Chapter 6 Introduction to Organic Chemistry: Alkanes PDF

Chapter 6 Introduction to Organic Chemistry: Alkanes Figure 6.1 Molecular model of cyclohexane in boat and chair forms Chapter Outlines 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Introduction to Organic Chemistry Classification of carbon atoms Classification of hydrocarbons Alkanes Nomenclature of alkan...

Chapter 6 Introduction to Organic Chemistry: Alkanes Figure 6.1 Molecular model of cyclohexane in boat and chair forms Chapter Outlines 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Introduction to Organic Chemistry Classification of carbon atoms Classification of hydrocarbons Alkanes Nomenclature of alkanes Cycloalkanes Physical properties of alkanes and cycloalkanes Chemical properties of alkanes and cycloalkanes Isomerism 200 Chapter 6 | Introduction to Organic Chemistry: Alkanes 6.1 Introduction to organic chemistry Organic chemistry is the study of the chemistry of the carbon compounds, nearly all of which also contain hydrogen atoms. The simplest of the organic compounds are hydrocarbons. They are compounds containing only carbon and hydrogen. Carbon is unique among the other elements in that its atoms can form stable covalent bonds with each other and with atoms of other elements in a multitude of variations. The resulting molecules can contain from one to millions of carbon atoms. We survey organic chemistry by dividing its compounds into families based on functional groups. Most organic chemicals are covalent compounds, which is why we introduce organic chemistry here. By convention, compounds containing carbonate ions and bicarbonate ions, as well as carbon dioxide and carbon monoxide, are not considered part of organic chemistry, even though they contain carbon. 6.2 Classification of carbon atoms Learning Objectives By the end of this section, you will be able to: • Classify different types of carbon atoms in organic molecules. Carbon atoms in organic compounds are classified by the number of other carbons bonded to them. A primary (1o) carbon is bonded to one other C atom; a secondary (2o) carbon is bonded to two other C atoms; a tertiary (3o) carbon is bonded to three other C atoms. A quaternary (4o) carbon is bonded to four other C atoms.  201 Chapter 6 | Introduction to Organic Chemistry: Alkanes  EXAMPLE 6.1 Classify each carbon atom in the compound below. Solution:  LEARNING CHECK 1. What types of carbon atoms are present in isobutane, (CH3)3CH? 2. How many tertiary hydrogens are there in Cl-CH2-C(CH3)2CH(CH3)-C(CH3)3? Answer o 1. 1 and 3o 2. 1 202 Chapter 6 | Introduction to Organic Chemistry: Alkanes 6.3 Classification of hydrocarbons Figure 6.2 Classification of hydrocarbons" 6.4 Alkanes Learning Objectives By the end of this section, you will be able to: • Write the molecular formulas for alkanes and cycloalkanes. • Draw structural formulas for alkanes by using expanded, line angle formulas. 6.4.1condensed Name and of Alkanes • Xxxxxxx 203 Chapter 6 | Introduction to Organic Chemistry: Alkanes 6.4.1 Naming alkanes Hydrocarbons, the simplest organic compounds, which are composed of carbon and hydrogen atoms only. There are several different kinds of hydrocarbons. They are distinguished by the types of bonding between carbon atoms and the properties that result from that bonding. Hydrocarbons with only carbon-carbon single bonds (C–C) and existing as a continuous chain of carbon atoms and bonded to hydrogen atoms are saturated hydrocarbons known as alkanes if they are acyclic (open chain, not cyclic), or as cycloalkanes if there are cyclic. Saturated hydrocarbons, in this case, it means that each carbon atom is bonded to four other atoms (hydrogen or carbon)—the most possible; there are no double or triple bonds in the molecules. Each alkane has a general formula CnH2n+2 also has a characteristic, systematic name depending on the number of carbon atoms in the molecule. These names consist of a stem that indicates the number of carbon atoms in the chain plus the ending -ane. The stem meth- means one carbon atom, so methane is an alkane with one carbon atom. Similarly, the stem eth- means two carbon atoms; ethane is an alkane with two carbon atoms. Continuing, the stem prop- means three carbon atoms, so propane is an alkane with three carbon atoms. Figure 6.3. Figure 6.3 Formulas and molecular models of the three simplest alkanes 204 Chapter 6 | Introduction to Organic Chemistry: Alkanes The VSEPR theory correctly predicts a tetrahedral shape for the methane molecule. Figure 6.4 The tetrahedral methane molecule Alkanes are named according to the number of carbon atoms. The first 10 members of this series are given in Table 12.1. Table 6.1 The first 10 straight chain alkanes Name Molecular formula (CnH2n+2) Condensed structural formula Stem methane ethane propane butane pentane hexane heptane octane nonane decane CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 CH4 CH3CH3 CH3CH2CH3 CH3CH2CH2CH3 CH3(CH2)3CH3 CH3(CH2)4CH3 CH3(CH2)5CH3 CH3(CH2)6CH3 CH3(CH2)7CH3 CH3(CH2)8CH3 meth eth prop but pent hex hept oct non dec Consider the series in Figure 12.4. The sequence starts with C3H8 and a CH2 unit is added in each step moving up the series. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a homologous series. The members of such a series, called homologs. 205 Chapter 6 | Introduction to Organic Chemistry: Alkanes Figure 6.5 Members of a homologous series 6.4.2 Branching in alkanes 1. Alkyl groups (R): An alkyl group is a group of atoms that results when one hydrogen atom is removed from an alkane. The group is named by replacing the ane suffix of the parent hydrocarbon with -yl. For example, the CH3 group derived from methane (CH4) results from subtracting one hydrogen atom and is called a methyl group. Table 6.2. Table 6.2 "Common alkyl groups" alkyl group structure methyl CH3– ethyl CH3CH2– propyl CH3CH2CH2– butyl CH3CH2CH2CH2– 2. Functional groups: a single atom, group of atoms or bonds that possess a characteristic chemical reactivity to the molecule. 206 Chapter 6 | Introduction to Organic Chemistry: Alkanes Figure 6.6 Common functional groups in organic compounds Table 6.3 Halogen functional groups in organic compounds Halogen F Cl Br I Prefix fluoro chloro bromo iodo Suffix fluoride chloride bromide iodide 6.4.3 Expanded, condensed and line-angle structural formulas for alkanes. A several kinds of formulas to describe organic compounds have been used. A molecular formula shows only the kinds and numbers of atoms in a molecule. For example, the molecular formula C4H10 tells us there are 4 carbon atoms and 10 hydrogen atoms in a molecule, but it does not distinguish between butane and isobutane. A structural formula shows all the carbon and hydrogen atoms and the bonds attaching them. Thus, structural formulas 207 Chapter 6 | Introduction to Organic Chemistry: Alkanes identify the specific isomers by showing the order of attachment of the various atoms. Unfortunately, structural formulas are difficult to type/write and take up a lot of space. Chemists often use condensed structural formulas to alleviate these problems. The condensed formulas show hydrogen atoms right next to the carbon atoms to which they are attached, as illustrated for butane: The ultimate condensed formula is a line-angle formula, in which carbon atoms are implied at the corners and ends of lines, and each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. For example, pentane (CH3CH2CH2CH2CH3) and isopentane [(CH3)2CHCH2CH3] are drawn as follows: Link to Learning Watch the video link of the different structural formulas Chapter 6 | Introduction to Organic Chemistry: Alkanes 208  EXAMPLE 6.2 Draw the condensed formula for each of the following structural formula. 1. 2. Solution 1. CH3CH2CH3 2. CH3CH2CH2CH2CH3  LEARNING CHECK 1. Give the condensed formula for the compound below. 2. Draw the Line-angle formula for the following condensed compound. (CH3)2CHCH2CH(CH2CH3)CH2CH2C(CH3)3 Answer 1. (CH3)2CHCH2CH(CH3)CH2CH3 2. Chapter 6 | Introduction to Organic Chemistry: Alkanes 209 6.5 Nomenclature of alkanes Learning Objectives By the end of this section, you will be able to: • • Use IUPAC system to name acyclic alkanes. Identify alkanes by using the common names. 6.5.1 IUPAC name of alkanes (International Union of Pure and Applied Chemistry) IUPAC rules for naming alkanes: 1. Choose the LCC (longest continuous chain) rather than the total number of carbon atoms. 2. Number the carbon atoms of the LCC, giving the carbon atoms with attached substituents the lowest numbers. 3. The LCC may be drawn in a straight line or bent line. 4. The name is written as Prefix + parent (LCC) + suffix (ane) 5. Name the substituent groups in alphabetical order preceding the name of the parent compound. Incorrect Correct 3-ethyl-5-methylheptane 6. 5-ethyl-3-methylheptane If the same group appears more than once, the number of identical groups is indicated by the Greek prefixes di-, tri-, tetra-, and so on. 210 Chapter 6 | Introduction to Organic Chemistry: Alkanes 2,3,5-trimethylhexane 7. These prefixes are not count in alphabetical order of the substituents. For example, ethyl is listed before dimethyl. The only prefixes that count is iso, neo and cyclo in alphabetical order. 6-ethyl-2,4-dimethyloctane 8. The last alkyl group named must be attached to the name of the parent alkane to form one word. 9. If the substituents have the same number from both ends, choose the one in alphabetically order. Correct Incorrect 10. If the first substituent has the same distance from both sides, give the third substituent the less number. 5-bromo-2-chloro-3-methylhexane Correct 2-bromo-5-chloro-4-methylhexane Incorrect 211 Chapter 6 | Introduction to Organic Chemistry: Alkanes 11. If two chains of equal length is obtained, then the chain with more substituents is the favored one. 3-ethyl-2,6-dimethylheptane (3 substituents) Correct. 2-methyl-5-isopropylheptane (2 substituents) Incorrect  EXAMPLE 6.3 What is the IUPAC name for each of the following compound? Solution 4-chloro-1-fluoro-2,2-dimethylpentane Chapter 6 | Introduction to Organic Chemistry: Alkanes 212  LEARNING CHECK What is the IUPAC name for each of the following compound? 1. 2. Answer 1. 3-Ethyl-2,4,4-trimethylhexane 2. 3,3-Dimethylnonane 6.5.2 Common name for alkanes 1. The prefix, n-, is usually given to the straight or unbranched alkane in common naming. CH3CH2CH2CH3 CH3CH2CH2CH2CH3 n- Butane n-Pentane 2. The prefix, iso-, is commonly given to 2-Methylalkane, i.e., for mono-substituted alkane that contain methyl branch at carbon number 2, while all the remaining chain is straight. Isobutane Isopentane Isohexane 213 Chapter 6 | Introduction to Organic Chemistry: Alkanes 3. The prefix, Neo, refers to 2,2-dimethylalkane (pentane at least, 5C), i.e., they contain two methyl substituents specifically at carbon number 2, while all the remaining chain is straight. Link to Learning Watch the video link of Naming alkanes 6.6 Cycloalkanes Learning Objectives By the end of this section, you will be able to: • Use IUPAC system to name cyclic alkanes. • . The cycloalkanes are cyclic hydrocarbons with only single bonds. named by adding the prefix cyclo- to the name of the open-chain compound having the same number of carbon atoms as there are in the ring. They usually have the formula CnH2n. The simplest of these cyclic hydrocarbons has the formula C3H6. Each carbon atom has two hydrogen atoms attached. 214 Chapter 6 | Introduction to Organic Chemistry: Alkanes 6.6.1 Nomenclature of cycloalkanes Prefix + cyclo + parent + suffix (ane) a. Mono-substituted Cycloalkanes Methylcyclohexane Butylcyclohexane b. Disubstituted Cycloalkanes 215 Chapter 6 | Introduction to Organic Chemistry: Alkanes c. Trisubstituted Cycloalkanes       EXAMPLE 6.4 What is the IUPAC name for the following compound? a. b. c. d. Chloroethylcyclohexane 2-Chloro-1-ethylcyclohexane 1-Chloro-2-ethylcyclohexane 1-Ethyl-1-chlorocyclohexane Solution c 216 Chapter 6 | Introduction to Organic Chemistry: Alkanes  LEARNING CHECK 1. Give the IUPAC name for the following compound? 2. Choose the IUPAC name for the compound below. A. B. C. D. 1-bromo-3-chloro-2,2,4-trifluorocyclobutane 2-bromo-4-chloro-1,1,3-trifluorocyclobutane 2-bromo-1,1,3-trifluoro-4-chlorocyclobutane 4-bromo-2-chloro-1,1,3-trifluorocyclobutane Answer 2. B Link to Learning Seethe video link of naming cycloalkanes 217 Chapter 6 | Introduction to Organic Chemistry: Alkanes 6.7 Physical properties cycloalkanes of alkanes and Learning Objectives By the end of this section, you will be able to: • Explain the trends of boiling and melting points as well as the solubility of alkanes. 1. Solubility: Alkanes and cycloalkanes are insoluble in water since they are non-polar, and water is a polar solvent. The insolubility of alkanes in water makes them good preservative for the metals. 2. Density: Alkanes and cycloalkanes are less dense than water (their density ranges from 0.6-0.8 g/ml while water density is 1g/ml.). That is why oil spills float on the water surface. 3. Boiling point: The boiling points for Alkanes and cycloalkanes increase as the carbon atoms increases. while decreases as branching increases in alkanes. 4. Physical states of alkanes: Alkanes from C1-4 are gases, whereas alkanes from (C5-C17) are liquids and higher (C 17) are solids. 218 Chapter 6 | Introduction to Organic Chemistry: Alkanes Table 6.4 Properties of some alkanes Name Molecular Formula Melting Point (°C) Boiling Point (°C) Density Physical State Methane CH4 –182 –164 0.668 g/L Gas Ethane C2H6 –183 –89 1.265 g/L Gas Propane C3H8 –190 –42 1.867 g/L Gas Butane C4H10 –138 –1 2.493 g/L Gas Pentane C5H12 –130 36 0.626g /mL Liquid Hexane C6H14 –95 69 0.659 g/mL Liquid Heptane C7H16 –90.6 98.4 0.684 g/mL Liquid Octane C8H18 –57 125 0.703 g/mL Liquid Nonane C9H20 –53.6 150.8 0.718 g/mL Liquid Decane C10H22 –30 174 0.730 g/mL Liquid 6.8 Chemical properties of alkanes 6.8.1 Combustion of alkanes Learning Objectives By the end of this section, you will be able to: • Describe the combustion and halogenation reactions of alkanes. Alkanes undergo complete Combustion that is, they burn in the presence of oxygen to form carbon dioxide CO2 and H2O. 𝜟 CnH2n+2 + excess O2 → 𝛥 CH4 + 2O2 → Methane nCO2 (g) + (n+1) H2O + energy CO2 (g) + 2H2O Chapter 6 | Introduction to Organic Chemistry: Alkanes 219 𝛥 C3H8+ 5O2 → Propane 3CO2(g) + 4H2O 𝛥 2C4H10 + 13O2 → 8CO2(g) + 10H2O Butane 6.8.2 Halogenation of alkanes and cycloalkanes: (Substitution reactions) Alkanes also react with the halogens, chlorine (Cl2) and bromine (Br2) in the presence of ultraviolet light or at high temperatures to yield chlorinated and brominated alkanes. For example, chlorine reacts with methane (CH4) to give methyl chloride (CH3Cl). With more chlorine, a mixture of products is obtained: CH3Cl, CH2Cl2, CHCl3, and CCl4. Fluorine (F2), the lightest halogen, combines explosively with most hydrocarbons. Iodine (I2) is relatively unreactive that why they have been excluded from halogenation of alkanes. Chlorination of methane 𝛥 CH4 + Cl2 → CH3Cl (Chloromethane) + HCl 𝛥 CH3Cl + Cl2 → CH2Cl2 (dichloromethane.) + HCl 𝛥 CH2Cl2 + Cl2 → CHCl3 (trichloromethane) + HCl 𝛥 CHCl3 + Cl2 → CCl4 (tetrachloromethane) + HCl Bromination of ethane  Bromination of propane 220 Chapter 6 | Introduction to Organic Chemistry: Alkanes Bromination of butane Bromination of isobutane Note that the Stability is inversely proportional to reactivity i.e., the more reactive, the less stable it is. So the reactivity of halogens are Cl2 < Br2 . Whereas the stability of halogens are Br2 < Cl2. The Ease o o o of abstraction of H-atoms is: 3 < 2 <1 221 Chapter 6 | Introduction to Organic Chemistry: Alkanes  EXAMPLE 6.5 Complete the following reaction giving all products and predict the major one Solution  LEARNING CHECK What do you expect when methylcyclohexane reacts with one mole of bromine in presence of UV light. Draw all possible products and determine the major one? 6.9 Isomerism Learning Objectives By the end of this section, you will be able to: • Distinguish the different types of isomers for alkanes. • Differentiate between the different types of geometrical isomers. Isomers are defined as two or more compounds that have the same molecular formula but different structures or arrangement of atoms in space. This phenomenon is called Isomerism. Isomers can be classified into: 222 Chapter 6 | Introduction to Organic Chemistry: Alkanes 6.9.1 Constitutional isomers Isomers have different structures and different connectivity, this can be classified into a. Chain isomers: Isomers that differs in chain length. Butane C4H10 exists as n-butane and 2-methylpropane. CH3CH2CH2CH3 & CH3CH(CH3)2 n-Butane 2-Methylpropane b. Positional isomers: Isomers that differ in the position of functional group. Chloropropane C3H7Cl exists as 1-chloropropane and 2-chloropropane. CH2(Cl)CH2CH3 & CH3CH(Cl)CH3 1-Chloropropane 2-Chlropropane c. Functional isomers: will be discussed in alcohols and ethers. Link to Learning Watch the video link of the Structural isomers 6.9.2 Stereoisomers Isomers have same connectivity; this can be classified into: a. Geometrical isomers b. Optical isomers Both will be discussed in alkenes and carbohydrates. Link to Learning See link: the classification of isomers 223 Chapter 6 | Introduction to Organic Chemistry: Alkanes  EXAMPLE 6.6 1. Two isomeric forms of a saturated hydrocarbon a. have the same structure. b. have different compositions of elements. c. have the same molecular formula. d. have a different content of the isotopes of hydrogen. 2. Which of the following hydrocarbons does not have isomers? a. C7H16 b. C3H8 c. C5H10 d. C4H8 Solution 1. c 2. b  LEARNING CHECK The common name of the alkane isomer of isohexane is: A. 2-methylpentane B. 3-methylpentane C. Cyclohexane D. n-hexane Answer: D 224 Chapter 6 | Introduction to Organic Chemistry: Alkanes PROBLEMS Choose the correct statement for each of the following questions: 1. Two isomeric forms of a saturated hydrocarbon A. have the same structure. B. have different compositions of elements. C. have the same molecular formula. D. have a different content of the isotopes of hydrogen. 2. Which of the following hydrocarbons does not have isomers? A. C7H16 B. C3H8 C. C5H10 D. C4H8 3. How many isomers of dibromocyclohexane exist? A. 6 B. 4 C. 3 D. 2 4. For which of the compounds below are positional isomers possible? CH3CHFCH2F CH3CH2CHF2 FCH2CH2CH2F (1) A. 2 and 3 B. 3 only C. 1,2 and 3 D. 1 and 2 (2) (3) 225 Chapter 6 | Introduction to Organic Chemistry: Alkanes 5. Which of the following does NOT exhibit isomerism? (Hint: draw them!) A. 1,2-Dibromoethane B. 1,2-Dichloropropane C. Propane D. Butane 6. How many isomeric alkanes of the molecular formula C5H12 are there? A. 1 B. 2 C. 3 D. 4 7. What is the general formula of a cycloalkane? A. CnH2n+2 B. CnH2n C. CnH2n-2 D. CnHn 8. What is the IUPAC name for the following compound? A. 2,4,4,4-tetramethylbutane B. 2,4,4-trimethylpentane C. 2,2,4-trimethylpentane D. 1,1,1,3-tetramethylbutane 226 Chapter 6 | Introduction to Organic Chemistry: Alkanes 9. What is the IUPAC name for? A. chlorotrimethylbutane B. 2-chloro-2,4-dimethylpentane C. 4-chloro-2,4-dimethylpentane D. 1-chloro-1,1-3trimethylbutane 10. The line angle drawing of CH3CH2CH(CH3)CH(CH3)CH2CH3 is: A. B. C. D. 11. Which compound is identical to this molecule? A. B. C. D. (CH3)2CHCH2CH2CH3 CH3(CH2)4CH3 CH3CH2CH(CH3)2 CH3(CH2)3CH(CH3)2 Chapter 6 | Introduction to Organic Chemistry: Alkanes 227 12. What is the IUPAC name of this molecule? 13. 14. 15. 16. A. 2,4-dimethyl-2-ethylhexane B. 3,3,5-trimethylheptane C. 2-ethyl-2,4-dimethylhexane D. 5-ethyl-3,5-dimethylhexane Which alkane among the following has the lowest boiling point? A. octane B. heptane C. pentane D. hexane The general molecular formula for cycloheptane is ____ A. C7H12 B. C7H16 C. C7H14 D. C6H12 Alkanes do not dissolve in water because …………… A. they are nonpolar. B. they have low boiling points. C. they have very high boiling points. D. they are polar. What is the IUPAC name of the following molecule? A. B. C. D. 3-ethyl-4,5,6-trimethylheptane 5-ethyl-2,3,4-trimethylheptane 4,5,6-trimethyl-3-ethyloctane 2,3,4-trimethyl-5-ethyloctane 228 Chapter 6 | Introduction to Organic Chemistry: Alkanes 17. What is the condensed formula for the following structure? A. B. C. D. (CH3)2CH(C2H5)CH2CH(CH3)CH2CH3 CH3CH2CH(CH3)(CH2)2CH(C2H5)CH(CH3)2 (CH3)2CH(C2H5)CH2CH2(CH3)CH2CH3 CH3CH2(CH3)(CH2)2CH(C2H5)CH(CH3)2 18. What is the IUPAC name of (CH3)(CH2)2CH(C2H5)CH(CH3)2? A. 3-isopropylhexane B. 3-ethyl-2-methylhexane C. 2-methyl-3-ethylhexane D. 4-isopropylhexane 19. How many positional isomers are there with C2H4Br2? A. 1 B. 2 C. 3 D. 4 20. What is the IUPAC name of the following? A. B. C. D. 1-bromo-3,6-diethyl-3-methylcyclohexane 2-bromo-1,4-diethyl-4-methylcyclohexane 5-bromo-1,4-diethyl-1-methylcyclohexane 3-bromo-1,4-diethyl-1-methylcyclohexane 229 Chapter 6 | Introduction to Organic Chemistry: Alkanes ADDITIONAL PROBLEMS 1. Which two products are produced by the complete combustion of fuels (C3H8)? 2. How many constitutional isomers are there with formula C4H9Cl? 3. Explain why these two molecules are not isomers: 4. Give the IUPAC name for the following compounds? 5. Draw the line formula for 2,5-dichloro-2,5-dimethylhexane. 6. Draw the structure for each of the following: a) 4-ethyloctane b) 3-ethyl-2-methylpentane c) 3,3,5-trimethylheptane d) 3-methylpentane e) 2,2,5-trimethylhexane f) 4-ethyl-3-methyloctane g) 2,3-dimethylbutane h) 4-ethyl-2-methylheptane Chapter 6 | Introduction to Organic Chemistry: Alkanes 230 7. How many carbon atoms are there in each molecule? a) methylbutane b) 3-ethylpentane c) 2,3-dimethylbutane d) 3-ethyl-2-methylheptane 8. What is the IUPAC name for each of the following? a) b) c) d) 9. Convert the following condensed formula to the line-angle formula. CH3CH2C(CH3)2CH(CH3)CH(CH3)CH2C(CH3)3 10. Give the IUPAC name for the following structure: a) b) 231 Chapter 6 | Introduction to Organic Chemistry: Alkanes RESOURCES 1. https://saylordotorg.github.io/text_introductory-chemistry/s20organic-chemistry.html 2. Janice G. Smith, (2015) General, Organic, & Biological Chemistry by, 3rd Edition, McGraw Hill Company, Boston, New York, USA