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Carbon Compounds: An Introduction to Organic Chemistry.
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Carbon Compounds: An Introduction to Organic Chemistry The Lewis structure for carbon shows 4 unpaired valence electrons. To fulfill the octet rule, a carbon atom needs 4 more electrons. A carbon atom may form 4 covalent bonds and is capable of forming long chains with single, double or triple bonds between carbon atoms. These chains may be straight or branched. The 2 ends of a chain can bond together to form a ring. Carbon compounds are divided into classes based on their chemical similarity.
Hydrocarbons Hydrocarbons are compounds containing hydrogen and carbon. Hydrocarbons may have different numbers of bonds between carbon atoms. The four hydrocarbon classes are: alkane (single bond), alkene, (double bond), alkyne (triple bond), aromatic (benzene ring). Alkanes contain only single C-C bonds. They contain as many hydrogen atoms as possible, and are said to be saturated. Hydrocarbons containing double or triple bonds are unsaturated. A homologous series is series of compounds that differ by a constant increment. Aromatic hydrocarbons include a benzene ring- 6 carbon atoms with all the bonds alternating between a single and a double bond.
The rules of nomenclature The rules of nomenclature determine the names for each organic molecule. The parent compound is named after the longest straight chain within the molecule. The names of the first ten straight-chain alkanes are be: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane. Branches are identified by a number which represents their position on the parent chain. An alkane branch then becomes an alkyl group. A suffix represents the hydrocarbon class: -ane for alkanes; -ene for alkenes; -yne for alkynes. An alkane forming a ring is called a cycloalkane.
Structural isomers Structural isomers, molecules having the same molecular formula but whose atoms are connected differently, can form in alkanes with 4 or more carbon atoms. Structural isomers are different compounds and have different physical and chemical properties.
Carbon atoms are classified as primary, secondary, tertiary, or quaternary based on the number of non-hyrogen groups attached to the sp3 carbon. The hydrogen atoms attached to these carbon atoms are given the same designation.
Propane has two distinct kinds of carbon atoms. The two carbon atoms on the end are bonded to only one carbon atom and are called primary carbons (1o). The central carbon atom is bonded to two other carbon atoms and is designated as a secondary (2o) carbon.
Properties of structural isomers Structural isomers • have the same molecular formula. • have different bond connectivities. • are different compounds. • have unique names. • have different physical properties. • may or may not have similar chemical properties. • are sometimes called "constitutional isomers".
Higher Alkanes We can continue adding carbon atoms to build up further members of the alkane series. Notice that each member increases the formula unit by CH2. A series made up of an orderly increase in a formula unit is a homologous series. The general formula for any alkane with n carbon atoms is CnH2n+2 The names and isomer number of the first 10 members of the alkane family are shown in the table. Observe how the number of isomers increases with increasing number of carbon atoms. A general formula has been developed that allows one to calculate the number of isomers of a given alkane. On this basis you can determine that eicosaneC20H42has 366,319 isomers.
Possible number of structural isomers • The alkanes have a general formula CnH2n+2. As one increases the number of carbon atoms in alkanes, the possible number of structural isomers increases astronomically.* There are three isomeric pentanes and five isomeric hexanes. • Decane (C10H22) has 75 possible isomers, eicosane (C20H22) has 366,319 isomers and tetracontane (C40H82) has 62,491,178,805,831 possible structural isomers!. • * H.R. Henze and C.M. Blair, "The Number of Isomeric Hydrocarbons of the Methane Series," J. Am. Chem. Soc., vol 53, p.3077, 1931.
Most organic compounds have other functional groups attached to the hydrocarbon molecule. A functional group replaces a hydrogen on a carbon atom and is responsible for the characteristic chemical behavior of the new molecule. The major classes of compounds based on addition of functional groups are listed at right. FUNCTIONAL GROUPS
Alkenes are molecules with one or more carbon-carbon double bonds.
Alkynes are molecules with one or more carbon-carbon triple bonds.
Chemicals from oil • How oil is formed • Oil is thought to have formed over millions of years from the break down of tiny dead creatures. Natural gas is formed alongside oil. • The dead organisms sank to the bottom of lakes or seas and became trapped in muddy sediments. As the sediments built up, the lower layers were under pressure. They eventually turned to rock. If there was no oxygen in the sediments, heat and pressure turned the remains of the organisms into oil and natural gas. • Some rocks are porous - they have a network of tiny holes in them. Sandstone and limestone are examples. Oil is a liquid so it seeps into porous rocks. Gas also diffuses into these rocks. • Porous rocks may also contain water. Gas and oil do not mix with water. They are less dense than water. This means they form layers above the water. • Sometimes the rock layers form so that the oil and gas are trapped under the rock such as shale that is not porous. Large amounts of oil and gas may collect in a porous rock. The pressure on the oil may build up so much that when a hole is drilled through the rock cap, oil gushes out.
Fractional distillation of crude oil • Crude oil is a mixture of many thousands of different compounds with different properties. They are called hydrocarbons because they only contain the elements hydrogen and carbon. • To make crude oil useful, batches of similar compounds with similar properties need to be sorted. These batches are called fractions and they are separated by fractional distillation. • The theory behind this technique is that some of the compounds in crude oil are easily vaporised, for example, they are volatile due to their low boiling points. Others are less volatile and have higher boiling points. • In fractional distillation, the crude oil is heated to make it vaporise. The vapour is then cooled. Different fractions of the oil are collected at different temperatures.
As the hydrocarbon molecule chain increases its boiling point increases, it becomes more viscous, becomes more difficult to light, the flame becomes sootier and it develops a stronger smell.
Products from crude oil Alkanes Physical properties: The chemistry of carbon compounds is called organic chemistry. There are millions of organic chemicals, but they can be divided into groups called homologous series. All members of a particular series will have similar chemical properties and can be represented by a general formula. The alkane series is the simplest homologous series. The main source of alkanes is from crude oil.
Alkanes are covalent compounds. They are hydrocarbons, which means they contain hydrogen and carbon. The general formula for an alkane is .
Properties and uses of alkanes: • The first four alkanes are gases at room temperature. • Alkanes with 5-17 carbon atoms are liquids. • Alkanes with 18 or more carbon atoms are solids. • As the number of carbon atoms increases, the melting points, boiling points and densities increases. • They are insoluble in water but dissolve in organic solvents such as benzene. • Their chemical reactivity is poor. The C-C bond and C-H bond are very strong so alkanes are not very reactive. • They will carry out combustion. Burning alkanes in air (oxygen) produces water and carbon dioxide. The reactions are very exothermic (give out heat energy), so alkanes in crude oil and natural gas are widely used as heating fuels. • For example: • If alkanes combust in too little air, carbon monoxide may form. This is dangerous and can cause death.
Cracking alkanes • The lighter fractions (for example, petrol) are in large demand. The heavier fractions are not so useful but unfortunately chemists have to be able to convert these heavier fractions into petrol and other useful products, due to supply and demand, by a method known as cracking. • Cracking breaks down molecules into smaller ones. Catalysts or heat may be used to crack the alkane chain into smaller ones. • Note, that one of the products that is formed when we crack naphtha contains a double bond between two carbon atoms. A hydrocarbon that possesses one double bond belongs to the next homologous series called alkenes.
Reforming • Another reaction that often occurs after fractional distillation is reforming. • Hydrocarbons of the same formula have different boiling points. Straight-chained alkanes have greater boiling points than the branched version. This means they catch light more easily - but this can be too much for the hot cylinder of the car engine. • Reforming converts straight-chained alkanes to branched.
Alkenes The members of this series contain a double bond. They are hydrocarbons. The general formula of the alkenes is CnH2n Most alkenes are formed when fractions from the fractional distillation of crude oil are cracked.
Properties of alkenes Like alkanes, the boiling point, melting point and densities increase with larger size molecules. They are insoluble in water. They combust like alkanes to produce carbon dioxide and water. However, they burn with sootier flames due to their higher percentage of carbon content to hydrogen. Chemically, alkenes are more reactive than alkanes. This is because they possess a double bond that can be broken open and added to in a reaction. For example: These reactions are called addition reactions.
