Chapter 11Hydrocarbons

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).

The unreactivity of alkanes may be explained on the basis of the non-polarity of the bonds present in them. The electronegativity values of carbon (2.6) and hydrogen (2.1) do not differ appreciably, and the bonding electrons in C–H and C–C bonds are almost equally shared. This makes alkanes almost non-polar. Reagents like acids, bases and oxidising agents find no reaction site in alkane molecules to which they could be attached.

That is why they are called paraffins ("little affinity"). They react only under harsh conditions: with chlorine in UV light, and with oxygen when ignited.

Methane and oxygen in air do not react at room temperature because the activation energy is high. A spark or flame supplies this energy, and the reaction is highly exothermic:

In a mixture, methane molecules are already in contact with oxygen everywhere, so once ignited, the whole mixture burns almost instantly. The heat released makes the gas products (CO₂ and steam) expand suddenly and violently, producing a pressure wave: an explosion. Mixtures of about 5-15 % methane in air are explosive.

Organic compounds have generally low melting and boiling points, while inorganic compounds (mostly ionic) have high ones.

Many organic compounds are gases or liquids at room temperature; most inorganic salts are high-melting solids.

Sunlight (UV light) supplies the energy needed to break the Cl–Cl bond, splitting chlorine molecules into very reactive chlorine atoms (free radicals):

A chlorine atom takes a hydrogen atom from methane (giving HCl and a methyl radical CH₃·), which then reacts with another Cl₂ molecule to give CH₃Cl and a new Cl atom, and the chain continues. Without light no chlorine atoms form and there is no reaction in the dark. These substitution reactions are carried out in the presence of UV light, so they are called photochemical substitution reactions.

n-Butane boils higher (−0.5 °C) than iso-butane (−12 °C). Both are C₄H₁₀ with the same number of electrons.

The straight-chain n-butane molecules are long and can lie close alongside each other, with a large area of contact, so the attractive forces between them are stronger. The branched iso-butane molecule is compact and nearly spherical, with a smaller surface of contact, so the forces are weaker and it boils at a lower temperature. Branching lowers the boiling point.

Water is a polar solvent whose molecules are held together by hydrogen bonds. Most organic compounds, especially hydrocarbons, are non-polar or only slightly polar covalent molecules.

Their molecules cannot attract water molecules strongly enough to break the hydrogen bonds between water molecules, so they do not mix ("like dissolves like"). They dissolve instead in non-polar organic solvents such as benzene, ether and kerosene.

Exceptions are organic compounds with polar groups that can hydrogen-bond with water: sugar, ethanol, acetic acid and urea dissolve well.

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.