Chapter 11: HydrocarbonsLong Questions with Answers

Food

Carbohydrates (starch, sugar), proteins, fats and vitamins in our food are all organic compounds; they give energy and build the body.

Fuels

Most of the fuels we use every day, natural gas, LPG, CNG, petrol, diesel and kerosene, are simple hydrocarbons. Coal and wood are also organic.

Clothing

Natural fibres (cotton, wool, silk) and synthetic fibres (nylon, polyester, acrylic) are organic.

Medicines

Pain relievers (aspirin, paracetamol), antibiotics (penicillin), vitamins and anaesthetics are organic compounds.

Materials

Hydrocarbons serve as a feedstock to prepare plastics, medicines, synthetic fibres, paints and varnishes. Rubber, paper, dyes, soaps, detergents, insecticides and fertilisers (urea) are organic too.

Life processes

Biomolecules like proteins, enzymes, carbohydrates, lipids, vitamins and nucleic acids (DNA, RNA) are organic; life is based on carbon compounds.

A huge number of compounds

The number of compounds formed by carbon is far more than the total formed by all the rest of the elements put together. Several million organic compounds are known, including all the compounds of living things. This is due to some unique properties of carbon.

1. Tetravalency

Carbon is at the centre of the periodic table; it is generally not possible for it to gain or lose electrons to form ionic bonds. Therefore it forms four covalent bonds.

2. Small size, strong bonds

Because of its small size, these covalent bonds are short and strong, enabling carbon to give strong and stable bonds with itself and with hydrogen, oxygen and nitrogen.

3. Catenation

The self-linking property of carbon is called catenation; due to it carbon forms long, straight and branched chains and rings. No other element does this to the same extent.

4. Multiple bonds

Carbon forms single, double and triple bonds (ethane, ethene, ethyne).

5. Isomerism

The same molecular formula can have different structures (n-butane and iso-butane), multiplying the number of compounds.

Because of all this, the chemistry of carbon compounds is studied as a separate branch, organic chemistry.

Functional group

A functional group is the atom or group of atoms in a molecule that decides its characteristic chemical reactions (for example –OH in alcohols, C=C in alkenes).

C–C single bond

In alkanes every carbon is saturated: all four valencies are used by strong single bonds. The C–C bond is non-polar (identical atoms) and strong (348 kJ mol⁻¹), so reagents find no reaction site. Alkanes are "paraffins", inert to acids, bases and oxidising agents. The C–C bond gives the molecule no special reaction: it is just the skeleton.

C=C double bond

In alkenes the double bond contains a second, weaker bond (a pi bond, about 266 kJ mol⁻¹ against 348 for the first) whose electrons are more exposed and easily attacked. Reagents such as H₂, Br₂, HCl and H₂O add across the double bond, turning it into a single bond:

So all alkenes show the same typical addition reactions because of their C=C: the double bond is their functional group. It also decolourises bromine water, a test for unsaturation that alkanes do not give.

Why a system

As a result of the great complexity and large number of organic compounds, it is not possible to name each compound individually. IUPAC (International Union of Pure and Applied Chemistry) devised a systematic way of naming them.

Parts of a name

Steps

Examples

Combustion

Alkanes burn in oxygen or air to form CO₂ and H₂O with the evolution of a large amount of heat.

Uses

Cautions

Incomplete combustion gives poisonous CO; CO₂ adds to global warming; gas leaks can cause explosions. Appliances must be well ventilated and serviced.

Reason

Natural gas is mostly methane. When it leaks it mixes with the air of the kitchen; between about 5 % and 15 % methane the mixture is explosive. A mixture of natural gas and air may explode when ignited. Any spark (a match, a lighter, a switch, a refrigerator motor) supplies the activation energy; because methane and oxygen are already mixed throughout the room, the whole mixture burns almost instantly and the hot gases expand violently.

Detecting a leak

Pure methane has no smell; gas companies add a strong-smelling compound (a mercaptan) so leaks can be noticed.

How to avoid

Neem (Azadirachta indica) is called the "village pharmacy" because almost every part of it contains useful organic compounds (such as azadirachtin and nimbin).

Neem also cleans the air and gives shade. Medicines from plants should still be used with a doctor's advice.

All contain carbon chains and rings with functional groups, made in laboratories or taken from plants and microbes.

Definition

Alkanes are the simplest family of hydrocarbons with only carbon-carbon and carbon-hydrogen single bonds. They are called saturated hydrocarbons because all four valencies of carbon are fully used by hydrogen atoms or other carbon atoms.

General formula

Structure

Each carbon forms four single covalent bonds. Methane, the simplest alkane, is called the parent hydrocarbon.

Unreactivity

Alkanes are called paraffins ("little affinity"). The electronegativities of C (2.6) and H (2.1) are close, so the bonds are almost non-polar; acids, bases and oxidising agents find no reaction site. They react only with chlorine in UV light (substitution) and with oxygen (combustion).

Physical properties

Non-polar, insoluble in water, soluble in organic solvents; melting and boiling points rise with chain length (C₁-C₄ gases, C₅-C₁₇ liquids, higher waxy solids).

1. Cracking of higher hydrocarbons

Hydrocarbons of higher molecular mass, less in demand, are broken into smaller ones, more in demand, by heating at high temperature with a catalyst. Naphtha is heated at around 500 °C with zeolite to give alkanes and alkenes with 5 to 10 carbon atoms.

2. Hydrogenation of alkenes and alkynes

Alkanes are prepared by reducing alkenes and alkynes with hydrogen gas in the presence of nickel. It is an addition reaction. Methane cannot be prepared by this method.

3. Reduction of alkyl halides

Alkyl halides are reduced to alkanes with hydrogen generated by the reaction of zinc with hydrochloric acid.

Substitution

Alkanes give substitution reactions: hydrogen of the alkane is replaced by an atom or group of atoms like a halogen. Alkanes react with halogens, especially chlorine, to give alkyl halides.

Photochemical

Since these reactions are carried out in the presence of UV light, they are called photochemical substitution reactions. Light splits Cl₂ into chlorine atoms.

Step by step

The reaction may proceed and all the hydrogen atoms of methane are successively replaced by chlorine atoms.

Structural formulas

In the dot-and-cross formula each carbon's four electrons (dots) pair with one electron from each hydrogen (crosses) or from the next carbon. Every carbon has an octet and every hydrogen a duplet.

Rule

An alkane CₙH₂ₙ₊₂ has (n − 1) C–C bonds and (2n + 2) C–H bonds, (3n + 1) bonds in all.

Comparison

Large number

Organic molecules are usually large and complex: biomolecules, pharmaceuticals and synthetic fibres.