10. Organic Chemistry
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http://chemmaster.co.in/showchapter.php?id=20&id2=161&title=Hydrocarbons
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Boiling Points:
http://www.colby.edu/chemistry/cmp/cmp.html
http://masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/
Origins
• Originally defined as the chemistry of living
materials or originating from living sources.
• Wohler synthesized urea from non organic
sources (1800 – 1882)
• Now generally defined as the chemistry of carbon
and its compounds.
• Why is carbon so special?
Carbon can form single, double and
triple bonds with itself
•
http://wps.prenhall.com/wps/media/objects/3311/3391006/blb0809.html
The bond length is defined as the distance between
the nuclei of the atoms involved in the bond.
Bond enthalpy is the enthalpy change,
H, for the breaking of a particular
bond in a mole of gaseous
substance.
Carbon can catenate
This property of linking of atoms of one element with
one another forming chains of identical atoms is
called catenation.
Hydrocarbons
Hydrocarbons are organic compounds that
contain mainly hydrogen and carbon
Types :




Alkanes
Alkenes
Alkynes
Aromatic
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http://www.ivy-rose.co.uk/Chemistry/Organic/Homologous-Series.php
• A Homologous Series is one in which
all the members have the same
general formula.
• The neighbouring members of the
series differ by CH2 and they show
similar chemical properties and a
gradual change in their physical
properties.
Naming Hydrocarbons
A series of prefixes are used to designate the
number of carbon atoms in a carbon chain:
meth
1C
hex
6C
eth
2C
hept
7C
prop
3C
oct
8C
but
4C
non
9C
pent
5C
dec
10 C
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Alkane Structures
Alkanes have only carbon to carbon single bonds.
General Formula
CnH2n+2
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Homologous Series:
CH4
Methane
C2H6
Ethane
C3H8
Propane
C4H10
Butane
C5H12
Pentane
A homologous
series of alkanes
Each compound in
this series differs
from the previous
compound by a
–CH2 –
• Methane
• Ethane
• Propane
• Butane
• Pentane
CH4
C2H6
C3H8
C4H10
C5H12
CH3CH3
CH3CH2CH3
CH3CH2CH2CH3
CH3CH2CH2CH2CH3
Isomers
• Structural Isomers are compounds with the same chemical
formula but a different structural formula.
• As the number of carbon atoms in an alkane molecule
increases, so do the possibilities for isomerism of this kind.
There are three isomeric pentanes, all with the C5H12, five
isomeric hexanes, C6H14
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General Properties of Alkanes
• The boiling points of alkanes are very low for
their molecular mass. This is because the
molecule is completely nonpolar( inertness)
• It increases with an increase in mass.
Structural Isomers
Boiling Points of Alkanes
http://wiki.chemeddl.org/index.php/8.2_Organic_Compounds:_Hydrocarbons
• The more spherical the isomer, the lower boiling point( less
dispersion forces) due to less surface area to form
intermolecular bonds.
Alkenes
•
Alkenes have one carbon to carbon
double bond C=C
•
Since there are fewer hydrogen atoms in
alkenes as a result of the double bond,
alkenes are referred to as unsaturated.
•
Alkanes on the other hand have the
maximum number of hydrogen atoms.
They are referred to as saturated.
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Alkene Structures
•
Like alkanes, alkenes can have branched or
consecutive chains. In the larger alkenes there are
also multiple locations for the C=C. Hence multiple
structural isomers are possible.
Straight chain.
Straight chain.
Branched
chain
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Alkynes
•
•
Alkynes have one (or more) carbon
to carbon triple bonds
Since there are fewer hydrogen
atoms in alkynes as a result of the
triple bond, alkynes like alkenes are
referred to as unsaturated.
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Alkyne Structures
•
Like alkanes and alkenes, alkynes can have
branched or consecutive chains. In the larger
alkenes there are also multiple locations for the
C=C. Multiple structural isomers are possible. The
branch cannot originate on one of the carbons
making up the triple bond
Straight chain. The
triple bond is
between the first and
second carbon
Straight chain. The
triple bond is
between the second
and third carbon
Branched chain. The triple
bond can occur in one of the
branches but branches cannot
be attached to any carbon in
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the triple bond
Functional Groups
• In organic chemistry , functional
groups are specific groups of atoms
within molecules, that are
responsible for the characteristic
chemical reactions of those
molecules. The same functional
group will undergo the same or
similar chemical reaction(s)
regardless of the size of the molecule
it is a part of.
•
http://www.3rd1000.com/chem301/chem301a.htm
Benzene
I-Alkanes
• Alkanes are very unreactive due to single C
C bonds. All combustion reactions are
highly exothermic.(low polarity &van der
waals)
I. Complete Combustion:
2C8 H18 + 25O2 => 16CO2 + 18 H2 O
II. Incomplete Combustion
C8 H18 + 9O2 => 2CO2 + 9 H2 O + C + 5CO
C and CO are emitted as particulates. Page
193 CC
Combustion of Hydrocarbons
• This is effectively a technical word for
burning.
• Complete combustion produces CO2
and H2O, incomplete combustion
produces CO, C and H2O ,where there
is
C3H8 + 5O2
=>
3CO2 + 4H2O
Resonance –Polarity of
Carboxylic acids
Ring Structures
• Hydrocarbons that exist in chains are
known as aliphatic hydrocarbons
• The ends of a chain may be joined to
form a ring structure.
• These compounds are known as
cyclic structures
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Aromatic Structures
• The benzene ring is a
common structure in organic
molecules
• It consists of 6 carbon atoms
and 6 hydrogen atoms.
• One would predict that there
should also be 3 C=C bonds
in a benzene ring
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Aromatic Structures
• Further investigation reveals
that the double bonds are not
distinct in benzene. Rather it
is a resonance hybrid.
• Either of these structures
could be used to represent
benzene.
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Aromatic Structures
• Research shows that
there are no differences
in the C to C bonds in
benzene.
• The current view of
benzene holds that there
are 6 C-C single bonds
and 3 pairs or 6
delocalized electrons
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Aromatic Structures
• The structure of benzene
is shown as either of these
two structures, or as a
circle in a hexagon which
depicts that the electrons
are delocalized
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Halogenoalkanes or Alkyl
Halides
• Halogenoalkanes are compounds in which one or
more hydrogen atoms in an alkane have been
replaced by halogen atoms (fluorine, chlorine,
bromine or iodine).
• Halogenoalkanes are commonly known as alkyl
halides
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Halogenoalkanes or Alkyl
Halides
• Depending on the location of the halogen atom,
halogenoalkanes may be primary secondary or
tertiary
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Boiling Points of
Halogenoalkanes
• The boiling point
depends on the
halide
Cl < Br < I
• The boiling
points increase
as the chain
length increases
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Solubility of Halogenoalkanes
• The halogenoalkanes are only very
slightly soluble in water.
• The attractions between the
halogenoalkane molecules (van der
Waals dispersion and dipole-dipole
interactions) are relatively strong
• Halogenoalkanes are only slightly polar
and do not effectively break the
hydrogen bonds between water
molecules.
• Halogenoalkanes are soluble in non
polar or less polar organic solvents
such as alcohol, ether, and benzene .
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Functional Groups
• http://www.ivy-rose.co.uk/Chemistry/Organic/HomologousSeries.php
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Alcohols
• http://www.physchem.co.za/OB12mat/organic2.htm#alkanols
Carboxylic Acids
• http://www.physchem.co.za/OB12mat/organic2.htm#acids
• Carboxylic acids have a higher boiling boint than the
alkanols with the same number of carbon atoms.
• This is not solely due to an increase in molar mass (14
units), but rather because carboxylic acids tend to be more
strongly hydrogen bonded than alkanols, and even have a
tendency to form dimers.
Aldehydes and Ketones
• http://www.physchem.co.za/OB12mat/organic2.htm#ketones
Amines and Amides
• http://www.physchem.co.za/OB12mat/organic2.htm#amines
Esters
• http://www.physchem.co.za/OB12mat/organic2.htm#acids
Halogenoalkanes
• Have the general
formula R-X where
R is an alkyl group
and X is a halogen
H
H
H
C
C
H
H
Chloroethane
Cl
Functional group Isomerism
•
http://www.creative-chemistry.org.uk/molecules/isomers.htm
• Functional isomerism, an example of structural
isomerism, occurs substances have the same
molecular formula but different functional groups.
This means that functional isomers belong to different
homologous series.
Volatility
• Is a measure of how easily a substance
evaporates. A high volatile substance evaporates
easily and has a low boiling point.
Volatile organic compounds (VOCs) are organic chemicals
that have a high vapor pressure at ordinary, roomtemperature conditions. Their high vapor pressure results
from a low boiling point, which causes large numbers of
molecules to evaporate from the liquid or solid form of the
compound and enter the surrounding air. An example is
formaldehyde, with a boiling point of –19 °C (–2 °F), slowly
exiting paint and getting into the air.
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Note the strong influence of hydrogen bonding which explains the
low volatility (high boiling point) of HF, H2O and NH3.
•
Also note that volatility generally decreases (B.Pt. increases)
down a group (e.g. CH4, SiH4, GeH4 etc) as the polarisability of the
molecules increases and their temporary dipoles get larger
leading to greater temporary dipole/temporary dipoles interactions
(van der Waals' forces).
VAPOUR PRESSURE
Vapor pressure is the pressure caused by a liquid's own vapor.
http://www.pkwy.k12.mo.us/west/teachers/anderson/pack5/boil/boil.html
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