Understanding Hydrocarbons: Alkanes, Alkenes, and Cycloalkanes, Lecture notes of Chemistry

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St Ninian’s High School Chemistry Department National 5 Chemistry Unit 2: Nature’s Chemistry Topic 1 Hydrocarbons Summary Notes Name____________________________________ 2 Learning Outcomes After completing this topic you should be able to : N5 Chemistry: Unit 2 Topic 1 1 state that a hydrocarbon is a compound made of hydrogen and carbon only 2 state that the alkanes, alkenes and cycloalkanes are each a subset of the set of hydrocarbons 3 identify an alkane from the -ane ending 4 name straight-chain alkanes from molecular formulae shortened and full structural formulae (C1 - C8) 5 write molecular formulae and draw shortened and full structural formulae given the names of straight-chain alkanes (C1 - C8) 6 derive and use the general formula for alkanes to work out molecular formulae 7 state that a homologous series is a set of compounds with the same general formula and similar chemical properties 8 explain that the alkane, alkene and cycloalkanes families are each an example of a homologous series 9 name branched-chain alkanes from shortened and full structural formula (only C4 - C8) 10 write molecular formulae and draw shortened and full structural given the systematic names of branched-chain alkanes (only C4 - C8) 11 identify an alkene from the –ene ending and the carbon to carbon double bond 12 state the name for each alkene up to octene and work out names from molecular, shortened or full structural formulae (C2 - C8) 13 state the name for straight and branched chain alkenes, incorporating the position of the double bond, from shortened and full structural formulae (C2 - C8) 14 derive and use the general formula for alkenes to work out molecular formulae 15 identify a cycloalkane from the name 16 name cycloalkanes from molecular formulae, shortened and full structural formulae (C3 - C8) 17 write molecular formulae and draw shortened and full structural given the names of cycloalkanes (only C3 - C8) 18 state that isomers are compounds with the same molecular formulae but different structural formulae 19 draw isomers for a given molecular formulae, shortened and full structural formulae 20 state that the alkanes and cycloalkanes are saturated hydrocarbons 21 state that saturated hydrocarbons contain only single carbon to carbon covalent bonds 22 state that the alkenes are unsaturated since they contain at least one carbon to carbon double covalent bond 23 state it is possible to distinguish an unsaturated hydrocarbon from a saturated hydrocarbon using bromine solution 24 state that an alkene reacts with hydrogen to form the corresponding alkane 25 state that the reactions of an alkene with bromine, hydrogen and water (hydration) are addition reactions. 5 Hydrocarbon Families Branched Alkanes So far we have only considered straight-chain alkanes where the carbon atoms are arranged one after another to give a straight-chain of carbon atoms. Branched alkane molecules also exist where there are one or more carbon atoms arranged as branches. Branched molecules are given a systematic name according to a convention. Consider the compound with molecular formula C5H12: there are several different molecules with this molecular formula. Note A is a straight-chain while B & C are branched chain molecules. Each molecule may have the same molecular formula but must be given a different name since the structure is different. Branches are named after the corresponding alkanes with the -ane ending changed to -yl. To name a branched molecule: i) select the longest continuous chain of carbon atoms and name it after the appropriate alkane ii) number the carbon atoms from the end of the chain nearest to the branch iii) name the branch(es) and indicate the position(s) of the branch(es) on the chain iv) use”di” and “tri”, etc, when the same branch is present more than once. N5 Chemistry: Unit 2 Topic 1 6 Hydrocarbon Families The Alkenes Both the alkanes and cycloalkanes contain only single carbon-to-carbon bonds. The alkenes are a family which contain one carbon-to-carbon double bond. The alkenes are another example of a homologous series. The first member of the alkene family is ethene (shown below) since at least 2 carbons are needed to have a double carbon-to-carbon bond, the molecule methene does not exist. The general formula for the alkenes is CnH2n. For some alkane molecules a number is used to indicate the position of the double carbon-to-carbon bond e.g. but-1-ene and but-2-ene. Branched Alkenes Alkenes, like alkanes, can also form branched chain molecules. Branched alkanes are named in a similar way to branched alkanes. i) Select the longest continuous chain of carbon atoms containing the double bond and name it after the appropriate alkene. ii) Number the carbon atoms from one end of the chain nearer the double bond and indicate the position of the double bond. iii) Name any branch(es) and indicate the position(s) of the branch(es) on the chain. Example: N5 Chemistry: Unit 2 Topic 1 7 Hydrocarbon Families Saturated and Unsaturated Molecules Hydrocarbon families are described as saturated if all the carbon-to-carbon bonds are single bonds. If the molecule contains any double (or triple) carbon-to-carbon bonds the molecules are referred to as unsaturated. The alkanes and cycloalkanes are examples of saturated hydrocarbon families. The alkenes are an example of an unsaturated hydrocarbon family. Isomers Isomers are compounds with the same molecular formula but a different structural formula. In other words isomers are molecules which have the same atoms but attached in a different way. Example 1: C3H6Cl Two different molecules can be created which have this molecular formula. In this example the chlorine atom is attached in a different location to create isomers. Example 2: C2H4Cl2 The two molecules here have the same molecule formula and the same structure and are therefore not isomers. (The chlorine atoms are still attached in the same way.) N5 Chemistry: Unit 2 Topic 1 10 Hydrocarbon Reactions Addition of Hydrogen (Hydrogenation) You have seen that alkenes react with bromine solution since an addition reaction occurs. Similar reactions also occur between the alkenes and hydrogen. In a similar way the hydrogen molecules will add to the alkene molecule with the loss of the carbon-to-carbon double bond. Alkenes will therefore react with hydrogen to produce the corresponding alkane. The addition of hydrogen can be called hydrogenation. Example: Ethene reacting with hydrogen to form ethane in an addition reaction. Alkenes will also react with chlorine in a similar way. Addition of Water (Hydration) Water will also react with alkene molecules causing the double bond to break. Water reacting with an alkene is also an example of an addition reaction but this is also called a hydration reaction. The alkene will react to form an alcohol molecule. Alcohols will be covered during Topics 2 and 3. Example: Ethene reacting with water to form ethanol. OH H2O N5 Chemistry: Unit 2 Topic 1 Ethanol (an alcohol) 11 Topic 1 Summary Statements A hydrocarbon is a compound made of hydrogen and carbon only. Alkanes, alkenes and cycloalkanes are each a subset of the set of hydrocarbons. An alkane can be identified from the -ane ending in the name. The alkanes are named as methane, ethane, propane, butane, pentane, hexane, heptane and octane. The general formula for the alkanes is CnH2n+2. A homologous series is a set of compounds with the same general formula, similar chemical properties and gradual changing physical properties. Alkanes, alkenes and cycloalkanes are each an example of a homologous series. Branched-chain alkanes can be named using a convention. An alkene has a name ending in –ene. All alkenes have at least one carbon to carbon double bond. The general formula for the alkenes is CnH2n. Cycloalkanes are a family of compounds where the carbon atoms are joined in a circle. The general formula for the cycloalkanes is CnH2n+2. Isomers are compounds with the same molecular formulae but different structural formulae. The alkanes and cycloalkanes are saturated hydrocarbons since they contain only single carbon-to- carbon bonds. The alkenes are unsaturated since they contain at least one carbon to carbon double covalent bond. It is possible to distinguish an unsaturated hydrocarbon from a saturated hydrocarbon using bromine solution. The bromine solution will be decolourised showing a molecule to be unsaturated. When an alkene reacts with hydrogen it will form the corresponding alkane. The reactions of an alkene with bromine, hydrogen (hydrogenation) and water (hydration) are examples of addition reactions. N5 Chemistry: Unit 2 Topic 1