fe hemistry Nature s hemistry Alkanes, Alkenes and ycloalkanes
ovalent Bonding The hydrogen and carbon in hydrocarbons are bonded covalently. A covalent bond is formed between non metal atoms. These atoms share electrons to achieve a full outer shell of electrons like their nearest noble gas and become stable. Lets use ydrogen as an example. ydrogen molecule 2 A molecule is two or more non-metal atoms joined together by covalent bonds. ydrogen can be described as a diatomic molecule as it is made from 2 atoms. (Di means 2) The atoms are held together by the electrostatic attraction between the positive nuclei of each atom and the negatively charged electrons.
ovalent Structure ovalent substances that are bonded covalently can exist in two different ways. ovalent Molecules ovalent molecules are compounds that are made up of a discrete number of atoms and can exist as solids, liquids or gases. Examples include water ( 2 O), carbon dioxide (O 2 ) and hydrocarbons. Water arbon Dioxide Propane ovalent molecules tend to have low melting and boiling points. This is because during melting and boiling the strong covalent bonds within the molecules are not broken. It is the weak forces of attraction between each molecule that is broken allowing the molecules to move further away from each other. This is the property which allows crude oil to be separated into fractions.
ovalent Networks ovalent networks are large 3-dimensional structures where all the atoms are covalently bonded together and are less common than covalent molecules. Examples include carbon in the form of diamond and graphite and sand (silicon oxide, SiO). Diamond () Silicon Oxide (SiO) Graphite () ovalent networks all have high melting and boiling points and are very strong structures because all covalent bonds have to be broken to turn them from solids into liquids or gases. ovalent substance do not conducts electricity as the electrons are localised in the bonds and are not free to move and conduct the charge. Graphite is the exception to the rule as each carbon only makes 3 bonds and delocalises the fourth electron allowing it to conduct a charge.
ydrocarbons rude oil is made up of compounds called hydrocarbons. They are molecules made from carbon and hydrogen atoms covalently bonded together. Alkanes The alkanes are a family of hydrocarbons, they have the same general formula and similar chemical properties. The smallest member of the family is methane, 4. Actual Shape arbon atoms always have 4 unpaired electrons in their outer shell and so can form 4 bonds. Tetrahedral The bonds keep as far apart as possible; the hydrogen atoms are at the corners of a pyramid. The normal bonds show two hydrogens are in the plane of the paper, the wedged bond shows the hydrogen is coming out of the paper and the dashed line shows one hydrogen is going back. Structural Formula It is much easier to draw flat pictures to show how each of the atoms are joined to each other. aving as many as 4 bonds allows carbon atoms to form chains, the alkane family is made up of chains where all the carbon atoms join together with a single bond. Due to all the bonds being single bonds, alkanes are described as saturated (nothing can be added).
Naming hydrocarbons The surname is ane to tell you it is a member of the alkane family, the first name tells you the number of carbon atoms in each molecule. Prefix Number of carbon atoms in the molecule meth- 1 eth- 2 prop- 3 but- 4 pent- 5 hex- 6 hept- 7 oct- 8 There are different mnemonics to help you remember the prefixes: 1 2 3 4 5 6 7 8 Monkeys Eat Peanut Butter Perched igh igh Overhead
No. of carbon atoms Name Full structural Formula Shortened Structural Formula Formula 1 Methane 4 4 2 Ethane 3 3 2 6 3 Propane 3 2 3 3 8 4 Butane 3 2 2 3 4 10 5 Pentane 3 2 2 2 3 5 12 6 exane 3 2 2 2 2 3 6 14
Branched hain Alkanes In a straight-chain hydrocarbon, all the carbon atoms in the molecule are linked one after the other in a single continuous chain. In a branched-chain hydrocarbon, the molecule has one or more side chains of carbon atoms coming from the main chain. As a result of branching, the same molecular formula can have a different structural arrangement, when this happens the molecules are called isomers. learly both of them cannot have exactly the same name. When writing shortened structural formulae for branched alkanes, the branches are often put in brackets, e.g. 3 ( 3 ) 2 3 or 3 ( 3 ) 2 3 Isomers are molecules with the same molecular formula but a different structural arrangement Examples Butane one has two isomers: Pentane has three isomers:
Naming Rules for Alkanes 1. The longest chain defines the main chain and the last part of the name 2. Numbering of the main chain starts from the end that gives the lower overall number positions for side branches 3. Side branch names end in -yl and depend on the number of carbon atoms in them: methyl for 1 carbon, ethyl for 2 carbon atoms, propyl for 3 carbon atoms, etc. 4. Alphabetical order is used if different side branches appear in the same structure (ethyl before methyl). 5. yphens are used before or after numbers that come next to letters within a name (2-ethyl-3-methyl..) 6. ommas are used between numbers showing more than one of the same side branch (2,2,3-trimethyl..)
omologous Series omologous Series is a group of molecules with the same general formula, similar chemical properties and graded physical properties. The alkanes are an examples of a homologous series of molecules. Each molecule differs from the previous molecule by the same amount, 2, ( homo- = same, logous = difference). This makes it possible to write a General Formula for the alkane family. General Formula: n 2n + 2 Where n is the number of carbon atoms in the chain. This means are always twice plus two hydrogens for any number of carbon atoms. As each molecule in the family varies by the same mass each molecule shows graded physical properties. This means they show a gradual increase in melting and boiling point and similar solubility. As they also have very similar structures they have similar chemical properties, meaning they all react in the same way.
Alkenes The alkenes are a family of hydrocarbons, they have the same general formula and similar chemical properties. The first and smallest member of the family is Ethene, 2 4. The Alkene family is made up of chains of carbon atoms where most of the carbon atoms join together with a single bond, but two of the carbon atoms will have joined with a double bond. Due to the double bond, alkenes are described as unsaturated. Actual Shape arbon atoms always have 4 unpaired electrons in their outer shell and so can form 4 bonds in total. Double Bond In the alkene family, two of the carbon atoms will share two pairs of electrons, as shown in the diagram. Structural Formula It is much easier to draw flat pictures to show how each of the atoms are joined to each other.
Naming Alkenes Alkenes are named using the same prefixes as the alkanes but the end section of the name is now ene. No. of carbon atoms Name Full structural Formula Shortened Structural Formula Formula 2 Ethene 2 = 2 2 4 3 Propene 2 = 3 3 6 4 Butene 2 = 2 3 4 8 5 Pentene 2 = 2 2 3 5 10 6 exene 2 = 2 2 2 3 6 12
Naming Alkenes 1. Select the longest continuous carbon chain containing the double bond and name it after the appropriate alkene. 2. Number the chain from the end nearest the double bond and indicate the position of the double bond between the prefix and ene, eg but-2-ene. 3. Name any branch(es) and indicate the position(s) of them. Examples
Isomers There are even more isomers possible in the alkene family. Again there are straight-chain alkenes and branched-chain alkenes. In addition, it is possible to change the position of the double bond to introduce even more different structural formulae. Examples But-1-ene has three isomers. The two straight chain molecules with the double bond moved and one branched chain molecule with the double bond. But-1-ene But-2-ene 2-methylprop-1-ene The alkenes have the following general formula: n 2n There are always twice as many hydrogens atoms for any number of carbon atoms. This is different from the alkane family. Since the alkenes have a double bond, there is no need for the two hydrogens on the ends of the molecule. As the alkenes are a homologous series, each molecule differs from the next one in the series by the same amount, they show similar chemical properties and graded physical properties.
ycloalkanes The cycloalkanes are another series of hydrocarbons made up of only carbon and hydrogen. They are made up of rings where all of the carbon atoms join together with a single bond. Molecules with single bonds only are described as saturated since they have the maximum number of hydrogens attached. The first and smallest member of the cycloalkanes is cyclopropane. Actual Shape arbon atoms are arranged in a ring with the hydrogens pointing out of the ring. The hydrogens are above and below the ring, which is why they can be drawn with wedged and dashed bonds. Structural Formula It is much easier to draw flat pictures to show how each of the atoms are joined to each other.
No. of carbon atoms Name Full structural Formula Shortened Structural Formula Formula 3 yclopropane 2 3 6 2 2 2 4 yclobutane 2 4 8 2 5 yclopentane 2 2 2 5 10 6 yclohexane 6 12 2 2 2 2 2 2 2 2 2 Naming ycloalkanes The same system is used to name the cycloalkanes as was used to name the alkanes. The surname is -ane to tell you it is like a member of the alkane family (only single bonds), the first name tells you the number of carbon atoms in each molecule. yclo tells you the carbons are in a ring.
Just like alkanes and alkenes, the cycloalkanes can also have branches and the same system for naming these branches is used. This molecule is 1-methylcyclopropane. 2 1 3 This molecule is 1,3-dimethylcyclohexane. NOT 1,4-dimethylcyclohexane. As the cycloalkanes are a homologous series they all have similar chemical properties, graded physical properties and the same general formula. The general formula is: n 2n The cylcoalkanes are similar to both the alkanes and the alkenes. They are saturated molecules (all single bonds) like the alkanes but due to the ring structure they have the same general formula as the alkenes.
Properties of the Alkanes, Alkenes and ycloalkanes Melting and Boiling Points As the number of carbons in the chain increases so does the boiling point. Flammability The flammability increases as the hydrocarbon chains get longer. Solubility Alkane molecules of all sizes, are insoluble (immiscible) in water but will dissolve (are miscible) in similar non-polar liquids. As most alkanes are liquids they form a layer on top of water as they are (usually) less dense than water.
Properties Explained The properties of the alkanes can all be explained by the weak forces of attraction that exist between molecules. As the molecules get bigger (more carbon and hydrogen atoms) the forces become stronger requiring more energy to overcome the interactions. As a result they have increasing boiling points and become less flammable and soluble as they get bigger. Increasing carbon chain, increasing intermolecular
Reactions of Alkanes, Alkenes and ylcoalkanes Bromine Test The alkanes, alkenes and cycloalkanes react differently with bromine and so this test can be used to identify each family. Bromine is an orange liquid and decolourises (becomes clear) when it reacts with an alkene but not with an alkane or cycloalkane. This test is really a test of saturation, as the unsaturated alkene will react but the saturated alkane and cycloalkane will not. Family General Formula Type of to bond Alkanes n 2n+2 Single (saturated) ycloalkanes n 2n+2 Single (saturated) Alkenes n 2n One Double (unsaturated) Result with bromine Bromine stays brown (decolourises slowly) Bromine stays brown (decolourises slowly) Bromine decolourises immediately
Addition Reactions The bromine test is an example of an addition reaction. An addition reaction is when two molecules react together to make one molecule. Eg ethene reacting with bromine These are important reactions that only molecules with = bonds (unsaturated) can do. They can be used to make many useful products. alogenation This is an addition reaction but with halogen molcules (F 2, l 2, Br 2 and I 2 ), or with hydrogen halide molecules (-F,-l,-Br,-I). In each case the molecule will break into two parts and they will add on to the atoms making up the = double bond. The alkenes are described as unsaturated because of the presence of the carbon to carbon double bond, whereas their products have only single bonds so are therefore saturated and have the ane ending to reflect this.
ydrogenation The alkenes can be made into alkanes by adding hydrogen and this reaction is called hydrogenation. The reaction is an addition reaction, the hydrogen molecule breaks and adds across the arbon to arbon double bond. This reaction is used by food scientists to produce margarine. Animal fats, like butter, contain mainly saturated fats (- single bonds) while vegetable oils have more unsaturated fats (= bonds) and are healthier for us. ydrogenation is used to convert some of the unsaturated fats in a liquid oil into saturated fats to produce a more solid form, margarine, which is still healthier than butter and has the advantage that it is spreadable. Butter has more saturated fats (- single bonds) Vegetable Oil has more unsaturated fats (= double bonds) Margarine has more saturated bonds than vegetable oil but less than butter.
racking The crude oil fractions with small hydrocarbon molecules, such as naptha, are more useful than the long chained molecules. These long chained molecules can be broken down by the process of cracking into smaller, more useful molecules. It is not possible for all the molecules produced from cracking to be alkanes as there is not enough hydrogens, as a result there will always be an alkene produced. There is also no way of controlling where the long chain molecule breaks meaning the alkanes and alkenes produced are random.
racking in the lab racking can be carried out in the lab to show the products of cracking. Aluminium Oxide In this experiment the vapour of liquid paraffin (a mixture of saturated hydrocarbons) is cracked by passing it over a heated catalyst, aluminium oxide. The mixture of gaseous short-chain hydrocarbons produced is collected over water and can then be tested for unsaturation with bromine water. There will be a positive bromine test (bromine water will change from orange to clear) as there will be an alkene present. To view a video clip of this experiment, go to: http://media.rsc.org/videoclips/demos/rackingahydrocaron.mpg
Uses of Alkanes, Alkenes and ycloalkanes The hydrocarbons have many different uses. As alkanes release large amounts of energy when burn, they are usually used as fuels. ydrocarbons, including alkanes, were used in many of the early dry-cleaners as they could dissolve many stains. Modern dry-cleaners use less flammable molecules derived from simple hydrocarbons. Though alkenes release large amounts of energy when burnt, they have much more important uses so are rarely used as fuels. Alkenes, particularly ethene are important in the manufacture of other chemicals such as plastics, ethanol, ethanoic acid, esters, halogen derivatives etc. Alkanes Alkenes plastics Examples Plastics bags Vinyl paints substituted alkanes alcohols Examples Anti-freeze and-gels Examples Propellants Refrigerants esters acids Examples Vinegar Aspirin Examples Flavourings Perfumes