Unit 2, Lesson 01: Introduction to Organic Chemistry and Hydrocarbons Organic Chemistry: is the branch of chemistry that deals with carbon-based covalent compounds. living organisms are made up of a huge variety of organic substances (proteins, carbohydrates, fats, nucleic acids etc, etc, etc). Because of their relationship to living things, organic compounds were believed to have a special life force. It was once believed that only living tissues could synthesize organic compounds. Of course, we now know this is not the case. organic compounds usually, but not always, contain C C and C H bonds ionic carbon compounds (compounds containing the ions C 4-, CN 1-, CO 3 2-, HCO 3 1-, SCN 1- etc.) are not considered to be organic compounds. Carbon monoxide and carbon dioxide are also not considered to be organic. 96% of all known compounds are organic more than nine million organic compounds are known this is known as the pigpen branch of chemistry because many organic molecules smell (stink) Why are there so many different organic compounds? Because carbon can bond so many different ways: each carbon atom can form four bonds carbon can bond with other carbon atoms to form long chains and rings carbon can form single, double and triple bonds with each other and a wide variety of other elements In order to study organic compounds, we have to be able to organize them somehow. Organic Compounds Hydrocarbons made of only carbon and hydrogen Hydrocarbon Derivatives made of carbon, hydrogen and other elements such as oxygen, nitrogen, halogens, sulfur etc
1. The Simplest Hydrocarbons: The Alkanes a) Straight chain alkanes These are straight, open chains of carbon and hydrogen with only single bonds. The naming system for all organic molecules begins here and is based on the number of carbon atoms in the chain. Structural formula Molecular formula Condensed formula IUPAC name CH 4 CH 4 methane C 2 H 6 CH 3 CH 3 ethane C 3 H 8 CH 3 CH 2 CH 3 propane C 4 H 10 CH 3 CH 2 CH 2 CH 3 butane C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane C 6 H 14 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 hexane If there are seven carbons bonded together- it is heptane: C 7 H 16 or CH 3 (CH 2 ) 5 CH 3 If there are eight carbons bonded together- it is octane: C 8 H 18 or CH 3 (CH 2 ) 6 CH 3 If there are nine carbons bonded together- it is nonane: C 9 H 20 or CH 3 (CH 2 ) 7 CH 3 If there are ten carbons bonded together- it is decane: C 10 H 22 or CH 3 (CH 2 ) 8 CH 3 Notice how the molecules increase in size by one (CH 2 ) group. This is a homologous series for the alkanes because the molecules all differ by one (CH 2 ) unit. The general formula for this series is: C n H 2n+2 Alkanes contain the maximum number of hydrogen atoms possible. For this reason they are considered to be saturated with hydrogen, or they are saturated hydrocarbons. You must memorize the names of the alkanes. You can remember the first four with the mnemonic: Monkeys Eat Peeled Bananas: Methane Ethane Propane Butane
b) Branched chain alkanes Carbon can bond with up to four other carbon atoms. This means that alkanes can have side chains and can be put together in more than one way. The three molecules below all have the molecular formula C 5 H 12 so they are all alkanes (remember, alkanes have the general formula C n H 2n+2 ). However, they all have different structural formulas so they are different compounds. Compounds that have the same molecular formulas but different structural formulas are called structural isomers. ( iso means same and mer means parts ) The greater the number of carbon atoms in a molecule, the greater the number of structural isomers that are possible. For example, the molecular formula C 30 H 62 has 4,111,846,763 structural isomers. Although isomers have the same molecular formulas, their different structural formulas give them very different physical properties. For example, the melting point of pentane is higher than the melting points of 5-carbon branched chain alkanes, because the straight chain means that there are stronger London dispersion forces between adjacent molecules. To name branched chain alkanes 1. Identify the longest continuous carbon chain; this determines the base name. 2. Number the carbon atoms along the main chain so that the side chains will have the smallest numbers possible. 3. The side chains are called alkyl groups and are named from their corresponding alkane: Structural formula Condensed formula IUPAC name CH 3 methyl CH 3 CH 2 ethyl CH 3 CH 2 CH 2 propyl 4. Indicate the positions of the side chains using the number of the carbon atom to which each is attached. A hyphen (dash) is used to separate the position number from the name of the alkyl group. 5. If there is more than one 'type' of side chain (substituent), they are named in alphabetical order. Ethyl is named first, methyl second and propyl third. Each is given a position number to indicate where it is found on the chain. 6. If there is more than one of the same 'type' of side chain, indicate this by giving their position numbers and a prefix to indicate how many of each side chain is in the molecule. eg. 2, 3-dimethyl or 5, 6, 7-tripropyl. When naming the side chains in alphabetical order, ignore the prefix.
Examples The longest continuous carbon chain is 5 carbons, so the base name is pentane. There is a methyl group (CH 3 ) on the third carbon in the chain, so this is 3-methyl pentane. The longest continuous carbon chain is 4 carbons, so the base name is butane. There is a methyl group (CH 3 ) on the second carbon in the chain, so this is 2-methyl butane. number of carbons in longest carbon chain: 6 base name: hexane full name: 3-methylhexane number of carbons in longest carbon chain: 4 base name: butane full name: 2,3-dimethylbutane number of carbons in longest carbon chain: 4 base name: butane full name: 2,2,3-trimethylbutane Is this a structural isomer of the molecule above? No. These molecules do not have the same molecular formulas. You try (answers on next page): number of carbons in longest carbon chain: 5 base name: pentane full name: 3-ethylpentane Is this a structural isomer of the molecule above? Yes. They have the same molecular formulas. 1. 4. 2. 5. 3. 6.
Answers for previous page: 1. 3-ethyl-2,2-dimethylpentane 4. 3-ethyl-2,4-dimethylpentane 2. 3-ethyl-2,3-dimethylhexane 5. 4-ethyl-2,2-dimethylhexane 3. 3-ethyl-4-methylhexane 6. 3-ethyl-2,5-dimethylhexane IUPAC Naming of Branched Side Chains Many organic compounds also have old, non-iupac names that you will still see. You need to be aware of iso names (eg. isopropyl alcohol). If a carbon chain has a side group on the second carbon of the main carbon chain, the old naming system was to name this side group iso and to name the molecule according to the total number of carbons. eg. isobutane the IUPAC name for the molecule to the right is 2-methylpropane the old iso name for the molecule is isobutane eg. isopentane the IUPAC name for the molecule to the right is 2-methylbutane the old iso name for this molecule is isopentane This system has been incorporated into the IUPAC system for naming branched side chains. You should know the following side chains: if a molecule has a branched three-carbon side chain, it is called an isopropyl group if a molecule has a four-carbon side chain with a branch on the second carbon, it is called an isobutyl group If a molecule has a four-carbon side chain with two branches on the second carbon it is a tert-butyl group To draw hydrocarbons: eg. Using the example: 3-ethyl-2,2-dimethylpentane 1. Determine the main carbon chain and draw a skeleton eg. the base name for the hydrocarbon is pentane, so draw 5 carbons 2. Use the names and position numbers of the side chains, draw them in eg. there is an ethyl group on the third carbon there are two methyl groups on the second carbon 3. Fill in the molecule with hydrogen atoms to complete stable octets 4. Double check your structure by naming it: 3-ethyl-2,2-dimethylpentane