Functional Groups and Homologous Series

Beyond Hydrocarbons — Functional Groups

Until now, we studied only hydrocarbons (C and H).

But carbon can bond with many other atoms: O, N, S, halogens (Cl, Br, I).

This vastly expands the variety of carbon compounds.

What is a Functional Group?

Functional Group = an atom or group of atoms attached to a hydrocarbon chain that determines the chemical properties of the compound.

The 'function' of these groups is to give specific chemical behaviour.

Why Functional Groups Matter

The functional group:

• Determines reactions a compound undergoes.
• Decides physical properties (b.p., m.p., solubility).
• Gives the compound its family (alcohols, acids, etc.).
• Often gives the compound its name.

Two compounds with same C-H skeleton but different functional groups → very different chemistry.

Major Functional Groups

The most important functional groups in Class 10 organic chemistry:

1. Halogen Group (—X)

X = F, Cl, Br, I (any halogen).

Examples:

• $CH_3Cl$ (chloromethane / methyl chloride)
• $CH_3Br$ (bromomethane)
• $CHCl_3$ (chloroform — old anaesthetic)
• $CCl_4$ (carbon tetrachloride — solvent)

Properties:

• Generally toxic.
• Used as solvents, refrigerants (CFCs).
• Some used as drugs.

2. Alcohol Group (—OH, hydroxyl)

The —OH group attached to a carbon.

Examples:

• $CH_3OH$ (methanol — wood alcohol)
• $C_2H_5OH$ (ethanol — drinking alcohol)
• $C_3H_7OH$ (propanol)

Properties:

• Polar (due to —OH).
• Often soluble in water.
• Higher b.p. than corresponding hydrocarbon.
• Used as solvents, fuels, drinks.

Section 8 will discuss ethanol in detail.

3. Aldehyde Group (—CHO)

The —CHO group: a C=O double bond + H.

Structure: R-C(=O)-H (R = alkyl group)

Examples:

• HCHO (formaldehyde / methanal)
• $CH_3CHO$ (acetaldehyde / ethanal)

Properties:

• Sharp pungent smell.
• Used in preserving biological specimens (formaldehyde in formalin).
• Important industrial chemicals.

4. Ketone Group (>C=O)

A C=O between two C atoms (no H on the C=O carbon).

Structure: R-C(=O)-R' (R, R' = alkyl groups)

Examples:

• $(CH_3)_2C=O$ (acetone / propanone)
• Used as solvent (nail polish remover!)

Properties:

• Polar.
• Pleasant smell sometimes.
• Used as solvents.

5. Carboxylic Acid Group (—COOH)

The —COOH group: C=O + O-H combined.

Examples:

• HCOOH (formic acid — in ant stings)
• $CH_3COOH$ (acetic acid — vinegar)
• $C_3H_7COOH$ (butyric acid — rancid butter smell)

Properties:

• Acidic (gives H⁺ in water).
• Sour taste.
• Soluble in water (small ones).

Section 8 will discuss ethanoic acid in detail.

Naming with Functional Groups

The compound is named based on:

• Number of C atoms in main chain.
• Functional group present.
• Position of the group.

(More on IUPAC naming in Section 6.)

Summary Table — Common Functional Groups

Memorise this table — fundamental for organic chemistry!

Functional Group	Symbol	Class	Suffix (IUPAC)	Example
Halogen	-F, -Cl, -Br, -I	Haloalkane	-halo (prefix)	CH₃Cl
Alcohol	-OH	Alcohols	-ol	CH₃OH
Aldehyde	-CHO	Aldehydes	-al	HCHO
Ketone	>C=O	Ketones	-one	CH₃COCH₃
Carboxylic acid	-COOH	Carboxylic acids	-oic acid	HCOOH
Double bond	C=C	Alkenes	-ene	CH₂=CH₂
Triple bond	C≡C	Alkynes	-yne	HC≡CH

Also (advanced — for reference):

Functional Group	Symbol	Class	Example
Ether	-O-	Ethers	CH₃-O-CH₃
Ester	-COO-	Esters	CH₃COOC₂H₅
Amine	-NH₂	Amines	CH₃NH₂
Amide	-CONH₂	Amides	CH₃CONH₂

Drawing the Structure

Each functional group has a specific bond arrangement:

Alcohol (—OH):

R — O — H

OH directly on C.

Aldehyde (—CHO):

       O
       ||
R — C — H

C=O at the end of chain, H attached to that C.

Ketone (>C=O):

       O
       ||
R — C — R'

C=O in the middle of chain.

Carboxylic acid (—COOH):

       O
       ||
R — C — O — H

C=O + OH together.

How Functional Groups Affect Properties

Alcohols:

• Polar (—OH).
• Form hydrogen bonds.
• Higher b.p. than corresponding alkane.
• Soluble in water (small ones).

Carboxylic Acids:

• Most polar (—COOH).
• Strong hydrogen bonding.
• Acidic (gives H⁺).
• Highly soluble in water (small ones).

Ketones, Aldehydes:

• Polar but no H on the C=O.
• Less hydrogen bonding than alcohols/acids.
• Some solubility in water.

Halogens (—X):

• Slightly polar.
• Higher density than water.
• Generally toxic.

Comparing Boiling Points (Same Carbon Count)

Compound	B.P.
Ethane (C₂H₆)	-89°C
Ethanol (C₂H₅OH)	78°C
Acetic acid (CH₃COOH)	118°C

Even though all have 2 C atoms:

• Ethane: no polar group → very low b.p.
• Ethanol: —OH polar → much higher b.p.
• Acetic acid: —COOH most polar → highest b.p.

Functional groups dramatically change properties!

Homologous Series

A 'family' of organic compounds with the same functional group but different number of C atoms.

Definition

Homologous Series: A series of compounds with:

• Same functional group.
• Same general formula.
• Successive members differ by CH₂ (or 14 amu).
• Similar chemical properties.
• Gradually changing physical properties.

'Homologous' = similar (Greek: 'homo' = same). Members of a homologous series are called 'homologues'.

Why Homologous Series?

Carbon's catenation allows us to:

• Add CH₂ groups one by one to make longer chains.
• Each new compound = previous + CH₂.
• Same functional group → similar chemistry.
• Different chain length → different physical properties.

Example 1: Alkanes

General formula: $C_nH_{2n+2}$.

n	Compound	Formula	B.P. (°C)
1	Methane	CH₄	-161
2	Ethane	C₂H₆	-89
3	Propane	C₃H₈	-42
4	Butane	C₄H₁₀	-1
5	Pentane	C₅H₁₂	36
6	Hexane	C₆H₁₄	69

Each member differs from the next by CH₂ (14 amu).

Pattern: as n increases, b.p. increases.

Example 2: Alcohols

General formula: $C_nH_{2n+1}OH$.

n	Compound	Formula	B.P. (°C)
1	Methanol	CH₃OH	65
2	Ethanol	C₂H₅OH	78
3	Propanol	C₃H₇OH	97
4	Butanol	C₄H₉OH	118
5	Pentanol	C₅H₁₁OH	138

Each differs by CH₂.

Example 3: Carboxylic Acids

General formula: $C_nH_{2n+1}COOH$ or $C_{n+1}H_{2n+2}O_2$.

n	Compound	Formula	B.P. (°C)
0	Formic acid	HCOOH	101
1	Acetic acid	CH₃COOH	118
2	Propanoic acid	C₂H₅COOH	141
3	Butanoic acid	C₃H₇COOH	164

Properties of Homologous Series

1. Same general formula. Members fit a single formula.

2. Successive members differ by CH₂ (or 14 amu). If one is C₂H₆, next is C₃H₈, then C₄H₁₀.

3. Same functional group. All alkanes have only C-C single bonds. All alcohols have -OH. All acids have -COOH.

4. Similar chemical properties. All members react similarly (same functional group). All alcohols + Na → produce H₂. All acids react with bases.

5. Gradual change in physical properties. Boiling point increases steadily. Density changes gradually. Solubility decreases as size increases.

This makes the series predictable and useful.

Why is This Useful?

Knowing one member of a series, we can predict properties of others:

• Same chemistry.
• Similar reactions.
• Just slightly different physical properties.

Saves enormous study time. Helps in industry to predict behaviour of new compounds.

All Common Homologous Series — At a Glance

1. Alkanes

Saturated hydrocarbons (single bonds only). General formula: $C_nH_{2n+2}$. Suffix: -ane.

Members: methane, ethane, propane, butane, pentane…

2. Alkenes

Unsaturated hydrocarbons with 1 double bond. General formula: $C_nH_{2n}$ (n ≥ 2). Suffix: -ene.

Members: ethene, propene, butene, pentene…

3. Alkynes

Unsaturated hydrocarbons with 1 triple bond. General formula: $C_nH_{2n-2}$ (n ≥ 2). Suffix: -yne.

Members: ethyne, propyne, butyne, pentyne…

4. Alcohols

Have -OH functional group. General formula: $C_nH_{2n+1}OH$. Suffix: -ol.

Members: methanol, ethanol, propanol, butanol…

5. Aldehydes

Have -CHO functional group. General formula: $C_nH_{2n}O$ or $C_nH_{2n+1}CHO$ (depending on counting). Suffix: -al.

Members: methanal (HCHO), ethanal (CH₃CHO), propanal…

6. Ketones

Have >C=O group (with C atoms on both sides). General formula: $C_nH_{2n}O$ (similar to aldehydes). Suffix: -one.

Members: propanone (acetone), butanone, pentanone…

7. Carboxylic Acids

Have -COOH group. General formula: $C_nH_{2n+1}COOH$ or $C_{n+1}H_{2n+2}O_2$. Suffix: -oic acid.

Members: formic acid, acetic acid, propanoic acid…

8. Halogenoalkanes

Have -X group (X = F, Cl, Br, I). General formula: $C_nH_{2n+1}X$.

Members: chloromethane, bromomethane…

Example Comparison — All 'Eth' Compounds

Compounds with 2 C atoms but different functional groups:

Compound	Formula	Functional Group	B.P. (°C)
Ethane	C₂H₆	None (alkane)	-89
Ethene	C₂H₄	C=C (alkene)	-104
Ethyne	C₂H₂	C≡C (alkyne)	-84
Ethanol	C₂H₅OH	-OH (alcohol)	78
Ethanal	CH₃CHO	-CHO (aldehyde)	21
Acetic acid	CH₃COOH	-COOH (carboxylic acid)	118
Chloroethane	C₂H₅Cl	-Cl (halogen)	12

Same C count, different functional groups → very different compounds!

A Beautiful Pattern

Each homologous series:

• Starts with a simple member.
• Adds CH₂ to make next member.
• Each addition increases:
• Molecular mass (by 14).
• Boiling point (by ~20-30°C usually).
• Density (slightly).
• All members do similar reactions.

This is the organising principle of organic chemistry.

How Functional Groups Determine Properties

Compounds Without Functional Groups

Pure hydrocarbons (alkanes, alkenes, alkynes):

• Generally non-polar.
• Insoluble in water.
• Lower boiling points.
• Less reactive (alkanes especially).

Adding Polar Groups

Adding -OH, -COOH, -NH₂, -CHO etc.:

• Makes molecule more polar.
• Increases water solubility.
• Raises boiling point.
• Makes compound more reactive.

Hydrogen Bonding

-OH and -COOH groups can form hydrogen bonds with water and other -OH/-COOH groups. This is why:

• Ethanol soluble in water (forms H-bonds).
• Acetic acid soluble in water.
• These compounds have higher b.p. than expected.

Trend in Boiling Points

For 2-carbon compounds:

Compound	B.P. (°C)	Why
Ethane	-89	Non-polar, weak forces
Ethene	-104	Non-polar, similar forces
Chloroethane	12	Polar (slight), some forces
Ethanal	21	Polar (C=O), no H-bonding
Ethanol	78	Strong H-bonding
Acetic acid	118	Strongest H-bonding

Note: not always perfectly ordered — depends on multiple factors.

Acidity

Different functional groups give different acid strength:

Compound	pKa (lower = stronger)
Methane	~50 (not acidic)
Ethanol	16 (very weak)
Acetic acid	4.76 (weak acid)
Sulphuric acid	-3 (very strong)

Carboxylic acids — weakly acidic. Alcohols — very weakly acidic (essentially neutral).

Important Reactions of Functional Groups

Alcohols:

• Combustion: burn in air to give CO₂ + H₂O.
• With Na: react to give H₂ gas.
• Oxidation: gives aldehydes, then acids.
• Esterification: with acids to form esters (perfumes!).

Carboxylic Acids:

• With base: neutralise → salt + water.
• With alcohol: esterification → ester.
• With metal: react to give H₂.

Aldehydes/Ketones:

• Oxidation: aldehydes → acids; ketones don't oxidise easily.
• Reduction: both → alcohols.

Halogens (Haloalkanes):

• Substitution: halogen replaced by other groups.
• Used to make many other compounds.

Why This Matters in Daily Life

Functional groups define the behaviour of:

• Drinking alcohol (ethanol, -OH group).
• Vinegar (acetic acid, -COOH).
• Sugar (multiple -OH groups).
• Anesthetics (often have specific functional groups).
• Plastics, dyes, perfumes — all defined by functional groups.

Understanding functional groups = understanding most of organic chemistry.

🧠 Memory Capsule

A quick glance just before the board exam.

1. Functional Group — Definition

Atom or group of atoms that determines the chemical properties of a compound.

2. Common Functional Groups

Group	Symbol	Class
Halogen	-Cl, -Br	Haloalkane
Alcohol	-OH	Alcohol (-ol)
Aldehyde	-CHO	Aldehyde (-al)
Ketone	>C=O	Ketone (-one)
Acid	-COOH	Carboxylic acid (-oic acid)

3. Examples of Each

Alcohol:

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)

Aldehyde:

• Methanal/Formaldehyde (HCHO)
• Ethanal/Acetaldehyde (CH₃CHO)

Ketone:

• Propanone/Acetone ((CH₃)₂CO)

Carboxylic acid:

• Methanoic/Formic acid (HCOOH)
• Ethanoic/Acetic acid (CH₃COOH)

4. Homologous Series — Definition

A 'family' of organic compounds with:

• Same functional group.
• Same general formula.
• Differ by CH₂ between members.
• Similar chemistry, gradually changing physical properties.

5. Common Homologous Series

Series	Formula	Suffix
Alkane	$C_nH_{2n+2}$	-ane
Alkene	$C_nH_{2n}$	-ene
Alkyne	$C_nH_{2n-2}$	-yne
Alcohol	$C_nH_{2n+1}OH$	-ol
Aldehyde	$C_nH_{2n+1}CHO$	-al
Ketone	$C_nH_{2n+1}COC_mH_{2m+1}$	-one
Acid	$C_nH_{2n+1}COOH$	-oic acid

6. Common Properties of Homologous Series

• Same general formula.
• Each member differs by CH₂.
• Same functional group → similar reactions.
• Gradual change in physical properties.

7. Boiling Point Order (2-C compounds)

Acetic acid (118) > Ethanol (78) > Ethanal (21) > Chloroethane (12) > Ethane (-89)

Reason: stronger functional group polarity → stronger intermolecular forces → higher b.p.

8. Board's 'Golden' Questions

1. Define functional group with example.
2. List 5 functional groups with their structures.
3. What is homologous series? Properties.
4. Why ethanol has higher b.p. than ethane?
5. Compare alcohols, aldehydes, acids.

Final Formula: Functional group decides chemistry; homologous series organises by chain length.

Solved Examples

Example 1: NCERT — What is a Functional Group?

Define functional group. Give 5 examples with their structures.

Solution:

Definition

'Functional Group' = an atom or group of atoms attached to a hydrocarbon chain that determines the chemical properties of the compound.

The 'function' = the chemistry it brings.

Two compounds with same C-H skeleton but different functional groups have very different properties.

5 Important Functional Groups

1. Hydroxyl Group (—OH) — Alcohols

Structure: R-O-H

Example: Methanol (CH₃-OH)

    H
    |
H — C — O — H
    |
    H

Properties:

• Polar.
• Forms hydrogen bonds.
• Soluble in water (small ones).

2. Aldehyde Group (—CHO)

Structure: R-C(=O)-H

Example: Methanal/Formaldehyde (HCHO)

         O
         ||
    H — C — H

Properties:

• Sharp pungent smell.
• Reactive.
• Used in preserving.

3. Ketone Group (>C=O)

Structure: R-C(=O)-R'

Example: Propanone/Acetone ((CH₃)₂C=O)

         O
         ||
CH₃ — C — CH₃

Properties:

• Polar.
• Used as solvents.

4. Carboxylic Acid Group (—COOH)

Structure: R-COOH

Example: Acetic acid (CH₃COOH)

         O
         ||
CH₃ — C — O — H

Properties:

• Acidic (gives H⁺).
• Sour taste.
• Highly polar.

5. Halogen Group (—X)

X = F, Cl, Br, I.

Example: Chloromethane (CH₃Cl)

    H
    |
H — C — Cl
    |
    H

Properties:

• Slightly polar.
• Often toxic.
• Used as solvents, refrigerants.

Why Functional Groups Matter

Comparing 2-carbon compounds:

Compound	Functional Group	Boiling Point
Ethane	None	-89°C
Ethanol	-OH	78°C
Acetic acid	-COOH	118°C

Same C count, different functional groups → very different properties!

This is why functional groups are central to organic chemistry — they define what a compound 'does'.

[NCERT — important]

Example 2: NCERT — Homologous Series

What is a homologous series? Explain with the example of alkanes.

Solution:

Definition

'Homologous Series' = a series of organic compounds with:

• Same functional group.
• Same general formula.
• Successive members differ by CH₂ (14 amu).
• Similar chemical properties.
• Gradually changing physical properties.

From Greek: 'homo' = same. Members are called 'homologues'.

Example: Alkanes

General formula: $C_nH_{2n+2}$

All alkanes have:

• Only C-C single bonds.
• Only C-H single bonds.
• Same general formula.
• Similar chemistry.

First 6 Alkanes

n	Compound	Formula	M.W.	B.P. (°C)
1	Methane	CH₄	16	-161
2	Ethane	C₂H₆	30	-89
3	Propane	C₃H₈	44	-42
4	Butane	C₄H₁₀	58	-1
5	Pentane	C₅H₁₂	72	36
6	Hexane	C₆H₁₄	86	69

Properties of Homologous Series (Verified)

1. Same general formula: ✓ All match $C_nH_{2n+2}$.

2. Successive members differ by CH₂:

• Ethane - Methane: C₂H₆ - CH₄ = CH₂ ✓
• Propane - Ethane: C₃H₈ - C₂H₆ = CH₂ ✓
• Each pair differs by CH₂ (= 14 amu).

3. Same functional group: ✓ All single bonds (C-C and C-H).

4. Similar chemical properties:

• All burn in oxygen → CO₂ + H₂O.
• All undergo substitution with halogens (under UV).
• All react similarly.

5. Gradual change in physical properties:

• B.P. increases steadily as n increases.
• Density increases gradually.
• State changes: gas (n=1-4) → liquid (n=5-17) → solid (n>17).

Why is the Series Useful?

Knowing alkanes:

• We can predict properties of new (unknown) alkanes.
• We know their reactions.
• We can plan industrial uses.

Example: If we discover a new alkane C₂₀H₄₂:

• We can predict it will be a solid at room temperature.
• It will burn cleanly.
• It won't react easily with other chemicals.

This predictive power makes homologous series fundamental to organic chemistry.

Other Homologous Series

Same principle applies to:

• Alkenes, alkynes (other hydrocarbons).
• Alcohols, aldehydes, acids (functional groups).
• Each forms a series with its own characteristic chemistry.

Around 30 different homologous series exist — each a 'family' of related compounds.

[NCERT — every year]

Example 3: NCERT — Compare Two Compounds

Compare ethanol (C₂H₅OH) and ethane (C₂H₆). Explain why their properties differ.

Solution:

The Two Compounds

Ethane (C₂H₆)

Saturated hydrocarbon (alkane). No functional group beyond C-H and C-C bonds.

Structure:

CH₃ — CH₃

Ethanol (C₂H₅OH)

Has -OH functional group (alcohol).

Structure:

CH₃ — CH₂ — OH

Comparison Table

Property	Ethane	Ethanol
Formula	C₂H₆	C₂H₅OH (or C₂H₆O)
Functional Group	None	-OH (hydroxyl)
State at 25°C	Gas	Liquid
Boiling Point	-89°C	78°C
Solubility in water	Insoluble	Highly soluble
Reaction with Na	None	Reacts → H₂ + sodium ethoxide
Smell	Almost none	Distinctive (alcoholic)

Why Such Different Properties?

Both have 2 C atoms. Yet very different properties!

Reason 1: Polarity

Ethane:

• Only C-H and C-C bonds.
• Non-polar molecule.
• Weak intermolecular forces (only van der Waals).

Ethanol:

• Has —OH group.
• O is more electronegative than H → polar O-H bond.
• Polar molecule.
• Stronger intermolecular forces.

Reason 2: Hydrogen Bonding

Ethane: No H-bonding possible (no electronegative atoms with H).

Ethanol:

• O-H can form H-bond with another O-H.
• Strong intermolecular bonds.
• Like a 'sticky' molecule.

Reason 3: Effect on Properties

Higher polarity + H-bonding =

• Higher b.p. (78°C vs -89°C — 167°C difference!).
• Liquid at room temperature (instead of gas).
• Soluble in water (forms H-bonds with water).
• Reactive with Na (acidic H of -OH).

Key Insight

Adding a single -OH group to ethane:

• Drastically changes physical properties.
• Adds new chemical behaviour.
• Makes the compound have a 'function' it didn't have before.

This is what 'functional group' means — it gives the compound a new function.

Why is this Important?

This shows the power of functional groups:

Same C-H backbone, but:

• Add -OH → alcohol (drinks, fuel, solvent).
• Add -COOH → acid (vinegar, biological molecules).
• Add -CHO → aldehyde (preservative, chemical).
• Add -Cl → haloalkane (refrigerant, anaesthetic).

Each functional group → completely different compound!

This is why organic chemistry has millions of compounds despite using only a few elements (C, H, O, N, S, halogens).

[NCERT — important]

Example 4: NCERT — Identify Functional Groups

Identify the functional group in each of the following: (a) CH₃Cl (b) C₂H₅OH (c) HCHO (d) CH₃COOH (e) CH₂=CH-CH₃

Solution:

Analysis

(a) CH₃Cl

Has chlorine (Cl) attached to carbon.

Functional Group: -Cl (halogen / halide) Class: Haloalkane (chloroalkane). Name: Methyl chloride / chloromethane.

(b) C₂H₅OH

Has -OH (hydroxyl) group attached to ethyl chain.

Functional Group: -OH (hydroxyl, alcohol) Class: Alcohol. Name: Ethanol / ethyl alcohol.

Structure: CH₃-CH₂-OH

(c) HCHO

Has -CHO group (C=O with H attached).

Functional Group: -CHO (aldehyde) Class: Aldehyde. Name: Formaldehyde / methanal.

Structure:

         O
         ||
    H — C — H

(d) CH₃COOH

Has -COOH group (carboxylic acid).

Functional Group: -COOH (carboxylic acid / carboxyl) Class: Carboxylic acid. Name: Acetic acid / ethanoic acid.

Structure:

         O
         ||
CH₃ — C — O — H

(e) CH₂=CH-CH₃

Has C=C double bond.

Functional Group: C=C (double bond) Class: Alkene. Name: Propene (or prop-1-ene).

Summary Table

Compound	Functional Group	Class	Name
CH₃Cl	-Cl	Haloalkane	Chloromethane
C₂H₅OH	-OH	Alcohol	Ethanol
HCHO	-CHO	Aldehyde	Methanal/Formaldehyde
CH₃COOH	-COOH	Carboxylic acid	Acetic acid/Ethanoic acid
CH₂=CHCH₃	C=C	Alkene	Propene

How to Identify Functional Groups

Practice steps:

1. Look for special atoms (O, N, S, halogens). 2. Look for double/triple bonds. 3. Look for groups like:

• -OH (alcohol)
• -CHO (aldehyde)
• -COOH (acid)
• -O- (ether, between Cs)
• -X (halogen)

4. If only C-H, C-C single bonds → alkane (no functional group). 5. If C=C → alkene; C≡C → alkyne.

Practical Importance

Identifying functional groups helps us:

• Predict chemical behaviour.
• Plan reactions.
• Identify unknown compounds.
• Design new molecules (medicines, materials).

[Board: 3-mark]

Example 5: NCERT — Properties of Homologous Series

State 4 properties of a homologous series with examples.

Solution:

Property 1: Same General Formula

All members of a homologous series fit a single general formula.

Example: Alkanes General formula: $C_nH_{2n+2}$

Members:

• CH₄ (n=1)
• C₂H₆ (n=2)
• C₃H₈ (n=3)
• C₄H₁₀ (n=4)

Each fits the formula.

Property 2: Successive Members Differ by CH₂

Each next member adds CH₂ (14 amu) to the previous.

Example:

• C₂H₆ - CH₄ = CH₂ (14 amu) ✓
• C₃H₈ - C₂H₆ = CH₂ ✓
• C₄H₁₀ - C₃H₈ = CH₂ ✓

This is true for all homologous series:

• Alcohols: CH₃OH → C₂H₅OH adds CH₂.
• Aldehydes: HCHO → CH₃CHO adds CH₂.
• Acids: HCOOH → CH₃COOH adds CH₂.

Property 3: Similar Chemical Properties

All members of a series react similarly because of the same functional group.

Example: All alcohols react with sodium: $$2R-OH + 2Na \rightarrow 2R-O^-Na^+ + H_2\uparrow$$

Whether R = methyl, ethyl, propyl, butyl… the reaction is the same.

All acids neutralise bases: $$R-COOH + NaOH \rightarrow R-COONa + H_2O$$

All alkanes burn: $$C_nH_{2n+2} + O_2 \rightarrow nCO_2 + (n+1)H_2O$$

Property 4: Gradual Change in Physical Properties

Boiling point, melting point, density change steadily.

Example: Boiling Points of Alcohols

Compound	Formula	B.P. (°C)
Methanol	CH₃OH	65
Ethanol	C₂H₅OH	78
Propanol	C₃H₇OH	97
Butanol	C₄H₉OH	118
Pentanol	C₅H₁₁OH	138

Pattern: each higher member has higher b.p. (~20°C higher each time).

Why? Larger molecule → more intermolecular forces → higher b.p.

Why This Pattern?

All members:

• Have same functional group → same chemistry.
• Larger molecule → stronger intermolecular forces (just from increased mass).
• Hence physical properties change gradually.

Practical Significance

This pattern helps:

1. Predict properties of unknown compounds. If we know methanol b.p. = 65°C, ethanol = 78°C, then propanol = ~97°C (close to actual).

2. Design experiments. Knowing the trend, we can plan separations and reactions.

3. Industrial decisions. Choose the right member of a series for a specific application.

Example: choosing an alcohol for a specific b.p. window in distillation.

Summary

Homologous series properties:

1. Same general formula.
2. Differ by CH₂.
3. Similar chemistry.
4. Gradual physical property changes.

This 'family' approach is the organising principle of organic chemistry — it brings order to a vast field.

[NCERT — important]

Example 6: NCERT — Why Are Carboxylic Acids Acidic?

Explain why carboxylic acids (like acetic acid) are weakly acidic, while alcohols are not.

Solution:

What Makes a Compound Acidic?

A compound is acidic if it can release H⁺ ions (Brønsted definition).

The H⁺ comes from a polar bond, usually H-O.

Carboxylic Acid (-COOH)

Structure:

       O
       ||
R — C — O — H

The -COOH group has:

• C=O double bond (strong attraction).
• Adjacent -O-H bond.

Why acidic:

• The C=O 'pulls' electrons away from O-H.
• O-H bond is weakened.
• H can be released as H⁺.
• Resulting carboxylate ion (R-COO⁻) is stabilised by spreading negative charge across both O atoms (resonance).

Example: $$CH_3COOH \rightleftharpoons CH_3COO^- + H^+$$

pKa ≈ 4.76 — weakly acidic.

Alcohol (-OH)

Structure:

R — O — H

Just an -OH group, no nearby C=O.

Why NOT acidic:

• O-H bond is strong (not weakened by anything).
• Releasing H⁺ would leave R-O⁻ (alkoxide).
• R-O⁻ is unstable — negative charge concentrated on one O.
• Hence H is not released easily.

pKa ≈ 16 (very weakly acidic, essentially neutral).

Comparison

Compound	pKa	Acid Strength
Methane (no OH)	~50	Not acidic
Ethanol (alcohol)	16	Very weak
Acetic acid	4.76	Weak
HCl	-7	Strong

Lower pKa = stronger acid.

Acetic acid is about 10¹¹ times more acidic than ethanol!

The Key — Resonance Stabilisation

Why does -COOH lose H⁺ easily?

After losing H⁺, the -COO⁻ ion has negative charge spread across two equivalent O atoms:

O ⁻       O ⁻
||   ↔    | 
C         C
|         ||
O⁻        O

This is resonance — the charge is delocalised. Stable structure → easier to lose H⁺.

Alcohols can't do this:

C — O⁻

Negative charge stuck on one O → unstable → H⁺ not released easily.

Practical Implications

Why acetic acid (vinegar) tastes sour:

• Releases H⁺ → activates sour-taste receptors on tongue.

Why acetic acid + base = neutralisation: $$CH_3COOH + NaOH \rightarrow CH_3COO^-Na^+ + H_2O$$

Why ethanol (drinking alcohol) doesn't taste sour:

• No H⁺ release.
• No sour activation.
• Ethanol is not an acid in everyday sense.

Other Carboxylic Acids

All carboxylic acids are weakly acidic:

• Formic acid (HCOOH): in ant stings.
• Acetic acid (CH₃COOH): in vinegar.
• Citric acid (C₆H₈O₇): in lemons (3 -COOH groups!).
• Lactic acid: in sour milk and tired muscles.
• Amino acids: in proteins.

Summary

The functional group determines acidity:

• -COOH: releases H⁺ (carboxylate stabilised by resonance).
• -OH alcohol: does NOT release H⁺ easily (alkoxide unstable).

This is one example of how functional groups define chemical behaviour.

[NCERT + Board]

Example 7: A Concluding Question

(a) Define functional group. (b) Define homologous series. (c) Write 5 important functional groups with suffixes. (d) Why does ethanol have higher b.p. than ethane?

Solution:

(a) Functional Group

'Functional Group' = an atom or group of atoms attached to a hydrocarbon chain that determines the chemical properties of the compound.

Examples:

• -OH (alcohol) — gives compound 'alcoholic' properties.
• -COOH (carboxylic acid) — gives 'acidic' properties.
• -CHO (aldehyde) — gives 'aldehyde' properties.

Two compounds with same hydrocarbon part but different functional groups have different chemistry.

(b) Homologous Series

'Homologous Series' = a 'family' of organic compounds with:

1. Same functional group. 2. Same general formula. 3. Successive members differ by CH₂ (14 amu). 4. Similar chemical properties. 5. Gradually changing physical properties.

Example: Alkane series — methane, ethane, propane, butane…

All have only C-C and C-H single bonds. Each adds one CH₂ to the previous. All burn similarly in O₂. B.P. increases gradually.

(c) 5 Important Functional Groups with Suffixes

Group	Symbol	Class	Suffix
1. Hydroxyl	-OH	Alcohol	-ol
2. Aldehyde	-CHO	Aldehyde	-al
3. Ketone	>C=O	Ketone	-one
4. Carboxyl	-COOH	Carboxylic acid	-oic acid
5. Halogen	-X (Cl, Br, I, F)	Haloalkane	-halo (prefix)

Examples:

• Ethanol: C₂H₅-OH (suffix '-ol' from -OH).
• Methanal: H-CHO (suffix '-al' from -CHO).
• Propanone: CH₃-CO-CH₃ (suffix '-one' from C=O).
• Ethanoic acid: CH₃-COOH (suffix '-oic acid' from -COOH).
• Chloromethane: Cl-CH₃ (prefix 'chloro' from -Cl).

(d) Why Ethanol's B.P. > Ethane's B.P.?

Ethane (C₂H₆)

• Boiling point: -89°C.
• Non-polar molecule.
• Only weak van der Waals forces between molecules.

Ethanol (C₂H₅OH)

• Boiling point: 78°C.
• Polar molecule (due to -OH).
• Forms hydrogen bonds between molecules.

The Reason

Hydrogen bonding makes ethanol molecules stick together strongly:

C₂H₅-O-H  ⋯  H-O-C₂H₅
     ↑
  Hydrogen bond

Each ethanol molecule can form H-bonds with multiple others. This forms a network that's hard to break.

To boil ethanol:

• Need to overcome H-bonds.
• Requires much more energy than just van der Waals.
• Hence high b.p. (78°C).

To boil ethane:

• Only weak van der Waals to break.
• Much less energy.
• Hence low b.p. (-89°C).

Quantitative Difference

Difference in b.p.: 78 - (-89) = 167°C!

Just adding -OH to a C₂H₅ chain increases b.p. by 167°C.

This is the power of functional groups.

Implications

This explains:

1. Why ethanol is liquid at room temperature (b.p. above 25°C). Useful as a drink, fuel, solvent.

2. Why ethane is a gas (b.p. below -25°C). Used as fuel in compressed form.

3. Why we can drink ethanol (not ethane). Ethanol is liquid → can be in beverages. Ethane is gas → can't be in beverages.

4. Industrial significance. Methanol (b.p. 65°C) is also liquid. Industrial uses depend on phase at room temperature.

Summary

Functional groups + homologous series = the organising framework of organic chemistry.

Understanding these:

• Predict properties of millions of compounds.
• Plan reactions efficiently.
• Design new molecules (drugs, materials).

This is why every organic chemistry student must master these concepts.

[Board: 5-mark mixed]