Organic Chemistry - Part I

We have seen in earlier chapters how carbon forms tetrahedral bonds. Organic chemistry is a branch of chemistry that deals with covalent bonded carbon only. We have seen earlier how carbon can form long chain or cyclic molecules. Organic compounds are classified into two group namely aliphatic or open chain compounds and aromatic or closed chain compounds. For the present syllabus we will be studying in details only the aliphatic compounds.

What we will study in this chapter
1. Classification of aliphatic compounds
2. Hydrocarbons (alkanes, alkenes, alkynes)
3. Naming of hydrocarbons
4. Physical and chemical properties of hydrocarbons
5. Alcohols
6. Organic acids
7. Esters

1. Classification of aliphatic compounds
Aliphatic compounds are derived from parent hydrocarbon. A hydrocarbon is a compound of hydrogen and carbon only. By replacing one or more hydrogen atoms from the hydrocarbon by a reactive atom of group of atom “X”, a new compound can be formed. For example, a hydrocarbon R----H can be made to be R-----X, R is the carbon-hydrogen grouping. X is called a functional group. A functional group is an atom or a group of atoms which defines the function or the mode of activity of a given carbon compound R----X. The functional group determines the properties of the compound.

Some of the functional groups are shown below :    

Structure of X	Name	Corresponding class of compounds
	hydroxyl	alcohols
	carbonyl	ketones
	carboxyl	carboxylic acids
	amino	amines

2. Hydrocarbons
Hydrocarbons are made up of carbon and hydrogen atoms only. Carbon, as you are aware forms sigma and pi bonds. Carbon forming sp³ hybridization has all sigma bonds. This type of carbon forms only single linear bonds and no double or triple bonds. The bond is also called a saturated bond. Hydrocarbons with saturated bonds are called alkanes. With sp² hybridization, a carbon has double bond and hydrocarbons with double and unsaturated bonds are called alkenes. With sp hybridization a carbon has triple bonds and hydrocarbons with triple and unsaturated bonds are called alkynes.

Alkane	Number of C-atoms	Number of H-atoms	Molecular formula
Methane	1	4	CH4
Ethane	2	6	C2H6
Propane	3	8	C3H8
Butane	4	10	C4H10
Pentane	5	12	C5H12
Hexane	6	14	C6H14

The structural formulae that is the actual arrangement of different atoms in space of these substances are shown below.  

For methane, ethane and propane the structural formula is simple and straight forward.  But for butane C₄H₁₀ and higher alkanes the structural representation can become complex as there can be various ways in which the carbon atoms can be arranged. For example, butane C₄H₁₀ can be shown by two different structural formulae.  

The straight chain butane is called normal butane or n-butane. The branched chain is called iso-butane or 2-methyl propane.

Pentane C₅H₁₂ can be represented by three different structural formulae.  

The simple straight chain pentane is called n-pentane, pentane with one branched chain is called iso-pentane or 2 -methyl butane; pentane with two branched chains is called neo-pentane or 2,2,dimethyl propane.

The way hydrocarbons are named we will see in the later section.  

Alkenes
Hydrocarbons which contain two hydrogen atoms less than the corresponding alkanes are called alkenes or olefins. They have one double bond and are unsaturated carbon compounds. Their general formula is C_nH_2n and their names are derived from the alkanes by changing the ending “ane’  of the alkane by “ene”, for example, ethene, propene, butene, etc. Since an alkene has one double bonded carbon atom, the simplest alkene is ethene with two carbon atoms. The table below gives names of the first three alkenes.  

Alkene	Known commonly as	Number of C-atoms	Number of H-atoms	Molecular formula
Ethene	ethylene	2	4	C2H4
Propene	propylene	3	6	C3H6
Butene	butylene	4	8	C4H8

The structural formula that is the actual arrangement of different atoms in space of the simplest alkene namely ethene (CH₂  CH₂)  is shown below.  

A molecule of ethene has five sigma bonds and one pi bond.

The next higher alkene is propene. (CH₃ CH  CH₂). This has one double bonded carbon atom.

Alkenes are called olefins because they react with chlorine to form oil-like derivatives (olefins means oil forming). These reactions we will see later in this chapter.  

Alkynes
Hydrocarbons that have two carbon atoms in a triple bond are called alkynes. They are unsaturated bonds. Their general formula is C_nH_2n-2 and their names are derived from the alkanes by changing the ending “ane’  of the alkane by “yne”, for example, ethyne, propyne, butyne, etc. The simplest of alkynes has two carbon atom in triple bond and is called ethyne. The table below gives names of the first three alkynes.  

Alkene	Known commonly as	Number of C-atoms	Number of H-atoms	Molecular formula
Ethyne	Acetylene	2	2	C2H2
Propyne	Methyl acetylene	3	4	C3H4
Butyne	Dimethyl acetylene	4	6	C4H6

The structural formula that is the actual arrangement of different atoms in space of the simplest alkyne namely ethyne (CH CH )  is shown below.    

A molecule of ethyne has three sigma bonds and two pi bond.

Golden Rule for remembering whether a given hydrocarbon is an alkane, an alkene or an alkyne. First count the number of carbon and hydrogen atoms present in a given molecular formula. Then :  
1. If the number of hydrogen atoms is 2 more than double the number of carbon atoms, then the compound is an alkane.
2. If the number of hydrogen atoms is same as double the number of carbon atoms, then the compound is an alkene.
3. If the number of hydrogen atoms is two less than double the number of carbon atoms, then the compound is called an alkyne.

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