Formulas, Functional Groups and the Naming of Organic Compounds

Full Notes

Functional Groups and Their Importance (RECAP)

A functional group is an atom or group of atoms within a molecule that gives the molecule its reactivity and determines its physical and chemical properties.

Two compounds with the same carbon skeleton but different functional groups behave very differently.

You need to know the following functional groups and be able to recognise them.

Types of Organic Formulas (RECAP)

Organic molecules can be written using different formula types and you need to be comfortable using and changing between the following:

Molecular Formula – Shows the actual number of atoms in a molecule.

ExampleHexane → C₆H₁₄

Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.

Example Hexane (C₆H₁₄) → C₃H₇

General Formula – Represents a homologous series using an algebraic expression.

Example Alkanes → C_nH_2n+2

Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.

Example Hexane → CH₃CH₂CH₂CH₂CH₂CH₃

Displayed Formula – Shows all bonds and atoms.

Example Hexane

Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.

Example Hexane

Matt’s exam tip

Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.

Systematic Nomenclature: Aliphatic Compounds

Aliphatic compounds are organic compounds that don’t have an aromatic ring in them. At this level, that means compounds that don’t contain a benzene ring.

We name aliphatic compounds follows IUPAC naming rules (as per in year one study):

• Prefix (number of carbon atoms): meth-, eth-, prop-, but-, pent-, hex-
• Infix (type of bonding): -an- (single), -en- (double), -yn- (triple)
• Suffix (functional group): -ol (alcohol), -al (aldehyde), -oic acid (carboxylic acid), -amine, -one, etc.

Advanced (year two) examples:

Example Esters

Name based on the carboxyl group and alkyl group. Following the form Alkyl-carboxylate

Example Amides

Name based on the amide and alkyl group(s) bonded to the nitrogen. Following the form N-alkyl amide.

Matt’s exam tip

Don’t forget to use N when naming amides. The N tells us the alkyl group is bonded to the nitrogen in the amide. For example, N-ethylpropanamide means an ethyl group is bonded to the N in the propanamide.


Just using ethylpropanamide would mean an ethyl group is bonded to the carbon chain in propanamide.

Systematic Nomenclature: Aromatic Compounds

Aromatic compounds are based on the benzene ring. Substituents on the ring are numbered to give the lowest possible numbers.

Examples:

• 2-nitrophenol: A nitro group at position 2 relative to the hydroxyl group.
• 3-nitrobenzoic acid: Nitro group on carbon 3 of the benzene ring with a carboxylic acid group at position 1.
• 2,4,6-tribromophenol: Bromine atoms at positions 2, 4, and 6; hydroxyl group at position 1.

Summary

• Functional groups determine the reactivity and properties of organic compounds.
• Different ways to represent molecules include general, structural, displayed, and skeletal formulas.
• Systematic naming follows IUPAC rules with prefixes, infixes, and suffixes.
• Esters and amides have special naming conventions — N notation is important for amides.
• Aromatic compounds are named by numbering substituents on the benzene ring to give the lowest possible positions.