Hydrocarbon and its Compounds

LPG in your kitchen, petrol in a motorbike, the wax of a candle, the plastic of a bucket, vinegar in a pickle and the alcohol in a hand sanitiser are all compounds of carbon. Carbon forms more compounds than all other elements together, because its atoms join to each other in long chains and rings. The study of these compounds is called organic chemistry. This chapter starts with the simplest organic compounds, the hydrocarbons, and then moves to two families that you meet in daily life: alcohols and carboxylic acids.

18.1 Alkanes, alkenes and alkynes

Why carbon forms so many compounds

• Tetravalency. Carbon has 4 valence electrons (2, 4) and forms four covalent bonds.
• Catenation. Carbon atoms join to each other in long chains, branched chains and rings.
• It forms single, double and triple bonds.

A hydrocarbon is a compound made only of carbon and hydrogen. Hydrocarbons are of two types:

• Saturated hydrocarbons contain only single bonds (alkanes).
• Unsaturated hydrocarbons contain at least one double or triple bond (alkenes and alkynes).

Alkanes (paraffins)

Alkanes have only C–C and C–H single bonds. The general formula is

n	Name	Molecular formula	Condensed formula	State at room temperature
1	Methane	CH₄	CH₄	gas
2	Ethane	C₂H₆	CH₃CH₃	gas
3	Propane	C₃H₈	CH₃CH₂CH₃	gas
4	Butane	C₄H₁₀	CH₃CH₂CH₂CH₃	gas
5	Pentane	C₅H₁₂	CH₃(CH₂)₃CH₃	liquid
6	Hexane	C₆H₁₄	CH₃(CH₂)₄CH₃	liquid

Names of the first four are historical (meth-, eth-, prop-, but-). From five onward the Greek number is used (pent-, hex-, hept-, oct-, non-, dec-). All alkane names end in -ane.

Properties of alkanes

• Physical: colourless; insoluble in water, soluble in organic solvents; boiling point rises with size; lighter than water.
• Chemical: fairly unreactive because all bonds are single and strong.

Reaction	Equation
Combustion (burning in excess air)	CH₄ + 2O₂ → CO₂ + 2H₂O + heat
Combustion of propane (LPG)	C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Substitution (with chlorine in sunlight)	CH₄ + Cl₂ --sunlight--> CH₃Cl + HCl

In substitution, one hydrogen atom is replaced by another atom or group.

Uses. Methane (biogas, natural gas) for cooking; propane and butane (LPG); petrol, kerosene and diesel as fuel; paraffin wax in candles; bitumen on roads.

Alkenes (olefins)

Alkenes have one C=C double bond. General formula:

n	Name	Formula	Structure
2	Ethene (ethylene)	C₂H₄	CH₂=CH₂
3	Propene	C₃H₆	CH₃CH=CH₂
4	Butene	C₄H₈	CH₃CH₂CH=CH₂

Names end in -ene.

Laboratory preparation of ethene. Ethanol is heated with excess concentrated sulphuric acid (a dehydrating agent) at 170 °C.

CH₃CH₂OH --conc. H₂SO₄, 170 °C--> CH₂=CH₂ + H₂O

Properties. Colourless gas with a faint sweet smell; slightly soluble in water. Alkenes are reactive because the double bond can break open.

Reaction	Equation
Combustion	C₂H₄ + 3O₂ → 2CO₂ + 2H₂O (burns with a smoky flame)
Addition of hydrogen (Ni catalyst, 200 °C)	CH₂=CH₂ + H₂ → CH₃CH₃
Addition of bromine	CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
Addition of HCl	CH₂=CH₂ + HCl → CH₃CH₂Cl
Polymerisation	n CH₂=CH₂ → –(CH₂–CH₂)ₙ– (polythene)

Uses. Making polythene, ripening of fruits, making ethanol, and in welding and cutting as a fuel gas.

Alkynes

Alkynes have one C≡C triple bond. General formula:

n	Name	Formula	Structure
2	Ethyne (acetylene)	C₂H₂	HC≡CH
3	Propyne	C₃H₄	CH₃C≡CH
4	Butyne	C₄H₆	CH₃CH₂C≡CH

Names end in -yne.

Laboratory preparation of ethyne. Calcium carbide reacts with cold water:

CaC₂ + 2H₂O → C₂H₂ ↑ + Ca(OH)₂

The gas is collected over water. It is colourless, with a garlic-like smell when impure (from traces of phosphine).

Reaction	Equation
Combustion	2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O (very sooty flame, very hot)
Addition of H₂ (Ni)	C₂H₂ + 2H₂ → C₂H₆
Addition of Br₂	C₂H₂ + 2Br₂ → CHBr₂–CHBr₂

Uses. Oxy-acetylene flame for welding and cutting metals (about 3000 °C); making plastics and ethanoic acid; old miners' lamps.

Tests to separate alkanes from alkenes and alkynes

Test	Alkane	Alkene / alkyne
Bromine water (red-brown)	stays red-brown	decolourised
Baeyer's reagent (cold dilute alkaline KMnO₄, pink-purple)	no change	purple colour disappears
Flame	clear or blue, little smoke	yellow, smoky (more in alkyne)

18.2 Homologous series

Characteristics

1. All members can be represented by one general formula (alkanes C_nH_{2n+2}).
2. Successive members differ by CH₂ (relative molecular mass 14).
3. Chemical properties are similar, because they have the same functional group.
4. Physical properties change gradually: melting and boiling points rise with the molecular mass.
5. They can be made by similar methods.

Functional group

A functional group is the atom or group of atoms that decides the chemical properties of a compound.

Homologous series	Functional group	General formula	First member
Alkane	none (C–C)	CₙH₂ₙ₊₂	methane
Alkene	C=C	CₙH₂ₙ	ethene
Alkyne	C≡C	CₙH₂ₙ₋₂	ethyne
Alcohol	–OH (hydroxyl)	CₙH₂ₙ₊₁OH	methanol
Carboxylic acid	–COOH (carboxyl)	CₙH₂ₙ₊₁COOH	methanoic acid

18.3 Alcohols

An alcohol is an organic compound in which one or more hydrogen atoms of an alkane is replaced by the hydroxyl group (–OH). General formula for the simple (monohydric) alcohols: CₙH₂ₙ₊₁OH. Names end in -ol.

Name	Formula	Structure
Methanol (methyl alcohol)	CH₃OH	CH₃–OH
Ethanol (ethyl alcohol)	C₂H₅OH	CH₃CH₂–OH
Propanol	C₃H₇OH	CH₃CH₂CH₂–OH

Ethanol

Preparation by fermentation. Sugar from molasses, rice, millet or grapes is changed to ethanol by the enzymes in yeast (zymase). Local drinks of Nepal such as jaand and raksi are made by fermentation.

C₆H₁₂O₆ --zymase--> 2C₂H₅OH + 2CO₂ ↑

Other methods: hydration of ethene (C₂H₄ + H₂O → C₂H₅OH, steam, phosphoric acid catalyst, high pressure).

Physical properties. Colourless liquid with a pleasant smell and burning taste; boiling point 78 °C; mixes with water in all proportions; a good solvent.

Chemical properties.

Reaction	Equation
Combustion	C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (burns with a blue, almost smokeless flame)
With sodium	2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂ ↑ (sodium ethoxide)
Oxidation (acidified K₂Cr₂O₇ or alkaline KMnO₄)	C₂H₅OH + 2[O] → CH₃COOH + H₂O
Dehydration (conc. H₂SO₄, 170 °C)	C₂H₅OH → C₂H₄ + H₂O
Esterification (with ethanoic acid)	CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (conc. H₂SO₄)

Ethanol does not change the colour of litmus (it is neutral).

Uses. Solvent for paints, perfumes and medicines; antiseptic and hand sanitiser (70 %); fuel (blended with petrol; gasohol); preparation of ethanoic acid and ether; alcoholic drinks.

18.4 Carboxylic acids

A carboxylic acid is an organic compound containing the carboxyl group –COOH. General formula: CₙH₂ₙ₊₁COOH (or R–COOH). Names end in -oic acid.

Name (common name)	Formula	Source
Methanoic acid (formic acid)	HCOOH	red ants, nettle sting (sisnu)
Ethanoic acid (acetic acid)	CH₃COOH	vinegar
Propanoic acid	C₂H₅COOH	preservative in bread

Ethanoic acid (acetic acid)

Preparation. Oxidation of ethanol by acidified potassium dichromate:

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Vinegar is made when bacteria (Acetobacter) oxidise dilute ethanol (as in fermented fruit juice or rice) with air. Vinegar contains 5 to 8 % ethanoic acid. Pure (100 %) ethanoic acid freezes at 16.6 °C into ice-like crystals and is called glacial acetic acid.

Physical properties. Colourless liquid with sharp pungent smell and sour taste; miscible with water; boiling point 118 °C.

Chemical properties. It is a weak acid.

Reaction	Equation
With litmus	turns blue litmus red
With metals	2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂ ↑
With carbonates / bicarbonates	2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ ↑ ; CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ ↑
With bases	CH₃COOH + NaOH → CH₃COONa + H₂O
With alcohols (esterification)	CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The brisk effervescence with sodium bicarbonate is the test that separates a carboxylic acid from an alcohol (alcohols give no gas with NaHCO₃).

Esterification. The reaction of an alcohol with a carboxylic acid in the presence of a little conc. H₂SO₄ gives a sweet-smelling ester and water. Esters are used as perfumes, in artificial fruit flavours and as solvents.

Uses. Vinegar for food and pickles; making dyes, plastics, rayon and medicines; preparing esters; making cellulose acetate; as a solvent.

	Ethanol	Ethanoic acid
Functional group	–OH	–COOH
Litmus	no effect	turns blue litmus red
NaHCO₃	no reaction	CO₂ with brisk fizz
Smell	sweet, spirit-like	pungent, vinegar
Nature	neutral	weak acid

Chapter summary

• Carbon is tetravalent and shows catenation; hydrocarbons contain only C and H.
• Alkanes CₙH₂ₙ₊₂ (single bonds, substitution); alkenes CₙH₂ₙ (C=C, addition and polymerisation); alkynes CₙH₂ₙ₋₂ (C≡C, addition).
• Unsaturated hydrocarbons decolourise bromine water and Baeyer's reagent; alkanes do not.
• A homologous series has the same functional group and general formula, and members differ by –CH₂– (14 u).
• Alcohols CₙH₂ₙ₊₁OH (–OH): ethanol is made by fermentation of sugar; it reacts with Na to give H₂; oxidises to ethanoic acid; methanol is poisonous.
• Carboxylic acids CₙH₂ₙ₊₁COOH (–COOH): ethanoic acid (vinegar) is a weak acid, gives CO₂ with NaHCO₃, and forms esters with alcohols.

Practice questions

Group A — Objective questions (1 mark each)

1. The general formula of alkenes is [1]aCₙH₂ₙ₊₂bCₙH₂ₙcCₙH₂ₙ₋₂dCₙHₙ
2. The functional group of alcohols is [1]a–COOHb–CHOc–OHdC=C
3. Which gas is produced when calcium carbide reacts with water? [1]amethanebethenecethynedhydrogen
4. Vinegar contains [1]aformic acidbethanoic acidccitric aciddlactic acid
5. Which compound will decolourise bromine water? [1]aC₂H₆bCH₄cC₂H₄dC₃H₈
6. The next member after C₃H₈ in the alkane series is [1]aC₄H₈bC₄H₁₀cC₄H₁₂dC₅H₁₂

Answers to Group A

1. (b) — CₙH₂ₙ.

2. (c) — hydroxyl.

3. (c) — CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.

4. (b) — 5 to 8 % acetic acid.

5. (c) — ethene is unsaturated.

6. (b) — add CH₂ to C₃H₈.

Group B — Short answer (2 marks each)

1. What is a homologous series? Give two characteristics. [2]
2. Write the molecular formula and structural formula of ethyne. [2]
3. How would you distinguish ethane from ethene in the laboratory? [2]
4. Write the equation for the reaction of ethanoic acid with sodium bicarbonate. [2]
5. What is esterification? Write an equation. [2]

Answers to Group B

1. A family of organic compounds with the same functional group and general formula, in which successive members differ by CH₂. Characteristics: gradual change in physical properties; similar chemical properties.

2. C₂H₂; H–C≡C–H.

3. Shake each with bromine water: ethene decolourises it; ethane does not.

4. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ ↑.

5. Reaction of an alcohol and a carboxylic acid in presence of conc. H₂SO₄ to give an ester and water: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.

Group C — Long answer (4 marks each)

1. Write the preparation, two properties and two uses of ethanol. [4]
2. Compare alkanes, alkenes and alkynes in terms of general formula, bond type, example and type of reaction. [4]
3. Describe the laboratory preparation of ethyne and write two of its reactions. [4]
4. Write the formulae of the alkane, alkene and alkyne with 6 carbon atoms and calculate the molecular mass of the alkane (C = 12, H = 1). [4]

Answers to Group C

1. Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ with yeast. Properties: colourless liquid, miscible with water; reacts with sodium giving H₂; burns with a blue flame. Uses: solvent, sanitiser, fuel.

2. Alkanes CₙH₂ₙ₊₂, single bond, ethane, substitution. Alkenes CₙH₂ₙ, double bond, ethene, addition. Alkynes CₙH₂ₙ₋₂, triple bond, ethyne, addition.

3. CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂, gas collected over water. Reactions: 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O; C₂H₂ + 2H₂ → C₂H₆ (Ni).

4. Alkane C₆H₁₄, alkene C₆H₁₂, alkyne C₆H₁₀. Mass of C₆H₁₄ = 72 + 14 = 86.