Carbon and its Compounds • Topic 3 of 3

Chemical Properties & Important Carbon Compounds

Carbon compounds undergo a small set of characteristic reactions, and two compounds — ethanol and ethanoic acid — are studied in detail at this level, along with soaps and detergents.

Combustion

Carbon and its compounds burn in oxygen (air) to release heat and light. For example, CH4 + 2O2 → CO2 + 2H2O + heat. Saturated hydrocarbons give a clean blue flame; unsaturated ones give a yellow, sooty flame because of incomplete combustion.

Oxidation

Alcohols can be oxidised to carboxylic acids by oxidising agents such as alkaline KMnO4 or acidified K2Cr2O7: ethanol is oxidised to ethanoic acid. These oxidising agents add oxygen to the substrate, so they are also called oxidising agents.

Addition reaction

Unsaturated hydrocarbons add hydrogen across the double or triple bond in the presence of a nickel or palladium catalyst — this is hydrogenation. For example, vegetable oils (unsaturated) are hydrogenated to make vanaspati ghee (saturated): C=C + H2 → C−C.

Substitution reaction

Saturated hydrocarbons react with chlorine in sunlight, where chlorine atoms replace hydrogen atoms one by one: CH4 + Cl2 → CH3Cl + HCl (in sunlight). This is a substitution reaction.

Ethanol (C2H5OH)

Ethanol is a colourless liquid used in alcoholic drinks, as a solvent and in medicines. With sodium: 2C2H5OH + 2Na → 2C2H5ONa + H2. Dehydration: heating ethanol with excess concentrated H2SO4 at about 170 °C removes water to give ethene: C2H5OH → C2H4 + H2O.

Ethanoic acid (CH3COOH)

Ethanoic acid is the acid in vinegar (5–8% solution). Pure ethanoic acid freezes in cold weather, so it is called glacial acetic acid. It reacts with bases (NaOH → sodium ethanoate + water) and with carbonates/hydrogencarbonates to release CO2: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2.

Esterification

An acid reacts with an alcohol, in the presence of a little concentrated H2SO4, to form a sweet-smelling ester: CH3COOH + C2H5OH → CH3COOC2H5 + H2O. Esters are used in perfumes and flavourings; on treatment with a base (saponification) they give back the alcohol and the sodium salt of the acid.

Soaps, detergents and micelles

A soap is the sodium or potassium salt of a long-chain carboxylic (fatty) acid. Each molecule has a long hydrophobic (water-hating) carbon tail and a hydrophilic (water-loving) ionic head. In water, soap molecules cluster into a micelle — tails pointing inwards trapping oil/grease, heads pointing outwards into the water — so the dirt is carried away in rinse water. Soaps do not work well in hard water (containing Ca2+ and Mg2+ ions) because they form an insoluble scum. Detergents are the ammonium or sulphonate salts of long-chain hydrocarbons; they do not form scum and clean effectively in both hard and soft water.

A soap micelle: hydrophilic heads face water, hydrophobic tails trap the oil dropletoil / greaseblue heads = hydrophilic (water-loving)lines = hydrophobic tails (point into the oil)Soap Micelle
Solved Examples
1
Worked Example
Write the balanced equation for the complete combustion of methane and state the type of flame for a saturated hydrocarbon.
Solution
  1. Methane burns in oxygen to give carbon dioxide and water.
  2. Balanced equation: CH4 + 2O2 → CO2 + 2H2O + heat.
  3. Saturated hydrocarbons burn completely, giving a clean blue flame.

Answer: CH4 + 2O2 → CO2 + 2H2O; a clean blue flame.

2
Worked Example
How is ethanol converted to ethanoic acid? Name the reaction and a reagent.
Solution
  1. Ethanol is treated with an oxidising agent such as alkaline KMnO4 (or acidified K2Cr2O7) and warmed.
  2. The oxidising agent adds oxygen to the −CH2OH group, converting it to −COOH.
  3. The product is ethanoic acid: C2H5OH → CH3COOH.

Answer: By oxidation with alkaline KMnO4 (or acidified K2Cr2O7); ethanol is oxidised to ethanoic acid.

3
Worked Example
What is hydrogenation? Give one industrial use.
Solution
  1. Hydrogenation is the addition of hydrogen across a C=C or C≡C bond using a nickel (or palladium) catalyst.
  2. It converts an unsaturated compound into a saturated one: C=C + H2 → C−C.
  3. Industrially, unsaturated vegetable oils are hydrogenated to make solid vanaspati ghee.

Answer: Hydrogenation is addition of H2 to an unsaturated bond (Ni catalyst); it is used to make vanaspati ghee from vegetable oils.

4
Worked Example
Write the esterification reaction between ethanoic acid and ethanol, naming the catalyst and the type of product.
Solution
  1. An acid and an alcohol react in the presence of a little concentrated H2SO4 as catalyst.
  2. Equation: CH3COOH + C2H5OH → CH3COOC2H5 + H2O.
  3. The product CH3COOC2H5 is a sweet-smelling ester (ethyl ethanoate).

Answer: CH3COOH + C2H5OH → CH3COOC2H5 + H2O; catalyst conc. H2SO4; product is an ester.

5
Worked Example
Explain how a soap micelle removes oily dirt from cloth.
Solution
  1. A soap molecule has a hydrophobic carbon tail and a hydrophilic ionic head.
  2. In water the molecules form a micelle: the tails point inward and dissolve in the oily dirt, while the heads point outward into the water.
  3. The oil is trapped inside the micelle and is washed away with the rinse water.

Answer: The hydrophobic tails trap the oil inside the micelle while the hydrophilic heads keep it suspended in water, so the dirt is rinsed away.

6
Worked Example
Why do soaps not lather well in hard water, and how do detergents overcome this?
Solution
  1. Hard water contains Ca2+ and Mg2+ ions.
  2. These ions react with soap to form an insoluble precipitate (scum), so less soap is left to clean.
  3. Detergents are sulphonate/ammonium salts whose calcium and magnesium salts are soluble, so they do not form scum and clean well in hard water.

Answer: Ca2+/Mg2+ in hard water form an insoluble scum with soap; detergents form soluble salts with these ions, so they lather and clean in hard water.

Key Points

  • Key reactions: combustion (burning in O2), oxidation (alcohol to acid by KMnO4/K2Cr2O7), addition/hydrogenation (H2 across C=C with Ni) and substitution (Cl replaces H in alkanes in sunlight).
  • Ethanol (C2H5OH) reacts with sodium to give H2 and is dehydrated by hot conc. H2SO4 to ethene.
  • Ethanoic acid (CH3COOH), the acid in vinegar (glacial acetic acid when pure), reacts with bases and with carbonates to release CO2.
  • Esterification: acid + alcohol (conc. H2SO4) give a sweet-smelling ester plus water; used in perfumes and flavours.
  • Soap is a sodium salt of a fatty acid that forms micelles (hydrophobic tails in, hydrophilic heads out); soaps form scum in hard water (Ca2+/Mg2+), but detergents clean in both hard and soft water.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Addition of hydrogen across a C=C double bond using a nickel catalyst is called:
Explanation: Hydrogenation adds H2 across an unsaturated bond, converting it to a saturated bond.
Q2.Pure ethanoic acid is called glacial acetic acid because it:
Explanation: Pure ethanoic acid freezes (forms ice-like crystals) just below room temperature, hence glacial acetic acid.
Q3.The reaction of an alcohol with a carboxylic acid to form a sweet-smelling compound is:
Explanation: Acid + alcohol (with conc. H2SO4) give an ester and water - this is esterification.
Q4.Soaps do not work well in hard water because hard water contains:
Explanation: Ca2+ and Mg2+ ions react with soap to form an insoluble scum, wasting the soap.
Practice more MCQs → 📝 Assignment · 30 marks