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1. Chemistry B11
Chapters 10-13
Alkanes, Alkenes, Alkynes and Benzene
Organic compounds: organic chemistry is the chemistry of carbon and only a few other
elements-chiefly, hydrogen, oxygen, nitrogen, sulfur, halogens, and phosphorus (from 116
elements).
Note: In the early days of chemistry, scientists though organic compounds were those
produced by living organisms and they could not synthesize any organic compound by
starting with only inorganic compounds. In 1828, Friedrich Wöhler synthesized the first
organic compound in his laboratory (urea).
O
heat
NH4Cl AgNCO H2N-C-NH2
+ +
AgCl
Ammonium
chloride
Silver
cyanate
Urea Silver
chloride
Today, chemists obtain organic compounds in two principal ways: 1.Isolation from nature and
2. Synthesis in the laboratory.
Note: Compounds made in the laboratory are identical in both chemical and physical
properties to those found in nature.
Why organic chemistry: we can find organic compounds everywhere around us (foods,
flavors, fragrances, medicines, toiletries, cosmetics, plastic, paints, our body, and etc.).
Chemist have discovered or synthesized more than 10 million of organic compounds.
However, 1.7 million inorganic compounds are discovered or synthesized (85% of all known
compounds are organic compounds).
Typical properties of organic compounds: 1. They contain carbon atom. 2. Bonding is
almost entirely covalent (covalent compounds). 3. They have low boiling points and low
melting points. 4. They are flammable (almost all burn). 5. They are soluble in nonpolar
compounds and most are insoluble in water. 6. Many are gases, liquids, or solids.
It is important to know:
••Carbon:: normally forms four covalent bonds and has no unshared pairs of electrons.
••Hydrogen: forms one covalent bond and has no unshared pairs of electrons.
•Nitrogen: normally forms three covalent bonds and has one unshared pair of electrons.
•Oxygen: normally forms two covalent bonds and has two unshared pairs of electrons.
••A Halogen: normally forms one covalent bond and has three unshared pairs of electrons.
Functional group: an atom or group of atoms within a molecule that shows a characteristic
set of predictable physical and chemical behaviors.
Functional groups are important in organic chemistry because:
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2. 1. They are sites predictable chemical behavior. A particular functional group, in whatever
compound it is found, undergo the same types of chemical reactions.
2. Determine in large measure the physical properties of a molecule.
3. Serve as the units by which we classify organic compounds into families.
4. Serve as a basis for naming organic compounds.
Hydrocarbons: a large family of organic compounds and they contain only carbon and
hydrogen.
Hydrocarbons are divided into two groups:
1. Saturated hydrocarbon: a hydrocarbon that contains only carbon-carbon single bonds
(alkanes, also called Aliphatic hydrocarbons). Saturated in this context means that each
carbon in the hydrocarbon has the maximum number of hydrogen atoms bonded to it.
2. Unsaturated hydrocarbon: a hydrocarbon that contains one or more carbon-carbon
double bonds, triple bonds, or benzene rings.
Alkanes: They are saturated hydrocarbons (they have only carbon-carbon single bonds). The
molecular formula of this group is CnH2n+2 (n is the number of carbon atoms).
Naming the unbranched alkanes: Chemists have adopted a set of rules established by the
International Union of Pure and Applied Chemistry (IUPAC). The IUPAC name for an alkane
consists of two parts: 1. A prefix that shows the number of carbon atoms (meth-, eth-, prop-,
but-, pent-, hex-, hept-, oct-, non- and dec-). 2. The suffix “-ane”.
CH4 Methane C2H6 Ethane C3H8 Propane C4H10 Butane
Note: we can represent the formula of an organic compound by the molecular formula or by
the structural formula. Structural formulas can be represented by three ways: Expanded
(Complete) structural formula, Condense structural formula, and Line-angle formula:
Expanded (complete) structural formula: to represent this model, the carbon atom is shown
attached to the hydrogen atoms (we show all connections).
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3. Condensed structural formula: to represent this model, the hydrogen atoms are grouped
with their carbon atom. The number of hydrogen atoms is written as subscript.
CH3-CH2-CH2-CH2-CH3 or CH3-(CH2)3-CH3
Line-angle formula: is a form of the structural formula. A line represents a carbon-carbon
bond and a vertex represents a carbon atom. A line ending in space represents a –CH3 group.
CH3-CH2-CH3 Propane
CH3-CH2-CH2-CH2-CH3 Pentane
Substituent groups: they are the branches in organic compounds. A substituent group
derived from an alkane by removal of a hydrogen atom is called an alkyl group (R-). Alkyl
groups are named by dropping the “-ane” from the name of the parent alkane and adding the
suffix “-yl”.
CH3- Methyl C2H5- Ethyl C3H7- Propyl
Note: some substituents derived from other elements or other spices than alkanes:
-F Fluoro -Cl Chloro -OH Hydroxyl -NO2 Nitro
Naming branched alkanes: 1. Write the alkane name of the longest continuous chain of
carbon atoms (parent chain or root chain). 2. Number carbon atoms starting from the end
nearest substituent. 3. Give the location and name of each substituent (alphabetical order) as a
prefix to the alkane name (main chain). Use a hyphen to connect the number to the name.
CH3
CH3-CH-CH2-CH3
1 2 3 4
In this example, the longest chain is Butane. We number carbon atoms starting from the end
nearest substituent (left to right). The location of subtituent is 2 and its name is methyl.
Therefore, the complete name of this compound is 2-Methylbutane.
Cl CH3
CH3-CH2-CH-CH-CH3 3-Chloro-2-methylpentane
5 4 3 2 1
Constitutional isomers: compounds with the same molecular formula but a different
connectivity of their atoms (different structural formulas).
CH3
Butane: C4H10 CH3-CH2-CH2-CH3 Methylpropane: C4H10 CH3-CH-CH3
Note: Constitutional isomers are different compounds and have different physical and
chemical properties.
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4. Cycloalkane: a saturated hydrocarbon that contains carbon atoms bonded to form a ring. A
hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon.
Cyclobutane Cyclopentane
Physical properties of alkanes: 1. They are nonpolar compounds (the electronegativity
difference between carbon and hydrogen is 2.5-2.1 = 0.4). 2. The only interactions between
their molecules are the very week London dispersion forces. 3. They are insoluble in water
(because water is polar) and they are soluble in nonpolar organic compounds. 4. They have
the lower density than water (their densities is between 0.7 and 0.8 g/mL). 5. They have the
low boiling points and the low melting points. 6. They can be gases (with 1 to 4 carbon
atoms), liquids (with 5 to 17 carbon atoms), or solids (with 18 or more carbon atoms).
Note: In general, both boiling and melting points of alkanes increase with increasing
molecular weight (the number of carbon atoms).
Note: In general, both boiling and melting points of alkanes decrease with increasing the
number of branches (for alkanes with the same molecular weights). As branching increases,
the alkane molecule becomes more compact and its surface area decreases (London dispersion
forces act over a smaller surface area).
Chemical properties of alkanes: in general, they have a low reactivity (inert). Their most
important reactions are combustion (reaction with oxygen) and halogenation (reaction with
halogens).
Combustion: alkanes react with oxygen (they are oxidized). In this reaction, CO2, H2O, and
energy (heat) are produced.
CH4 + 2O2 ® CO2 + 2H2O + energy (heat)
Halogenation: alkanes react with chlorine and bromine if we heat the mixture or if we expose
the mixture to light (in the dark at room temperature, nothing happens).
heat or light
CH4 + Cl2 CH3Cl + HCl
Chloromethane
If chlromethane is allowed to react with more chlorine:
heat or light
CH3Cl + Cl2 CH2Cl2 + HCl
Dichloromethane
heat or light
CH2Cl2 + Cl2 CHCl3 + HCl
Trichloromethane
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5. heat or light
CHCl3 + Cl2 CCl4 + HCl
Tetrachloromethane
Sources of alkanes: the two major sources of alkanes are natural gas and petroleum. Natural
gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other
relatively low-boiling alkanes (propane, butane and 2-methylpropane). Petroleum is a thick,
viscous, liquid mixture of thousands of compounds, most of them hydrocarbons, formed the
decomposition of marine plants and animals. The fundamental separation process in refining
petroleum is fractional distillation. All crude petroleum that enters a refinery goes to
distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into
fractions.
Alkene: an unsaturated hydrocarbon that contains one or more carbon-carbon double bonds.
The molecular formula of this group is CnH2n (n is the number of carbon atoms). Alkenes
have less hydrogen atoms than alkanes.
C2H4 CH2=CH2 C3H6 CH2=CH-CH3
Alkynes: an unsaturated hydrocarbon that contains one or more carbon-carbon triple bonds.
The molecular formula of this group is CnH2n-2 (n is the number of carbon atoms). Alkynes
have less hydrogen atoms than alkanes and alkenes.
C2H2 CH CH C3H4 CH C-CH3
Naming unbranched alkenes and alkynes: we use the IUPAC system of naming for
alkanes. For alkenes, we replace the suffix “-ane” of alkanes by “-ene”. For alkynes, we
replace the suffix “-ane” of alkanes by “-yne”.
Note: Some alkenes and alkynes (particularly those of low molecular weight) are known
almost exclusively by their common names (we show them here in the parenthesis).
Naming branched alkenes and alkynes: 1. Name the longest carbon chain that contains the
double or triple bond. 2. Number the carbon chain starting from the end nearest the double or
triple bond. 3. Give the location and name of each substituent (alphabetical order) as a prefix
to the alkene or alkyne name.
CH3-CH=CH-CH3 2-butene
C C-CH2-CH3 1-butyne
CH2-CH3
CH3-CH2-C=CH-CH3 3-ethyl-2-pentene
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6. Cis and trans isomers: isomers that have the same connectivity of their atoms (and the same
molecular formulas) but a different arrangement of their atoms in space. Specifically, cis and
trans stereoisomers result from the presence of either a ring or a carbon-carbon double bond
(not a carbon-carbon triple bond). Cis and trans isomers are different compounds and have
different physical and chemical properties.
H3C H CH3
C = C C = C
H3C
H H H
CH3
trans-2-Butene cis-2-Butene
Note: Both boiling and melting points of the cis isomers are lower than the trans isomers
(because the surface areas of cis isomers are smaller and molecules are more compact).
Physical properties of alkenes and alkynes: their physical properties are similar to those of
alkanes with the same carbon skeletons.
Chemical properties of alkenes and alkynes: these organic compounds are more reactive
than alkanes. The most characteristic reaction of alkenes (alkynes) is addition to the carbon-carbon
double bond (triple bond): The double bond is broken and in its place single bonds
form to two new atoms or groups of atoms. We can name four important chemical reactions
for them: 1. Addition of hydrogen (Hydrogenation or Reduction). 2. Addition of hydrogen
halides (Hydrohalogenation). 3. Addition of water (Hydration). 4. Addition of bromine and
chlorine (Halogenation).
Hydrogenation or Reduction (addition of hydrogen): atoms of hydrogen add to the carbons
in a double or triple bond to form alkanes. A catalyst as platinum (Pt), nickel (Ni), or
palladium (Pd) is added to speed up the reaction. The transition metal catalysts used in this
hydrogenation are able to absorb large quantities of hydrogen onto their surfaces, probably by
forming metal-hydrogen bonds.
H H H H
Pt
HC = CH + H2 HC - CH
H H
Halogenation (addition of chlorine and bromine): chlorine and bromine react with alkenes
(alkynes) at room temperature by addition of halogen atoms to the carbon atoms of the double
bond (triple bond). We do not need any catalysts for this reaction (in general, we use an inert
solvent, such as dichloromethane, CH2CH2).
H H H H
Pt
HC = CH + Cl2 HC - CH
Cl Cl
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7. Note: Addition of bromine is a useful qualitative test for the presence of an alkene (or an
alkyne). If we dissolve bromine in carbon tetrachloride, the solution is red. In contrast,
alkenes (alkynes) and dibromoalkanes are colorless. The disappearance of the red color as
bromine adds to the double bond (triple bond) tells us that alkene (alkyne) is, indeed present.
Note: addition of halogen to an alkene (an alkyne) is an addition reaction and two atoms of
halogens are added to the carbon atoms of the double bond (triple bond). However, addition
of halogen to an alkane is a substitution reaction and only one atom of halogen is replaced
by one hydrogen atom.
Benzene: a molecule of benzene consists of a ring of six carbon atoms with onr hydrogen
atom attached to each carbon. It has the molecular formula C6H6. The real molecule of
benzene is a resonance hybrid of the two Lewis structures (a unique feature that makes
benzene chemically stable).
C
C
H H
C C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
Aromatic compounds: they are unsaturated hydrocarbons that contain one or more benzene
rings in their structures.
Arene: a compound containing one or more benzene-like rings.
Naming aromatic compounds:
1. When one substituent group is attached to benzene, we write the name of the group in front
of benzene.
Cl +
Chlorobenzene
NO2 +
Nitrobenzene
CH2CH3
Ethylbenzene
Note: The IUPAC system retains certain common names for several of the simpler
monosubstituted alkylbenzenes:
O
O
OH NH2 C-H
OCH3 C-OH
Phenol Anisole Aniline Benzaldehyde Benzoic acid
CH3
Toluene
2. When two or more substituents are attached to benzene, the ring is numbered give the
lowest numbers to the substituents. The substituents are listed alphabetically.
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8. Note: if we have two substituents, we can also use the prefix “ortho-”, “meta-”, and “para-”
to show the position of the substituents (these names are common names).
COOH
Br
CH3
3
4
3 2
CH3
CH2CH3
1
Cl
1
1
2
2-Bromobenzoic acid
(o-Bromobenzoic acid)
2
1,3-Dimethylbenzene
(m-Xylene)
1-Chloro-4-ethylbenzene
(p-Chloroethylbenzene)
Note: The susbtituent group derived by loss of an H from benzene is called phenyl group,
C6H5-.
Chemical properties of benzene and its derivatives: The most important characteristic
reaction of aromatic compounds is substitution at a ring carbon (aromatic substitution). We
can name for them these three reactions: 1.Halogenation 2. Nitration 3. Sulfonation.
Halogenation: in the present of an iron catalyst, chlorine reacts rapidly with benzene to give
chlorobenzene and HCl.
FeCl3 + Cl + HCl
H Cl2
Benzene Chlorobenzene
Nitration: when we heat benzene or one of its derivatives with a mixture of concentrated
nitric and sulphuric acids, a nitro (-NO2) group replaces one of the hydrogen atoms bonded to
the ring. In this reaction, sulfuric acid is a catalyst and it is added to speed up the reaction.
+ NO2 + H2O
H HNO3
H2SO4
Nitrobenzene
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