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WEEK 7
ORGANIC CHEMISTRY
NATURE OF ORGANIC CHEMISTRY
Organic chemistry is that branch of chemistry that deals with certain carbon
compounds. There are certain essential differences between organic and inorganic
chemistry:
INORGANIC CHEMISTRY
1. Molecules are relatively
small
2. Molecules are relatively
ionizable.
3. Reactions involve a major
change in the molecule
4. Many reactions occur
instantaneously
5. Four type of compounds
ORGANIC CHEMISTRY
1. Molecules are relatively
large
2. Molecules are relatively
nonionizable
3. Reactions involve a change in
only a small part of the molecule
4. Most reactions take place slowly
5. At least 12 types of compounds
PROPERTIES OF CARBON
The properties and reactions of organic compounds are ultimately the function of
the elements of which they are made. Let us examine some of the characteristics of the
element carbon.
COMBINING CAPACITY – Carbon has a combining capacity of 4. Carbon has
four valence electrons in its outermost energy level. When we say that carbon has a
combining capacity of 4, we mean that it forms four bonds. We can represent the bonds
as follows:
C
Carbon may combine with four univalent atoms (atoms that need one electron to fill their
outermost shell). An example of a univalent atom is hydrogen:
H
C
H
H
H
Carbon may combine with two divalent atoms like oxygen. Oxygen needs two electrons
to fill its outer shell.
O
C
O
Carbon can combine with two univalent atoms and one divalent atom:
H
H
C
O
Carbon may also combine with one univalent and one trivalent atom (ex: Nitrogen):
HC N
THE CARBON-TO-CARBON BOND
Carbon has a great tendency to combine with itself to form large molecules. Carbon may
be bound to adjacent carbon atoms by one, two, or even three bonds. Carbon has a
combining capacity of 4, but the maximum number of bonds between two adjacent
carbon atoms is 3.
C C
C
C
C
C
Organic compounds form covalent bonds because the electrons between the bonds are
shared.
FUNCTIONAL GROUPS
Combining capacities are easy to remember in organic chemistry, because they are one,
two, three, and four: hydrogen, 1; oxygen, 2; nitrogen, 3; carbon, 4. The essential
compounds are combinations of these elements and are organized into 12 elementary
groups. These groups are identified by what are called FUNCTIONAL GROUPS. The
twelve functional groups are:
1. Alkanes contain only C-H and C-C single bonds
2. Alkenes
C
C
3. Alkynes
C
C
4. Arenes
C
C
C
C
C
C
5. Alcohols
C OH
6. Ethers
C O
C
7. Aldehydes
O
C
8. Ketones
C O
C
9. Carboxylic acids
O
C
OH
10. Esters
O
C
O
11. Amines
H
H
N
N
N
H
12. Amides
O
C
NH2
O
O
C NH
C
N
The functional groups are drawn with open bonds and do not represent complete
compounds. The open bonds indicate where other atoms or groups of atoms attach to the
functional group. The site of most organic reactions is the functional group. The rest of
the molecule usually does not participate in the reaction. The functional groups alkanes,
alkenes, alkynes, and arenes are collectively referred to as HYDROCARBONS because
they only contain hydrogen and carbon.
Another way to write the functional groups is by combining them with an “R” group or
an alkyl group. We will discuss alkyl groups more later. These are called condensed
structures. The condensed structures that we need to be concerned with are as follows:
Alcohols
Ethers
Aldehydes
Ketones
Carboxylic acids
Esters
Amines
ROH
ROR
RCHO
RCOR
RCOOH
RCOOR
RNH2, R2NH, R3N
FORMULAS
Three types of formulas are used in organic chemistry.
1. MOLECULAR FORMULAS – These represent the actual number of each
constituent atom in the molecule. For example: The molecular formula for
formaldehyde is CH2O. The molecular formula for methyl alcohol is CH4O.
2. STRUCTURAL FORMULAS – These represent the spatial relationship of the
constituent atoms to each other. The structural formula for formaldehyde is:
O
H
C
H
The structural formula for methyl alcohol is:
OH
H
C
H
H
3. LINE FORMULAS – These are used to conserve space over structural formulas
and are nothing more than condensations of them. The line formula for
formaldehyde is HCHO. The line formula for methyl alcohol is CH3OH.
WEEK 7
HYDROCARBONS
The hydrocarbons are organic compounds containing only hydrogen and carbon as
constituent elements. The physical state of the hydrocarbons depends on the length of the
carbon chain.



C to C4 are gases
C5 to C17 are liquids
C18 and higher are solids
Hydrocarbons are the simplest of the organic compounds containing only two elements,
hydrogen and carbon. Adjacent carbons may be joined together by one, two, or three
bonds.
ALKANES
Hydrocarbons in which carbons are joined together by single bonds are called
ALKANES. They are named according to the number of carbon atoms in the chain and
an –ane suffix. The number of carbon atoms are named according to the following:
1 – Meth
2 – Eth
3 – Prop
4 – But
5 – Pent
6 – Hex
7 – Hept
8 – Oct
9 – Non
10 – Dec
Thus the structural formula of methane is:
H
C
H
H
H
and the molecular formula is CH4. The structural formula for ethane is:
H
H
H
C
C H
H
H
and the molecular formula is C2H6. The structural formula of propane would be:
H
H
H
H
C
C
C
H
H
H
H
and the molecular formula is C3H8. Notice that carbon always has four bonds attached to
it. The molecular formula is determined by counting the number of carbons and the
number of hydrogens. There is a fixed ratio between carbon and hydrogen in alkanes that
can be expressed by a TYPE FORMULA: CnH2 (n)+2 (n is equal to the number of
carbons). If we know the number of carbons, we can determine the number of
hydrogens. For example, if we have 5 carbons the number of hydrogens is 2(5) +2 = 12.
This alkane would be pentane because it has 5 carbons and the molecular formula would
be C5H12. Alkanes are also called SATURATED HYDROCARBONS because single
bonds between adjacent carbon atoms allow attachment of the maximum number of
hydrogen atoms. Remember a saturated solution contains the maximum number of solute
the solvent can hold.
ALKYL GROUPS
Alkanes are represented by the type formula CnH2n+2. If a hydrogen atom is removed
from an alkane, the type formula becomes CnH2n+1. The new structure is no longer a
complete compound but now is a group of atoms capable of attaching to another atom at
the place where the hydrogen was removed. This structure is called an ALKYL GROUP.
They are named by replacing the –ane ending of the parent hydrocarbon with –yl.
Methane
Ethane
Propane
Butane
Pentane
CH4
C2H6
C3H8
C4H10
C5H12
becomes
becomes
becomes
becomes
becomes
methyl
ethyl
propyl
butyl
pentyl
Structurally an alkyl group would look like the following:
H
H C
H
Methyl
H
H
H
H C
C
C
H
H
H
Propyl
CH3
C2H5
C3H7
C4H9
C5H11
Notice the missing hydrogen in both compounds.
ALKYL HALIDES
Alkyl groups may be attached to Group 7A (halogen) elements. The substances that are
formed are called ALKYL HALIDES. Alkyl halides have the general formula
RX
Alkyl group
Halogen
In this formula, “R” is from the German word Radikal, which means, “group”.
The simplest alkyl halide occurs if we replace one hydrogen on a methane with a
chlorine:
H
C Cl
H
H
methyl chloride
(chloromethane)
We can continue to obtain more alkyl halides by replacing additional hydrogens on the
methane. Two of the more common compounds are:
H
Cl
C Cl
Cl
C
Cl
Cl
Cl
Cl
Chloroform
Carbon tetrachloride
ISOMERS
Alkanes tend to form ISOMERS. Two or more substances having the same molecular
formula but different structural formulas are Isomers. Butane is the first member of the
alkanes to form an isomer.
H
H
H
H
H C
C
C
C
H
H
H
H
H
H
H
H
H
C
C
C H
H
H
H C H
Normal butane
(n-butane)
H
Isobutane
The molecular formula for both normal and isobutane is C4H10. However, their structural
formulas show that there is a difference between the two compounds. The line formulas
also show a difference. The line formula for Normal butane is CH3CH2CH2CH3 and the
line formula for isobutane is CH3CHCH3CH3.
ALKENES
When multiple bonds are present in a carbon chain, the substance is referred to as an
UNSATURATED HYDROCARBON. It is called this because it contains less hydrogen
atoms than it could if only single bonds were present. The simplest unsaturated
hydrocarbons contain one double bond in the chain. Alkenes are hydrocarbons
containing only double bonds. The simplest alkene is ethene:
H2C
CH2
The eth- indicates the two carbons and the –ene indicates the double bond. When naming
alkenes we use the same prefixes learned for alkanes (meth, eth, prop, but...) but we use
the –ene ending to let us know we have a compound with a double bond. As we did for
alkanes and the alkyl group, we can write a type formula for alkenes. The formula is
CnH2n.
ALKYNES
Another form of unsaturated hydrocarbons contains a triple bond between adjacent
carbon atoms. The first member of this series is ethyne or acetylene:
H
C
C H
The type formula for the Alkynes is CnH2n-2. Alkynes are named using the prefixes we
learned (meth, eth, prop...) followed by the suffix –yne. Alkynes contain only one triple
bond. If we have two triple bonds we then have a diyne and three triple bonds would be a
triyne.
CYCLIC HYDROCARBONS
So far the hydrocarbons we have looked at have all been open-chain compounds.
Hydrocarbons also exist as ring or cyclic compounds. They are named by adding the
prefix cyclo- to the name of the corresponding open chain compound. The type formula
for the cycloalkanes is CnH2n and for cycloalkenes is CnH2n-2. Cyclopropane is an
example of a cyclic hydrocarbon.
H2
C
CH2
C
H2
AROMATIC HYDROCARBONS
A major way to classify all organic compounds is as ALIPHATIC or AROMATIC. The
word aliphatic means “fatty”. The alkanes, alkenes, alkynes, and their cyclic derivatives
are aliphatic compounds. The connection between the word “fatty” and these compounds
is that fats contain a hydrocarbon portion that often resembles long-chain alkanes or
alkenes. Compounds classifies as aromatic contain a benzene ring or a system of these
rings. These compounds, also known as ARENES, are called aromatic because the first
ones discovered had pleasant odors. Examples are vanilla and oil of wintergreen.
STRUCTURE OF BENZENE
The molecular formula of benzene is C6H6. It is a ring compound. In order for each
carbon to have four bonds, chemists who first studied benzene described it as a ring
compound with alternating double and single bonds. This cannot be true since all the
bond lengths in a benzene ring are equal. Carbon- to- carbon double bonds are shorter
than carbon-to-carbon single bonds. If benzene did have alternating double and single
bonds, then it would undergo typical alkene reactions. This is not the case. Benzene
consists of six equal carbon-to-carbon bonds that are intermediate in properties between a
double and single bond. The structure of benzene is represented as follows:
The arrow indicates that the molecule’s true structure is a blend of what are drawn as two
contributing structures. Each of the corners of the hexagons represents a carbon atom
bonded to a hydrogen. The only difference between the two structures is the placement
of the double bonds. Whenever two or more structures that differ only in the position of
electrons can be drawn for a compound, that compound has the property of
RESONANCE. This is characteristic of all aromatic compounds. Another way which
we can represent benzene is:
This is now the more accepted structure of benzene. This structure represents the fact
that the electrons in the double bonds are free to move about the benzene molecule and
are not just “stuck” between the carbon atoms forming the double bond. This explains
why the bond lengths are all the same in benzene. The circle represents electron
delocalization (the electrons are not “stuck” in one place) and is a stabilizing factor for
benzene. The molecule is unreactive. The major reactions of benzene generally occur
only under strenuous conditions and involve substituting other atoms for the hydrogens.
Benzene is a good organic solvent and the starting material for the synthesis of many
widely used compounds.
DERIVATIVES OF BENZENE
We will focus on only the monosubstituted (only one hydrogen replaced) and
disubstituted (two hydrogens are replaced) benzenes.
Some monosubstituted benzenes are known by common names. Important
compounds in this classification are Toluene (methylbenzene), phenol (hydroxybenzene),
and aniline (aminobenzene). Toluene is an extremely good solvent for paints and
varnishes. It is the starting material for trinitrotoluene (TNT). Their structures are as
follows:
OH
NH2
Phenol
Aniline
CH3
Toluene
Because all six carbon-to-hydrogen bonds are equivalent in benzene, only one
form of each of the monosubstituted benzenes exists. The story is different for the
disubstituted benzenes. Three isomers of each exist. Consider the compound
dichlorobenzene. There are three possible ways to draw its structure.
Cl
Cl
Cl
Cl
Cl
ortho-dichlorobenzene
meta-dichlorobenzene
Cl
para-dichlorobenzene
Each of these represents an individual compound with different properties. To
distinguish among them, chemists use a nomenclature with the prefixes ortho, meta,
para. If two substituents on a benzene ring are on adjacent carbons, the compound is the
ORTHO isomer. When one carbon separates the positions of the two substituents, the
molecule is the META form. Finally, if the substituents are directly across the ring from
each other the compound is the PARA isomer.