Isomers
Definition: Isomers are special kinds of chemical compounds that contain the same types and number of atoms, meaning they share the same molecular formula. However, what makes them different is the way those atoms are connected or arranged. This different arrangement changes their shape and affects how they behave in terms of physical properties (like boiling point or smell) and chemical properties (like how they react with other substances).
Structure difference: Isomerism exists because the atoms in a molecule can be joined together in more than one way. Even though two molecules are made of the same atoms, they can be arranged or linked differently, which leads to different chemical structures.
Types of Isomerism
Chain isomers: Chain isomers are molecules that have the same number and type of atoms, but their carbon atoms are connected in different ways. Some have long straight chains of carbon atoms, while others have branches, like the branches of a tree.
Chain isomer examples: For instance, butane (C₄H₁₀) can look like a straight chain called n-butane, or it can have a branch and be called 2-methylpropane. Another example is pentane (C₅H₁₂), which can form three different structures, each with a unique shape of carbon connections.
Isomer count trend: As the number of carbon atoms in a compound increases, the number of ways those carbon atoms can be connected also increases. So, the more carbon atoms there are, the more chain isomers can exist.
Alternate name: Sometimes, chain isomers are also called skeletal isomers because the carbon skeleton (or backbone) of the molecule changes from one isomer to another.
Position isomers: Position isomers have the same carbon chain and the same functional group (the reactive part of the molecule), but the functional group is located on a different carbon atom in the chain. This small change in position can lead to different physical and chemical properties.
Position isomer examples: For example, in alcohols, propan-1-ol and propan-2-ol both have three carbon atoms and an -OH group, but the -OH is attached to a different carbon in each one. Another example is but-1-ene and but-2-ene, where the double bond is between different carbon atoms in the chain.
Applicability: Position isomerism is commonly found in molecules with functional groups such as the double bond (C=C) in alkenes, the triple bond (C≡C) in alkynes, and the hydroxyl group (-OH) in alcohols.
Functional group isomers: Functional group isomers have the same atoms in the same number, but they belong to different families of compounds because they have different functional groups. This means they act differently in chemical reactions.
Examples: One good example is ethanoic acid and methyl methanoate. They both have the formula C₂H₄O₂, but ethanoic acid has a carboxylic acid group (-COOH), while methyl methanoate has an ester group (-COO-). Another example is butan-1-ol (an alcohol with an -OH group) and methyl propyl ether (an ether with an -O- group). Even though they contain the same atoms, their different functional groups make them behave differently.
IUPAC Nomenclature
Definition: The IUPAC (International Union of Pure and Applied Chemistry) nomenclature is a system used by scientists to name organic compounds in a clear and standardised way. It helps avoid confusion and ensures that the name shows the structure of the compound.
General Rules
Longest chain: First, find the longest continuous chain of carbon atoms in the molecule. This becomes the main part of the name (the parent chain).
Carbon numbering: Number the carbon atoms in the main chain in such a way that the attached groups (branches or functional groups) get the smallest possible numbers.
Naming substituents: Identify all side groups or functional groups that are attached to the main chain. These are called substituents, and each one must be named.
Prefix and order: Add the names of substituents as prefixes to the main name. Put them in alphabetical order and include position numbers. If the same group appears more than once, use prefixes like di-, tri-, etc.
Cyclic compounds: If the compound forms a ring (a cycle), number the carbon atoms in the ring to give the lowest possible numbers to substituents.
Priority order: If there are several functional groups, use the one with the highest priority to name the compound and give it the lowest number.
Naming format: A complete name follows this format: position number – substituent – parent chain name. For example: 2-methylpropane.
Specific Nomenclature for Homologous Series
Alkanes
Structure: Alkanes are organic compounds made of carbon and hydrogen only. They are saturated hydrocarbons because they have only single bonds between carbon atoms (C–C).
Formula: The general formula of alkanes is CₙH₂ₙ₊₂.
Naming: Use a prefix that tells how many carbon atoms there are, followed by the suffix -ane.
Examples: Some examples of alkanes are methane (1 carbon), ethane (2 carbons), propane (3), butane (4), pentane (5), and hexane (6).
Branched naming: For branched alkanes, choose the longest chain for the main name and name the side chains as substituents, like in 2-methylbutane.
Alkenes
Structure: Alkenes are a type of hydrocarbon, which means they are made only of carbon and hydrogen atoms. What makes them special is that they are unsaturated, meaning they do not have as many hydrogen atoms as possible. This is because they contain at least one double bond (C=C) between two carbon atoms. This double bond is very important because it makes the molecule more reactive than an alkane, which only has single bonds.
Formula: The general formula for alkenes is CₙH₂ₙ, where “n” stands for the number of carbon atoms. This formula tells us how many hydrogen atoms are in the molecule depending on how many carbon atoms it has.
Naming: When naming an alkene, we start with the name of the corresponding alkane and then change the ending from -ane to -ene to show that a double bond is present. We also number the carbon atoms in the chain so that the double bond gets the lowest number possible. This helps everyone know exactly where the double bond is located.
Examples: Some common examples of alkenes are:
- Ethene – the simplest alkene with 2 carbon atoms and one double bond.
- Propene – with 3 carbon atoms.
- But-1-ene – with 4 carbon atoms and the double bond starting on the first carbon.
Branching rule: When alkenes have side branches (called substituents), we must make sure to number the double bond position first before numbering the side groups. This helps to keep the name clear and consistent.
Alkynes
Structure: Alkynes are also unsaturated hydrocarbons, but instead of a double bond like alkenes, they have at least one triple bond (C≡C) between carbon atoms. This triple bond makes alkynes even more reactive than alkenes.
Formula: The general formula for alkynes is CₙH₂ₙ₋₂. This means that alkynes have even fewer hydrogen atoms than alkenes for the same number of carbon atoms.
Naming: Just like with alkenes, we take the name of the matching alkane and change the ending from -ane to -yne to show there is a triple bond. We also number the carbon atoms so that the triple bond gets the smallest possible number.
Examples: Here are some examples of alkynes:
- Ethyne – 2 carbon atoms with a triple bond.
- Propyne – 3 carbon atoms.
- But-1-yne – 4 carbon atoms with the triple bond at carbon number 1.
Alcohols
Structure: Alcohols are organic compounds that include a group called the hydroxyl group (-OH). This group is made up of one oxygen atom and one hydrogen atom and is attached to a carbon atom. The presence of this group changes how the compound behaves.
Formula: The general formula for alcohols is CₙH₂ₙ₊₁OH, which means they follow the same pattern as alkanes but with one hydrogen atom replaced by the -OH group.
Naming: We name alcohols by using the alkane name as a base and replacing the ending -e with -ol. We also add a number in the name to show which carbon the -OH group is attached to. This is important because different positions of the -OH group can make different compounds.
Examples:
- Methanol – 1 carbon with the -OH on the only carbon.
- Ethanol – 2 carbons.
- Propan-1-ol and Propan-2-ol – both have 3 carbons, but the -OH is on a different carbon atom.
Priority: When numbering the carbon chain in alcohols, we always start at the end that gives the -OH group the lowest possible number because it has the highest naming priority.
Carboxylic Acids
Structure: Carboxylic acids are a type of organic compound that contains a special group called the carboxyl group (-COOH). This group is made of one carbon atom double-bonded to an oxygen atom and also bonded to an -OH group. It always appears at one end of the carbon chain.
Formula: The general formula is CₙH₂ₙ₊₁COOH. This shows a carbon chain with a carboxyl group at the end.
Naming: We take the name of the alkane and replace the -e with -oic acid. The carbon in the -COOH group is always carbon number 1, so we start counting from that carbon.
Examples:
- Methanoic acid – 1 carbon.
- Ethanoic acid – 2 carbons.
- Propanoic acid – 3 carbons.
Substituent position: If there are any other groups attached to the chain, we number them starting from the -COOH carbon as position 1.
Esters
Structure: Esters are compounds formed when a carboxylic acid reacts with an alcohol. Their structure includes a -COOC- group, which combines parts from both the acid and the alcohol.
Formula: The general formula is CₙH₂ₙ₊₁COOCₕH₂ₕ₊₁, where one part comes from the acid (left side) and the other part comes from the alcohol (right side).
Naming: When naming esters, we start with the name of the group that came from the alcohol. This part is written first and ends in -yl (like methyl or ethyl). Then, we name the part that came from the acid, which ends in -oate.
Examples:
- Methyl methanoate – made from methanol and methanoic acid.
- Ethyl ethanoate – made from ethanol and ethanoic acid.
IUPAC alternative: Esters can also be named as alkyl alkanoates, which means naming them based on the alkyl group (from the alcohol) and the alkanoate part (from the acid).