Homologous series, functional group and isomerism

Organic chemistry has millions of compounds, but they are sorted into families called homologous series. A homologous series is a family of organic compounds that share five key features:

1. Same general formula — every member fits one formula (e.g. all alkanes fit CₙH₂ₙ₊₂).
2. Same functional group — the same reactive part, so they react in the same way.
3. Similar chemical properties — because they have the same functional group.
4. A gradual trend in physical properties — melting point, boiling point and viscosity change smoothly as the chain gets longer.
5. Adjacent members differ by CH₂ — each member has one more carbon and two more hydrogens than the one before it.

The prefix 'homo-' means 'same' — same chemistry, just different chain lengths.

Two homologous series you must know for Double Award:

Alkanes — general formula CₙH₂ₙ₊₂.

• Examples: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
• They contain only single C–C bonds (no functional group).

Alkenes — general formula CₙH₂ₙ (n ≥ 2).

• Examples: ethene C₂H₄, propene C₃H₆, butene C₄H₈.
• They contain one C=C double bond — this is their functional group.

Why the trend in physical properties is smooth. Each time you add a CH₂ group the molecule gets a bit heavier and bigger. Larger molecules have more electrons and a larger surface area, so the forces of attraction between molecules are stronger. This means more energy is needed to separate them — so boiling point rises steadily as the chain gets longer.

A functional group is the reactive part of an organic molecule. It is the atom or group of atoms that gives a homologous series its characteristic chemical reactions.

Because every member of a series has the same functional group, they all react in the same way — only the chain length is different.

Examples of functional groups:

Series	Functional group	Why it matters
Alkenes	C=C (double bond)	reactive site for addition reactions (e.g. decolourises bromine water)
Alcohols	−OH (hydroxyl)	makes alcohols burn, react with sodium, etc.
Carboxylic acids	−COOH (carboxyl)	makes them weakly acidic

The key idea. Two compounds in the same series react in the same way because they share a functional group. For example, all alkenes have a C=C bond, so all alkenes decolourise bromine water — ethene, propene and butene all behave the same way.

Alkanes have no functional group (just strong single bonds), which is why they are fairly unreactive apart from burning.

IUPAC names are built from a prefix (how many carbon atoms) plus a suffix (which functional group).

Chain-length prefixes (memorise the first four):

Number of C atoms	Prefix
1	meth-
2	eth-
3	prop-
4	but-

Functional-group suffixes:

Series	Functional group	Suffix
Alkane	(none)	-ane
Alkene	C=C	-ene

The first four alkanes (CₙH₂ₙ₊₂):

Name	Formula
methane	CH₄
ethane	C₂H₆
propane	C₃H₈
butane	C₄H₁₀

The first members of the alkenes (CₙH₂ₙ):

Name	Formula
ethene	C₂H₄
propene	C₃H₆
butene	C₄H₈

Note: there is no 'methene' — an alkene needs at least two carbons to have a C=C double bond, so the series starts at ethene (C₂H₄).

How to name a compound:

1. Count the carbon atoms in the longest chain → choose the prefix (meth-, eth-, prop-, but-).
2. Identify the type of compound → choose the suffix (-ane for an alkane with single bonds only; -ene for an alkene with a C=C).

Worked examples:

• 3 carbons, single bonds only → prop + ane → propane (C₃H₈).
• 2 carbons, with a C=C → eth + ene → ethene (C₂H₄).
• 4 carbons, single bonds only → but + ane → butane (C₄H₁₀).

Structural isomers are compounds with the same molecular formula but a different structural formula — the same atoms are joined together in a different way.

They have:

• the same number of each type of atom (same molecular formula, e.g. C₄H₁₀);
• a different arrangement of those atoms (straight chain vs branched);
• slightly different physical properties (such as boiling point).

Why isomers exist. Once a molecule has four or more carbon atoms, there is more than one way to join them. The carbons can form a straight chain or a branched chain. As the chain gets longer, the number of possible isomers grows quickly:

Molecular formula	Number of structural isomers
CH₄ (methane)	1
C₂H₆ (ethane)	1
C₃H₈ (propane)	1
C₄H₁₀	2
C₅H₁₂	3
C₆H₁₄	5

The two isomers of C₄H₁₀.

Isomer 1 — butane (straight chain).

  H   H   H   H
  |   |   |   |
H-C - C - C - C-H
  |   |   |   |
  H   H   H   H

Condensed formula: CH₃CH₂CH₂CH₃. A straight chain of four carbons.

Isomer 2 — methylpropane (branched).

        H
        |
        C-H (this is a CH₃ branch)
        |
  H   H | H
  |   | | |
H-C - C - C-H
  |   |   |
  H   H   H

Condensed formula: CH(CH₃)₃ — a three-carbon chain with a methyl (CH₃) branch on the middle carbon.

Both have the molecular formula C₄H₁₀, but different structures — so they are structural isomers.

How to draw isomers without making mistakes:

1. First draw the straight chain (all carbons in a row).
2. Then shorten the main chain by one carbon and attach that carbon as a branch somewhere along the remaining chain.
3. Check it is genuinely different — count the carbons in the longest chain and see what is attached where. A structure that is the same chain just 'flipped over' is not a new isomer.

Effect of branching on boiling point. Straight-chain isomers have a slightly higher boiling point than branched ones, because straight chains pack closely together (more contact between molecules → stronger forces). Branched molecules are more compact, touch less, and so boil at a lower temperature.