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The influence of heteroatom substituents on the reactivity of carbonyl functions toward nucleophiles was discussed earlier with respect to carboxylic acid derivatives. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The main source of these liquid alkane fuels is crude oil, a complex mixture that is separated by fractional distillation. The International Union of Pure and Applied Chemistry (IUPAC) has devised a system of nomenclature that begins with the names of the alkanes and can be adjusted from there to account for more complicated structures. The names and structures of these and several other alkyl groups are listed in Figure 4. Conjugation extends the dipolar character of the carbonyl group to the double bond (or aromatic ring) so that the beta-carbon atom shares the positive character of the carbonyl carbon. The number of substituents of the same type is indicated by the prefixes di- (two), tri- (three), tetra- (four), and so on (for example, difluoro- indicates two fluoride substituents). Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. We use the term normal, or the prefix n, to refer to a chain of carbon atoms without branching. The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be pentene. The values cited here are for pure liquid or CCl4 solution spectra. Chemical Bonding and Molecular Geometry, 7.5 Strengths of Ionic and Covalent Bonds, Chapter 8. The carbonyl stretching absorption is colored blue, and characteristic overtones near 3400 cm-1, which are only prominent in liquid phase spectra, are colored orange. The location of an alkyl group on a hydrocarbon chain is indicated in the same way as any other substituent: Alkanes are relatively stable molecules, but heat or light will activate reactions that involve the breaking of C–H or C–C single bonds. The largest database [1] of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. There are several sets of answers; one is: (a) $\text{C}_5\text{H}_{12}$ Want more practice naming alkanes? 2. The infrared spectrum of aniline is shown beneath the following table. When this happens, the C-C bonds of the ring assume greater p-character and the C-O sigma bond has correspondingly greater s-character. The double bond of the carbonyl group is therefore shorter and stronger, and exhibits a larger stretching frequency. In acetylene, the bonding uses sp hybrids on carbon atoms and s orbitals on hydrogen atoms. The simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. Draw Lewis structures for these compounds, with resonance structures as appropriate, and determine the hybridization of the carbon atoms in each. (a) 1 mol of 1-butyne reacts with 2 mol of iodine. Q:-In a reaction A + B2 → AB2 Identify the limiting reagent, if any, in the following reaction mixtures. Each of the six bonds between its carbon atoms is equivalent and exhibits properties that are intermediate between those of a C–C single bond and a $\text{C}\;=\;\text{C}$ double bond. 1.5 Measurement Uncertainty, Accuracy, and Precision, 1.6 Mathematical Treatment of Measurement Results, Chapter 3. Teflon is prepared by the polymerization of tetrafluoroethylene. Structure of Alkynes 2. The buttons beneath the spectrum will display spectra for propionic acid as a pure liquid and in the gas phase. Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas). The 30 to 40 cm-1 decrease in frequency is illustrated by the following examples. As a consequence, alkanes are excellent fuels. Aromatic hydrocarbons contain ring structures with delocalized π electron systems. Each carbon atom is converted into the end of a line or the place where lines intersect. Ethylene (the common industrial name for ethene) is a basic raw material in the production of polyethylene and other important compounds. The spectrum of vanillin shows this for the phenolic hydroxyl, which is hydrogen bonded to the adjacent ether oxygen. Preparation. Advanced Theories of Covalent Bonding, 9.2 Relating Pressure, Volume, Amount, and Temperature: The Ideal Gas Law, 9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions, 10.6 Lattice Structures in Crystalline Solids, Chapter 13. In ether solvents a sharper hydrogen bonded monomer absorption near 3500 cm-1 is observed, due to competition of the ether oxygen as a hydrogen bond acceptor. (a) 2,2-dibromobutane; (b) 2-chloro-2-methylpropane; (c) 2-methylbutane; (d) 1-butyne; (e) 4-fluoro-4-methyl-1-octyne; (f) trans-1-chloropropene; (g) 5-methyl-1-pentene. Over 135 million tons of ethylene were produced worldwide in 2010 for use in the polymer, petrochemical, and plastic industries. Explain why these two molecules are not isomers: How does the carbon-atom hybridization change when polyethylene is prepared from ethylene? The green hydrogen atom in 2-methylpropane is bonded to a carbon atom bonded to three other carbon atoms. Hydrocarbons with the same formula, including alkanes, can have different structures. Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. The chlorine at position 1 will be described by adding 1-chloro-, resulting in the name of the molecule being 2-bromo-1-chlorobutane. The O-H stretching absorption band is therefore split into two (shaded orange and blue). Thus, the stretching frequency of a free O-H bond is 3600 cm-1, but the O-D equivalent is lowered to 2600 cm-1. Assume a 100% yield of product. Answers to Chemistry End of Chapter Exercises. This compound contains 16 hydrogen atoms for a molecular formula of C8H16. The bond energy (and force constant) of the conjugated C=O group is correspondingly reduced, and this results in a lower stretching frequency. Fractional distillation takes advantage of differences in the boiling points of the components of the mixture (see Figure 5). The ending -o replaces -ide at the end of the name of an electronegative substituent (in ionic compounds, the negatively charged ion ends with -ide like chloride; in organic compounds, such atoms are treated as substituents and the -o ending is used). ; (c) $\text{C}_6\text{H}_{12}$ The sulfuric acid serves as the dehydration agent. Gasoline is a liquid mixture of continuous- and branched-chain alkanes, each containing from five to nine carbon atoms, plus various additives to improve its performance as a fuel. A smaller peak at higher frequency (light blue) is presumed due to less associated clusters. By clicking on any of the ten structural terms in the first column, a corresponding spectrum will be displayed beneath the table. They are equivalent because each is bonded to a carbon atom (the same carbon atom) that is bonded to three hydrogen atoms. Stretching absorptions are marked in blue, bending absprptions in green. The O-H stretching absorption is exclusively monomeric in the gas phase, but in moderately dilute CCl4 solution both monomeric and hydrogenbonded absorptions are evident. Explain why unbranched alkenes can form geometric isomers while unbranched alkanes cannot. Lewis structures that look different may actually represent the same isomers. In benzene, the carbon atoms are sp2 hybridized. Watch this brief video tutorial to review the nomenclature process. The hybridization of the carbon atoms in the double bond in an alkene changes from sp2 to sp3 during an addition reaction. Such cases will be discussed as carboxylic acid derivatives. Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties. The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene. Naming Substituted Alkanes Check Your Learning The reaction of acetylene with bromine is a typical example: Acetylene and the other alkynes also burn readily. ; (f) $\text{C}_6\text{H}_{10}$, 11. Crude oil is the remains of an ancient biomass consisting mainly of plankton that was buried in mud. 3. Crude oil is … Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. ; 3. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. (i) 300 atoms of A + 200 molecules of B (ii) 2 mol A + 3 mol B (iii) 100 atoms of A + 100 molecules of B (iv) 5 mol A + 2.5 mol B Dimeric clusters shift the absorption frequency to 3500 cm-1, but polymeric associations are shifted further, 3200 to 3500 cm-1, and broadened. The equation on the right describes the major factors that influence the stretching frequency of a covalent bond between two atoms of mass m1 and m2 respectively. The broad O-H bend at 935 cm-1, for example is typical of the dimeric species. Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid. Electron donating substituents on the carbonyl group stabilize the ionic resonance contributor, and increase the single bond character of the C=O bond. Carbon atoms 1 and 4 have four single bonds and are thus tetrahedral with sp3 hybridization. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes. Two different alkyl groups can be formed from each of these molecules, depending on which hydrogen atom is removed. The location of the double bond is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond: Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs. This effect, which may shift the carbonyl stretching frequency up or down, is particularly important when an alkyl substituent is replaced by a hetero atom such as N, O or X (halogen). The increase in frequency ranges from 30 to 45 cm-1 for a 5-membered ring, to 50 to 60 cm-1 for a 4-membered ring, and nearly 130 cm-1 for a 3-membered ring. Geometric isomers of alkenes differ in the orientation of the groups on either side of a $\text{C}\;=\;\text{C}$ bond. Used in isomerization of alkenes; isomerization of amines; a form of high surface area sodium metal. Electron withdrawing groups have an opposite influence, and increase the stretching frequency of the carbonyl group. Molecules having both hydrogen bonding donors and acceptors located so that intramolecular hydrogen bonding is favored, display slightly broadened O-H stretching absorption in the 3500 to 3600 cm-1 range. Identify the chemical formula of the molecule represented here: All alkanes are composed of carbon and hydrogen atoms, and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of CnH2n+2. The two purple hydrogen atoms in propane are of a second type. Three factors are known to perturb the carbonyl stretching frequency: 1. (a) 2,2,4-trimethylpentane; (b) 2,2,3-trimethylpentane, 2,3,4-trimethylpentane, and 2,3,3-trimethylpentane: 21. Starting with the spectrum of cyclohexanone, infrared spectra of six illustrative ketones will be displayed below on clicking the "Toggle Spectra" button. Amines are bases, and their corresponding conjugate acid "onium" salts are often the form in which they are commonly encountered. They are identical because each contains an unbranched chain of four carbon atoms. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules. By clicking the "Toggle 1°-Amine" button, solution and gas phase spectra will be displayed sequentially, along with the spectrum of cyclohexylamine, an aliphatic 1°-amine. Chemically, the alkynes are similar to the alkenes. These characteristics are listed below. Stoichiometry of Chemical Reactions, 4.1 Writing and Balancing Chemical Equations, Chapter 6. The differing geometries are responsible for the different properties of unsaturated versus saturated fats. Not all molecular vibrations lead to observable infrared absorptions. No carbon-carbon bonds are broken in these reactions, and the hybridization of the carbon atoms does not change. For example, the reaction between ethane and molecular chlorine depicted here is a substitution reaction: The C–Cl portion of the chloroethane molecule is an example of a functional group, the part or moiety of a molecule that imparts a specific chemical reactivity. These operations have been in effect since the 1970s and have made the production of some plastics among the most efficient industrial operations today. The enantioselectivities of these processes, when the reactions were carried out using rat and human liver microsomal preparations, were modest and dependent on both P450 composition and substrate concentrations. (a) $\text{CH}_3\text{CH}_2\text{CBr}_2\text{CH}_3$, (b) $(\text{CH}_3)_3\text{CCl}$, (d) $\text{CH}_3\text{CH}_2\text{C}\;{\equiv}\;\text{CH\;CH}_3\text{CH}_2\text{C}\;{\equiv}\;\text{CH}$, (g) $(\text{CH}_3)_2\text{CHCH}_2\text{CH} = \text{CH}_2$, (a) $(\text{CH}_3)_2\text{CHF}$, (b) $\text{CH}_3\text{CHClCHClCH}_3$, (d) $\text{CH}_3\text{CH}_2\text{CH} = \text{CHCH}_3$, (f) $(\text{CH}_3)_3\text{CCH}_2\text{C}{\equiv}\text{CH}$. How many kilograms of ethylene is produced by the pyrolysis of 1.000 × 103 kg of ethane, assuming a 100.0% yield? Again, this absorption appears at slightly higher frequency when the nitrogen atom is bonded to an aromatic ring. We begin counting at the end of the chain closest to the double bond—in this case, from the left—the double bond spans carbons 2 and 3, so the name becomes 2-pentene. Check Your Learning In the saturated hydrocarbon, an existing C–H bond is broken, and a bond between the C and the Br can then be formed. 1. Write Lewis structures and IUPAC names for all isomers of $\text{C}_4\text{H}_9\text{Cl}$. Aromatic compounds more readily undergo substitution reactions than addition reactions; replacement of one of the hydrogen atoms with another substituent will leave the delocalized double bonds intact. Infrared spectra of many carboxylic acid derivatives will be displayed in the figure below the table by clicking the appropriate buttons presented there. Signal word Danger. Write the Lewis structure and molecular formula for each of the following hydrocarbons: Write the chemical formula, condensed formula, and Lewis structure for each of the following hydrocarbons: Give the complete IUPAC name for each of the following compounds: Butane is used as a fuel in disposable lighters. Since deuterium has a mass = 2, the mass term in the equation changes fron 1 to 1/2, and the frequency is reduced by the square root of 2. Figure 2 shows three different ways to draw the same structure. The chemistry of these compounds is called organic chemistry. Provide the IUPAC names for the reactant and product of the halogenation reaction shown here: Solution In the gas phase and in dilute CCl4 solution (0.01 M) small to moderate sized alcohols exhibit a sharp absorption in the 3620 to 3670 cm-1 region. In a substitution reaction, another typical reaction of alkanes, one or more of the alkane’s hydrogen atoms is replaced with a different atom or group of atoms. As illustrated by the following resonance equation, this not only explains conjugate addition reactions of nucleophiles, but also suggests that the carbonyl double bond has slightly more single bond character than does an unconjugated function. Nature Chemistry, DOI: 10.1038/s41557-019-0409-4, published online 10 February 2020 To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown in Figure 10. Polymers can be natural (starch is a polymer of sugar residues and proteins are polymers of amino acids) or synthetic [like polyethylene, polyvinyl chloride (PVC), and polystyrene]. Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the major alkene: (i) 1-Bromo-1-methylcyclohexane (ii) 2-Chloro-2-methylbutane (iii) 2,2,3-Trimethyl-3-bromopentane. Under ideal conditions the carbon atom of a carbonyl group is essentially sp2 hybridized, which implies that the bond angles will be 120° and the C-O sigma bond has 33% s-character. Write the structures for all the isomers of the $-\text{C}_5\text{H}_{11}$ alkyl group. In aromatic amines these absorptions are usually 40 to 70 cm-1 higher in frequency. If you are uncertain about the IUPAC rules for nomenclature you should review them now. The O-H stretching absorption of the hydroxyl group is sensitive to hydrogen bonding. The simplest member of the alkyne series is ethyne, C2H2, commonly called acetylene. Styrene is used to produce the polymer polystyrene. The product of the halogenation reaction will have two chlorine atoms attached to the carbon atoms that were a part of the carbon-carbon double bond: This molecule is now a substituted alkane and will be named as such. Properties such as melting point and boiling point (Table 1) usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane. Does this explanation involve the macroscopic domain or the microscopic domain? Write the chemical formula and Lewis structure of the following, each of which contains five carbon atoms: What is the difference between the hybridization of carbon atoms’ valence orbitals in saturated and unsaturated hydrocarbons? These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Stretching absorptions are marked in blue, bending absprptions in green. Thus, alkenes undergo a characteristic reaction in which the π bond is broken and replaced by two σ bonds. Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials. This reaction is called an addition reaction. Most of the monomers that go into common plastics (ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate) are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling. In the analogy of a spring, it corresponds to the spring's stiffness. To understand the functioning of these factors consider the following analyses: 1. The following are typical examples of substituted benzene derivatives: Toluene and xylene are important solvents and raw materials in the chemical industry. Alkynes contain one or more carbon-carbon triple bonds. One possible isomer created by a substitution reaction that replaces a hydrogen atom attached to the aromatic ring of toluene with a chlorine atom is shown here. We use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. In the unsaturated hydrocarbon, the only bond broken in the hydrocarbon is the π bond whose electrons can be used to form a bond to one of the bromine atoms in Br2 (the electrons from the Br–Br bond form the other C–Br bond on the other carbon that was part of the π bond in the starting unsaturated hydrocarbon). Electronic Structure and Periodic Properties of Elements, 6.4 Electronic Structure of Atoms (Electron Configurations), 6.5 Periodic Variations in Element Properties, Chapter 7. The "Toggle 2°-Amine" and "Toggle 3°-Amine" buttons display illustrative spectra for these classes of amines. Saper et al. Safety & Documentation. Composition of Substances and Solutions, 3.2 Determining Empirical and Molecular Formulas, 3.4 Other Units for Solution Concentrations, Chapter 4. Overtones are colored orange. Since there are two carbon-containing groups attached to the two carbon atoms in the double bond—and they are on the same side of the double bond—this molecule is the cis-isomer, making the name of the starting alkene cis-2-pentene. Condensed structural formulas for ethane and pentane are shown at the bottom of Figure 1, and several additional examples are provided in the exercises at the end of this chapter. You have likely heard of unsaturated fats. Acetylene is a very weak acid; however, it will react with moist silver(I) oxide and form water and a compound composed of silver and carbon. C-N stretching absorptions are found at 1200 to 1350 cm-1 for aromatic amines, and at 1000 to 1250 cm-1 for aliphatic amines. Name and write the structures of all isomers of the propyl and butyl alkyl groups. The C-N absorptions are found in the same range, 1200 to 1350 cm-1 (aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1°-amines. Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. ; (d) $\text{C}_6\text{H}_{12}$ Write Lewis structures and name the five structural isomers of hexane. The carbonyl stretching frequency of the dimer is found near 1710 cm-1, but is increased by 25 cm-1 or more in the monomeric state. This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. Transition Metals and Coordination Chemistry, 19.1 Occurrence, Preparation, and Properties of Transition Metals and Their Compounds, 19.2 Coordination Chemistry of Transition Metals, 19.3 Spectroscopic and Magnetic Properties of Coordination Compounds, 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters, Appendix D: Fundamental Physical Constants, Appendix F: Composition of Commercial Acids and Bases, Appendix G: Standard Thermodynamic Properties for Selected Substances, Appendix H: Ionization Constants of Weak Acids, Appendix I: Ionization Constants of Weak Bases, Appendix K: Formation Constants for Complex Ions, Appendix L: Standard Electrode (Half-Cell) Potentials, Appendix M: Half-Lives for Several Radioactive Isotopes. Approximate Infrared Stretching Frequencies. Three valence electrons in the sp2 hybrid orbitals of each carbon atom and the valence electron of each hydrogen atom form the framework of σ bonds in the benzene molecule. 3. Kerosene, diesel oil, and fuel oil are primarily mixtures of alkanes with higher molecular masses. Safety Information. This shift also occurs in the presence of the previous conjugative lowering of the stretching absorption. The C-N absorptions are found in the same range, 1200 to 1350 cm-1(aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1°-amines. Salts of 1° and 2°-amines also exhibit strong bending absorptions in the range of 1500 to 1600 cm-1, but the corresponding band from 3°-ammonium salts is relatively weak. Primary aliphatic amines display two well-defined peaks due to asymmetric (higher frequency) and symmetric N-H stretching, separated by 80 to 100 cm-1. The hydrogen atoms can be replaced by many different substituents. The name of an alkene is derived from the name of the alkane with the same number of carbon atoms. (a) What is the empirical formula of the compound of silver and carbon? Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. Adding functional groups to the structure of a polymer can result in significantly different properties (see the discussion about Kevlar later in this chapter). Name the molecule whose structure is shown here: The four-carbon chain is numbered from the end with the chlorine atom. Draw the skeletal structures for these two molecules: Solution They are the same compound because each is a saturated hydrocarbon containing an unbranched chain of six carbon atoms. The longest carbon chain runs horizontally across the page and contains six carbon atoms (this makes the base of the name hexane, but we will also need to incorporate the name of the branch). Hydrogen atoms are not drawn if they are attached to a carbon. Equilibria of Other Reaction Classes, 16.3 The Second and Third Laws of Thermodynamics, 17.1 Balancing Oxidation-Reduction Reactions, Chapter 18. These derivatives display strong, broad N-H stretching absorptions in the 2250 to 3000 cm-1 region, with 1°-ammonium salts absorbing at the high frequency end, where overlap with C-H absorption occurs. Other characteristic stretching and bending absorptions are marked in the spectrum. What mass of 2-bromopropane could be prepared from 25.5 g of propene? The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities. The cis isomer, on the other hand, has a more hindered hydroxyl group which adopts some conformations having smaller hydrogen bonded clusters. Name the following molecule: We call a substituent that contains one less hydrogen than the corresponding alkane an alkyl group. In general, a vibration must cause a change in the charge distribution within a molecule to absorb infrared light. A π bond, being a weaker bond, is disrupted much more easily than a σ bond. Since the six-carbon ring with alternating double bonds is necessary for the molecule to be classified as aromatic, appropriate isomers can be produced only by changing the positions of the chloro-substituent relative to the methyl-substituent: Check Your Learning Identifying isomers from Lewis structures is not as easy as it looks. The stretching frequency is therefore decreased, as noted in the right hand example below. Identify the chemical formula of the molecule represented here: Solution On a microscopic level, how does the reaction of bromine with a saturated hydrocarbon differ from its reaction with an unsaturated hydrocarbon? The types of functional groups present in an organic molecule are major determinants of its chemical properties and are used as a means of classifying organic compounds as detailed in the remaining sections of this chapter. Describe the geometry and hybridization of the carbon atoms in the following molecule: Solution Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code (see Figure 8). Polyethylene is a commodity plastic used primarily for packaging (bags and films). C=O stretch1860 ± 20 cm-11800 ± 151800 ± 15, Acid Anhydride, (RCO)2Oacyclic6-membered ring5-membered ring, C=O stretch (2 bands)1750 & 1820 cm-11750 &18201785 & 1865, Esters & Lactones (RCOOR')esters6-membered lactone5-membered lactone4-membered lactone, C=O stretch 1740 cm ± 10 cm-11740 cm ± 101765 cm± 51840 cm ± 5, Amides & Lactams (RCONR2)1° & 2°-amides3°-amides6-membered lactams5-membered lactams4-membered lactams, C=O bands1510 to 1700 cm-1 (2 bands)1650± 15 (one band)1670 ± 10 (one band)1700 ± 15 1745 ± 15. (a) $\text{CH}\;{\equiv}\;\text{CCH}_2\text{CH}_3\;+\;2\text{I}_2\;{\longrightarrow}\;\text{CHI}_2\text{CI}_2\text{CH}_2\text{CH}_3$, (b) $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\;+\;8\text{O}_2\;{\longrightarrow}\;5\text{CO}_2\;+\;6\text{H}_2\text{O}$. Check Your Learning The force constant (f) is proportional to the strength of the covalent bond linking m1 and m2. The green hydrogen atom in 2-methylpropane differs from the other nine hydrogen atoms in that molecule and from the purple hydrogen atoms in propane. Benzene does not, however, exhibit the characteristics typical of an alkene. In this case, we want to number from right to left (as shown by the blue numbers) so the branch is connected to carbon 3 (imagine the numbers from left to right—this would put the branch on carbon 4, violating our rules). The fourth valence electron of each carbon atom is shared with an adjacent carbon atom in their unhybridized p orbitals to yield the π bonds.