Classification by reaction mechanism

In modern organic chemistry organic reactions can be classified by their mechanisms. A detailed description of the changes in structure and bonding that take place in the course of a reaction, and the sequence of such events is called the reaction mechanism. A reaction mechanism should include a representation of possible electron reorganization, as well as the identification of any intermediate species that may be formed as the reaction progresses. In studying the reaction mechanisms, we ascertain the order and the way old chemical bonds are broken and new ones are made in the course of a reaction. When we classify reactions by their mechanisms, our attention is attracted above all by the way a covalent bond in a reacting molecule is cleaved Since chemical reactions involve the breaking and making of bonds, a consideration of the movement of bonding (and non-bonding) valence shell electrons is essential to this understanding. If a covalent single bond is broken so that one electron of the shared pair remains with each fragment,В this bond-breaking is called homolysis. If the bond breaks with both electrons of the shared pair remaining with one fragment, this is called heterolysis. The products of bond breaking are not stable in the usual sense, and cannot be isolated for prolonged study. Such species are referred to as reactive intermediates.

Reaction mechanisms provide details on how atoms are shuffled and reassembled in the formation of products from reactants.В

Chain reactions. Chain reactions occur in a sequence of steps, in which the product of each step is a reagent for the next. Chain reactions generally involve three distinct processes: an initiation step that begins the reaction, a series of chain-propagation steps, and, eventually, a termination step.

ВВ Polymerization reactions are chain reactions, and the formation of Teflon from tetrafluoroethylene is one example. In this reaction, a peroxide (a compound in which two oxygen atoms are joined together by a single covalent bond) may be used as the initiator. Peroxides readily form highly reactive free-radical species (a species with an unpaired electron) that initiate the reaction.

ВВ Photolysis reactions. Photolysis reactions are initiated or sustained by the absorption of electromagnetic radiation. One example is the decomposition of ozone to oxygen in the atmosphere. Another example is the synthesis of chloromethane from methane and chlorine, which is initiated by light. This reaction, coincidentally, is also a chain reaction. It begins with the endothermic reaction of a chlorine molecule (Cl₂) to give chlorine atoms, a process that occurs under ultraviolet irradiation. When formed, some of the chlorine atoms recombine to form chlorine molecules, but not all do so. If a chlorine atom instead collides with a methane molecule, a two-step chain propagation occurs. The first propagation step produces the methyl radical (CH₃). This free-radical species reacts with a chlorine molecule to give the product and a chlorine atom, which continues the chain reaction for many additional steps. Possible termination steps include combination of two methyl radicals to form ethane (CH₃CH₃) and a combination of methyl and chlorine radicals to give chloromethane.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. How do we call a process when one or more substances are converted into one or more different substances? 2. When did the concept of a chemical reaction appear? 3. What is synthesis? 4. How is the ratio of reactants and products in a chemical reaction called? 5. What reaction is said to be endothermic? 6. What conditions are necessary to begin a reaction? 7. What factors influence the reaction rate?

Ex.2. Match the words with their definitions:

1. precipitate	a. a large molecule consisting of chains or rings of linked monomer units, usually characterized by high melting and boiling points;
2. reduction	b. a chemical reaction in which the solute and solvent react to form a new compound;
3. elimination	c) an insoluble solid that emerges from a liquid solution;
4. fermentation	d. any chemical reaction that involves the gaining of electrons;
5. polymer	e. a polysaccharide that is composed of glucose monomers and is the main constituent of the cell wall of plants;
6. cellulose	f. any of a group of chemical reactions induced by microorganisms or enzymes that split complex organic compounds into relatively simple substances
7. solvolysis	g. a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism.

Ex.3. Say whether the following statements are true or false:

1. A chemical reaction rearranges the constituent atoms of the reactants to create different substances. 2. In reactions under normal conditions matter is neither created nor destroyed. 3. One mole of any element or compound contains different number of atoms or molecules. 4. Entropy is a measure of the number of ways in which energy can be distributed in any system. 5. Starch and cellulose are members of a class of compounds called hydrocarbons. 6. If the bond breaks with both electrons of the shared pair remaining with one fragment, this is called homolysis. 7. A detailed description of the changes in structure and bonding that take place in the course of a reaction, and the sequence of such events is called the reaction mechanism.

Ex.4. Insert the necessary word:

1. When making a new substance from other substances, chemists say that they carry out a.... 2. In an... reaction, energy as heat is evolved. 3. Chemical reactions commonly need an initial input of... to begin the process. 4. Several factors influence... rates, including the concentrations of reactants, the temperature, and the presence of catalysts. 5. The... that separates is called a precipitate. 6. Simple... reactions include the reactions of an element with oxygen. 7.... reactions occur in a sequence of steps in which the product of each step is a reagent for the next. (energy, synthetic, chain, reaction, redox, exothermic, solid)

UNIT III TYPES OF BONDS

Chemical bonds allow all of the elements to combine in a variety of ways to create everything on Earth. Without these chemical bonds life as we know it would not be able to exist as every living organism is created from compounds of elements that work together in specific ways.

ВВВВВВВВ A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The strength of chemical bonds varies considerably; there are "strong bonds" such as covalent or ionic bonds and "weak bonds" such as Dipole-dipole interaction, the London dispersion force and hydrogen bonding.

ВВ Since opposite charges attract via a simple electromagnetic force, the negatively charged electrons that are orbiting the nucleus and the positively charged protons in the nucleus attract each other. An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond. A chemical bond is a region that forms when electrons from different atoms interact with each other. The electrons that participate in chemical bonds are the valence electrons, which are the electrons found in an atom's outermost shell.

ВВ In general, strong chemical bonding is associated with the sharing or transfer of electrons between the participating atoms. The atoms in molecules, crystals, metals and diatomic gases are held together by chemicalbonds, which dictate the structure and the bulk properties of matter. The number of bonds an atom forms corresponds to its valence. The amount of energy required to break a bond and produce neutral atoms is called the bond energy. All bonds arise from the attraction of unlike charges according to Coulomb's law. The principal types of a chemical bond are the ionic, covalent, metallic, and hydrogen bonds. The ionic and covalent bonds are idealized cases, however; most bonds are of an intermediate type.

Ionic Bonds

ВВ An ionic bond is formed when one atom accepts or donates one or more of its valence electrons to another atom. These chemical bonds are created between ions where one is a metal and one is a non-metal element. When an atom (or group of atoms) gains or loses one or more electrons, it forms an ion. Ions have either a net positive or net negative charge. Positively charged ions are attracted to the negatively charged 'cathode' in an electric field and are called cations. Anions are negatively charged ions named as a result of their attraction to the positive 'anode' in an electric field. Every ionic chemical bond is made up of at least one cation and one anion.

ВВВВВВВВ The atoms of metallic elements, e.g., those of sodium, lose their outer electrons easily, while the atoms of nonmetals, e.g., those of chlorine, tend to gain electrons. The highly stable ions that result retain their individual structures as they approach one another to form a stable molecule or crystal. Sodium chloride (NaCl) is the classic example of ionic bonding

ВВВВВВВВ Ionic bonding is not limited by simple binary systems, however. An ionic bond can occur at the center of a large covalently bonded organic molecule such as an enzyme. In this case, a metal atom, like iron, is both covalently bonded to large carbon groups and ionically bonded to other simpler inorganic compounds (like oxygen).

ВВ Ionic bonds form many compounds every day.

Covalent Bonds

ВВВВВВВВ A single covalent bond is created when two atoms share a pair of electrons. If the atoms share more than two electrons, double and triple bonds are formed, because each shared pair produces its own bond.

ВВВВВВВВВВВВВВВ By sharing their electrons, both atoms are able to achieve a highly stable electron configuration corresponding to that of an inert gas. Multiple covalent bonds are common for certain atoms depending upon their valence configuration. For example, a double covalent bond, which occurs in ethylene (C₂H₄), results from the sharing of two sets of valence electrons. Atomic nitrogen (N₂) is an example of a triple covalent bond.

ВВВВВВВВ The polarity of a covalent bond is defined by any difference in electronegativity of the two participating atoms. Bond polarity describes the distribution of electron density around two bonded atoms. For two bonded atoms with similar electronegativities, the electron density of the bond is equally distributed between the two atoms. This is a nonpolar covalent bond. The electron density of a covalent bond is shifted towards the atom with the largest electronegativity. This is a polar covalent bond.

ВВВВВВВВ A coordinate covalent bond is formed when one atom donates both of the electrons to form a single covalent bond. These electrons originate from the donor atom as an unshared pair. Some elements form very large molecules by forming covalent bonds. When these molecules repeat the same structure over and over in the entire piece of material, the bonding of the substance is called network covalentВВВВВВВВ Covalent bonds are of particular importance in organic chemistry because of the ability of the carbon atom to form four covalent bonds. These bonds are oriented in definite directions in space, giving rise to the complex geometry of organic molecules.

Metallic and Hydrogen Bonds

ВВВВВВВВ Unlike the ionic and covalent bonds, which are found in a great variety of molecules, the metallic and hydrogen bonds are highly specialized.

ВВВВВВВВ The metallic bond is responsible for the crystalline structure of pure metals. This bond cannot be ionic because all the atoms are identical, nor can it be covalent, in the ordinary sense, because there are too few valence electrons to be shared in pairs among neighboring atoms. Instead, the valence electrons are shared collectively by all the atoms in the crystal. This is the strongest of the three major bonds because the electrons are shared in more than just the first shells. The more shells involved in sharing electrons, the stronger the bond. Compounds formed by metallic bonds do not completely break until the metal is boiled, usually at a very high temperature. Steel is an example of a metallic bond. Without metallic bonds and the properties of metal modern life would not be possible. Steel is used extensively in every modern society. Metal is also used in electronics that form the basic components in computers and other essential modern conveniences

ВВВВВВВВ Hydrogen bonding occurs between a hydrogen atom and an electronegative atom, e.g., oxygen, fluorine, chlorine. It's a strong electrostatic attraction between two independent polar molecules, i.e., molecules in which the charges are unevenly distributed. The bond is muchВ weaker than an ionic or a covalent bond, but stronger than van der Waals forces. A hydrogen bond is classified as a type of weak chemical bond. It is responsible for the structure of ice.

ВВВВВВВВ The reason hydrogen bonding occurs is because the electron is not shared evenly between a hydrogen atom and a negatively-charged atom.

ВВВВВВВВ Hydrogen bonding does not occur in molecules with nonpolar covalent bonds. Any compound with polar covalent bonds has the potential to form hydrogen bonds.

ВВВВВВВВ Hydrogen bonds account for some important qualities of water:

В· Вwater remains liquid over a wide temperature range;

В· Вwater has an unusually high heat of vaporization.

В· Вwater has a much higher boiling point than other hydrides.

ВВВВВВВВ Hydrogen bonding is most significant between hydrogen and highly electronegative atoms. The length of the chemical bond depends upon its strength, pressure, and temperature. The bond angle depends on the specific chemical species involved in the bond. The strength of hydrogen bonds ranges from very weak to very strong.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What is a chemical bond caused by? 2. What is bond energy? 3. What are the principal types of a chemical bond? 4. When is an ionic bond formed? 5. How is the polarity of a covalent bond defined? 6. What bond is responsible for the crystalline structure of pure metals? 7.What is the reason of hydrogen bonding?

Ex.2. Match the words with their definitions:

1. attraction	a. a measure of the combining power of an element with other atoms when it forms chemical compounds or molecules;
2. charge	b. a positively charged ion that is attracted to the cathode in electrolysis;
3. valence	c. a protein that functions as a catalyst for a chemical reaction;
4. cation	d. the physical property of matter that causes it to experience a force when placed in an electromagnetic field;
5. bonding	e.a system that uses only two values (as 0 and 1 or yes and no) to represent codes and data;
6.binary system	f. the force attracting atoms to each other and binding them together in a molecule;
7. enzyme	g. an interaction that accounts for the association of atoms into molecules, ions, crystals and other stable species that make up the familiar substance.

Ex.3. Say whether the following statements are true or false:

1. A chemical bond is a region that forms when electrons from different atoms interact with each other. 2. The bond is caused by the electrostatic force of attraction between similar charges. 3. The negatively charged electrons orbiting the nucleus and the positively charged protons in the nucleus repel each other. 3. The atoms in molecules, crystals, metals are held together by chemical bonds which dictate the properties of mater. 4. Covalent bonds are created between ions where one is a metal and one is a non-metal element. 5. The polarity of a covalent bond is defined by any difference in electropositivity. 6. A coordinate covalent bond is formed when one atom donates both of the electrons to form a single covalent bond. 7. Metallic bond is the strongest of the three major bonds because the electrons are shared in more than just the first shells.

Ex.4. Insert the necessary word:

1. The extreme mobility of the electrons in a metal explains its high thermal and electrical.... 2. Hydrogen bonding does not occur in molecules with nonpolar... bonds. 3. Chemical bonds allow all of the elements to... in a variety of ways to create everything on the Earth. 4. Covalent bonds are of practical importance in... chemistry because of the ability of the carbon atom to form four covalent bonds. 5.... covalent bonds are common for certain atoms depending upon their valence configuration. 6. An ionic bond can occur at the centre of a large covalently bonded organic molecule such as.... 7. All bonds arise from the... of unlike charges. (multiple, combine, attraction, conductivity, enzyme, covalent, organic)ВВВВВВВВВВВВВВВВВВВВВВВВВ

 

UNIT IV Isomerism

Isomerism is the phenomenon when certain compounds, with the same molecular formula, exist in different forms owing to their different organizations of atoms. The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry.

The molecular formula of organic compounds tells you how many atoms of each element are present in a molecule. The different structures lead to different properties.

Two molecules that have the same molecular formula but differ in the way atoms are arranged are called isomers.

Isomers are distinct compounds with different physical properties and often different chemical properties too. The occurrence of isomers (isomerism) is very common in carbon compounds because of the great variety of ways in which atoms carbon can form chains and rings, but you will meet examples in inorganic chemistry too.

The atoms are bonded together in a different order in each isomer. These are called structural isomers.

The order of bonding in the isomer is the same; but the arrangement of atoms in space is different in each isomer. These are called stereoisomers.

The Isomerism tree

Structural Isomerism	Stereoisomerism
Chain Isomerism	Position Isomerism	Functional group Isomerism	GeometricВ Isomerism	Optical Isomerism

Structural isomerism. В “Structural” isomers are widely called “conformational” isomers. The latter term is preferred in the IUPAC system of nomenclature. Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism (where there are two isomers are known as the keto and enol isomers) that will not be introduced here.

Chain isomerism occurs when the way carbon atoms are linked together is different from compound to compound. It is also called nuclear isomerism. Carbon chains can be straight or branched. Butane and methylpropane are examples of chain isomerism.

There are three chain isomers of C₅H₁₂ shown below. Note that these isomers have the same empirical formula as pentane, but different conformations.

Position isomerism can occur when molecule with one (or more) functional groups are situated in different positions on the same carbon
chain in the molecule. For example, there are two isomeric compounds with the molecular formula C₃H₇CI. The –C_l functional group is situated at two different places on the hydrocarbon chain. Positional isomers of alcohols, alkenes, and aromatics are common.

Functional group isomerism. It is sometimes possible for compounds with the same molecular formula to have different functional groups. As well as showing different physical properties (such as boiling point), they have quite different chemical properties because they belong to different homologous series. There are two functional group isomers of which you need to be aware:

• alcohols and ethers

• aldehydes and ketones.

Stereoisomers have identical molecular formula and the atoms are held together in the same order, but the arrangement of atoms in space is different in each isomer. Stereoisomers may possess quite different physical properties, such as melting point, density and solubility in water (e.g. maleic acid and fumaric acid). Ring structures and other steric factors also result in geometric isomerism. They have different spatial arrangements and their molecules are not superimposable. There are two different ways this can happen: geometric isomerism and optical isomerism.

Geometric isomerism can occur in compounds that contain a C=C double bond. That does not allow free rotation about the double bond (unlike a C-C single bond). They are not superimposable. For example, there are two isomers of 1,2-dichloroethane (C2H2C12)depending on whether the chlorine atom are on the same, or opposite sides of the double bond. They are called cis- and trans- isomer respectively.

Optical isomerism occurs when a molecule is asymmetric, i.e. it does not have a center or plane of symmetry. Optical isomerism involves an atom, usually carbon, bonded to four different atoms or groups of atoms. They exist in pairs. It means that there will be two different forms of the molecule that are mirror images of each other – rather like your right and left hand. Such compounds are called optical isomers or enantiomers. The central carbon atom, to which four different atoms or groups are attached, is called an asymmetrical carbon atom. Enantiomers have identical physical constants, such as melting points and boiling points, but are said to be optically active since they can be distinguished from each other by their ability to rotate the plane of polarised light in opposite directions. A mixture of enantiomers in equal proportions is optically inactive, and is called a racemic mixture. The isomers are often labeled D- or compounds too, particularly in the complexes of transition-metal ions with ligands.

Other types of isomerism exist outside this scope. Topological isomers called topoisomers are generally large molecules that wind about and form different shaped knots or loops. Molecules with topoisomers include catenanes and DNA. Topoisomerase enzymes can knot DNA and thus change its topology. There are also isotopomers or isotopic isomers that have the same numbers of each type of isotopic substitution but in chemically different positions. In nuclear physics, nuclear isomers are excited states of atomic nuclei.

REVISION EXERCISES

Ex.1. Answer the following questions:

1.What is the phenomenon of isomerism? 2. How are isomers classified according to the order of atoms or their arrangement in space? 3. Describe the isomerism tree. 4. Characterize each type of isomerism. 5. When are isomers super imposable? 6. What discoveries were made by Woehler and Pasteur to prove isomerism? 7. Who introduced the term isomerism?

Ex.2. Match the words with their definitions:

1. occur	a. to cause (two or more people or things) stop being together, joined or connected
2. arrangement	b. to connect, to combine, to unite
3. introduce	c. the act or process of moving or turning around a central point
4. link	d. the amount of a substance that will dissolve in a given amount of another substance
5.separate	e. to cause something to begin to be used for the first time
6. solubility	f. to happen, to be found or met with
7. rotation	g. the way that things are organized for a particular purpose or activity

Ex.3. Say whether the following statements are true or false:

1.Topological isomers called topoisomers are generally large molecules that wind about and form different shaped knots or loops. 2. Geometric isomers are superimposable. 3. The central carbon atom, to which four different atoms or groups are attached, is called an asymmetrical carbon atom. 4. Chain isomerism occurs when the way carbon atoms are linked together is the same. 5.Two molecules that have the same molecular formula but differ in the way atoms are arranged are called asymmetrical molecules. 6.There are three types of structural isomerism: chain isomerism, positional isomerism and functional isomerism.

Ex.4. Insert the necessary word:

1. The roots of the word isomer are Greek—isos plus meros, or “equal parts.” 2. Stated colloquially, isomers are chemical … that have the same parts but are nonetheless not the same. 3. To make a crude analogy, two bracelets, each consisting of five red and five green beads, could be … in many different isomeric forms, depending on the order of the colours. 4. Each bracelet would have the same parts—that is, the five red and five green beads—but each … would be different. 5. One could also imagine … of those same beads in which pendant chains were attached to a bracelet in

a variety of ways. 6. One might imagine two bracelets of the same red-green order but with …chains attached in different orientations. 7. Such structures also would be analogous to isomers. 8. In a more subtle …, one’s hands can be seen as isomeric. 8. Each hand possesses the same kinds of fingers, but a right … can never be superimposed perfectly on a left hand; they are different. (identical, variation, hand, arranged, combinations, compounds, analogy).

Ex.5. Translate the following text:

History of isomerism

Isomerism was first noticed in 1827, when Friedrich Woehler prepared cyanic acid and noted that although the elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be different only when they had different elemental compositions. After additional discoveries of the same sort were made, such as Woehler's 1828 discovery that urea had the same atomic composition as the chemically distinct ammonium cyanate, Jöns Jakob Berzelius introduced the term isomerism to describe the phenomenon.

In 1849, Louis Pasteur separated tiny crystals of tartaric acid into their two mirror-image forms. The individual molecules of each were the left and right optical stereoisomers, solutions of which rotate the plane of polarized light in opposite directions.ВВВВВ

 

В

 

В UNIT V Hydrocarbons

Simple Hydrocarbons. The simplest hydrocarbons are those that contain only carbon and hydrogen. These simple hydrocarbons come in three varieties depending on the type of carbon-carbon bonds that occur in the molecule. Alkanes are the first class of simple hydrocarbons and contain only carbon-carbon single bonds. The alkanes are named by combining a prefix that describes the number of carbon atoms in the molecule with the root ending “ane”. The names and prefixes for the first ten alkanes are given in the following table:

	Meth-	Methane	CH4	CH4
	Eth-	Ethane	C2H6	CH3CH3
	Prop-	Propane	C3H8	CH3CH2CH3
	But-	Butane	C4H10	CH3CH2 CH2CH3
	Pent-	Pentane	C5H12	CH3CH2CH2 CH2CH3
	Hex-	Hexane	C6H14	CH3CH2CH2 CH2CH2CH3
	Hept-	Heptane	C7H16	CH3CH2CH2 CH2CH2CH2CH3
	Oct-	Octane	C8H18	CH3CH2CH2 CH2CH2CH2CH2CH3
	Non-	Nonane	C9H20	CH3CH2CH2CH2 CH2CH2CH2CH2CH3
	Dec-	Decane	C10H22	CH3CH2CH2CH2 CH2CH2CH2CH2CH2CH3

The chemical formula for any alkane is given by the expression C_nH_2n+2. The structural formula, shown for the first five alkanes in the table, shows each carbon atom and the elements that are attached to it. This structural formula is important when we begin to discuss more complex hydrocarbons. The simple alkanes share many properties in common. All enter into combustion reactions with oxygen to produce carbon dioxide and water vapour. In other words, many alkanes are flammable. This makes them good fuels. For example, methane is the principle component of natural gas, and butane is common lighter fluid.

CH₄ + 2O₂ в†’ CO₂ + 2H₂O

The combustion of methane.

The second class of simple hydrocarbons, the alkenes, consists of molecules that contain at least one double-bonded carbon pair. Alkenes follow the same naming convention used for alkanes. A prefix (to describe the number of carbon atoms) is combined with the ending “ene” to denote an alkene. Ethene, for example is the two-carbon molecule that contains one double bond. The chemical formula for the simple alkenes follows the expression C_nH_2n. Because one of the carbon pairs is double bonded, simple alkenes have two fewer hydrogen atoms than alkanes. Alkynes are the third class of simple hydrocarbons and are molecules that contain at least one triple-bonded carbon pair. Like the alkanes and alkenes, alkynes are named by combining a prefix with the ending “yne” to denote the triple bond. The chemical formula for the simple alkynes follows the expression C_nH_2n-2, e.g. ethyne.

Hydrocarbons. These are compounds composed of carbon and hydrogen.They are generally insoluble in water although those with lighter molecular masses are gases and are slightly soluble. Examples of hydrocarbons include methane - the gas we burn as natural gas, propane (also called liquid petroleum gas) and petroleum jelly.

The carbon atoms join together to form the framework of the compound; the hydrogen atoms attach to them in many different configurations. Hydrocarbons are the principal constituents of petroleum and natural gas. They serve as fuels and lubricants as well as raw materials for the production of plastics, fibers, rubbers, solvents, explosives, and industrial chemicals.

Many hydrocarbons occur in nature. In addition to making up fossil fuels, they are present in trees and plants, as, for example, in the form of pigments called carotenes that occur in carrots and green leaves. More than 98 percent of natural crude rubber is a hydrocarbon polymer, a chainlike molecule consisting of many units linked together. The structures and chemistry of individual hydrocarbons depend in large part on the types of chemical bonds that link together the atoms of their constituent molecules.

Nineteenth-century chemists classified hydrocarbons as either aliphatic or aromatic on the basis of their sources and properties. Aliphatic (from Greek aleiphar, “fat”) described hydrocarbons derived by chemical degradation of fats or oils. Aromatic hydrocarbons constituted a group of related substances obtained by chemical degradation of certain pleasant-smelling plant extracts. The terms aliphatic and aromatic are retained in modern terminology, but the compounds they describe are distinguished on the basis of structure rather than origin.

Aliphatic hydrocarbons are divided into three main groups according to the types of bonds they contain: alkanes, alkenes, and alkynes. Alkanes have only single bonds, alkenes contain a carbon-carbon double bond, and alkynes contain a carbon-carbon triple bond. Aromatic hydrocarbons are those that are significantly more stable than their Lewis structures would suggest; i.e. they possess “special stability.” They are classified as either arenes, which contain a benzene ring as a structural unit, or non-benzenoid aromatic hydrocarbons, which possess special stability but lack a benzene ring as a structural unit.

This classification of hydrocarbons serves as an aid in associating structural features with properties but does not require that a particular substance be assigned to a single class. Indeed, it is common for a molecule to incorporate structural unit characteristic of two or more hydrocarbon families. A molecule that contains both a carbon-carbon triple bond and a benzene ring, for example, would exhibit some properties that are characteristic of alkynes and others that are characteristic of arenes.

Hydrocarbons Classification

Hydrocarbons and compounds derived from them generally fall into three large categories.

Aliphatic hydrocarbons consist of chainof carbon atoms that do not involve cyclic structures. They are often referred to as open-chain or acyclic structures, e.g. propane, pentane, hexane.

Alicyclic or simply cyclic hydrocarbons are composed of carbon atoms arranged in a ring or rings, e.g. cyclopropane, cyclopentane, cyclohexane.

Aromatic hydrocarbons are a special group of cyclic compounds that usually have six-membered rings with alternative single and double bonds. They are classed separately from aliphatic and alicyclic hydrocarbons because of their characteristic physical and chemical properties, e.g. benzene, naphthalene.

Addition of hydrogen: hydrogenation. The addition of hydrogen to a carbon-carbon double bond, called hydrogenation, reduces an alkene to an alkane. The process requires the presence of a metal catalyst, and for this reason, it is also called catalytic reduction. Catalytic reduction of alkenes is a very important reaction in the laboratory. In hydrogenation, both hydrogen atoms are added to the same side of the alkene molecule.

Addition of water: hydration. In the presence of an acid catalyst, often 60% aqueous sulfuric acid, water adds to alkenes to produce alcohols. Hydrogen adds to the carbon of the double bond with the greater number of hydrogen; OH adds to alkenes in accordance with Markovnikov^,s rule.

The addition of water to an alkene is called hydration. Hydration of alkenes is a very important reaction both in the chemical industry and in biological systems.

Sources and uses of hydrocarbons. В Petroleum and its associated natural gases are now the major source of hydrocarbons. Natural gas is composed principally of methane (CH₄). Ethane (C₂H₆) and propane (C₃H₆) typically represent 5 to 10 per cent of the total, along with traces of C₄ and C₅ hydrocarbons. The gas is freed of various unwanted contaminants and then it is utilized almost exclusively as fuel. Valuable side products of petroleum cracking provide raw materials for the petrochemical industry. Ethylene (C₂H₄) and propylene (C₃H₆) are principal starting points for the manufacture of chemicals, medicines, and polymers.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What are naturally occurring hydrocarbons? 2. What classifications are hydrocarbons divided into? 3. What purposes do the classifications serve to? 4. What are hydrocarbons composed of? 5. What are alkanes? Give examples. 6. What is the chemical formula of alkanes? 7. What are the chemical reactions of alkanes? 8. What are the three classes of hydrocarbons? 9. What are the rules of naming them? 10. What other classifications are hydrocarbons divided into? 11. What is hydrogenation? 12.Why is hydrogenation called catalytic reduction? 13. What is hydration? 14. What are sources and uses of hydrocarbons?

Ex.2. Match the words with their definitions:

1..liquid	a. a material (such as coal, oil, or gas) that is burned to produce power;
2. medicine	b. to cause to combine until there is no further tendency to combine;
3. fuel	c. a special part or characteristic, quality, structure, form;
4. bond	d. a substance that is used in treating disease or relieving pain, andВ that is usually in the form of a pill or liquid;
5. feature	e. capable of flowing freely like water: not a gas or a solid;
6. saturate	f. an attractive force that holds together atoms, ions, or groups of atoms in a molecule or crystal;

Ex.3. Say whether the following statements are true or false:

1. The structures and chemistry of individual hydrocarbons do not depend in large part on the types of chemical bonds that link together the atoms of their constituent molecules. 2. The addition of hydrogen to an alkene is called hydration. 3. Petroleum and its associated natural gases are now the major source of hydrocarbons. 4. The addition of hydrogen to a carbon-carbon double bond, called hydrogenation, reduces an alkene to an alkane. 5. Aromatic hydrocarbons are a special group of cyclic compounds that usually have six-membered rings with alternative triple and double bonds. 6. Aliphatic hydrocarbons are divided into three main groups according to the types of bonds they contain: alkanes, alkenes, and alkynes.

Ex.4. Insert the necessary word:

1. Alkanes are described as saturated hydrocarbons, while …, alkynes, and aromatic hydrocarbons are said to be unsaturated. 2. Hydrocarbons with single carbon-carbon bonds are referred to as being …whilst any hydrocarbon that contains a double bond is said to be …. 3.Saturated hydrocarbons are also called the …, while the unsaturated hydrocarbons include both those molecules that contain carbon-carbon double bonds (referred to as the alkenes) and those that contain carbon-carbon triple bonds (referred to as the …). 4. … and …are natural products that have resulted from the … of organic compounds from plants and animals that lived millions of years ago. 5. They are found today as petroleum, which are … of hydrocarbons containing up to 30 or 40 carbon atoms. 6. Different components of …ВВ can be isolated by fractional distillation. 7. These hydrocarbons are good sources of fuels, the so-called „fossil fuels‟. 8. As mentioned previously, the global production of such fossil fuels is 3 billion tonnes. 9. As they are produced in such large quantities,ВВ … of the environment with these fossil fuels is of concern. 10. The major route of entry into the environment isn’t through spectacular … such as the oil spills from ships, but rather through our daily activities. 11. Pumping fuel into cars, and oil spilled onto the road as a result of old faulty cars are major contributors. (disasters, alkanes (2), mixtures, decay, saturated, alkynes, pollution, alkenes (2), petroleum, unsaturated).