Chapter 11: HydrocarbonsShort Questions with Answers

Organic chemistry is the chemistry of carbon compounds.

The number of compounds formed by carbon is far more than the total number formed by all the rest of the elements put together.

Natural gas (methane) burnt completely gives CO₂ and water. Under controlled oxidation (limited oxygen, a catalyst and suitable temperature) it gives useful products:

By catalytic cracking. Naphtha, a mixture of liquid hydrocarbons from the fractional distillation of petroleum, is heated at around 500 °C in the presence of a catalyst called zeolite, to give hydrocarbons which have five to ten carbon atoms (alkanes and alkenes).

It is an isomer of n-butane (CH₃–CH₂–CH₂–CH₃): same molecular formula, different structure.

Glucose (fruits, honey), sucrose (sugar cane), starch (wheat, rice, potatoes), cellulose (cotton, wood), proteins (milk, meat), methane (natural gas), citric acid (lemons), urea (urine), ethanol (fermented fruit).

IUPAC name: 2-methylbutane (C₅H₁₂, common name isopentane).

They increase with the number of carbon atoms. Bigger molecules have more electrons and a larger surface, so the attractive forces between molecules (London forces) are stronger and more energy is needed to separate them.

So C₁-C₄ are gases, C₅-C₁₇ liquids and higher alkanes waxy solids (paraffin wax).

A structural formula shows the symbols for the atoms present in a compound connected by short lines which represent the bonds. It shows how the atoms are joined, unlike the molecular formula. Ethane: H₃C–CH₃ with every C–H bond drawn.

Zinc. It loses electrons: Zn → Zn²⁺ + 2e⁻ (it becomes ZnCl₂). The electrons, via atomic hydrogen, reduce the alkyl halide. Oxidation and reduction happen together.

Broken: C–C and C–H bonds of ethane and O=O bonds of oxygen. Formed: C=O bonds of CO₂ and O–H bonds of water. 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. More energy is released in forming the new bonds, so the reaction is exothermic.

Hydrogen chloride (HCl) in every step, and further substitution products: dichloromethane CH₂Cl₂, trichloromethane (chloroform) CHCl₃ and tetrachloromethane (carbon tetrachloride) CCl₄. (Traces of ethane also form when two methyl groups join.)

The chemistry of carbon compounds. Organic compounds contain carbon as an essential element; most also contain hydrogen and oxygen.

Carbonates, cyanides, carbides, cyanates and the oxides of carbon (CO, CO₂) are classified as inorganic compounds.

The self-linking property of carbon; because of it carbon forms long, straight and branched chains and rings.

Carbon is at the centre of the periodic table and it is generally not possible for it to gain or lose four electrons to form ionic bonds. Therefore it forms four covalent bonds, which are short and strong because of its small size.

Compounds in which carbon forms single bonds with other atoms: C₂H₆, CH₃Cl, CH₃OH, CH₃NH₂.

Organic compounds containing carbon and hydrogen only. Fuels: natural gas, LPG, CNG, petrol, diesel and kerosene oil.

Alkanes (single bonds), alkenes (C=C), alkynes (C≡C) and aromatic hydrocarbons (benzene rings).

Because all four valencies of carbon in them are fully used either by hydrogen atoms or by other carbon atoms through single bonds. In ethane both carbons are fully saturated.

CₙH₂ₙ₊₂. For n = 6: C₆H₁₄ (hexane).

Root (number of carbons in the longest chain), suffix (class of compound, -ane), prefix (branches and their positions).

To give the branch the lowest possible number, so every compound has one correct name: 2-methylbutane, not 3-methylbutane.

Both are C₄H₁₀. n-Butane is a straight chain, CH₃–CH₂–CH₂–CH₃; iso-butane (2-methylpropane) is branched, CH₃–CH(CH₃)–CH₃. Such compounds are isomers.

Each carbon shares one pair with the other carbon and one pair with each of three hydrogens: 7 shared pairs in all.

Mainly as a fuel, and to make hydrogen gas, carbon black, carbon disulphide, chloroform and hydrocyanic acid.

A process in which hydrocarbons of higher molecular mass, which are less in demand, are broken into smaller hydrocarbons, more in demand, by heating at high temperature with a catalyst. It increases the supply of petrol and balances demand with supply.

By hydrogenation: hydrogen gas is added in the presence of nickel at 200 °C. CH₂=CH₂ + H₂ → CH₃–CH₃. It is an addition (reduction) reaction.

A reaction in which atoms (such as hydrogen) are added to an unsaturated compound, so that a double or triple bond becomes a single bond. Alkanes cannot give it.

Reduction of the alkyl halide with atomic hydrogen produced by zinc and hydrochloric acid.

Atomic hydrogen, [H], produced at the moment of reaction (as when zinc reacts with HCl). It is much more reactive than ordinary H₂ gas, so it can reduce alkyl halides.

By hydrogenation of vegetable oils: hydrogen is added to the C=C double bonds of the oil using a nickel catalyst; the liquid oil becomes a solid fat.

Paraffin means "little affinity". Alkanes show little chemical affinity for other substances and are chemically inert to most laboratory reagents.

A reaction in which one atom or group of atoms is replaced by another atom or group of atoms, such as hydrogen of an alkane by chlorine.

Because it is carried out in the presence of UV light. Light provides the energy that splits Cl₂ molecules into reactive chlorine atoms.

CH₃Cl chloromethane, CH₂Cl₂ dichloromethane, CHCl₃ trichloromethane (chloroform), CCl₄ tetrachloromethane (carbon tetrachloride), with HCl at each step.

Carbon monoxide (CO), soot (carbon particles) and water, when the oxygen supply is too small. CO is poisonous.

A mixture of natural gas (methane) and air may explode when ignited: the whole mixture burns almost instantly, and the hot gases expand violently.

Chloroform (CHCl₃) is a solvent and was once used as an anaesthetic. Carbon tetrachloride (CCl₄) is a solvent and was used in fire extinguishers and dry cleaning (now limited because it is toxic).