UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
HYDROCARBONS
SATURATED UNSATURATED
ALKANES ALKENES ALKYNES
AROMATICS
H
H
H
H
H
H C C
H
HH
H
CC
HH CC
H
H
H
H
H
HC
C
CC
CC
RING WITH CONJUGATED DOUBLE BONDS
CONJUGATED DOUBLE BONDS
ISOLATED DOUBLE BONDS
PARAFFINS
CYCLOALKANES
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Chemistry 21A Dr. Dragan Marinkovic
ethene (ethylene) propene (propylene)
1-butene (1-butylene)chloroethene (chloroethylene)
vinyl chloride
cis-2-butene trans-2-butene
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Chemistry 21A Dr. Dragan Marinkovic
Muscalure, or cis-9-tricosene, is the sex pheromone of the female common
housefly (Musca domestica)
C23H46
cyclopropene
cycloheptene
cyclooctene
butadiene
cyclopentadiene
propan-1,2-diène
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Alkane R–CH2–CH2–R CnH2n+2 This is the maximum H/C ratio for a given number of carbon atoms.
Alkene R–CH=CH–R CnH2n Each double bond reduces the number of hydrogen atoms by 2.
Alkyne R–C≡C–R CnH2n-2 Each triple bond reduces the number of hydrogen atoms by 4.
Each ring reduces the number
of hydrogen atoms by 2
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
In sp2 hybridization the 2s orbital is mixed with only two of the three available 2p orbitals:
forming a total of 3 sp2 orbitals with one p-orbital remaining. In ethylene (ethene) the two carbon atoms form a σ bond by overlapping two sp2 orbitals and each carbon atom forms two covalent bonds with hydrogen by s–sp2 overlap all with 120° angles.
The π bond between the carbon atoms perpendicular to the molecular plane is formed by 2p–2p overlap.
π-bond
Ethylene (ethene), showing the pi bond in green
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Since sp2 hybrid orbitals are always in the same plane, the entire ethylene molecule is planar.
cis-2-butene trans-2-butene
Cis-/trans- isomerism are the consequence of the planarity of all atoms bonded to double-bonded (C=C) carbon atoms.
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
cyclopropene
cycloheptene
CH2
CH2
CH
CH2 CHCH2
CH2
All are cis
CH3
H H
CH3CH3
H CH3
H CH3
CH3 H
H
cis-2-butene trans-2-butene methylpropene
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Elaidic acid is a trans unsaturated fatty acid often found in partially hydrogenated vegetable oils.
Oleic acid is a cis unsaturated fatty acid that comprises 55–80% of olive oil.
Stearic acid is a saturated fatty acid found in animal fats and is the intended product in full hydrogenation. Stearic acid is neither cis nor trans because it has no double bonds.
C18H34O2
C18H36O2
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Chemistry 21A Dr. Dragan Marinkovic
Trans fatty acids turn out to increase total cholesterol levels and LDL cholesterol levels, and to reduce HDL cholesterol levels. In other words, trans fatty acids are detrimental to cardiac health.
Unsaturated fatty acids become rancid relatively quickly.
To combat the instability of unsaturated fatty acids, manufacturers began to "hydrogenate" them, a process that makes them more stable. The result was a more solid and longer lasting form of vegetable oil, called "partially hydrogenated" oil, in which a new type of fatty acid is formed. This new type of fatty acid is called trans fatty acid.
Elaidic acid C18H34O2
Chemistry 21A Dr. Dragan Marinkovic
Wachenroder isolated β-carotene from carrots in 1831 giving the extracted crystals the name 'carotene'.
β-carotene
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Ripe gac fruits
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Physical properties of alkenes (and alkynes) are very similar to the physical properties of alkanes with the same number of carbons (and skeletal structure).
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Addition of hydrogen (hydrogenation) across carbon-carbon double bond converts alkenes into alkanes.
CH2
CH2 CH2
CH2
CH
CH3
CH2
CH2 CH2
CH2
CH2
CH
CH3
Additions of halogens (Cl2, Br2, I2, etc) also called halogenation: Bromine and chlorine add readily to ethene to form 1,2-dibromoethane and
1,2-dichloroethane respectively. The reaction occurs even in the dark at room temperature.
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Markovnikov's Rule In the addition of an unsymmetrical reagent such as
HCl, HBr and H2O to a double bond of an alkene, the hydrogen atom adds to the carbon of the double bond
with the greatest number of hydrogen atoms.
CH2
CH
CH2
CH3 CH3
CH C
H2
CH3
Br
+ HBr
CH2
CH2
CH
CH2CH3
CH2
CH2
CH2
CH2CH3
OH
+ H2Oacid
1-butene 2-bromobutane
1-methylcyclopentene 1-methylcyclopentanol
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Chemistry 21A Dr. Dragan Marinkovic
Poly(ethene) (polythene or polyethylene)
an example of addition polymerisation
Temperature: about 200°C
Pressure: about 2000 atmospheres
Initiator: a small amount of oxygen as an impurity
The number of molecules joining up is very variable, but is in the region of 2000 to 20000.
High density poly(ethene) is used to make things like plastic milk bottles and similar containers, washing up bowls, plastic pipes and so on. Look for the letters HDPE near the recycling symbol.
Low density poly(ethene) is used for familiar things like plastic carrier bags and other similar low strength and flexible sheet materials.
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
It has uses in packaging - for example, in plastic film for shrink wrapping food. There are also medical uses - for example, in medical tubing and for medical bags and pouches.
Poly(propene) (polypropylene): PP
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Poly(chloroethene) is commonly known by the initials of its old name (polyvinyl chloride), PVC.
Poly(tetrafluoroethene): PTFEYou may have come across this under
the brand names of Teflon or Fluon.
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
The chemical bonding in compounds such as alkynes with triple bonds is explained by sp hybridization.
In this model the 2s orbital mixes with only one of the three p-orbitals resulting in two sp orbitals and two remaining unchanged p orbitals. The chemical bonding in acetylene (ethyne) (C2H2) consists of sp–sp overlap
between the two carbon atoms forming a σ
bond and two additional π-bonds formed by p–p overlap. Each carbon also bonds to hydrogen in a sigma s–sp overlap at 180° angles.
“it takes alkynes to make a world”Chemist humor
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
acetylene (ethyne)
CH CHHH C C
In acetylene, the H-C≡C bond angles are 180°. By virtue of this bond angle, alkynes tend to be rod-like.
sp-sp s bond
Two pairs of p-orbitals
forming a π-bond
Acetylene is a colorless, unpleasant-smelling gas (bp -28.1°C), which burns in air to produce a very sooty flame. In the presence of pure oxygen, however, it burns at very high temperatures (up to 2800°C), and is used in welding and cutting torches.
C2H2
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
two depictions of propyne
the naturally-occurring 1-phenylhepta-1,3,5-heptatriyne the strained cycloheptyne
Capillin is an antifungal agent. Pargyline is an antihypertensive agent, sold
under the trade names Eudatin® and Supirdyl®
IUPAC Nomenclature 1. alkane name - “ane” + “yne” ethyne (acetylene) butynes
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
R-C≡C-R + H2 & Lindlar catalyst ——> cis R-CH=CH-R
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Discovery in 1825 by Michael Faraday
In 1858, August Kekule proposed a rapid oscillation (I & II) between the 3 C=C and the 3 alternating C-C of hexagonal benzene (C6H6).
C6H6
Linus Pauling proposed resonance theory in 1931
BENZENE
AROMATIC COMPOUNDS
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
All the C-atoms in benzene are sp2-hybridized. Two sp2-hybrid orbitals of each C-atom overlap with two sp2-hybrid orbital of two other C-atoms to form sigma bonds. In this way there are six sigma bonds are formed between six C-atoms which are 120o apart. Remaining six sp2-orbital of six C-atoms overlap with 1s orbital of six H-atoms individually to form six sigma bonds. Since sigma bond results from the overlap of above said planar orbital, all H and C atoms are in the same plane and their generate a hexagonal ring of C-atoms.
Each C-atom in benzene also has an unhybrid 2pz-orbital containing one electron. These 2pz-orbital are perpendicular to the plane of sigma bonds
The overlap of these 2pz-orbital result in the formation of a fully delocalized pi-bond, which extends all over the six C-atoms of benzene nucleus.
Actually these 2pz-orbital produce a pi-molecular orbital containing six electrons. One half of this pi-molecular orbital lies above the plane of hexagonal ring and remaining half below the ring like a sandwich.
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
π-electrons in benzene
chemical formulas of benzene
Kekule
Pauling
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Why is Trinitrotoluene (TNT) Explosive? Because nitrogen in the nitro group (NO2) has a charge of +1, the nitro group has a very strong tendency to withdraw (pull) electrons from the aromatic ring. Therefore, attaching three nitro groups to a benzene ring leads to an extremely unstable or explosive compound.
heterocyclic aromatic compound
heterocyclic aromatic compound
naphthalene
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
C6H5-
C6H5CH2- OH
phenylcyclohexane
benzyl alcohol
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Benzene boils at 80°C - rather higher than other hydrocarbons of similar molecular size (pentane and hexane, for example). This is presumably due to the ease with which temporary dipoles can be set up involving the delocalized electrons.
Methylbenzene (toluene) boils at 111°C. It is a bigger molecule and so the van der Waals dispersion forces will be bigger.
You might have expected that methylbenzene's melting point would be higher than benzene's as well, but it isn't - it is much lower! Benzene melts at 5.5°C; methylbenzene at -95°C.
Molecules must pack efficiently in the solid if they are to make best use of their intermolecular forces. Benzene is a tidy, symmetrical molecule and packs very efficiently.
The methyl group sticking out in methylbenzene tends to disrupt the closeness of the packing. If the molecules aren't as closely packed, the intermolecular forces
don't work as well and so the melting point falls.
The arenes (aromatic compounds) are insoluble in water.
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
Benzene is resistant to addition reactions. Adding something new to the ring would need you to use some of
the delocalized electrons to form bonds with whatever you are adding. That results in a major loss of stability as the delocalization is broken.
Instead, benzene mainly undergoes substitution reactions – replacing one or more of the hydrogen atoms by something new.
That leaves the delocalized electrons as they were.Br
+ Br2 + HBr
UNSATURATED HYDROCARBONS
Chemistry 21A Dr. Dragan Marinkovic
The term carcinogen refers to any substance, radionuclide or radiation that is an agent directly involved in the promotion of cancer or in the increase of its propagation
mutagen is a physical or chemical agent that changes the genetic material
teratology generally refers to disfiguring birth defects or malformations
Polycyclic aromatic hydrocarbons (PAHs) are chemical compounds that consist of
fused aromatic rings and do not contain heteroatom or carry substituents. PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as carcinogenic, mutagenic, and teratogenic. PAHs are also found in foods. Studies have shown that most food intake of PAHs comes
from cereals, oils and fats. Smaller intakes come from vegetables and cooked meats.
Chemistry 21A Dr. Dragan Marinkovic
However, please, keep in mind that most aromatic compounds are beneficial (many medical drugs are aromatic) and some even essential in our diet and nutrition (some amino acids, vitamins…)
phenylalanine tryptophan tyrosine tyroxine
Pyridoxine (Vitamin B6)
Riboflavin (vitamin B2)
Alpha-tocopherol (Vitamin E)
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