Introduction and key concepts  (Page 2/2)

• Carbon has four valence electrons which means that each carbon atom can form four bonds with other atoms. Because of this, long chain structures can form. These chains can either be unbranched ( [link] ) or branched ( [link] ). Because of the number of bonds that carbon can form with other atoms, organic compounds can be very complex.

  

  

• Because of its position on the Periodic Table, most of the bonds that carbon forms with other atoms are covalent . Think for example of a C-C bond. The difference in electronegativity between the two atoms is zero, so this is a pure covalent bond. In the case of a C-H bond, the difference in electronegativity between carbon (2.5) and hydrogen (2.1) is so small that C-H bonds are almost purely covalent. The result of this is that most organic compounds are non-polar. This affects some of the properties of organic compounds.

Representing organic compounds

There are a number of ways to represent organic compounds. It is useful to know all of these so that you can recognise a molecule however it is shown. There are three main ways of representing a compound. We will use the example of a molecule called 2-methylpropane to help explain the difference between each.

Molecular formula

The molecular formula of a compound shows how many atoms of each type are in a molecule. The number of each atom is written as a subscript after the atomic symbol. The molecular formula of 2-methylpropane is:

C ${}_4$ H ${}_{10}$

Structural formula

The structural formula of an organic compound shows every bond between every atom in the molecule. Each bond is represented by a line. The structural formula of 2-methylpropane is shown in [link] .

Condensed structural formula

When a compound is represented using its condensed structural formula, each carbon atom and the hydrogen atoms that are bonded directly to it are listed as a molecular formula, followed by a similar molecular formula for the neighbouring carbon atom. Branched groups are shown in brackets after the carbon atom to which they are bonded. The condensed structural formula below shows that in 2-methylpropane, there is a branched chain attached to the second carbon atom of the main chain. You can check this by looking at the structural formula in [link] .

CH ${}_3$ CH(CH ${}_3$ )CH ${}_3$

Representing organic compounds

1. For each of the following organic compounds, give the condensed structural formula and the molecular formula .

   1. 

   2. 

2. For each of the following, give the structural formula and the molecular formula .
   1. CH ${}_3$ CH ${}_2$ CH ${}_3$
   2. CH ${}_3$ CH ${}_2$ CH(CH ${}_3$ )CH ${}_3$
   3. C ${}_2$ H ${}_6$
3. Give two possible structural formulae for the compound with a molecular formula of C ${}_4$ H ${}_{10}$ .

Isomerism in organic compounds

It is possible for two organic compounds to have the same molecular formula but a different structural formula . Look for example at the two organic compounds that are shown in [link] .

If you were to count the number of carbon and hydrogen atoms in each compound, you would find that they are the same. They both have the same molecular formula (C ${}_4$ H ${}_{10}$ ), but their structure is different and so are their properties. Such compounds are called isomers .

Isomer

In chemistry, isomers are molecules with the same molecular formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently.

Isomers

Match the organic compound in Column A with its isomer Column B:

Column A	Column B
CH ${}_3$ CH(CH ${}_3$ )OH	CH ${}_3$ CH(CH ${}_3$ )CH ${}_3$
	C ${}_3$ H ${}_7$ OH

Functional groups

All organic compounds have a particular bond or group of atoms which we call its functional group . This group is important in determining how a compound will react.

Functional group

In organic chemistry, a functional group is a specific group 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.

In one group of organic compounds called the hydrocarbons , the single, double and triple bonds of the alkanes, alkenes and alkynes are examples of functional groups. In another group, the alcohols, an oxygen and a hydrogen atom are bonded to each other to form the functional group for those compounds (in other words an alcohol has an OH in it). All alcohols will contain an oxygen and a hydrogen atom bonded together in some part of the molecule.

[link] summarises some of the common functional groups. We will look at these in more detail later in this chapter.

Name of group	Functional group	Example	Diagram
Alk ane		Ethane
Alk ene		Ethene
Alk yne		Ethyne (acetylene)
Halo-alkane		Chloroethane
Alcoh ol / alkan ol		Ethanol
Carboxylic acid		ethanoic acid
Amine		Glycine