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Introduction to organic chemistry
What is Organic chemistry?
Is the branch of chemistry that deals with the study of compounds of carbon
All organic compounds contain carbon together with one or more other elements such as hydrogen, oxygen, chlorine, Nitrogen and sulphur
These organic compounds range from simple molecules such as methane, ethene, ethyne, giant molecules such as Deoxyribonucleic acid (DNA),rubber, cotton, drugs, insecticides, explosives and photographic films- of which all these compounds are of great importance in everyday life
Organic chemistry is also the foundation for basic studies in Botany, nutrition, agriculture, pharmacy, technology, forestry, zoology etc
UNIQUENESS OF CARBON IN ORGANIC COMPOUNDS
- Carbon behaves differently from the rest of the group members due:
- Small Atomic radius
- High electronegativity
- Lack of vacant orbital
- Some of the properties in which Carbon differs from the rest of the members include:
- It forms a stable multiple bonds with itself and with other non-metals.
- Carbon can form a number of hydrocarbons because it can catenate. Catenation is the ability of the element to form bonds between atoms of the same element.
- It forms gaseous oxides unlike other members
- The halides are stable to hydrolysis.
CHARACTERISTIC PROPERTIES OF ORGANIC COMPOUNDS
Most organic compounds are:
Covalently bonded and generally gases, volatile liquids and low melting point solids with few elements such as carbon, hydrogen, sulphur, nitrogen and halogens
Low melting and boiling point compounds
Generally insoluble in water except those with polar functional groups such as
REACTANTS AND REACTIONS
In chemical reactions, the reactants molecules are that are attacked are called SUBSTRATE but the
general term given to the attacking substrate is called a REAGENT.
Substrates and Reagents usually interact to yield the products involve redistribution of electrons.
Generally there are two ways in which a covalent bond can be broken include:
(a) Homolytic cleavage
In this type, the bond is broken symmetrically and each atom remains with one electron to form
a radical
Example include
These types of reactions are usually catalyzed by:
– Ultraviolet light or short wavelength light or sunlight
– High temperatures
– Peroxide
(b) Heterolytic cleavage
In this of the reactions, the bonds are broken unsymmetrically generating cation and anion
If the molecule contains different atoms with different electronegativity, the tendency of ions
formation differs.
These breakages usually take place in solvents of high ionizing power which is catalysed by ionic
polar catalysts
Example include
If A is more electronegative than B, then
These are formed when fragments of a molecule in which a group and the bonding electrons are
removed from the carbon chain
Carbocations are divided into 1, 2 and 3 ddegrees
The stability of the Carbocations depend on the number of the electron releasing group e.g. CH3 and is
expressed as 3 > 2 > 1 in degrees
ELECTROPHILES AND NUCLEOPHILES
Electrophiles are species which are electron loving since they are electron deficient while Nucleophiles
are species which are electron rich
Examples of Electrophiles and Nucleophiles include:
HYDROCARBONS
These compounds that contain carbon and hydrogen as the only elements present.
They are classified as:
(a) Saturated hydrocarbons
These are hydrocarbons that contain a carbon- carbon single bond e.g. in Alkanes
(b) Unsaturated hydrocarbons
These are hydrocarbons that contain multiple bonds between the carbon atoms e.g Alkenes
Further classification includes:
(a) Aliphatic Hydrocarbons
These are hydrocarbons that contain low carbon-hydrogen ratio. Usually on burning, they burn with a non-sooty flame.
(b) Aromatic hydrocarbons
These are hydrocarbons that contain a high carbon- hydrogen ratio. Usually on burning, they burn with a sooty flame. These compounds are usually considered as derivatives of benzene.
N.B Also Alicyclic hydrocarbons which are hydrocarbons in which carbon atoms are arranged in the ring
other than benzene ring e.g
FUNCTIONAL GROUPS AND HOMOLOGOUS SERIES
Homologous series is a series in which organic compounds can be placed. The characteristics of
homologous series include;
– Member of a homologous series can be represented by a general formula e.g. CnH2n+2 for Alkanes, CnH2n for Alkenes
– Each member of a series has a similar method of preparation
– Members in a given series has a similar chemical properties to each other
– As a series is ascended down, a methylene group is added to each successive member therefore
physical properties such as boiling and melting points change slightly.
– Several homologous series are regarded as alkane derivatives since the hydrogen atom in the alkane is replaced by a functional group
Functional group is atom or group of atoms which impart specific chemical properties to the chemical compounds containing then regardless of the nature of the hydrocarbon part of the molecule.
Generally Organic compounds are classified based upon the functional group because compounds having the same functional groups have the similar chemical properties.
The table below shows some of the examples of functional groups:
TYPES OF ORGANIC REACTIONS
Organic reactions are usually initiated by Electrophiles or Nucleophiles or free radical attack upon the
substrate. These reactions include:
(a) Substitution or Displacement reactions
These reactions involve the direct displacement of an atom or group by another atom or group.
The number of atoms displaced depends on the nature of the reactants and conditions under
which the reaction is carried out.
Generally they are represented as;
Substitution reactions are divided into two classes:
(i) Nucleophilic substitution reaction (SN).
These are reactions whereby electron rich species simply displaces an atom or group. These reactions are undergone by Alkyl halides.
(ii) Electrophilic substitution reaction (SE) These are reactions where by electron deficient group displaces an atom. These reactions are undergone by Benzene where hydrogen atom is replaced by an electron deficient group
According to the number of species involved in the formation of the activated complex in the rate
determining step, Nucleophilic substitution reaction is divided into two:
(i) Nucleophilic substitution unimolecular (SN1) where a single species is involved.
(ii) Nucleophilic substitution Bimolecular (SN2) where two species are involved.
(b) Elimination reactions
These are reactions involve the removal of atoms or groups of atoms from two adjacent atoms to form a multiple bond or unsaturated compound.
These reactions are usually encountered to yield alkenes and alkynes Generally they are represented as;
Elimination reaction is also divided into two types based upon the number of species involved in the
formation of activated complex in the rate determining step.
(i) Elimination Unimolecular (E1)e.gdehydrohalogenation and dehydration of 30 Alkyl halides and Alcohols respectively
(ii) Elimination Bimolecular (E2)e.gdehydrohalogenation and dehydration of 20 Alkyl halides and Alcohols.
(c) Addition reactions
These are reactions wherebythe attacking reagent simply adds across the unsaturated bond of the reactant to saturated product.
These reactions are usually the reverse of the elimination reactions and the unsaturated reactants act as Nucleophiles and consequently more vulnerable to attack by Electrophiles.
These are reactions are generally represented as:
Example include
Addition reactions are divided into classes:
(i) Electrophilic addition reactions (AE)
These are reactions where the electron loving species simply adds itself to unsaturated
compound e.g. addition reactions of alkenes and alkynes.
(ii) Nucleophilic addition reactions (AN)
These are reactions where the electron rich species simply adds itself to the
unsaturated bond of the bond e.g. addition reactions of carbonyl compounds
(d) Rearrangement reactions
These are reactions which involve the migration of an atom or group from one site to another within the same molecule. It involve the migration of a functional group from one position to another within the same molecule
Example include
MECHANISM OF ORGANIC REACTIONS
Mechanisms of reaction are sequence steps by which the reactions take place from start to the end. The mechanism is worked out from a study of the kinetics of the reactions. Other method include spectroscopy and use of radioactive isotopes
ISOMERISM
This is the existence of different compounds with the same molecular formulae but different structural formulae.
Compounds with same molecular formulae but different structural formulae are known as Isomers. Isomerism is divided into two classes:
(a) Structural Isomerism
This is the type of isomerism which shows the sequence in which the atoms in a molecule are bonded or linked.
It is divided into three classes:
(i) Chain Isomerism
This is the type of structural isomerism where by the isomers differ in the carbon chain. The isomers usually have the same functional group and belong to the same homologous series.
It can be divided into classes’i.e. Straight chain and branch chain
Example included:Q has molecular formula C4H10, write down possible chain structural formula ofQ. Since the M.F. is C4H10isomers are:
(ii) Positional Isomerism
This is the isomerism where the substituent groups are in different positions in the same carbon skeleton. The isomers are chemically similar because they possess the same functional groups
(iii) Functional Isomerism
This is the isomerism where by the isomers in different homologous series have different functional groups.
Examples include:
(a) A compound Q has Molecular formula C2H6O
(b) Stereoisomerism
This is the type of isomerism where by the compounds have the same molecular formula and
same structural formula but differ in the spatial arrangement of the bonds.
Stereoisomerism is divided into two classes:
(i) Cis-trans Isomerism (Geometrical Isomerism)
This occurs in compounds in which free rotation is prevented by the presence of multiple bonds,
a ring or steric factors.
Geometrical Isomers differ in spatial arrangement giving rise to Cis isomers which has identical
atoms or groups on the same side of the multiple bond and a Trans isomers which has the atoms
or groups in opposite side.
Examples include:
(ii) Optical Isomerism
This occurs in compounds with four atoms or groups attached to the same carbon such that the
isomers obtained are not super imposable on the mirror images. These isomers which are not super imposable are referred as to as Enantiomers and the structures are said to be chiral.
Example includes:
Enantiomers have same physical properties but may be distinguished from each other by the ability they rotate polarized light due to optical activity.
If the rotation of the plane is in clockwise direction (to the right), the substance is dextrorotatory (with +sign) while if the rotation is in the anticlockwise (to the left), the substance is laevorotatory (with – sign). The degree of rotation is determined by use of a polarimeter.
METHODS OF RELATIVE MOLECULAR MASS DETERMINATION OF ORGANIC COMPOUNDS
The four commonly used methods for determining the relative molar mass of organic compounds include:
(a) Vapour density measurements using gas syringe or Victor Meyer’s method Example
A hydrocarbon Q contains 91.3% carbon and the rest hydrogen.
(a) Determine the empirical formula of Q (C=12, H=1)
(b) The vapour density of Q is 39 at a constant temperature and pressure.
Determine the:
(i) Molecular mass of Q
(ii) Molecular formula of Q
(c) Q burns with a sooty flame, write down the possible isomer(s) of Q
Solution
Example
When 0.29g of substance Q was burnt in oxygen, 0.66g of carbondioxide and 0.27g of water were formed.
(a) Calculate the empirical formula of Q
(b) Q forms a yellow solid with 2, 4-dintrophenylhydrazine. The vapour density of Q is 29.
(i) Write the structural formulae of the isomers and names of Q
(ii) What type of structural isomerism is exhibited by Q.?
(iii) How can the structural isomers be differentiated
Solution:
(ii) Functional Isomerism
(iii) By using Ammoniacal Silver nitrate solution
Example
(a) Define the hydrocarbon
(b) A compound X, on combustion gave 0.629g of carbondioxide and 0.257g of water. Determine the empirical formula of X
(c) When 0.10g of X was vapourised, it occupied a volume of 40cm3 at s.t.p
(i) Calculate the molecular mass of X
(ii) Determine the molecular mass of X hence write the structural formulae of the possible isomers
(a) A compound that contains carbon and hydrogen as the only elements present
(b) Colligative Properties of dilute solutions containing non-volatile solutes
These include lowering of vapour pressure, cryoscopy, ebullioscopy and osmotic pressure Cryoscopicmethod has an advantage over Ebullioscopic method that it is not affected very much by pressure changes of which boiling points depend on the atmospheric pressure
Molecular mass of polymers is determined using Osmotic pressure because they are easily denatured by heat and a amount of polymer changes the osmotic pressure of the solvent significantly.
Example
A compound W contains Carbon, Hydrogen and oxygen. When 1.5g of W was burnt in excess oxygen, 1.467g of carbon dioxide and 0.297g of water were formed.
(a) Determine the empirical formula of W
(b) 7.5g of W was dissolved in 204.4g of water to a solution .The prepared boiled at 100.212oC. The boiling point constant of water is 0.52oC per mole per kg. Determine the:
(i) Molecular mass of W
(ii) Molecular formula
(iii) Structural formula of w
(c) State the condition(s) employed in order for W to react with following compounds.
In each case write the equation of reaction
(i) Sulphuric acid
(ii) Potassium manganate (VII) solution
(d) Name the reagent that can be used to distinguish between W and Methanoic acid.
Solution
(b) (i) 204.4g of water contains 7.5g of W
(d) Ammoniacal Silver nitrate solution
Example
(a) Differentiate between Diffusion and Osmosis
(b) The osmotic pressure of a solution containing 4.00gdm-3 of polyvinylchloride,
In dioxin is 65Pa at 20oC. Determine the number of monomers, n, in the polymer.
(c) State two conditions under which the Osmotic laws cannot be obeyed
Solution
(a) Diffusion is the movement of molecules from high concentrated area to a low concentrated area while Osmosis is the movement of solvent molecules from a dilute solution to concentrated
solution across a semi-permeable membrane
(c) When the solution is concentrated When the solute associate or dissociate in the solvent
(c) Eudiometry method or Gaseous explosion. This method is suitable for gaseous hydrocarbon only
Example
(a) State Gay-Lussac’s law.
(b) 30cm3 of a gaseous hydrocarbon Y burnt at constant temperature and pressure produced 120cm3 of carbon dioxide and 120cm3 of steam.
(i) Determine the formula of Y.
(ii) Write structures and names of all the isomers of Y.
Solution
(a) When gases combine together at a constant temperature and pressure they do so in volumes which bear a simple ratio to each other and to the volume of gaseous product.
(b) (i)
Example
30cm3 of gaseous hydrocarbon were mixed with 140cm3 of excess oxygen and exploded. After cooling to room temperature the residual gases occupied 95cm3. By absorption with potassium hydroxide solution a diminution of 60cm3 was produced. Determine the molecular mass of the hydrocarbon
Solution
(d) Steam Distillation
This is the technique of separating a volatile substance that is immiscible with water from a nonvolatile component at a temperature below its boiling point. This method helps in separating organic compounds that decompose near their boiling point and distil at a temperature below their boiling points.
It uses the principle of
Example
(a) When 4.9g of an organic compound X was burnt in excess oxygen, 14.78g of carbondioxide and 5.38g of steam was formed. Calculate the simplest formula of X
Note: Check Physical Chemistry Textbook for more questions in order to practice