Organic Chemistry Module 3 PDF

PALAWAN STATE UNIVERSITY College of Sciences Chem 2/L - ORGANIC CHEMISTRY MODULE 3 Alkanes, Alkenes, Alkynes and their Cyclic Analogs  Learning Outcomes After going through in this module, you should be able to: ✔ Demonstrate understanding on hydrocarbons and their class...

PALAWAN STATE UNIVERSITY College of Sciences Chem 2/L - ORGANIC CHEMISTRY MODULE 3 Alkanes, Alkenes, Alkynes and their Cyclic Analogs  Learning Outcomes After going through in this module, you should be able to: ✔ Demonstrate understanding on hydrocarbons and their classes, properties and sources. ✔ Draw structures of specific class of organic molecule (hydrocarbon) given IUPAC/Common name and vice-versa. 2 Page 57  Overview There are more than 37 million known organic compounds. Each of these compounds has its own physical properties, such as melting point, and each has its own chemical reactivity. Chemists have learned through years of experience that organic compounds can be classified into families according to their structural features and that the members of a given family often have similar chemical reactivity. Instead of 37 million compounds with random reactivity, there are a few dozen families of compounds whose chemistry is reasonably predictable. We’ll study the chemistry of specific families of organic molecules throughout this book, beginning in this chapter with a look at the simplest family, the alkanes. In this module, we will talk about the different 3 classes of organic compounds based on their functionality, nomenclatures, structures, stereochemistry, physical properties, sources, uses, preparation, analysis reactions and mechanisms. Let us witness together the beauty of hydrocarbons. Page 58  Discussion 3.1 Hydrocarbons Hydrocarbons are compounds that are composed entirely of carbon and hydrogen atoms bonded to each other by covalent bonds. Fossil fuels—natural gas, petroleum, and coal—are the principal sources of hydrocarbons. Figure 3.1 Classes of Hydrocarbons 4 Page 59  Discussion 3.2 Saturated Hydrocarbons: Alkanes The alkanes, also known as paraffins, are straight- or branched-chain hydrocarbons with only single covalent bonds between the carbon atoms. Alkanes are hydrocarbons because they contain only carbon and hydrogen atoms; saturated because they have only C—C and C—H single bonds and thus contain the maximum possible number of hydrogens per carbon. They have the general formula CnH2n+2, where n is any integer. 5 Page 60  Discussion 3.2.1 Isomerism Think about the ways that carbon and hydrogen might combine to make alkanes. With one carbon and four hydrogens, only one structure is possible: methane, CH4. Similarly, there is only one possible combination of two carbons with six hydrogens (ethane, CH3CH3) and only one possible combination of three carbons with eight hydrogens (propane, CH3CH2CH3) (see figure 3.2.a). If larger numbers of carbons and hydrogens combine, however, more than one kind of molecule can form. For example, there are two ways that molecules with the formula C4H10 can form: the four carbons can be in a row (butane), or they can branch (isobutane) (see figure 3.2.b). Similarly, there are three ways in which C5H12 molecules can form, and so on for larger alkanes (see figure 3.2.c). 6 Figure 3.2.a Page 61  Discussion Figure 3.2.b Figure 3.2.c ©McMurry, Chemistry 7th ed. 7 Page 62  Discussion Compounds like the two C4H10 molecules and the three C5H12 molecules, which have the same formula but different structures, are called isomers, from the Greek isos + meros, meaning “made of the same parts.” Isomers have the same numbers and kinds of atoms but differ in the way the atoms are arranged. 8 Page 63  Discussion If a hydrogen atom is removed from an alkane, the partial structure that remains is called an alkyl group. Alkyl groups are named by replacing the -ane ending of the parent alkane with an -yl ending. For example, removal of a hydrogen atom from methane, CH4, generates a methyl group, -CH3, and removal of a hydrogen atom from ethane, CH3CH3, generates an ethyl group, -CH2CH3. Similarly, removal of a hydrogen atom from the end carbon of any n- alkane gives the series of n-alkyl groups shown in Table 3.3. 9 ©McMurry, Chemistry 7th ed. Page 64  Discussion 3.2.2 Nomenclature A chemical name typically has four parts in the IUPAC system of nomenclature: prefix, parent, locant, and suffix. The prefix specifies the location and identity of various substituent groups in the molecule, the parent selects a main part of the molecule and tells how many carbon atoms are in that part, the locant gives the location of the primary functional group, and the suffix identifies the primary functional group. STEP 1: Find the parent hydrocarbon. 10 (a) Find the longest continuous carbon chain in the molecule and use the name of that chain as the parent name. The longest chain may not always be obvious; you may have to “turn corners.” Page 65  Discussion (b) If two chains of equal length are present, choose the one with the larger number of branch points as the parent. STEP 2: Number the atoms in the main chain. Beginning at the end nearer the first branch point, number each carbon atom in the parent chain. 11 The first branch occurs at C3 in the proper system of numbering but at C4 in the improper system. Page 66  Discussion STEP 3: Identify and number the substituents. Assign a number, called a locant, to each substituent to specify its point of attachment to the parent chain. If there are two substituents on the same carbon, assign them both the same number. There must always be as many numbers in the name as there are substituents. Page 67  Discussion STEP 4 Write the name as a single word. Use hyphens to separate the various prefixes and commas to separate numbers. If two or more different side chains are present, cite them in alphabetical order. If two or more identical side chains are present, use the appropriate multiplier prefixes di-, tri-, tetra-, and so forth. Don’t use these prefixes for alphabetizing, though. Full names for some examples follow: Page 68  Learning Check Practice Exercise 3.1 Draw structures corresponding to the following IUPAC names: (a) 3,4-Dimethylnonane (b) 3-Ethyl-4,4-dimethylheptane (c) 2,2-Dimethyl-4-propyloctane (d) 2,2,4-Trimethylpentane 14 Page 69  Discussion 3.2.3 Reactions of Alkanes One type of alkane reaction has inspired people to explore equatorial jungles, endure the heat and sandstorms of the deserts of Africa and the Middle East, mush across the frozen Arctic, and drill holes—some more than 30,000 feet deep—on land and on the ocean floor! These strenuous and expensive activities have been undertaken because alkane, as well as other hydrocarbons, undergo combustion with oxygen with the evolution of large amounts of heat energy. Example: Some important non-combustion reactions of alkanes are: 15 1. Halogenation (a substitution reaction). A halogen is substituted for a hydrogen atom. When a specific halogen such as chlorine is used, the reaction is called chlorination; RH is an alkane (alkyl group + H atom) that reacts with halogens in this manner: Page 70  Discussion 2. Dehydrogenation (an elimination reaction). Hydrogen is lost from an alkane during dehydrogenation: 3. Cracking (breaking up large molecules to form smaller ones): 16 4. Isomerization (rearrangement of molecular structures): Page 71  Discussion 3.2.4 Sources of Alkanes The two main sources of alkanes are natural gas and petroleum. Natural gas is formed by the anaerobic decay of plants and animals. Its main component is methane (80-95%), the balance being varying amounts of other hydrocarbons, hydrogen, nitrogen, carbon monoxide, carbon dioxide, and in some locations, hydrogen sulfide. Economically significant amounts of methane are now obtained by the decomposition of sewage, garbage, and other organic waste products. Petroleum, also called crude oil, is a viscous black liquid consisting of a mixture of hydrocarbons with smaller amounts of nitrogen and sulfur-containing organic compounds. It is formed by the decomposition of plants and animals over millions of years. Major petroleum product is 17 gasoline. At the rate that natural gas and petroleum are being used, these sources of hydrocarbons are destined to be in short supply and virtually exhausted in the not-too-distant future. Alternative sources of fuels must be developed. Page 72  Discussion 3.2.5 Cycloalkanes Cyclic/closed-chain/cycloalkanes also exist. Their names are formed by adding the prefix cyclo- to the name of the open-chain alkane with the same number of carbon atoms. Its general formula is CnH2n. Figure 3.3 Cycloalkanes. In the line representations, each corner represents -CH2 group. 18 Page 73  Discussion Naming Cycloalkanes Step 1. Find the parent. Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane. If the number of carbon atoms in the largest substituent is greater than the number in the ring, the compound is named as a cycloalkyl-substituted alkane. 19 Page 74  Discussion Step 2. Number the substituents and write the name. For substituted cycloalkanes, start at a point of attachment and number around the ring. If two or more substituents are present, begin numbering at the group that has alphabetical priority and proceed around the ring so as to give the second substituent the lowest number. 20 Page 75  Learning Check Practice Exercise 3.2: Give IUPAC names for the following cycloalkanes: Draw structures corresponding to the following IUPAC names: (a) 1-tert-Butyl-2-methylcyclopentane (b) 1,1-Dimethylcyclobutane (c) 1-Ethyl-4-isopropylcyclohexane (d) 3-Cyclopropylhexane 21 Page 76  Discussion 3.3 Unsaturated Hydrocarbons: Alkenes And Alkynes Because of their multiple bond, alkenes and alkynes have fewer hydrogens per carbon than related alkanes and are therefore referred to as unsaturated. Alkenes, sometimes called olefins, are hydrocarbons that contain a carbon–carbon double bond. The open-chain alkene has a general formula of CnH2n.Alkynes are hydrocarbons that contain a carbon–carbon triple bond with a general formula CnH2n-2. Alkenes occur abundantly in nature, but alkynes are much less common. Ethylene, for instance, is a plant hormone that induces ripening in fruit, and α-pinene is the major component of turpentine. Life itself would be impossible without such compounds as β-carotene, a polyalkene that contains 11 double bonds. An orange 22 pigment responsible for the color of carrots, -carotene is a valuable dietary source of vitamin A. It was once thought to offer some protection against some types of cancer, but that has now been shown not to be true. Page 77  Discussion 3.3.1 Nomenclature of Alkenes and Alkynes Step 1: Name the parent hydrocarbon. Find the longest carbon chain that contains the double bond, and name the compound using the suffix -ene in place of -ane. Step 2: Number the carbon atoms in the chain. Begin numbering at the end nearer the double bond, or, if the double bond is equidistant from the two ends, begin at the 23 end nearer the first branch point. This rule ensures that the double-bond carbons receive the lowest possible numbers. Page 78  Discussion Step 3: Write the full name. Number the substituents on the main chain according to their position, and list them alphabetically. Indicate the position of the double bond by giving the number of the first alkene carbon and placing that number directly before the -ene suffix. If more than one double bond is present, give the position of each and use the appropriate multiplier suffix -diene, -triene, -tetraene, and so on. 24 Page 79  Learning Check Practice Exercise 3.3 Give IUPAC names for the following compounds: Alkenes Alkynes 25 Cycloalkenes Page 80  Discussion 3.3.3 Physical and Chemical Properties of Alkenes Both alkenes and alkynes have fewer than the maximum of four atoms bonded per carbon. These molecules are more reactive than the corresponding alkanes and readily undergo 26 addition and oxidation reactions. Page 81 Discussion 27 Page 82  Discussion Examples of Hydrogenation Reaction In hydrogenation reaction the two hydrogen atoms form a sigma bond with each of the carbon atoms with a double bond. The double bond is converted to a single bond. An alkene is converted to an alkane. An alkyne needs two moles of hydrogen atoms to become an alkane. 28 Page 83  Discussion Example of Halogenation Reaction In halogenation reaction, each Bromine atom form a covalent bond with each of the doubly bonded C atom. The double bond becomes a single bond =>. Example of Hydration of Alkene 29 Page 84  Discussion Examples of Hydrohalogenation Reaction For alkynes, two moles of each reactant are needed to completely convert an alkyne to an alkane. 30 Oxidation Reaction Page 85  Discussion 3.3.4 Physical and Chemical Properties of Alkynes Physical Properties Acetylene is a colorless gas, with little odor when pure. The disagreeable odor we associate with it is the result of impurities (usually PH3). Acetylene is partially water soluble and is a gas at normal temperature and pressure (bp = - 84⁰C). As a liquid, acetylene is very sensitive and may decompose violently (explode), either spontaneously or from a slight shock. The reaction is as follow: To eliminate the danger of explosions, acetylene is dissolved under pressure in acetone and is packed in cylinders that contain a porous inert material. 31 Chemical Properties Certain reactions are unique to alkynes. Alkynes are capable of reacting at times when alkenes will not. Acetylene, with certain catalysts, reacts with HCN to form CH2 = CHCN (acrylonitrile). This chemical is used industrially to manufacture Orlon, a polymer commonly found in clothing. It is also used to form the superabsorbent that are used in disposable diapers as well as in soil additives to retain water. Page 86  Evaluation Modified True or False Write TRUE if the statement is correct. If the statement is false, correct the term that makes the statement wrong. Write your answer at the provided space. ____________ 1. Hydrocarbons are composed of carbon and hydrogen atoms only. ____________ 2. Alkenes are considered to be saturated, since they contain the maximum number of hydrogen atoms attached to the carbon backbone. ____________ 3. Isomers are chemicals with the same molecular formulas, but with different structural formulas. ____________ 4. Oxygen is always needed for a combustion reaction to occur. 32 ____________ 5. The series of hydrocarbons that contain only single carbon to carbon bonds with an open chain is called alkanes. Page 87  Evaluation ____________ 6. Based on their physical properties, hydrocarbons (alkanes, alkenes and alkynes) are all nonpolar substances. ____________ 7. The study of hydrocarbons is the study of a class of organic compounds that contain mostly carbon and hydrogen. ____________ 8. Alkenes and Alkynes react with halogens in an addition reaction at room temperature. ____________ 9. Alkanes are highly reactive and highly combustible. ____________10. Carbon-carbon double and triple bonds are more reactive than carbon-carbon single bonds. Supply Type 33 A. Write the IUPAC names of the following hydrocarbons. 1. 2. Page 88  Evaluation 3. 4. CH3CH2CH2CH2CH2CH2CH=CH2 5. 6. 34 7. 8. CH3CH2C(CH3)2CH≡CH Page 89  Evaluation 9. 10. B. Draw the structure of each compound: 1. 2-methyl-2-pentene 2. cyclohexene 3. 3-ethyl-3-methyl1-pentene 4. 4-ethyl-2-heptyne 35 Page 90  Evaluation C. Write the product of the following reactions: 1. CH2 = CH-CH2CH3 + H2 2. CH2 = CH-CH2CH3 + HCl 3. CH3 CH=CH CH3 + H2O 4. + Br2 5. C4H10 + O2 36 Page 91  Evaluation Refer to the Modelling Activity Sheet that accompanies this module and do the Modelling Activity 3. 37 Page 92  References Textbooks: 1. McMurray, J.E. 2011. Seventh Edition: Fundamentals of Organic Chemistry. Cengage Learning. 20 Davis Drive Belmont, CA 94002-3098 USA. 2. Stoker, H.S. 2013. Sixth Edition: General, Organic and Biological Chemistry. Cengage Learning. 20 Davis Drive Belmont, CA 94002-3098 USA. Retrieved from: 1. https://www.alpha.chem.umb.edu/chem 2. www.siyavula.com 3. https://www.britannica.com/science 38 Page 93