Unit VI alkanes, alkenes, alkynes

Alkanes

A chain of carbon atoms that contains the maximum number of hydrogen atoms has the general formula C_nH_2n+2 (n = number of carbon atoms). This type of hydrocarbon is called an alkane (a historical name is paraffin). Alkanes are saturated hydrocarbons, they consist only ofВ carbon and hydrogen atoms and all bonds are single bonds. The simplest possible alkane is methane CH₄.В Alkanes can exist in straight chains, chains with branching, and cycles. The carbon atom in an alkane forms four single bonds that are equidistant from each other. The shape is referred to as tetrahedral. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated and is a hydrocarbon. When the carbon atoms of an alkane are connected in a single continuous sequence, the alkane is known as a normal hydrocarbon. These alkanes are sometimes called linear or straight-chain hydrocarbons, but the names are misleading. Really, the carbon chains are kinked, twisted and are linear only in symbolic representations. A succession of normal hydrocarbons which differ by one methylene (-CH₂-) group is known as homologous series. Alkanes belong to a homologous series of organic compounds in which the members differ by a molecular mass. Alkanes with more than three carbon atoms can be arranged in various different ways, forming structural isomers. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n -isomer (n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms.

The two major sources of alkanes are natural gas and petroleum. Natural gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other relatively low-boiling alkanes (propane, butane and 2-methylpropane). Petroleum is a thick, viscous, liquid mixture of thousands of compounds, most of them hydrocarbons, formed by the decomposition of marine plants and animals.

Knowledge about the names of alkanes is extremely important because the names of alkanes form the basis for the names of many other types of organic compounds. The trivial (non-systematic) name for alkanes is paraffins. Together, alkanes are known as the paraffin series. AВ system of nomenclature for organic compounds has been developed by the International Union of Pure and Applied Chemistry (IUPAC ). The IUPAC name for an alkane consists of two parts: a prefix that shows the number of carbon atoms and the suffix "ane". The names of the first ten alkanes are: CH₄ - methane,В CH₃CH₃ - ethane, CH3CH₂CH₃ - propane, CH₃CH₂CH₂CH₃ - butane,В C₅H_I2 - pentane, C₆H_I4 - hexane, C₇H₁₆ - heptane, C₈H_l8 - octane, C₉H₂₀ - nonane,В C₁₀H₂₂ - decane.

Physical Properties. The first four alkanes (C₁ to C₄) are all gases at room temperature. Natural gas is mostly methane. Compounds from C₅ to C₁₇ are oily liquids. Petroleum contains a wide variety of alkanes. Natural gas and petroleum alkanes are used mainly for fuels. Compounds greater than C₁₇ are solids at room temperature and pressure. They are nonpolar compounds. The only interactions between their molecules are the very week London dispersion forces. They have the low boiling points and the low melting points. As the number of carbon atoms increases, the molecules become larger and heavier. This means that their melting points and boiling points increase. In general, both boiling and melting points of alkanes decrease with increasing the number of branches (for alkanes with the same molecular weights). Alkanes are insoluble in water, but they are soluble in non-polar solvents such as toluene and tetrachloromethane. Their density usually increases with increasing number of carbon atoms but is less than that of water. The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to pack well in the solid phase. Alkanes do not conduct electricity, nor are they substantially polarized by an electric field. All alkanes are colourless and odourless.

Chemical properties. Alkanes are relatively inert. Most reactions require specialized catalysts for breaking the carbon-carbon bonds. Although they are relatively unreactive, alkanes do undergo two important kinds of reactions: reactions with oxygen (combustion) and substitution of hydrogen atoms with chlorine and other halogens (halogenation).В Burning, oxidation of alkanes, is a very important reaction in using alkanes as fuel. The fuels used in engines require additives to improve the combustibility for smooth and efficient burning. In burning, the alkanes react with the oxygen in air to produce carbon dioxide and water. Methane and other alkanes react with chlorine in sunlight or at high temperature in a substitution reaction. The reaction of an alkane with other halogens (such as bromine) results in a range of substitution products called haloalkanes. Alkanes are useful as solvents for a wide range of organic substances.

Alkenes

Alkene is an unsaturated hydrocarbon that contains one or more carbon-carbon double bonds. The compounds are said to be unsaturated because they do not have the maximum number of atoms each carbon is able to accommodate. The molecular formula of this group is C_nH_2n (n is the number of carbon atoms). Alkenes have less hydrogen atoms than alkanes. Ethene (or ethylene as it is sometimes called) is the simplest alkene. Alkenes are often referred to as olefins, an old term derived from the fact that chlorine adds to gaseous ethylene to form an oily product.

Naming alkenes, we replace the suffix “-ane” of the corresponding alkane by “-ene”.

Physical properties of alkenes are similar to those of alkanes with the same carbon skeletons. The low-molecular-weight alkenes of importance in the petrochemical industry are gases. Most common alkenes found in the chemical laboratory are pungent-smelling liquids. Like alkanes, alkenes are relatively nonpolar compounds which are insoluble in water.

The alkenes are generally more reactive than the alkanes. The carbon-carbon double bond is an unsaturated linkage to which many chemical reagents can add. Chlorination and hydrogenation are two of the addition reactions that are typical of alkenes. The reaction of ethene with bromine is a simple example of an addition reaction. An example of hydrogenation is the reaction of propylene with hydrogen. Alkenes also undergo combustion reactions. For example, propene burns in a supply of air to give carbon dioxide and water. If there is not enough oxygen available, propene will burn with a yellow flame and produce soot (carbon) and carbon monoxide (CO).

An alkene that contains two double bonds is called a diene. If the two double bonds are separated by only one single bond, such as in 1,3-butadiene, the double bonds are said to be in conjugation.

Alkenes may be oxidized to ketones or carboxylic acids by an oxidizing agent. If the carbon in the double bond is attached to two alkyl groups, a ketone is the product. If the carbon has only one alkyl group, a carboxylic acid is the product.

ВEthene (ethylene) and propene (propylene) are present in small quantities in natural gas and petroleum. The chemical industry prepares enormous quantities of these compounds by heating ethane and propane. Ethylene and propylene are used to manufacture plastics and a number of organic chemicals, including ethanol (alcohol), acetic acid, and ethylene glycol (antifreeze).

Alkynes

Alkynes areВ unsaturated hydrocarbons which contain one or more carbon-carbon triple bonds. The molecular formula of this group is CnH2n-2 (n is the number of carbon atoms). Alkynes have less hydrogen atoms than alkanes and alkenes. Alkynes are more unsaturated than alkenes. The simplest alkyne is ethyne (or acetylene (C2H2) as it is sometimes called) Alkynes are often referred to as acetylenes, a non IUPAC name (International Union of Pure and Applied Chemistry). They show greater tendency to polymerize or oligomerize than alkenes do. The resulting polymers, called polyacetylenes (which do not contain alkyne units) are conjugated and can exhibit semiconducting properties. Naming unbranched alkynes, we replace the suffix "-ane" of alkanes by "-yne".

The physical properties of alkynes are similar to those of alkenes and alkanes. These unsaturated compounds undergo additions similar to alkenes. Hydrogenation of alkynes produces alkanes and alkenes.

Acetylene, the dominant alkyne, is used as a fuel and a precursor to other compounds, e.g., acrylates. Hundreds of millions of kilograms are produced annually by partial oxidation of natural gas. Acetylene is widely used in the oxy-acetylene welding torch because it burns in oxygen with a hot flame. Ethyne (trivial name acetylene) burns in air with a luminous flame and produces a lot of soot. If a good supply of oxygen is available, ethyne burns with a colourless flame. Propyne, also industrially useful, is also prepared by thermal cracking of hydrocarbons. Most other industrially useful alkyne derivatives are prepared from acetylene, e.g. via condensation with formaldehyde.

Specialty alkynes are prepared by dehydrohalogenation of vicinal alkyl dihalides or vinyl halides. Via the Fritsch-Buttenberg-Wiechell rearrangement, alkynes are prepared from vinyl bromides. Alkynes can be prepared from aldehydes using the Corey-Fuchs reaction and from aldehydes or ketones by the Seyferth–Gilbert homologation. In the alkyne zipper reaction, alkynes are generated from other alkynes by treatment with a strong base.

Halogenation of alkynes gives the vinyl dihalides or alkyl tetrahalides. The hydroboration of alkynes gives vinylic boranes which oxidize to the corresponding aldehyde or ketone. Hydrohalogenation gives the corresponding vinyl halides or alkyl dihalides, again depending on the number of HX added.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What is a normal hydrocarbon? 2. In what physical states do alkanes exist? 3. Where are alkanes soluble? 4. What is a non IUPAC name of alkenes? 5. How is an alkene with two double bonds called? 6. How are alkynes prepared? 7. What does hydrogenation of alkynes result in?

Ex.2. Match the words with their definitions:

1. saturated hydrocarbon	a. a chemical reaction that involves the reaction of a compound with halogen and results in the halogen added to the compound;
2. sequence	b. a chemical compound or substance that may speed up or slow down a chemical reaction without itself being changed or consumed at any time during the reaction;
3. isomer	c. the simplest hydrocarbon species which is composed entirely of single bonds;
4. catalyst	d. an ordered collection of elements in which repetitions are allowed;
5. halogenation	e. any chemical reaction that involves the moving of electrons;
6. combustion	f. a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen;
7. oxidation	g. one of two or more compounds, radicals, or ions that contain the same number of atoms of the same elements but differ in structural arrangement and properties.

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

1. Alkanes are unsaturated hydrocarbons as they consist of carbon and hydrogen atoms and all bonds are double bonds. 2. The alkanes are sometimes called linear hydrocarbonsВ because the carbon chains are straight. 3. Boiling and melting points of alkanes with the same molecular weights decrease with increasing the number of branches. 4. Most common alkenes found in the chemical laboratory are pungent-smelling liquids. 5. Alkenes may be oxidized to ketones if the carbon in the double bond is attached to two alkyl groups. 6. Alkynes are often referred to as olefins, a non IUPAC name. 7. Alkynes are unsaturated hydrocarbons which contain one or more carbon-hydrogen triple bonds.

Ex.4. Insert the necessary word:

1. The carbon atom in an... forms four single bonds that are equidistant from each other. 2. The... of alkanes increases with increasing the number of carbon atoms but is less than that of water. 3. Most reactions of alkanes require specialized... for breaking the carbon-carbon bonds. 4. Chlorination and hydrogenation are two of the... reactions that are typical of alkenes. 5. The low-molecular-weight alkenes of importance in the petrochemical industry are.... 6. The parent... of alkynesВ must include the triple bond even if it is not the longest possible carbon chain in the molecule. 7.... of alkynes produces alkanes and alkenes. (gases, density, hydrogenation, alkane, chain, catalysts, addition)

 

ВUNIT VII HALOGENS

The halogens are the family of chemical elements that includes fluorine (atomic symbol F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Chlorine was the first halogen to be discovered in 1774, followed by iodine, bromine, fluorine, and astatine was discovered last in 1940. The halogens make up Group VIIA of the Periodic Table of the elements. Elemental halogens are diatomic molecules. The bonds in these diatomic molecules are non-polar covalent single bonds. Due to their high reactivity, the halogens are never found in nature in native form. Compounds containing more than one kind of halogen are possible. As a general rule, fluorine is the most reactive halogen and astatine is the least reactive. The name "halogen" is derived from the Greek roots hal- ("salt") and -gen ("to form"). Together these words combine to mean "salt former", referencing the fact that halogens form salts when they react with metals. The salinity of the oceans on Earth is due in large part to such halogen salts (halides) as sodium chloride (NaCl) and potassium iodide (KI).

ВВ Halogens display physical and chemical properties typical of nonmetals. They have relatively low melting and boiling points that increase steadily down the group. At room temperature, the halogens demonstrate all of the physical states: fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. All of the elements are colored, with the color becoming more intense moving down the group. Fluorine gas is pale yellow, and chlorine gas is a yellowish green. Liquid bromine and its vapors are brownish red. Solid iodine appears as shiny, dark gray crystals, and the vapors are a deep purple. The halogens are poor thermal and electrical conductors in all phases, and as solids they are brittle and crumbly. The halogens have distinctive, unpleasant odors, and are toxic.

The neutral atoms of the halogens possess seven outer electrons, they only require one additional electron to form a full octet. This characteristic makes them more reactive than other non-metal groups. An additional electron can be added to halogen atoms to form singly charged negative ions. These ions have a closed outer-shell configuration. Electronegativity is a measure of the ability of an atom of one element to remove an electron from an atom of another element. As a group, the halogens are among the most electronegative elements. Fluorine has the highest electronegativity of all the elements. Halogens are so reactive that all the elements except helium and neon have been found to react with at least one of the halogens.

Elements

Fluorine. Fluorine has an atomic number of 9 and is denoted by the symbol F. Elemental fluorine was first discovered in 1886 by isolating it from hydrofluoric acid. Fluorine exists as a diatomic molecule in its free state (F2) and is the most abundant halogen found in the Earth's crust. Fluorine is the most electronegative element in the periodic table. It appears as a pale yellow gas at room temperature. Fluorine also has a relatively small atomic radius. Its oxidation state is always -1 except in its elemental, diatomic state (in which its oxidation state is zero). Fluorine is extremely reactive and reacts directly with all elements except helium (He), neon (Ne) and argon (Ar). In H2O solution, hydrofluoric acid (HF) is a weak acid. Although fluorine is highly electronegative, its electronegativity does not determine its acidity; HF is a weak acid due to the fact that the fluoride ion is basic (pH>7). In addition, fluorine produces very powerful oxidants. For example, fluorine can react with the noble gas xenon and form the strong oxidizing agent Xenon Difluoride (XeF2).

В Chlorine. Chlorine has the atomic number 17 and the chemical symbol Cl. Chlorine was discovered in 1774 by extracting it from hydrochloric acid. In its elemental state, it forms the diatomic molecule Cl2. Chlorine exhibits multiple oxidation states, such as -1, +1, 3, 5, and 7. At room temperature it appears as a light green gas. Since the bond that forms between the two chlorine atoms is weak, the Cl2 molecule is very reactive. Chlorine reacts with metals to produce salts called chlorides. Chloride ions are the most abundant ions that dissolve in the ocean. Chlorine also has two isotopes: 35Cl and 37Cl. Sodium chloride is the most prevalent compound of the chlorides.

Bromine. Bromine has an atomic number of 35 with a symbol of Br. It was first discovered in 1826. In its elemental form, it is the diatomic molecule Br2. At room temperature, bromine is a reddish- brown liquid. Its oxidation states vary from -1, +1, 3, 4 and 5. Bromine is more reactive than iodine, but not as reactive as chlorine. Also, bromine has two isotopes: 79Br and 81Br. Bromine consists of bromide salts, which have been found in the sea. The world production of bromide has increased significantly over the years, due to its access and longer existence. Like all of the other halogens, bromine is an oxidizing agent, and is very toxic.

Iodine. Iodine has the atomic number 53 and symbol I. Iodine has oxidation states -1, +1, 5 and 7. Iodine exists as a diatomic molecule, I2, in its elemental state. At room temperature, it appears as a violet solid. Iodine has one stable isotope: 127I. It was first discovered in 1811 through the use of seaweed and sulfuric acid. Currently, iodide ions can be isolated in seawater. Although iodine is not very soluble in water, the solubility may increase if particular iodides are mixed in the solution. Iodine has many important roles in life, including thyroid hormone production.

Astatine. Astatine is a radioactive element with an atomic number of 85 and symbol At. Its possible oxidation states include: -1, +1, 3, 5 and 7. It is the only halogen that is not a diatomic molecule and it appears as a black, metallic solid at room temperature. Astatine is a very rare element, so there is not much known about this element. In addition, astatine has a very short radioactive half-life, no longer than a couple of hours. It was discovered in 1940 by synthesis. Also, it is thought that astatine is similar to iodine. However, these two elements are assumed to differ by their metallic character.

Applications of Halogens

Fluorine. Although fluorine is very reactive, it serves many industrial purposes. For example, it is a key component of the plastic polytetrafluoroethylene (called Teflon-TFE by the DuPont company) and certain other polymers, often referred to as fluoropolymers. Chlorofluorocarbons (CFCs) are organic chemicals that were used as refrigerants and propellants in aerosols before growing concerns about their possible environmental impact led to their discontinued use. Hydrochlorofluorocarbons (HFCs) are now used instead. Fluoride is also added to toothpaste and drinking water to help reduce tooth decay. Fluorine also exists in the clay used in some ceramics. Fluorine is associated with generating nuclear power as well. In addition, it is used to produce fluoroquinolones, which are antibiotics.

Chlorine. Chlorine has many industrial uses. It is used to disinfect drinking water and swimming pools. Sodium hypochlorite (NaClO) is the main component of bleach. Hydrochloric acid, sometimes called muriatic acid, is a commonly used acid in industry and laboratories. Chlorine is also present in polyvinyl chloride (PVC), and several other polymers. PVC is used in wire insulation, pipes, and electronics. In addition, chlorine is very useful in the pharmaceutical industry. Medicinal products containing chlorine are used to treat infections, allergies, and diabetes. The neutralized form of hydrochloride is a component of many medications. Chlorine is also used to sterilize hospital machinery and limit infection growth. In agriculture, chlorine is a component of many commercial pesticides: DDT (dichlorodiphenyltrichloroethane) was used as an agricultural insecticide, but its use was discontinued.

Bromine. Bromine is used in flame retardants because of its fire-resistant properties. It is also found in the pesticide methyl bromide, which facilitates the storage of crops and eliminates the spread of bacteria. However, the excessive use of methyl bromide has been discontinued due to its impact on the ozone layer. Bromine is involved in gasoline production as well. Other uses of bromine include the production of photography film, the content in fire extinguishers, and drugs treating pneumonia and Alzheimer's disease.

Iodine. Iodine is important in the proper functioning of the thyroid gland of the body. If the body does not receive adequate iodine, a goiter (enlarged thyroid gland) will form. Table salt now contains iodine to help promote proper functioning of the thyroid hormones. Iodine is also used as an antiseptic. Solutions used to clean open wounds likely contain iodine, and it is commonly found in disinfectant sprays. In addition, silver iodide is important for photography development.

Astatine. Because astatine is radioactive and rare, there are no proven uses for this halogen element. However, there is speculation that this element could aid iodine in regulating the thyroid hormones. Also, it has been used in mice to aid the study of cancer.

Halogen derivatives

Compounds derived from hydrocarbons by the replacement of one or more hydrogen atoms by the corresponding number of halogen atoms are known as halogen derivatives. The halogen derivatives are classified on the basis of nature of hydrocarbon from which they are obtained as:

- alkyl halides (halogen derivatives of alkanes)

- alkenyl halides (halogen derivatives of alkenes)

- alkynyl halides (halogen derivatives of alkynes)

- aryl halides (halogen derivatives of arenes)

The alkyl halides (also known, as halogenoalkanes or haloalkanes) are a group of chemical compounds derived from alkanes containing one or more halogens. They have the general formula RX, where R is an alkyl or substituted alkyl group and X is a halogen (F, Cl, Br, I). Haloalkanes are widely used commercially and, consequently, are known under many chemical and commercial names. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Many halocarbons are serious pollutants and toxins. Haloalkanes which contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases.

ВВВВВВВВ While most haloalkanes are human-produced, non-artificial haloalkanes do occur on Earth, mostly through enzyme-mediated synthesis by bacteria, fungi, and especially sea macroalgae (seaweeds). More than 1600 halogenated organics have been identified, with bromoalkanes being the most common haloalkanes.

ВВВВВВВВ Haloalkanes generally resemble the parent alkanes in being colorless, relatively odorless, and hydrophobic. Their boiling points are higher than those of the corresponding alkanes. They are less flammable than alkanes, and some are used in fire extinguishers. Haloalkanes are better solvents than the corresponding alkanes because of their increased polarity. Haloalkanes containing halogens other than fluorine are more reactive than the parent alkanes.

ВВВВВВВВ The most readily available alkenyl halide is chloroethene (vinyl chloride). Chloroethene is produced in large quantities for the production of polymers. These polymers commonly are described as PVC plastics or less specifically as "vinyl". They are materials that may be either flexible or rigid according to what they are mixed with, and they are used in the manufacture of many familiar articles such as plastic curtains, rainwear, floor tile, synthetic leather goods, upholstery, table mats, insulation, plastic pipes, tubing and packaging materials.

ВВВВВВВВ Alkynyl halides belong to one of the most useful and well-investigated classes of substituted alkynes. By analogy with alkyl and alkenyl halides these compounds can be expected to be valuable precursors in the synthesis of various alkynes by acetylenic nucleophilic substitution. Alkynyl halides can be effective building blocks in organic synthesis.

ВВВВВВВВ The stability and physical and chemical properties of alkynyl halides vary in a broad range depending on the nature of the halogen. Alkynyl fluorides are the least stable and the chemistry of these compounds is quite different from that of other alkynyl halides. The first synthesis of alkynyl fluorides was only reported in 1959. Only a few examples of alkynyl fluorides are known, namely fluoroethyne, difluoroethyne, fluorochloroethyne, perfluoropropyne, t-butyl fluoroethyne, and fluoropropiolyl fluoride. All of these compounds are highly unstable and require special caution in their preparation and handling.

ВВВВВВВВ Alkynyl chlorides, bromides and iodides are relatively stable and have been known since the 19-th century. A large variety of alkynyl chlorides are known. The first preparation of the parent representative of the class, monochloroethyne, was reported in1880 by the pyrolysis of dichloroacrylic acid. A number of more convenient synthetic procedures based on dehydrochlorination of polychlorinated ethenes with different bases have been developed more recently.

ВВВВВВВВ Alkynyl iodides are best prepared by the direct iodination of terminal alkynes or the corresponding alkynylides. Iodine in liquid ammonia has been found to be very effective for the direct iodination of free alkynes.

ВВВВВВВВ The newest member of the family of alkynyl halides, alkynyliodonium salts, only became readily available in the 1980s.В In the decade 1993-2003, there has been a substantial interest in the synthetic application of alkynyl halides (mainly bromides, iodides and alkynyliodonium salts), which are valuable precursors in the synthesis of various alkynes reactions.

ВВВВВВВВ In organic chemistry, an aryl halide (also known as haloarene or halogenoarene) is a compound in which the halogen is attached directly to an aromatic ring. Carbon-halogen bonds in aryl halides are shorter and stronger than carbon-halogen bonds in alkyl halides. Because the carbon-halogen bond is stronger, aryl halides react more slowly than alkyl halides when carbon-halogen bond breaking is rate determining.

ВВВВВВВВ Aryl halides areВ colourless oily liquids having characteristic odour of aromatic compounds. In spite of the fact that aryl halides are polar molecules they are insoluble in water. This is because aryl halides cannot form hydrogen bond with water. However aryl halides are soluble in organic solvents like alcohol, ether etc. The aryl halides in general are heavier than water.

ВВВВВВВВ Aryl halides occur widely in nature, most commonly produced by marine organisms that utilize the chloride and bromide in ocean waters. Chlorinated and brominated aromatic compounds are also numerous, e.g. derivatives of tyrosine, tryptophan, and various pyrrole derivatives. Some of these naturally occurring aryl halides exhibit useful medicinal properties.

ВВВВВВВВ Aryl halides react with metals to give more reactive derivatives that behave as sources of aryl anions. Magnesium aryl halides are Grignard reagents, which are useful in organic synthesis of other aryl compounds. At high temperatures, aryl groups react with ammonia to give anilines.

ВВВВВВВВ Aryl halides most often are synthetic intermidiates for the production of other useful substances.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. In what physical states do halogens occur? 2. Why are halogens never found in nature in their native form? 3. What are the elements of the halogen family? 4. Where are organic compounds containing halogens used? 5. How are halogen derivatives classified? 6. Why are aryl halides insoluble in water? 7. What do physical and chemical properties of alkynyl halides depend on?

Ex.2. Match the words with their definitions:

1. halogens	a. the lightest halogen which exists as a highly toxic, pale yellow diatomic gas;
2. nonmetal	b. a compound added to manufactured materials that inhibit, suppress or delay the production of flame;
3. electronegativity	c. a chemical element that mostly lacks metallic attributes;
4. fluorine	d. a group in the periodic table consisting of five chemically related elements (chlorine, bromine, etc.);
5. propellant	e. a binary compound of which one part is a halogen atom and the other part is an element or radical;
6. flame retardant	f. a pressurized gas that is used to create movement of a fluid;
7. halide	g. a measure of the ability of an atom of one element to remove an electron from an atom of another element.

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

1. Iodine is the most reactive halogen. 2. The halogens are poor thermal and electrical conductors in all phases. 3. Halogens require one additional electron to form a full octet. 4. Elemental halogens are diatomic molecules with non-polar covalent triple bonds. 5. Fluorine is the least electronegative element in the Periodic Table. 6. Astatine is widely distributed in nature. 7. Compounds containing more than one kind of halogen are possible.

Ex.4. Insert the necessary word:

1. Halogens have relatively low melting and boiling points that... steadily down the group. 2. The halogens have distinctive, unpleasant..., and are toxic. 3. Halogens are so... that all the elements except helium and neon react with at least one of the halogens. 4. Bromine consists of bromide... which have been found in the sea. 5. The... of iodine may increase if particular iodides are mixed in the solution. 6. Medicinal products containing... are used to treat infections, allergies, and diabetes. 7.... which contain chlorine, bromine, and iodine are a threat to the ozone layer. (solubility, odours, chlorine, salts, haloalkanes, increase, reactive)

ВВВВВВ Unit VIII NITRO COMPOUNDS

A wide variety of organic compounds contain nitrogen. In fact, the types of nitrogen compounds are so numerous and diverse that we shall be unable to consider them all.

Nitro compound is any of a family of chemical compounds in which the nitro group (в?’Oв?’N=O) forms part of the molecular structure. The most common examples are organic substances in which a carbon atom is linked by a covalent bond to the nitrogen atom of the nitro group. The structure of a nitro group can be represented as a resonance hybrid of two equivalent zwitter ionic polar structures. The hybrid structure has a positively charged nitrogen and two equivalent negatively charged oxygens.