Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Introduction to the Names and Structure of Organic Molecules Homologous series The diversity of life owes much to the diverse chemistry of one element - carbon. Over 90% of known compounds contain carbon. Carbon’s ability to form this vast range of compounds has led to an equally vast range of living things composed of carbon-based molecules. Because the major source of carbon compounds is living or once-living material, it was originally thought that only living things had an ‘organic vital force’ needed to produce carbon-based compounds. The synthesis of urea ((NH2)2CO) in 1828 by Friedrich Wöhler (1800–1882) showed this to be untrue, but the name ‘organic’ is still applied to the branch of chemistry dealing with the study of carbon-based compounds (excluding such compounds as CO, CO2 and the carbonates). Several factors contribute to the vast range of carbon compounds: • Carbon has four outer-shell electrons allowing each carbon atom to form up to four covalent bonds. • Carbon can form single, double or triple bonds. • Carbon can bond to itself, forming long chains and cyclic (ring) molecules. • Carbon can bond to a range of other elements, including hydrogen, oxygen, nitrogen, sulfur and chlorine. The study of organic chemistry is central to understanding the chemistry (and biology) of living systems. Knowledge of organic chemistry is used in the manufacture of drugs, foods, pesticides, fertilisers and other chemicals used by humans in agriculture, nutrition and medicines. Production of other materials of huge importance in society, such as fuels, solvents and polymers, also requires a sound knowledge of organic chemistry. Features of a homologous series To classify such a vast range of compounds, chemists look at various ‘families’ of carbon compounds that have common characteristics. The usual starting point is hydrocarbons, as they contain only two elements: hydrogen and carbon. A ‘family’ of carbon compounds with common characteristics is called a homologous series. A homologous series is a group of carbon compounds with the same general formula. Each member of a homologous series differs from the previous member by one CH2 group. Questions 1 (M06/S/2) State two characteristics of a homologous series. 1|P age Alkanes The most basic non-cyclic (straight chained) hydrocarbons are those where only single bonds occur. The simplest of these is methane (CH4), which is the first in a series of compounds called the alkanes. The term ‘saturated’ is also applied to alkanes. Saturated hydrocarbons have only single carbon–carbon bonds and therefore have the maximum number of hydrogen atoms; that is, they are saturated with hydrogen. The general formula for the homologous series of alkanes is CnH2n + 2. For example, if a molecule has five carbon atoms (n = 5), the alkane formed is C5H12 (2 × 5 + 2 = 12). Table 1 shows the first six straight chain alkanes. The naming of carbon compounds is very systematic. Both the prefix and suffix provide information. The prefix indicates the number of carbon atoms and the suffix ‘–ane’ indicates an alkane. Table 1: Complete the table first six members of the alkane homologous series. Molecular Empirical Name Structural formula Properties formula Formula Uses CH4 non-polar, gas, boiling point Bpt –164ºC cooking, Bunsen burners, gas heating ethane C2H6 non-polar, gas, Bpt –87ºC Producing ethene propane C3 H 8 non-polar, gas, Bpt –42ºC liquid petroleum gas (LPG) C4H10 Non-polar, gas, Bpt –0.5ºC LPG, cigarette lighters, camp stoves Non-polar, liquid, Bpt 36.1 ºC Present in natural gas, cigarette lighters and aerosol propellants methane butane Pentane 2|P age C5H12 Hexane C6H14 Non-polar, liquid, Bpt 68.7 ºC Present in petrol and many solvents The molecular formula shows the actual numbers of all the atoms in a molecule of the compound, but gives no information about the way the atoms are arranged, whereas the structural formula shows the way in which the atoms are bonded to each other and the shape of the molecule. A structural formula is much more useful when you need to visualise the molecule. The empirical formula is the lowest whole number ratio of elements in a compound, so the empirical formula of ethane would be CH3 and for butane would be C2H5. As the length and complexity of molecules increase, drawing full structural formulas becomes laborious. In those cases, more useful representations are semi-structural formulas, also known as condensed structural formulas, in which each carbon atom is listed alongside its attached hydrogen atoms. Condensed Structural formulas of pentane is: CH3CH2CH3CH2CH3. Question 2. Draw the structural and condensed formulas of heptane, C7H16 Drawing in 2d or 3d It is difficult to draw molecules to accurately represent their three-dimensional structure. Using pentane as an example, each carbon atom has four bonds. The expected tetrahedral arrangement exists around each of these carbon atoms. However, for simplicity, as the size and complexity of the molecules increases, many molecules are simply drawn in two dimensions. Three dimensional structures are drawn for simple inorganic and organic molecules wherever possible. 3d structural formula for a tetrahedral molecule 3|P age Question 3. Draw the Lewis structure for the methane and ethane molecule in their 2d shape. Give a reason for shape using VSEPR theory. The standard way of naming compounds (nomenclature) was developed by the International Union of Pure and Applied Chemistry; and is referred to as the IUPAC system. The IUPAC organization was formed in 1919 by chemists around the world with the goal of fostering a common language for communicating chemistry using of clear, concise and unambiguous terminology. The advantage of this system is that the name gives clues as to the structure of the molecule. Using this IUPAC system, it is possible to write the structural formula of any organic compound from its name. It is gradually replacing the older, less systematic way of naming organic compounds. For example, the chemical used to preservative animal samples is called ‘methanal’. Like methane, this compound has one carbon atom. It is an aldehyde (see later) and so has the ending ‘-al’. The name methanal is a more useful and informative name than the older name, ‘formalyn’. Table 2: Prefixes used for carbon compounds in IUPAC system Number of carbon atoms 1 2 3 4 5 6 7 4|P age Prefix MethEthProButPentHexHept- 8 9 10 OctNonDec- Structural Isomers The structural formulas shown in table 1 show only one possible arrangement of the atoms in the molecular formula. Molecules with the same molecular formula and different structural formulas are called isomers or structural isomers. The first member of the alkane homologous series to exhibit structural isomerism is butane. Methane, ethane and propane can each only take on one structure. Butane has two isomers called butane and 2-methylpropane. Structural isomers of butane: Butane 2-methylpropane Using the IUPAC system, branched alkanes are named by considering them as straight-chain alkanes with side groups attached. Side groups are named using the prefix for the appropriate number of carbon atoms, and the suffix ‘–yl’. Thus a CH3– side group is a methyl group; a CH3CH2– side group is an ethyl group and so on. Question 4. Draw the three structural isomers of pentane and their condensed structural formula. 5|P age Table 2 Naming hydrocarbons Steps CH3CH2CH2CH2CH2 CH3 CH3 CH3 Name this alkane Identify the longest carbon chain. Number the carbon of the longest chain so as to give the carbon atoms attached to the side group or the branch the lowest sum. CH3CH2CH2CH2CH2 CH3 CH3 CH3 1 2 3 4 5 6 CH3CH2CH2CH2CH2 CH3 CH3 CH3 Name the side branches and main chain. 2, 3 - dimethyl Combine to write the full name. 2, 3-dimethylhexane In cases where identical side groups are present on the hydrocarbon stem, they are given numbers, and the prefix ‘di-’, ‘tri-’, ‘tetra-’ and so on, is added to the hydrocarbon stem name. When numbering the carbon chain, numbering is such that the side groups or branch have the lowest possible numbers. It is important to inspect a formula carefully, because the most ‘obvious’ carbon chain is not always the longest one. The name is written as a single word, using hyphens to separate the different prefixes and using commas to separate numbers. If two or more different substituents are present, they are cited in alphabetical order. Prefixes like ‘di’, ‘tri’ etc are not used for alphabetising purposes. 6|P age Question 5. Draw and name the five structural isomers of hexane. 7|P age Cycloalkanes In cycloalkanes the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane. If you count the carbons and hydrogens, you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose two hydrogen atoms. All the cycloalkanes from cyclopentane upwards exist as "puckered rings". Cyclohexane, for example, has a ring structure which looks like this: 8|P age Alkenes Another homologous series of hydrocarbons is the alkenes, formed when the molecule contains one carbon–carbon double bond. Because at least two carbon atoms are required for a carbon-carbon double bond to be present, the simplest is ethene, C2H4. The alkenes have the general formula CnH2n. Alkenes are an example of an unsaturated hydrocarbon. Unsaturated hydrocarbons contain at least one carbon-carbon double bond, so also contain less than the maximum number of hydrogen atoms. Table 3: Number carbon atoms 2 Complete the table for the first six alkenes Molecular formula Name C2H4 Ethene 3 4 C4H8 5 C5H10 6 Structural formula CH2=CH2 Boiling point (°°C) –104 Propene –47 Butene –6 30 Hexene CH2=CHCH2CH2CH2 CH3 64 Alkenes also exhibit structural isomerism, the double bond may occur in different places along the chain, or branching may occur. There are two straight chain isomers of butene called but-1-ene and but-2-ene. The number indicates the position of the double bond in the chain. But-1-ene has its double bond in the first position along the chain – between carbons 1 and 2, whereas but-2-ene has its double bond in the second position – between carbons 2 and 3. When naming branched alkenes the carbon atoms of the double bond are given the lowest number. Questions 6. Draw and name the two straight chain structural isomers of butene. 9|P age 7. Draw and name the two branched chain isomer of butene. 8. a) Classify each of the following as alkanes or alkenes. C12H24 b) C16H32 c) C15H32 d) C72H146 9. a) Name the hydrocarbon molecules whose semi-structural (condensed structural) formulas are given below. CH3CH(CH3)CH2CH2CH3 b) CH2CHCHCH3 c) CH3CH2C(CH3)CH3 d) CH3C(CH3)2CHCHCH2CH3 i) ethene, C2H4 ii) ethane, C2H6 10 | P a g e Functional Groups Alkanes and alkenes only carbon and hydrogen. Introducing other types of atoms into a hydrocarbon allow a vast range of compounds to be formed. Atoms or groups of atoms attached to a hydrocarbon chain are called functional groups. These functional groups are significant because they change the reactivity of the molecule and can change the chemical properties of the compound. When we are focussing on the presence of a functional group in a hydrocarbon, it is common to write the general formula with the symbols of the functional group written as they appear in the structure of the molecule and a single letter R to represent the alkyl group (the rest of the hydrocarbon). If there is more than one alkyl group, then R′′ represents the second alkyl group and R′′′ may represent a third alkyl group. The structural formula of the important homologous series containing functional groups 11 | P a g e Alcohols The alcohol functional group is made up of an oxygen atom bonded to a hydrogen atom. This –OH group is known as a hydroxyl group and replaces one hydrogen in the structure of an alkane. The prefixes that are used to indicate the number of carbon atoms still apply to alcohols, however the suffix (ending) of the name of an alcohol is always –ol. The general formula of the alcohol homologous series is CnH2n+1OH or R-OH. Table 1 The first six members of the alcohol series. Condensed Name structural Structural formula formula Boiling Point (ºC) Solubility in water methanol CH3OH 65 Soluble ethanol C2H5OH 78 Soluble propanol C3H7OH 97 Slightly soluble butanol C4H9OH 118 Insoluble pentanol C5H11OH 138 Insoluble hexanol C6H13OH 156 Insoluble The position of the hydroxyl group is indicated by a number corresponding to the position in the carbon chain of the carbon atom to which the hydroxyl group is bonded. As with all isomers, the carbon atoms are counted from the end closest to the hydroxyl group. 12 | P a g e 10. Draw structural formulas and state the names of all the alcohols with the molecular formula C5H12O. 11. Draw the structural formulas for ethanol and hexan-1-ol. Explain why it is necessary to write the ‘1’ in hexan-1-ol, but no number is given with ethanol. 12. Draw the Lewis dot diagram for methanol and deduce the shape and bond angle with respect to the carbon and oxygen atoms. 13 | P a g e Aldehydes and Ketones Aldehydes and ketones are both homologous series containing the carbonyl functional group (C=O). The general formula of aldehydes is CnH2nO or RCHO. To avoid confusion with alcohols, the aldehyde functional group is written as -CHO in a formula indicating that the H is not directly bonded to the O, whereas the alcohols functional group is written as -OH. Ketones are similar to aldehydes in that they have a single carbonyl group (C=O), however the difference is that in an aldehyde, the carbonyl group is bonded to a carbon that is on the end of the carbon chain, whereas in a ketone, the carbonyl group is bonded to a carbon atom that is not on either end of the carbon chain. The general formula of ketones is RCOR′′ where R and R′′ may or may not be the same alkyl group. The consequence of this structure is that the smallest ketone, propanone, has three carbon atoms in its chain. The names of aldehydes and ketones follow the same pattern as other hydrocarbons. The number of carbon atoms in the chain including that in the carbonyl group is indicated by the prefix methan-, ethan-, propan- and the presence of the aldehyde or ketone functional group is indicated by the suffix. The names of aldehydes finish with the suffix –al and those of ketones finish with the suffix – one. The position of the carbonyl group in a ketone is indicated by a number just before the suffix (one). This number indicates which carbon atom is part of the carbonyl group. Table 2 Complete the table for aldehydes with up to 6 carbon atoms. Boiling Point (°C) Solubility in water Name Molecular formula Methanal HCHO -21 soluble Ethanal CH3CHO 21 soluble 46 Slightly soluble C2H5CHO 14 | P a g e Structural formula 75 Very slightly soluble Pentanal 103 insoluble Hexanal 119 insoluble Butanal C3H7CHO Table 3 Complete the table with Ketones with up to 6 carbon atoms. Condensed Name structural Structural formula formula Boiling Point (°C) Solubility in water Propanone CH3COCH3 56 soluble Butanone C2H5COCH3 80 Slightly soluble Pentan-2-one C3H7COCH3 102 insoluble Pentan-3-one C2H5COC2H5 100-102 insoluble Hexan-2-one C4H9COCH3 127 insoluble Hexan-3-one C3H7CO C2H5 123 insoluble 15 | P a g e Questions 13. (N05/S/1) Which compound is a member of the aldehyde homologous series? A. CH3COCH3 B. CH3CH2CH2OH C. CH3CH2COOH D. CH3CH2CHO 14. (M07/H/2/9) Draw the functional group isomers of C3H6O. (2 marks) Carboxylic Acids Carboxylic acids are organic acids. The carboxylic acid functional group is called the carboxyl group, has the formula –COOH and is made up of a carbonyl group (C=O) and a hydroxyl (-OH) group. Naming of carboxylic acids follows the same pattern as other hydrocarbons. The number of carbons in the carbon chain, including the carbon in the functional group is indicated by the prefix: methan-, ethan-, propan- and the presence of the carboxyl functional group is indicated by the suffix –oic acid. 16 | P a g e Table 4 Complete the table for Carboxylic acids with up to 6 carbon atoms. Boiling Point (°C) Solubility in water HCOOH 101 Soluble CH3COOH 118 Soluble 141 Soluble C3H7COOH 164 Soluble C4H9COOH 186 Slightly soluble 202 Slightly soluble Name Molecular formula Methanoic acid Propanoic acid Butanoic acid Hexanoic acid Structural formula Works Cited Clark, Jim. Chem Guide. 2008. 5 January 2008 <http://www.chemguide.co.uk/>. Derry, Lanna, Maria Connor and Carol Jordan. Chemistry for use with the IB Diploma Standard Level. Melbourne: Pearson Education, 2008. Clugston, Michael and Rosalind Flemming. Advanced Chemistry. Oxford: Oxford University Press, 2000. Organisation, International Baccalaureate. Online Curriculum Centre. <http://occ.ibo.org/ibis/occ/guest/home.cfm>. —. "Chemistry Data Booklet." International Baccalaureate Organisation, March 2007. —. "Chemistry Guide." International Baccalaureate Organisation, March 2007. 17 | P a g e