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# Key concepts in organic chemistry

## Organic chemistry definition

Organic chemistry studies compounds that contain carbon.

There is a great variety of different carbon-based compounds. There are several reasons for this:

• Carbon has four valence electrons (electrons on the outer shell that form bonds). As a result, a carbon atom can form four bonds with other atoms.
• Carbon atoms can be combined in long chains that are chemically stable.
• Carbon can engage in single, double and even triple bonds with other carbon atoms.

Organic chemicals have a wide variety of uses.

Common fuels, fertilisers, medicines and plastics are organic chemicals.

Apart from carbon, organic chemicals usually contain a number of other elements, such as hydrogen, oxygen, nitrogen and chlorine.

Many objects in daily life consist of organic materials. The natural environment is also full of organic compounds.

## Functional groups

A functional group is a group of atoms that occurs in different molecules and causes specific reactions and behaviours.

The carboxyl group ($\ce{-COOH}$, highlighted in red) is a functional group. Compounds containing the group act as weak acids.

To save time and space, chemists shorten an unimportant section of a molecule to a simple "R" (for remainder). This R-group is not a single atom.

The R-group could stand for any combination of atoms. Often it represents a hydrocarbon chain.

When the focus is on the alcohol group of propanol, one can abbreviate the carbon chain as an R-group.

## Homologous series

In organic chemistry, a homologous series is a group of similar compounds. Compounds in a homologous series have the same functional group (for example an alcohol group).

They differ only in the length of their carbon chains.

The first four members in a simple straight-chained alcohol (containing $\ce{\bond{-}OH}$) homologous series.

A general formula gives the molecular formula of members of a homologous series as a function of the number of carbon atoms.

The general formula for alkanes, a homologous series of organic molecules, is $\text{C}_\text{n}\text{H}_{2\text{n}+2}$ where $n$ is the number of carbon atoms.

Homologous series General formula
Alkane $\text{C}_\text{n}\text{H}_{2\text{n}+2}$
Alkene $\text{C}_\text{n}\text{H}_{2\text{n}}$
Alcohol $\text{C}_\text{n}\text{H}_{2\text{n}+1}\text{O}\text{H}$
Carboxylic acid $\text{C}_n\text{H}_{2\text{n}+1}\text{C}\text{O}\text{O}\text{H}$

The molecules in a homologous series have similar chemical properties. In other words, they react in similar ways.

The physical properties change in line with the length of the carbon chain. Molecules with longer carbon chains generally have higher melting and boiling points than those with shorter chains.

## Isomers

Two compounds are isomers if they have the same atomic composition but different structures.

Butane has the chemical formula $\ce{C4H10}$. It has one linear isomer and one branched isomer.

The linear isomer of $\ce{C4H10}$ (left) and the branched isomer (right).

The different structures make isomers different compounds. They may have completely different chemical and physical properties.

The following molecules are all isomers with the formula $\ce{C3H8O}$. Two of the isomers are alcohols while one is not.

Isomers do not have to have the same functional groups.

The first two isomers boil at over $80^{\circ}\text{C}$. The last isomer boils at $7^{\circ}\text{C}$.