Chapter 4: Carbon and its Compounds (Chemistry)

Carbon needs 4 more electrons and each oxygen needs 2. Carbon shares two pairs with each oxygen, forming two double bonds, so all three atoms complete their octets.

O=C=O, with two shared pairs between C and each O, and two lone pairs on each O.

Sulphur has 6 valence electrons. In S₈, each S atom shares one pair with each of its two neighbours (2 bonds), which gives it an octet with two lone pairs left over.

An eight-membered ring (crown shape) of S atoms joined by single bonds, each S carrying two lone pairs.

Pentane, C₅H₁₂, has three structural isomers:

Three: n-pentane, 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane).

• Catenation: carbon can link with other carbon atoms by strong covalent bonds to form long chains, branched chains and rings.
• Tetravalency: with a valency of 4, carbon can bond with four other atoms, of carbon or of elements such as H, O, N, S and Cl. Its bonds with most elements are strong, so the compounds are stable.

Catenation and tetravalency.

Cyclopentane is a ring of five carbon atoms, each joined to two H atoms. Its formula is C₅H₁₀ (a cycloalkane, CₙH₂ₙ).

C₅H₁₀; a five-carbon ring in which every C shares one pair with each neighbouring C and one pair with each of its two H atoms.

Yes, bromopentane has structural isomers. The Br atom can be on different carbons of the chain (1-bromopentane, 2-bromopentane, 3-bromopentane), and the carbon chain itself can be branched (e.g. 1-bromo-2-methylbutane).

See the structures; yes, isomers are possible by changing the position of Br or branching the chain.

1. Two carbons with a bromine: bromoethane
2. One carbon with the –CHO group: methanal (formaldehyde)
3. Six carbons with a triple bond at the end: hexyne (hex-1-yne)

(i) Bromoethane (ii) Methanal (iii) Hexyne

$\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{OH}\underset{\mathrm{or}\mathrm{acidified}\mathrm{K}{}_2\mathrm{Cr}{}_2\mathrm{O}{}_7}{\overset{\mathrm{alkaline}\mathrm{KMnO}{}_4+\mathrm{heat}}{\to }}\mathrm{CH}{}_3\mathrm{COOH}$

In this change, ethanol gains an oxygen atom and loses two hydrogen atoms (CH₃CH₂OH → CH₃COOH). Gain of oxygen (or loss of hydrogen) is oxidation. The oxygen is supplied by the oxidising agent, potassium permanganate or potassium dichromate.

Because ethanol gains oxygen (and loses hydrogen) when it changes into ethanoic acid.

Welding needs a very high temperature (about 3000 °C). With pure oxygen, ethyne burns completely and gives this very hot, clean flame. Air contains only about 21% oxygen, so ethyne burns incompletely, giving a sooty, much cooler flame that cannot melt metals.

In air, ethyne burns incompletely with a sooty flame that is not hot enough; pure oxygen gives complete combustion and the very high temperature needed.

Add a little sodium hydrogencarbonate (or sodium carbonate) to each. The carboxylic acid gives brisk effervescence of carbon dioxide, which turns lime-water milky:

$\mathrm{CH}{}_3\mathrm{COOH}+\mathrm{NaHCO}{}_3\to \mathrm{CH}{}_3\mathrm{COONa}+\mathrm{H}{}_2\mathrm{O}+\mathrm{CO}{}_2\uparrow$

The alcohol gives no effervescence. (Also, the acid turns blue litmus red; the alcohol does not.)

With sodium hydrogencarbonate, only the carboxylic acid gives CO₂ (effervescence).

Substances that supply oxygen to (or remove hydrogen from) other substances are called oxidising agents. Examples: alkaline potassium permanganate and acidified potassium dichromate, which oxidise alcohols to carboxylic acids.

Substances that can add oxygen to other substances, e.g. alkaline KMnO₄ and acidified K₂Cr₂O₇.

No. Detergents form lather easily with both soft and hard water, because their calcium and magnesium salts are soluble and do not form scum. So a detergent shows no difference. Soap is used instead: with hard water it forms scum and little lather.

No, because detergents lather well in both hard and soft water; soap must be used.

Soap molecules form micelles that trap the oily dirt in their centre. Beating, scrubbing or agitating helps the soap solution reach the dirt between the fibres and loosens the dirt particles, so they are taken up by the micelles and rinsed away with water.

It loosens the dirt and helps the soap micelles trap it so it can be washed away.

Six C–H bonds and one C–C bond: $6+1=7$.

(b) 7 covalent bonds

The ending "-one" shows the ketone group, >C=O.

(c) ketone

Incomplete combustion gives a sooty flame, and the unburnt carbon deposits on the vessel. The air holes of the stove should be cleaned.

(b) the fuel is not burning completely.

Carbon (2, 4) needs 4 more electrons, hydrogen needs 1, and chlorine (2, 8, 7) needs 1. None of them can easily gain or lose electrons, so they share them. Carbon shares one pair with each of three hydrogen atoms and one pair with the chlorine atom.

Each shared pair is a single covalent bond. Carbon now has 8 electrons, each H has 2, and Cl has 8. Because the electrons are shared and no ions form, the bonds are covalent, and CH₃Cl exists as separate molecules with weak forces between them.

Carbon forms four single covalent bonds by sharing one electron pair each with three H atoms and one Cl atom.

See the four electron-dot structures above.

A homologous series is a series of compounds with the same functional group and similar chemical properties, in which each member differs from the next by a –CH₂– unit (14 u in molecular mass).

Example, the alcohols: CH₃OH (methanol), C₂H₅OH (ethanol), C₃H₇OH (propanol), C₄H₉OH (butanol). All have the –OH group and react alike, while their physical properties, such as boiling point, change gradually as the chain gets longer.

A series of compounds with the same functional group, differing by CH₂, e.g. methanol, ethanol, propanol.

The sodium hydrogencarbonate test is the easiest: only ethanoic acid gives CO₂.

A soap molecule has an ionic (hydrophilic) head that dissolves in water and a long hydrocarbon (hydrophobic) tail that does not. In water, the tails keep away from the water by clustering together inside, while the heads face outward towards the water. This cluster is a micelle.

No, micelles do not form in ethanol. Ethanol is an organic solvent in which the hydrocarbon tail also dissolves, so there is no reason for the tails to cluster together.

Micelles form because the hydrocarbon tails avoid water while the ionic heads stay in it; in ethanol both parts dissolve, so no micelles form.

• They burn in air to give out a lot of heat and light.
• Most of them burn with a clean flame when the air supply is enough.
• They have a suitable ignition temperature, so they burn steadily and their burning can be controlled.
• They are easily available (coal, petroleum, natural gas, wood) and easy to store and transport.

Because they release a large amount of heat and light on burning, burn at a controllable rate, and are easily available.

Hard water contains calcium and magnesium salts. Soap (the sodium salt of a long-chain fatty acid) reacts with these ions to form calcium and magnesium salts of the fatty acid, which are insoluble in water. They separate out as a sticky white solid called scum, and much of the soap is wasted.

Soap reacts with the Ca²⁺ and Mg²⁺ ions of hard water to form insoluble salts, which separate as scum.

Soap solution is basic (alkaline). It turns red litmus blue; blue litmus stays blue.

Red litmus turns blue; blue litmus is unchanged.

Hydrogenation is the addition of hydrogen to an unsaturated compound (with C=C or C≡C bonds) in the presence of a catalyst such as nickel or palladium:

$\mathrm{R}{}_2\mathrm{C}=\mathrm{CR}{}_2+\mathrm{H}{}_2\stackrel{\mathrm{Ni}}{\to }\mathrm{R}{}_2\mathrm{CH}-\mathrm{CHR}{}_2$

Application: vegetable oils, which are unsaturated, are hydrogenated to make solid fats such as vanaspati ghee.

Addition of hydrogen with a nickel catalyst; used to turn vegetable oils into vanaspati ghee.

Addition reactions happen in unsaturated hydrocarbons. C₃H₆ (propene, CₙH₂ₙ) has a double bond and C₂H₂ (ethyne, CₙH₂ₙ₋₂) has a triple bond. C₂H₆, C₃H₈ and CH₄ are saturated alkanes (CₙH₂ₙ₊₂).

C₃H₆ and C₂H₂

• Bromine water test: add bromine water (reddish-brown) to each. An unsaturated hydrocarbon decolourises it at once, because bromine adds across the double or triple bond. A saturated hydrocarbon does not.
• Burning test: unsaturated hydrocarbons burn with a yellow, sooty flame; saturated ones usually burn with a clean blue flame.

Bromine water is decolourised by unsaturated hydrocarbons but not by saturated ones.

• Most dirt is oily, and oil does not dissolve in water.
• A soap molecule has a hydrophobic hydrocarbon tail and a hydrophilic ionic head.
• In soapy water, the tails dissolve in the oily dirt while the heads stay in the water. The molecules arrange themselves around the oil drop with tails inside and heads outside, forming a micelle that traps the dirt in its centre.
• Because the heads are charged, the micelles repel each other and stay dispersed in the water as an emulsion instead of joining up.
• Rinsing with water washes away the micelles with the dirt in them, so the cloth becomes clean.

Soap molecules form micelles around oily dirt (tails in the oil, heads in the water), and the micelles are washed away with water.