SSS 1: ORGANIC CHEMISTRY (I)

Organic chemistry is defined as the chemistry of carbon compounds. Apart from the main element, carbon, organic compounds may contain hydrogen, oxygen, and sometimes nitrogen, halogens, phosphorus, and sulphur. Organic compounds are covalent.

                          Catenation 

Catenation is the combination of the atoms of an element to form straight, branched, or ring-chain compounds.

Examples of elements that can catenate include carbon, silicon, boron, and sulphur.

           Hybridization in a carbon atom

Hybridization is the mixing of two or more orbitals of the same principal quantum number to give new sets of orbitals that are exactly equivalent. The new orbitals are known as hybrid orbitals.

              Hybridization of carbon

               

               Types of hybridization 

The different types of hybridization based on the mixing of the orbitals of carbon are: 

1. SP³ hybridization 

2. SP² hybridization 

3. SP¹ hybridization 

                     SP³ hybridization 

When one S-orbital and three P-orbital of an excited carbon atom are in the same shell, SP³ hybridization is formed. The arrangement is tetrahedral with a bond angle of 109.5°.

Examples: methane (CH₄), tetrachloromethane (CCl₄), etc

                      SP² hybridization

A carbon atom is SP² hybridized when bonding takes place between one S-orbital with two P-orbitals. There is a formation of two single bonds and one double bond between three atoms. The hybrid orbitals are placed in a triangular arrangement with 120° angles between bonds.

Example: graphite, ethene, etc

                       SP¹ hybridization

 A triple bond is formed when only one unpaired P-orbital and 2S¹ orbital of an excited carbon atom is hybridized. When the hybridization occurs the molecules have a linear arrangement of the atoms with a bond angle of 180°.

Example: carbon(IV)oxide_, Ethyne, etc

                   Sigma and Pi bond 

Sigma bonds (σ): are the type of covalent bond, formed by the overlap of atomic orbitals head-to-head. They are found in single, double, and triple bonds. 

Pi bonds (π): are the types of covalent bonds, formed by lateral overlap of p orbitals side-to-side. They only exist in double and triple bonds.

        Sigma(σ) and pi(π) bond of ethyne

Example: 

What is the total number of sigma and pi bonds in the following molecules?

(a) Ethene, C₂H₄  (b) Ethyne, C₂H₂

${}_{\mathrm{Solution}}$

(a) C₂H₄

Sigma= 5, Pi= 1 

${}_{\mathrm{(b)}}$C₂H₂

Sigma= 3, Pi= 2

Class activities:

What is the total number of sigma bonds and pi bonds in the following molecules?
${\mathrm{1.}}^{}{}_{\mathrm{C2H3}}Cl$

$\mathrm{2.}{}_{\mathrm{CH2}}C{l}^{}$₂

${\mathrm{3.\; H3C-HC=CH-C\equiv C-H}}^{}$

          Naming of organic compounds 

                                Alkane 

General formula: C_nH_2n+2 

Members of the alkane family are:

1. Methane      n=1,  CH₄

2. Ethane         n=2,  C₂H₆

3. Propane      n=3,  C₃H₈

4. Butane        n=4,   C₄H₁₀

5. Pentane.     n=5,   C₅H₁₂

Skeletal structure of some alkanes 

Class activities: 

 Write the structures of the following:

(I)   2- methylpropane 

(ii)  2, 2-dimethylbutane 

(iii) 2, 2, 4-trimethyl pentane 

(iv) 2, 3-dimethyl-4-ethyl hexane 

                     Alkyl group

The Alkyl group is a group derived from the alkanes by the loss of a hydrogen atom.

General formula: C_nH_2n+1

Members of the alkyl group are:

1. Methyl         n=1,   CH₃

2. Ethyl            n=2,   C₂H₅

3. Propyl         n=3,   C₃H₇

4. Butyl           n=4,   C₄H₉

5. Pentyl         n=5,   C₅H₁₁

        The Haloalkanes (Alkylhalides)

The haloalkanes are alkanes containing one or more halogen substituents. 

The halogens in haloalkanes are addressed as follows:

Florine(Floro), Chlorine(Chloro), Bromine(Bromo) and Iodine(Iodo).

Write out the structures of the following:

(I). 2-chloropropane 

(II) 2-bromo- 3-Iodo pentane

(II) 2,2-dibromo-4-methyl heptane 

                      Cycloalkane

Cycloalkanes are a class of hydrocarbons that have a ring-like structure.

Some common examples of cycloalkanes are cyclopropane, cyclobutane, cyclopentane, cyclohexane cycloheptane, cyclooctane, etc 

                               Alkene 

General formula: C_nH_2n

Members of the alkene family are:

1. Ethene      n=2,  C₂H₄      

2. Propene   n=3,   C₃H₆

3. Butene     n=4,   C₄H₈

4. Pentene.  n=5,   C₅H₁₀

Skeletal structures of some alkenes

Class activities: 

1. Write the structures of the first ten members of the alkene family

2. Write the structures of the following:

(I)   2- methylpropene 

(ii)  3,3-dimethyl pentene

(iii) 2, 2, 3-trimethyl pentene 

(iv) 2, 3-dimethyl-4-ethyl hexene 

(v) 2-chlorobut-2-ene 

(vi) 3-methylhex-2, 4-diene 

                          Alkyne 

General formula: C_nH_2n-2

Members of the alkyne family are:

1. Ethyne      n=2,  C₂H₂      

2. Propyne   n=3,   C₃H₄

3. Butyne     n=4,   C₄H₆

4. Pentyne.  n=5,   C₅H₈

Skeletal structure of some alkynes

Class activities: 

1. Write the structures of the first ten members of the alkyne family

2. Write the structures of the following:

(I)  2- methylpropyne 

(ii)  2, 2-dimethyl pent-diene 

(iii) 2, 2, 4-trimethyl heptyne 

(iv) 2, 3-dimethyl-4-ethyl hexyne 

(v)  2-chlorobutyne 

(vi) 3-methylhexyne 

                     Benzene 

Compounds of Benzene:

            Homologous series 

Homologous series is a family of organic compounds that follows a regular structural pattern in which each successive member differs in its molecular formula by a -CH₂- group 

 Characteristics of the homologous series

(i) They can be represented by a general molecular formula
 (ii) They have similar chemical properties
 (iii) They have same functional group
 (iv) They can be prepared by a general method
 (v) Each consecutive member differ by –CH₂ group

 (vi) Increase intensity of physical properties with increase in molar mass 

The difference between ethane and methane: C₂H₆ -  CH₄ = CH₂

                  Functional group 

A functional group is an atom, a radical, or a bond common to a homologous series, and which determines the main chemical properties of the series.

                    Stereochemistry 

Stereochemistry is the study of chemical structures in three dimensions. 

Isomerism: Isomerism is the existence of two or more isomers with the same molecular formula but different molecular structures.

                  Types of isomerism 

1. Structural isomerism 

2. Stereoisomerism

                Structural isomerism 

The functional groups and the atoms in the molecules of these isomers are linked in different ways.

Examples:

I. Butane has 2 isomers 

II. Pentane has 3 isomers 

III. Hexane has 5 isomers 

                   Isomers of butane

Class activities:

Write out the structures and names of the isomers of:

1. Pentane 

2. Hexane

                Stereoisomerism 

This type of isomerism arises in compounds having the same chemical formula but different orientations of the atoms belonging to the molecule in three-dimensional space. The compounds that exhibit stereoisomerism are often referred to as stereoisomers.

The two types of stereoisomerism are:

1. Geometric isomerism 

2. Optical isomerism

              Geometric isomerism 

Geometric isomerism is the existence of compounds with the same molecular formula but different spatial arrangements of the compound atoms.

It is popularly known as cis-trans isomerism. It is common among alkenes.

Geometric isomers are compounds with the same molecular formula but a different orientation in space. e.g.

                  Optical isomerism 

 Optical isomerism is the existence of two or more compounds with the same molecular formula but with different configurations.

Due to molecular symmetry, it can rotate plane polarised light.

A compound that rotates plane polarised light is said to be optically active. For a compound to be optically active, it must have a carbon atom that is substituted by four different groups e.g. 

1. Lactic acid(2-hydroxypropanoic acid)

2. Alanine (2-aminopropanoic acid)

3. Isobutanol

   Note:

When the compound rotates the plane-polarised light 

1. to the right (clockwise), it is called Dextro-rotatory (d+)

2. to the left (anti-clockwise), it is called Laevorotatory (L-).