2
Hydrocarbon containing twocarbon-carbon double bonds
NOMENCLATURE
.
cis,cis –2,4-hexadiene
(2Z,4Z)-2,4-hexadiene
(2Z,4E)-2,4-hexadiene
cis,trans-
C CC C
CH3H3C
H
HH
HC C
C CCH3
H3C
HH
H
H
•Suffix: ne diene• Cis-trans isomers:
STRUCTURE OF DIENES
. 4 C atoms are sp2- hybridized.
C2-C3 σbond: sp2-sp2overlap C
H
H
C CC
H
HH
H
1,3-Butadiene:
πbond: 2p-2p overlap
C2-C3 partially overlap by 2p-2p orbital
4 C atoms are coplanar
CH
H
C CC
H
HH
H
4 πelectrons are delocalized over 4 C atomsC
Delocalization of πelectronslowers the energy.
CONFORMATION
.Two possible planar conformation of 1,3-butadiene:
s-Cis conformation s-Trans conformation
REACTIONS Conjugated dienes have enhanced stability as
compared to molecules without conjugated double bonds due to resonance. In general, this makes them slightly less reactive than other types of alkenes in general and dienes specifically. However, many reactions proceed through high-energy cation or radical intermediates; in these cases the resonance stabilization of the intermediate allyl species makes conjugated dienes more reactive than non-conjugated dienes or simple alkenes.
Hydrobromination: Example: Butadiene + HBr--> 3-bromobutene (Low
Temperature) + 1-bromobut-2-ene (High Temperature) + 1-bromobutene (Not Observed)
REACTIONS
Diels-Alder Reaction One of the most important of all diene
reactions is the Diels-Alder Reaction, in which a conjugated diene reacts with an dienophile to form a cyclohexene.
Requirements: The diene must be able to access the s-cis conformation for the reaction to take place.
Example:H2C CH C
O
H H2C CH C
O
OCH2CH3 H2C CH C N
HC C COOCH3
DINEOPHILES - - A Dienophile must contain a double or triple
bond. Typically, an electron withdrawing group is conjugated to the dienophile to make it electron-poor (nitriles, ketones, and esters are common electron withdrawing groups). Because the reaction is highly stereospecific, the configuration of the dieneophile will determine the relative stereochemistry of the cyclohexene product.
The Diels-Alder reaction occurs most effectively with an electron-poor dieneophile and an electron-rich diene ('normal demand'). 'Inverse demand' Diels-Alder reactions can also be carried out, in which the dienophile is electron-rich and the diene electron-poor.
A species which likes to attack Dienes
ALKYNES- PROPERTIES Alkynes contain carbon-carbon triple
bonds. The carbon in an alkyne is sp, has a
bond angle of 180o, and a linear shape. A carbon-carbon triple bond contains one sigma bond and two pi bonds.
A terminal alkyne contains at least one hydrogen attached to the carbon-carbon triple bond. An alkyne that is not terminal contains two alkyl groups attached to the carbon-carbon triple bond.
SYNTHESIS AND REACTION
Synthesis of AlkynesVicinal dihalides are dehydrohalogenated twice with base to form alkynes. Geminal dihalides are dehydrohalogenated twice with base to form alkynes.
Reactions of AlkynesHydrogen halide adds across a triple bond, via Markovnikov addition and with anti or syn addition, to form dihaloalkanes. Halogen adds across a triple bond to form a tetrahaloalkane. Hydrogen adds across a triple bond to make an alkane.
PHYSICAL PROPERTIES Alkynes are compounds which have low
polarity, and have physical properties that are essentially the same as those of the alkanes and alkenes.
They are insoluble in water. They are quite soluble in the usual organic
solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.).
They are less dense than water. Their boiling points show the usual increase
with increasing carbon number. They are very nearly the same as the
boiling points of alkanes or alkenes with the same carbon skeletons.
PREPARATION
Alkynes Preparation The carbon-carbon triple bond of the alkynes
is formed in the same way as a double bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons.
W X W XHC - CH ==> HC = CH ==>
HCCH X X Alkane Alkene
Alkyne
The groups that are eliminated and the reagents used are essentially the same as in the preparations of alkenes.
ALKYNE REDUCTION H3C C C CH3
xs H2
Pt
CH3CH2CH2CH3
H2/Pd/BaSO4
Quinoline(Lindlar's Catalyst)
H3CC
H
C
H
CH3
Na NH3 (liq)
H3CC
H
CH
CH3
H3C C C H
HXH3C C C H
H
+
X-
H3C C C H
H
X
Markovnikov addition
a vinyl halide
H3C C C H
H
X
HXH3C C C H
H
HX
+ X-
a heteroatom stabilized carbocation
H3C C C H
H
HX+
H3C C C H
H
X H
X
a geminal dihalide
HYDROGEN HALIDE ADDITION
HYDRATION H3C C C H
H2O/H2SO4
HgSO4H3C C
O
CH3
H3C C C H H+H3C C C H
H
+a vinyl cation
H2O
H+ H3C C C H
H
OHan enol
H3C C C H
H
OH
H3C C
O
CH3
enol ketone
Keto-enol tautomerization
HALOGENATION
Formation of Vicinal tetrahalides C C
an alkyne
X2 C C
X X
X X
a vicinal tetrahalide
carbontetrachloride
SUBSTITUTION R C CH
NaR C C Na
+
R C C Na+ + R'CH2 X R C C CH2R' + NaX
+ H2
TerminalAlkyne
Methylor
PrimaryAlkylHalide
Alkynyl Anion Synthesis of Alkynes via Bimolecular Nucleophilic Substitution
DIELS-ALDER REACTION
Six-membered Ring formation (4+2)
Diene + Dienophile
C
C
CC
C
C
CLEAVAGE REACTIONS hot KMnO4
R C C H R C
O
OH +
H C C Hozonolysis
2(reductive workup)
ozonolysis
(reductive workup)H C
O
OH
H C
O
OH
R C C H R C
O
OH +
H C C H
ozonolysis
2(oxidative workup)
ozonolysis (oxidative workup)
CO2
CO2
orhot KMnO4
or