CH221 CLASS 5 CHAPTER 3: ORGANIC COMPOUNDS: ALKANES AND CYCLOALKANES Synopsis. Class 5 begins with a brief review of functional groups, followed by a preliminary discussion of the alkanes, including isomerism, alkyl groups, primary, secondary carbons (etc) and finally, nomenclature. Functional Groups There are presently (2005) almost 20 million known organic compounds in the CAS registry – each with its own physical and chemical properties. Luckily, like the elements of the periodic table, all organic compounds can be arranged into a relatively small number of families whose chemistry is reasonably predictable. The compounds of each of these families are characterized by the presence of particular groupings of atoms, known as functional groups. It is the functional group that determines the chemical behavior of a family of compounds and the chemistry of every organic molecule, regardless of size or complexity, is related to the functional group it contains. Compare the reaction between the simple alkene, ethylene, and bromine with the reaction of the more complex cycloalkene, menthene, with the same reagent: H H C Br2 C H Br H CH3 H C C Br H H CH3 Br2 CH(CH3)2 H Br Br CH(CH3)2 Both molecules possess an alkene C=C functional group: both undergo an addition reaction A summary of the common functional groups, according to type, is given below and on the next page. Family name Functional group Simple example Alkane C-C and bonds only Alkene Arene Alkyne C=C Aromatic C=C CH2=CH2 C6H6 Ethene (ethylene) Benzene CHCH CH3CH2Br Ethyne (acetylene) Halide CC R-Halogen Alcohol R-OH (hydroxyl) CH3CH2OH Ethanol alcohol) Ether R-O-R CH3OCH3 Amine >N- CH3NH2 Nitrile (cyanide) R-CN CH3CN Dimethyl ether (methoxymethane) Methylamine (aminomethane) Ethanenitrile (acetonitrile, methyl cyanide) Nitroso Nitro Thiol (mercaptan) R-N=O R-NO2 R-SH C6H5NO CH3NO2 CH3CH2SH Nitrosobenzene Nitromethane Ethanethiol (ethyl mercaptan) CH3-S-CH3 Dimethyl sulfide Sulfide (thioether) R-S-R C-H CH3CH3 Continued on Next page! Name Ethane Bromoethane (ethyl bromide) (ethyl Family name Functional group Simple example Sulfoxide Sulfone Aldehyde R2S=O R2S(=O)2 R-CH=O (CH3)2SO (CH3)2SO2 CH3CHO Ketone R2C=O (CH3)2CO Carboxylic acid R-C=O OH CH3COOH Ethanoic (acetic acid) Ester R-C=O OR CH3COOC2H5 Ethyl ethanoate (ethyl acetate) CH3CONH2 Ethanamide (acetamide) CH3COCl Ethanoyl chloride (acetyl chloride) (CH3CO)2O Ethanoic anhydride (acetic anhydride) Dimethyl Amide Carboxylic chloride Carboxylic anhydride R-C=O NR2 acid R-C=O Hal acid (R-C=O)2O Carbonate ester (R-O)2C=O (CH3O)2CO Name Dimethyl sulfoxide Dimethyl sulfone Ethanal (acetaldehyde) Propanone (acetone) acid carbonate The Alkanes The alkanes contain carbon and hydrogen only, bonded via single bonds only. They are thus called saturated hydrocarbons because of their elemental content and because their molecules possess the maximum number of hydrogen atoms per carbon atom. The general formula is CnH2n+2. Another general name for alkanes, or groups derived from alkanes (alkyl groups – see later), is aliphatic, from the Greek aleiphos, meaning “fat”. This arises from the fact that many long chain alkyl groups have been found in animal fats. CH2 O CO (CH2)16CH3 CH O CO (CH2)16CH3 CH2 O CO (CH2)16CH3 Glycerol Stearic acid This is a typical component of animal fat: it is a triglyceride ester called glyceryl tristearate. The long chain part is derived from stearic acid, a "fatty acid" Alkane Isomers The alkanes can be built from the simplest member, methane (CH 4) by formally inserting successive CH2 (methylene) units in C-H bonds. For the first two alkanes after methane - ethane (C2) and propane (C3) – there is only one way that this can be done: H H C H Methane H H H C H Ethane H CH2 H H C CH2 CH2 H H Propane However, beyond propane, there are at least two ways in which to extend the carbon chain: either in a linear fashion or in a branched fashion, as shown for C4H10 and C5H12 overleaf. H CH3 extend by CH2 CH3CHCH2 H CH3CH2CH2CH3 and Butane CH3CHCH3 2-Methylpropane (isobutane) Propane C4H10 C 3H 8 extend by CH2 CH3CH2CH2CH3 CH3 CH3CH2CH2CH2CH3 and CH3CH2CHCH3 Pentane CH3 CH3CHCH3 Butane and isobutane C4H10 extend by CH2 2-Methylbutane CH3 CH3CCH3 CH3 2,2-Dimethylpropane C5H12 Butane and pentane, with no branches, are called straight chain or normal (n) alkanes, whereas all the others are called branched-chain alkanes. Compounds that have the same molecular formula are called isomers. For example, butane is the straight chain isomer and 2-methylpropane is the branched-chain isomer with the molecular formula C4H10. Isomers like these, whose atoms are connected in different ways are called constitutional isomers or structural isomers. You will see later that there is another, more subtle, type of isomerism, called stereoisomerism. There are three main types of constitutional isomerism, as summarized in the table on the next page. Alkyl Groups A general name for a hydrocarbon group in organic molecules is alkyl group. Saturated alkyl groups are named by replacing the ending –ane of the parent hydrocarbon with the ending –yl. E.g. CH4 methane CH3methyl As in CH3-I methyl iodide or iodomethane CH3CH2CH2CH3 butane CH3CH2CH2CH2butyl or n-butyl As in CH3CH2CH2CH2-Li n-butyllithium Primary, Secondary, Tertiary and Quaternary Atoms By inspection of some of the foregoing examples, it will be noticed that in the alkyl groups of organic molecules there are different types of carbon atoms, depending on the number of C-C bonds to those carbon atoms. This gives rise to the nomenclature primary (1o), secondary (2o), tertiary (3o) and quaternary (4o), as illustrated below (R designates unspecified groups) for hydrocarbon groups. H R C H H R C H R R H R R C H R C R R R 1 C-C bond 2 C-C bonds 3 C-C bonds 4 C-C bonds primary (1o) secondary (2o) tertiary (3o) quaternary (4o) methyl methylene methine These terms are routinely applied to all organic compounds, not just the hydrocarbon groups above: OH CH3CH2 1o OH CH3 CH 2o CH3 CH3 CH3 C Cl 3o CH3 CH3 CH3 + N 4o CH3 CH3 The terms are important because, although within a family, functional groups at 1o, 2o and 3o carbon atoms will have a similar chemistry, it will not be exactly the same for each. For example, 1-propanol (1o) may be more or less reactive in certain reactions than 2-propanol (2o). Naming Alkanes Trivial and Systematic Nomenclature The original names of organic compounds are now known as trivial names. These names describe some aspect of the compound, often its origin, and are non-systematic (i.e. they cannot be logically derived from the molecular structure). Many compounds are still preferentially given trivial names, examples including formic acid, derived from ants (Latin: Formica) and salicylic acid, which was first discovered as a component of the bark of willow trees (Latin: Salix). As more and more complex organic compounds were discovered, the need for a more systematic kind of naming system became urgent: a system that will allow the organic chemist to draw the structure of the molecule from its name, and viceversa. Nowadays, the most widely used naming system is that of the International Union of Pure and Applied Chemistry (IUPAC). IUPAC names are generally composed of at least two, and usually three parts: PREFIX Gives type and positions of substituents ROOT SUFFIX Gives number of carbon atoms Indicates family The names of the first four straight chain alkanes take their roots from trivial sources: methane, ethane, propane and butane. After butane the root is derived from the Greek for the number of carbon atoms: pentane (C5), hexane (C6), heptane (C7), etc. Naming more complex (branched) alkanes requires more thought: carry out the following steps. 1 Find the parent hydrocarbon: the longest continuous chain of carbon atoms gives the ROOT of the name. E.g. 2 1 CH2CH3 Name is based on hexane CH3CH2CH2CHCH3 6 5 4 3 If two different chains of equal length are present, choose the one with the greater number of branch points. CH3 NOT 5 6 3 1 4 E.g. CH3CHCHCH2CH2CH3 CH3 2 CH3CHCHCH2CH2CH3 CH2CH3 CH2CH3 Name based on hexane with two substituents 2 Identify and number the substituents. Number the main chain so that substituents take the lowest possible numbers: give EACH substituent a number E.g. Substituent: 3-methyl CH3 6 5 4 2 1 CH3CH2CH2CHCH2CH3 Root: hexane 3 CH3 1 3 4 5 6 CH3CHCHCH2CH2CH3 2 CH2CH3 Substituent: 2-methyl Root: hexane Substituent: 3-ethyl 3 Write the name as one word, with hyphens (-) between numbers and words, and with substituents in alphabetical order. E.g. For the above examples, 3-methylhexane and 3-ethyl-2-methylhexane, respectively. If there are two substituents of the same type, use the prefix di (for three substituents, tri, etc), and a comma. 2 CH 33 4 5 E.g. CH3CCH2CH2CH3 1 CH3 2,2-dimethylpentane For alkanes with a more complex (e.g. branched) substituent, the substituent is named as if it were a compound itself. Its name appears in brackets. E.g. 1 CH3 3 6 10 CH3CHCHCH2CH2CHCH2CH2CH2CH3 2 CH3 CH2CHCH3 } 2-methylpropyl CH3 2 2,3-dimethyl-6-(2-methylpropyl)decane Class Question 1. Name the following alkanes according to the IUPAC system. 8 CH2CH3 2 1 (a) CH3CHCH2CH2CH2CHCH3 6 CH3 2,6-dimethyloctane (b) CH3 1 2CH3 CH3CCH2CH2CHCH2CH3 5 7 CH3 2,2,5-trimethylheptane 2. Draw structures corresponding to the following IUPAC names. (a) 2,2-Dimethyl-4-propyloctane (b) 3-Ethyl-4,4-dimethylheptane (a) 2,2-dimethyl-4-propyloctane 8 carbon atoms in longest chain two CH3 branches at position 2 1 CH3 one propyl branch at position 4 CH3 CH2CH2CH3 C 2 CH2 CH 4 CH2 CH2 CH2 8 CH3 CH3 (b) 3-ethyl-4,4-dimethylheptane 7 carbon atoms in longest chain one ethyl branch at position 3 CH3 1 CH3 CH2 two CH3 branches at position 4 CH2 CH 2 CH3 3 C CH2 CH3 CH2 7 CH3