Functional groups and homologous series: Question bank

The parent gave you the machinery: a functional group is the reactive atom-cluster; a homologous series is the same group carried on chains that grow by one $CH_{2}$ (mass $14$ ) at a time. Everything below just stress-tests those two ideas at their edges.

Figure — Functional groups and homologous series

Before the traps, keep two pictures in your head. First, the three oxygen groups people always confuse — the ether (oxygen wedged between two carbons, $C - O - C$ , with no $O - H$ bond), the aldehyde (a $C = O$ at the chain end, so it also carries one H, written $- C H O$ ), and the ketone. The ketone shorthand $> C = O$ deserves a word: the " $>$ " is not a maths symbol here — it is a little picture of two bonds coming off the same carbon on the left, and the " $= O$ " is the double bond to oxygen on the right. So $> C = O$ literally means a carbon holding two other carbons plus a double-bonded oxygen. Study the highlighted atoms in the figure below.

True or false — justify

Every alkane belongs to a homologous series.

True — alkanes share the general formula

C_{n} H_{2 n + 2}

and consecutive members differ by exactly one

CH_{2}

, which is the definition of a series.

All members of one homologous series have the same molecular formula.

False — they share the same general formula (a pattern in

n

), but each member has a different actual formula because

n

changes; only the type of formula is shared.

Two compounds with the same molecular formula must react the same way with sodium.

False — ethanol (

C_{2} H_{5} O H

) and dimethyl ether (

C H_{3} O C H_{3}

) are both

C_{2} H_{6} O

, yet only ethanol's

- O H

releases

H_{2}

with Na; reactivity follows the functional group, not the formula (see Isomerism).

Adding more

CH_{2}

units always makes a molecule chemically more reactive.

False — extra

CH_{2}

is inert skeleton; it changes physical properties (boiling point, solubility) and only tunes the group slightly via inductive effects, but the reaction type stays the same.

Boiling point always rises as you go up a homologous series.

True as a trend — longer chains have more electrons and larger surface, so stronger London forces raise boiling point; the rise is gradual ("gradation"), which is why we call it a series (see Intermolecular Forces).

C_{n} H_{2 n}

can only be an alkene.

False —

C_{n} H_{2 n}

is also the general formula of a cycloalkane (a ring costs the same 2 H as one double bond), so the formula alone doesn't pin the functional group.

An aldehyde and a ketone have the same functional group.

False — both contain

C = O

, but the aldehyde (

- C H O

) has that carbonyl at a chain end with an H attached, while the ketone (

> C = O

) has it between two carbons; the environment differs, so they are distinct groups with distinct suffixes (

- a l

- o n e

Members of a homologous series are always structural isomers of one another.

False — isomers share the same molecular formula; series members differ by

CH_{2}

, so they have different formulas and are never isomers of each other.

Spot the error

C H_{3} O C H_{3}

has an

- O H

group, so it is an alcohol."

Error — there is no

O - H

bond here; the oxygen sits between two carbons (

C - O - C

), making it an ether (defined and drawn above). Alcohols need oxygen bonded to both a carbon and a hydrogen.

"Ethene (

C_{2} H_{4}

) and ethane (

C_{2} H_{6}

) are consecutive members of one homologous series."

Error — they differ by

H_{2}

, not

CH_{2}

, and they carry different functional groups (

C = C

vs none). Consecutive members must differ by exactly one

CH_{2}

and share the group.

"The suffix for a carboxylic acid is

- a l

Error —

- a l

is the aldehyde suffix; carboxylic acids (

- C O O H

) take

- o i c a c i d

. Mixing these swaps a mild aldehyde for a proton-donating acid (see IUPAC Nomenclature).

C_{3} H_{8}

and

C_{5} H_{12}

are consecutive alkanes because both fit

C_{n} H_{2 n + 2}

Error — they are in the same series but not consecutive;

C_{4} H_{10}

sits between them, so they differ by two

CH_{2}

units, not one.

"Since the carbon chain is inert, it plays no role at all in a molecule's behaviour."

Error — "chemically lazy" is not "irrelevant"; the chain controls physical properties and, through the inductive effect, slightly modulates the group's reactivity (see Inductive and Mesomeric Effects).

"Methanol has no homologues because it's the smallest alcohol."

Error — being smallest just means it has no member below it; its homologues (

C_{2} H_{5} O H

C_{3} H_{7} O H

, …) all lie above it, so it very much belongs to the alcohol series.

Why questions

Why does a small functional group, not the big chain, control reactivity?

Because

C - C

and

C - H

bonds are strong and nonpolar (no electron imbalance), while the group carries a

π

-bond, lone pair, or polar bond — the electron-rich or electron-poor site where reactions actually start.

Why do consecutive members differ by a fixed mass of

14

Because inserting one

CH_{2}

adds one carbon (mass

12

) and two hydrogens (mass

2 \times 1

), giving

12 + 2 = 14

every single step.

Why do all members of a series show the same chemical reactions?

Because the reactive site — the functional group — is identical across the whole family; only the inert skeleton changes length, so the chemistry it drives is unchanged.

Why do boiling points rise gradually rather than jumping randomly?

Because each added

CH_{2}

adds a small, roughly constant amount of polarizable surface and electrons, so London forces increase in even steps — a smooth gradation is a hallmark of a homologous series (see Intermolecular Forces).

Why can we predict a brand-new molecule's chemistry just by spotting its group?

Because the group is transferable —

- C O O H

is acidic and

- O H

H-bonds and reacts with Na wherever they appear, so recognising the group imports all its known behaviour at once.

Why does the alcohol → aldehyde → carboxylic acid ladder change chemistry at each rung?

Because each oxidation step alters the functional group itself (adds oxygen / removes hydrogen), and a new group means new characteristic reactions, not just a longer chain (see Oxidation of Alcohols).

Edge cases

C H_{4}

(methane) a valid "first member" even though it has no

CH_{2}

to remove below it?

Yes — a series only requires each member to differ from the next by

CH_{2}

; the first member has nothing below it, which is fine and expected.

Can one molecule contain two different functional groups at once?

Yes — e.g. an amino acid carries both

- N H_{2}

and

- C O O H

; such a molecule then shows the characteristic reactions of both groups, so you classify it by whichever the question probes.

Does a straight-chain and a branched isomer with the same group belong to the same series?

They belong to the same type (same general formula and group), but they are structural isomers of each other, not consecutive members — series membership is about the

CH_{2}

ladder, not branching (see Isomerism).

What happens to the boiling-point trend for a series member with heavy branching?

Branching lowers boiling point relative to the straight-chain isomer (more compact shape, less surface contact, weaker London forces), so the smooth trend holds only when you compare like shapes.

Is a cycloalkane a "degenerate" case that breaks the

C_{n} H_{2 n + 2}

rule?

Yes — closing a ring removes 2 H, giving

C_{n} H_{2 n}

, the same formula as an alkene; the ring itself is not a classical functional group, so identity must be confirmed by structure, not formula alone.

Recall One-line survival summary

Group decides what reaction; chain length decides how sticky (boiling point). Same formula ≠ same chemistry; same series ≠ same formula; $+ 14$ up the ladder every step.

Connections

Parent topic (Hinglish)
Isomerism — same formula, different group (the sodium trap)
IUPAC Nomenclature — suffix traps
Intermolecular Forces — the boiling-point gradation
Inductive and Mesomeric Effects — how the chain does matter a little
Oxidation of Alcohols — the group-changing ladder
Hydrocarbons — the inert skeleton itself