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10Aug/090

Chemistry – Alkenes to Alkanes

Simple Organic Compounds Containing Carbon, Hydrocarbons With Functional Groups

Carbon (C) is present in most compounds, both inorganic and organic. Carbon is fairly unreactive, but at high temperatures is forms compounds with hydrogen, oxygen and various metals. Carbon is the only element with the ability to form chains and cyclical compounds of carbon atoms that line up next to each other in various lengths. This makes carbon the basis of organic chemistry. Thanks to carbon, more than 10 million known organisms survive, even thrive, on this Earth. In addition, there are around 200,000 known inorganic compounds which contain carbon.

Carbon is an important rock-forming mineral, forming carbonates. As carbon dioxide (CO2), it can dissolve in water and is also found in the atmosphere. It is an important component of all plants and animals, of all living organisms. Those organisms which died in the early years of our planet's history have helped to create a huge supply of carbon and carbon-based fossil fuels, such as coal, oil and natural gas.

In organic material which contains carbon, its atoms are bonded together in simple, single bonds (in saturated compounds) or in double and triple bonds (in unsaturated compounds). Carbon chains are the result. The sites which are not used for direct carbon-to-carbon bonding can be used for bonds with hydrogen (hydrocarbons) or with other elements.

According to the type of carbon chain present, we can differentiate between compounds with open chains (linear or branched - aliphatic or acyclic) and cyclic compounds. Aliphatic compounds are categorised in the ranks of branched carbon-containing compounds. Cyclical carbon-containing compounds are distinguished by their carbon atoms being arranged in a circle, in a closed cycle. Of these, the most important are aromatic carbon compounds, beginning with the founding member of the aromatic compounds, benzene (C6H6). In it, carbon atoms form a circle together, with the individual bonds between them showing both single and double bond character, a sort of hybrid between the two. Some of the more important organic compounds are fats, proteins and hydrocarbons.

Hydrocarbons

Hydrocarbons are composed exclusively of atoms of carbon and hydrogen. They are the simplest of all organic compounds. There are three types of homologous families of hydrocarbons: alkanes, alkenes and alkynes. Alkanes contain only single bonds between carbon atoms. Alkenes contain at least one double bond. Alkynes contain at least one triple bond. Most of these types of hydrocarbons can exist with the same chemical formula in different form or chemical structure. When a compound has the same chemical formula but two possible structures, these two structures are called isomers.

Hydrocarbon molecules can also contain what are called functional groups. These are groups which contain at least one atom which is neither carbon nor hydrogen. These functional groups can affect the chemical behaviour of the molecule that contains them by giving that molecule special chemical properties. One example is ethanol - CH3CH2OH. Here, the functional group is -OH, with oxygen the determining atom.

Stereochemistry

Stereochemistry is simply the three-dimensional arrangement of a molecule. Organic molecules of the same chemical formula can have their atoms arranged differently in space. When they do, they often have significantly different chemical properties.

Isomers are those types of compounds which have the same chemical formula but different atomic arrangements in space. Isomers can be divided into stereoisomers and structural isomers.

Stereoisometric molecules change their atomic arrangement as a result of changes in pressure or temperature. All bonds and types of bonds (single, double, triple) are conserved in the same original fashion, however.

Structural isomers have atoms which change their position in a molecule. One example is a linear compound (where all of the carbon atoms are lined up in linear fashion), compared to the same chemical formula compound with a shorter linear structure and branching (chain isomerism). Functional groups can change their position (functional isomerism), or can differ from another isomer in the position of a double or triple bond (bond isomerism).

The number of carbon atoms in a hydrocarbon determines how many forms that compound can take. The number of possible isomers in a compound rises as the number of carbon atoms it contains rises.

Alkanes, Alkenes, Alkynes

Hydrocarbons are composed exclusively of oxygen and hydrogen. There are three types of homogeneous hydrocarbons (whose members differ by one CH2 unit): alkanes, alkenes and alkynes. The difference between these three groups is in the bond types between carbon. Alkanes form only single bonds, alkenes form double bonds, and in alkynes there is at least one triple bond.

The simplest alkane is methane. It is formed from one atom of carbon which is bonded with four atoms of hydrogen. If a CH2 group is added, the second alkane compound is formed. The naming of alkanes, as with all other hydrocarbons, is based on the rules of IUPAC (International Union of Pure and Applied Chemistry). Alkane names all end with -ane (from alkan). In front of this ending is a prefix which describes the amount of carbon atoms, corresponding with either a Greek or Latin number. The first four alkanes are named according to historical convention.

Methane: CH4, ethane: C2H6, propane: C3H8, butane: C4H10, pentane: C5H12. The formula of all alkanes can be calculated according to the simple formula CnH2n+2. The number of carbon atoms is the defining factor as to which alkane is which. The alkanes, despite how many carbon atoms they contain, all share some common characteristics. For example, it is typical for all alkanes that they are not highly reactive, they burn well, and they react analogously with halogens in photochemical substitution reactions (exchange reactions). With increasing size of the molecule in the alkane family, alkanes begin to differ from one another in a fundamental way. The first four alkanes are found in the gaseous state of matter. Alkanes containing 5-16 carbon atoms are liquids, and alkanes with 17 or more carbon atoms are solids. Boiling and melting points rise with increasing atomic number.

Branched alkanes are first named according to the amount of carbon atoms they contain in a row. If a radical is contained in an alkaline compound, the -ane ending is replaced by -yl. The branch must be denoted in some way, so as to pinpoint its location on the main carbon chain. For this reason, carbon atoms are numbered from left to right from least to greatest number, so that the branch is arbitrarily assigned the lowest number possible. The main chain has to be the longest one in the molecule. If there are multiple chains in the molecule, they are assigned letters of the alphabet.

Properties and Reactivity

The bond between carbon and hydrogen in an alkane molecule is a weak, polar atomic bond. For this reason, the individual atoms of alkanes carry only a very weak partial charge. These partial charges cancel each other out over the molecule, since it is perfectly symmetrical. The result is a molecule which is non-polar overall. This is not to say one molecule of an alkane does not interact electrostatically with other atoms of its own kind. Weak van der Waals intermolecular forces are found between non-poplar molecules, causing them to mutually attract and repel each other in a weak way. The size of these forces increases as molecule size increases. According to this idea, the characteristics of unbranched alkanes change with increasing size of the carbon chain.

At room temperature, the first four alkanes are found in the gaseous state of matter. Pentane is the first of the liquid alkanes. Until hexane (16), alkane compounds become more and more viscous (parafin oil), because their viscosity rises as the strength of van der Waals forces increases. From heptadecane (17), the alkanes are solids (parafins). Their melting and boiling points rise as a function of the number of carbons in their chains.

Alkanes burn readily. When they do burn, carbon dioxide and water are the products. With increasing chain size, alkanes, given the same amount of oxygen, burn less easily, so that more carbon soot (elementary carbon) is formed with increasing chain size. In alkane molecules, all bonds are said to be saturated. For this reason, alkanes are not very reactive. They do tend to form compounds with halogens.

Van der Waals Forces

Because molecules carry a partial charge, there are forces and attractions between neighbouring molecules. These forces between molecules are very small, but they are big enough to hold the molecule together. The longer the carbon chain of a molecule, the more atoms can take part in these mutual forces, and the greater the resultant attractive force. If the inner forces in smaller alkanes are small, they may not be strong enough to hold the molecule together at room temperature. With increasing carbon chain size, however, these intramolecular forces do increase. At a chain length of 17 carbon atoms, the van der Waals forces are so strong that the individual molecules are held together in the solid state of matter.

Alkenes

Alkenes (olefíns) are unsaturated compounds of carbon with hydrogen which contain one or two double bonds between atoms of carbon. They burn to form carbon soot and carbon dioxide and water. They are more reactive than alkanes because of the fact that they contain double bonds.

Multiple bonds (double, triple bonds) are energetically less advantageous for atoms than corresponding single bonds. For this reason, the atoms in a compound will attempt to break multiple bonds to form single bonds, which are more advantageous energetically. This explains why compounds which contain double and triple bonds are so much more reactive than those which contain single bonds. The alkenes include ethene: C2H4, propene: C3H6, butene: C4H8 and pentene: C5H10. Up to butene, the alkenes occur as gases. Up to hexadecene (C16H32) they are liquids, with higher alkenes found in the solid state of matter. Their general chemical formula is CnH2n.

Alkynes

Alkynes (acetylenes) are unsaturated necyclical hydrocarbons which contain one or more triple bonds between atoms of carbon. When they burn, they tend to form carbon soot. When oxygen is present during burning, high temperatures can be reached. The general formula for alkynes is CnH2a-2. Among these are acetylene: C2H2, propyne: C3H4 and butyne:C4H6.

Alkenes and Alkynes, Unsaturated Hydrocarbons

The carbon atoms of hydrocarbons can be arranged in circles. These cyclical hydrocarbons with single bonds are called cycloalkanes. Benzene and its derivatives, however, are called aromatic hydrocarbons. They contain double bonds. Benzene (first called benzol) was discovered in 1825 by M. Faraday. The name benzol was coined by J. von Liebig. Because benzene is not an alcohol, we call it benzene, not benzol. Benzene is a colourless liquid which refracts light and has an aromatic odour. This characteristic smell was the reason why benzene's group is called the aromatic compounds. Benzene is less dense than water and does not mix with water. On the other hand, it does mix with, or dissolve in, non-polar solvents. Benzene can itself dissolve fats, resins and rubber. Its boiling point is 80.1° C, lower than that of water. At 5-6° C, benzene solidifies and begins to crystallise. When it is burned, benzene releases carbon soot. In its pure form, benzene can be dangerous for human health. If humans are exposed to benzene for long periods of time, their livers, kidneys and bone marrow can be harmed. Benzene is a carcinogen, but it is a useful material in chemistry, serving as a reactant in the synthesis of a number of organic compounds.

Cyclic Hydrocarbons

Cyclic hydrocarbons can be differentiated from aliphatic hydrocarbons. The cycloalkanes, which are composed of multiple CH2 groups and have no double bonds, form a homologous group of compounds. The first member is cyclopentane. The same as the next member cyclohexane, it is very unstable. Because cycloalkanes are saturated compounds, they, like linear alkanes, are not very reactive. They also share a number of properties. The aromatic hydrocarbons are derived from benzene. Group members have six free valence electrons which are distributed in a circle in the form of a charged cloud. Because of the presence of these valence electrons, we can predict that the reactivity of these aromatic compounds will be similar to other unsaturated hydrocarbons. This time, however, our prediction is incorrect: Benzene is much less reactive than other unsaturated hydrocarbons. Only at high temperatures and in the presence of a catalyst can benzene take on another hydrogen atom. When it does, cyclohexane is the resultant product.

The Molecular Structure of Benzene and Cyclohexane

Benzene (benzol), which was discovered as early as 1825, was described by A.F. Kekule von Stradonitz for the first time in 1865. According to Kekule's description, benzene was a circular compound with six atoms of carbon. The benzene circle contained three double bonds which alternate with three single bonds. Kekule believed that these double bonds were fixed in one place in the molecule. He thought that there were two isomeres of benzene which existed side-by-side.

Modern models of benzene's structure show that each carbon atom has associated with it one unpaired electron, a free electron. These unpaired electrons are divided among the circle in the form of a charged cloud. They do not have one certain position in the formation of double bonds. This strange electron arrangement is called mezomeric. It is the reason why benzene is not as reactive as we might expect as compared to other compounds which contain double bonds.

Cyclohexane belongs to the cyclic hydrocarbon family of single-bonded compounds between carbon atoms. It is made of six carbons, each having two hydrogens associated with it.

Noble Gases, Halogen-Substituted Alkanes

The noble gases are found in Group VIII of the main group elements, the A groups. They have a full outermost electron shell and are therefore nearly unreactive. The lighter noble gases do not form compounds at all, and the heavier ones form very few, these being able to be formed and exist only under certain conditions. The elements of the noble gas group include: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Ra). All occur in the gaseous phase of matter. It is possible to produce them through the distillation of condensed air (at temperatures of around -200° C).

The noble gases are not flammable. Helium is used in hot air balloons and other balloons, because it is lighter than air. Radon is the product of the fission reaction of the radioactive element radium. The other noble gases are used in numerous types of lighting because they do not react (light bulbs, neon tubes).

Halogens are found in the seventh main group of elements. They have seven electrons in their outermost electron shell. They can react with other elements and form covalent bonds as well as being able to react to form ionic bonds. They occur in nature in compounds. Smaller halogens, the ones at the top of the periodic table, are more reactive than the halogens in the lower portion of the table, so the smaller halogens can take the place of larger ones in compounds, replacing them or substituting for them. All halogens are poisonous. The halogens are: fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Fluorine and chlorine are gases at room temperature. Fluorine corrodes and attacks almost all other materials, including glass. Chlorine is highly poisonous. Other halogens are either liquids or solids at room temperature, based on their size, where the largest halogens are solids. In the gaseous form all halogens are highly poisonous.

Substitution Generally

The substitution of halogens with alkanes is another way besides burning that they can react. In a substitution reaction, one atom of hydrogen is replaced by one atom of a halogen. This type of reaction is called a halogenation. The halogenation of alkanes occurs in the presence of light, making it a photochemical reaction.

Methane (C2H4) reacts with chlorine (which occurs as a two-atom molecule Cl2) in the presence of light to produce methyl chloride, CH3Cl, and hydrogen chloride (HCl).

These compounds can be differentiated according to various criteria, including:

1. The type of halogen, for example fluoro-, chloro-, bromo-, and iodo-.

2. The type of carbon chain: open, closed, aromatic, saturated, unsaturated.

3. The number of atoms in the halogen: mono-, di- and poly halogen compounds.

The name of the compound is based on the number of carbon atoms present, and where the substitution of a halogen for a hydrogen atom has taken place. Before the name of the hydrocarbon the names of the substitued halogens are given, in alphabetical order if possible. Each carbon atom is assigned a number so as to place the substituted halogen at as low a number as possible. Then the number of the carbon which has been substituted is placed before the halogen prefix. For example:

The carbon chain is always numbered in such a way so that the substituting groups are assigned the lowest numbers. If, however, there are multiple substitutions or some larger group has been substituted, a functional group, that is, it is assigned the lowest possible number.

Fluorine is the first of the halogen group, which means that it is able to substitute for all of the other halogens in a chemical bond. For this reason, hydrocarbons containing fluorine are very stable, non-flammable, and are not poisonous. They are used as an ingredient in aerosol sprays or as the refrigerant liquid in refrigerators, and as a solvent. Their use has become less popular in recent years because of the damage they do in the atmosphere to the ozone layer.
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