Basic concepts of organic chemistry

In a nutshell

Organic compounds can be represented in different ways. All organic compounds belong to a homologous series/group and have a systematic name following the IUPAC rules.

Representing organic compounds

Organic compounds are classed as aliphatic or aromatic compounds, depending on their carbon skeleton. Compounds with a non-aromatic ring are classed as alicyclic. There are many ways to represent organic compounds.

formula	definition	example (using pentanoic acid)
General formula	The algebraic expression for a homologous group of compounds.	$C_nH_{2n+1}COOH$
Empirical formula	The simplest whole number ratio of atoms of each element present in a compound. This may be the same as the molecular formula.	$C_5H_{10}O_{2} \ or \ C_4H_9COOH$
Molecular formula	The actual number of atoms of each element present in a compound.	$C_5H_{10}O_{2} \ or \ C_4H_9COOH$
Structural formula	Shows the composition and arrangement of atoms in a compound, without displaying the bonds.	$CH_3CH_2CH_2CH_2COOH \\ or \\ CH_3(CH_2)_3COOH$
Displayed formula	Shows the composition and arrangement of atoms, including the bonds, in a compound.
Skeletal formula	Shows the composition and arrangement of atoms using a carbon skeleton. The carbons and hydrogens are not displayed.

Homologous series

A homologous series is a group of organic compounds that have the same functional group and general formula and have similar properties. Members in a homologous series differ by $-CH_2-$ .

Here are a list of homologous series you need to know.

homologous series	Prefix/suffix	example
Alkanes	-ane	ethane
Branched alkanes (alkyl groups)	alkyl-/-yl	$2-$ methylbutane
Cycloalkanes	cyclo-...ane	cyclopentane
Alkenes	-ene	ethene
Halogenoalkanes (haloalkanes; alkyl halides)	chloro-; bromo-; iodo-	chloromethane
Alcohols	hydroxy-/-ol	e.g.1/ methanol e.g.2/ $2-$ hydroxybutanoic acid
Aldehydes	-al	propanal
Ketones	-one	propanone
Carboxylic acids	-oic acid	pentanoic acid

Nomenclature

Nomenclature is the way an organic compound is named. The IUPAC system is used internationally for chemistry and each organic compound can be given a systematic name using this system.

Before you can begin naming the organic compound, you need know how to name the parent chain.

no. of carbons	$1$	$2$	$3$	$4$	$5$	$6$	$7$	$8$	$9$	$10$
name of parent chain	meth-	eth-	prop-	but-	pent-	hex-	hept-	oct-	non-	dec-

procedure

1.	Count the number carbon atoms in the longest continuous chain. This will be the parent chain.
2.	Identify the homologous series by determining the main functional group. This will allow you to correctly use the most appropriate prefix or suffix.
3.	Number the longest chain so the main functional group has the lowest possible carbon number.
4.	Identify the rest of the substituents and the carbon number they are on, in the parent chain.
5.	If the same substituent appears more than once they can be indicated by the prefix, di-, tri-, tetra-, etc.
6.	If there are two or more different substituents, list them in alphabetical order.
7.	Place a hyphen between numbers and letters and a comma between numbers.

Example

Chemistry; Organic chemistry I; KS5 Year 12; Basic concepts of organic chemistry

1. The longest chain is four carbons long so the parent chain is but-

2-3. The main functional group is the carboxylic acid which is on carbon one.

4. There is an alcohol on carbon two but the suffix is taken by the carboxyl so you will need to use the prefix hydroxy-. There is also a methyl on carbon two.

5. There are no duplicates of any substituents.

6. The alcohol group will be named before the methyl because the letter h (hydroxy-) comes before m in the alphabet.

The name of the molecule is $\underline{2-hydroxy-2-methylbutanoic \ acid}$ .