CHAPTER ONE Chemical Bonding 1. 8 CONSTITUTIONAL ISOMERS In the introduction we noted that both Berzelius and wohler were fascinated by the fact isomer"is derived from the that two different compounds with different properties, ammonium cyanate and urea, pos- Greek word meros, meaning sessed exactly the same molecular formula, CH4N2O. Berzelius had studied examples of part,"share,or"por- imilar phenomena earlier and invented the word isomer to describe different compounds om Greek(isos, " the that have the same molecular formula e"). Thus ers are dif. We can illustrate isomerism by referring to two different compounds, nitromethane and methyl nitrite, both of which have the molecular formula Ch3, Nitromethane, H used to power race cars, is a liquid with a boiling point of 101C. Methyl nitrite is a gas boiling at C, which when inhaled causes dilation of blood vessels. Isomers that dif- fer in the order in which their atoms are bonded are often referred to as structural iso- mers. A more modern term is constitutional isomer. As noted in the previous section the order of atomic connections that defines a molecule is termed its constitution and we say that two compounds are constitutional isomers if they have the same molecular formula but differ in the order in which their atoms are connected PROBLEM 1.14 There are many more isomers of CH3NO2 other than nitromethane and methyl nitrite Some such as carbamic acid, an intermediate in the commercial preparation of urea for use as a fertilizer, are too unstable to iso- late. Given the information that the nitrogen and both oxygens of carbamic acid are bonded to carbon and that one of the carbon-oxygen bonds is a double bond, write a Lewis structure for carbamic acid PROBLEM 1.15 Write structural formulas for all the constitutionally isomeric compounds having the given molecular formula (c) CaH (b)C3HO SAMPLE SOLUTION (a)Begin by considering the ways in which two carbons and one oxygen may be bonded. There are two possibilities: C-C-o and C-O-C Add the six hydrogens so that each carbon has four bonds and each oxygen two There are two constitutiona lers:ethyl alcohol and dimethyl ether. H-C-C-O-H duced In Chapter 3 another type of isomerism, called stereoisomerism, will be intro- Stereoisomers have the same constitution but differ in the arrangement of atoms Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
1.8 CONSTITUTIONAL ISOMERS In the introduction we noted that both Berzelius and Wöhler were fascinated by the fact that two different compounds with different properties, ammonium cyanate and urea, possessed exactly the same molecular formula, CH4N2O. Berzelius had studied examples of similar phenomena earlier and invented the word isomer to describe different compounds that have the same molecular formula. We can illustrate isomerism by referring to two different compounds, nitromethane and methyl nitrite, both of which have the molecular formula CH3NO2. Nitromethane, used to power race cars, is a liquid with a boiling point of 101°C. Methyl nitrite is a gas boiling at 12°C, which when inhaled causes dilation of blood vessels. Isomers that differ in the order in which their atoms are bonded are often referred to as structural isomers. A more modern term is constitutional isomer. As noted in the previous section, the order of atomic connections that defines a molecule is termed its constitution, and we say that two compounds are constitutional isomers if they have the same molecular formula but differ in the order in which their atoms are connected. PROBLEM 1.14 There are many more isomers of CH3NO2 other than nitromethane and methyl nitrite. Some, such as carbamic acid, an intermediate in the commercial preparation of urea for use as a fertilizer, are too unstable to isolate. Given the information that the nitrogen and both oxygens of carbamic acid are bonded to carbon and that one of the carbon–oxygen bonds is a double bond, write a Lewis structure for carbamic acid. PROBLEM 1.15 Write structural formulas for all the constitutionally isomeric compounds having the given molecular formula. (a) C2H6O (c) C4H10O (b) C3H8O SAMPLE SOLUTION (a) Begin by considering the ways in which two carbons and one oxygen may be bonded. There are two possibilities: C±C±O and C±O±C. Add the six hydrogens so that each carbon has four bonds and each oxygen two. There are two constitutional isomers: ethyl alcohol and dimethyl ether. In Chapter 3 another type of isomerism, called stereoisomerism, will be introduced. Stereoisomers have the same constitution but differ in the arrangement of atoms in space. Dimethyl ether H±C±O±C±H H W W H H W W H Ethyl alcohol H±C±C±O±H H W W H H W W H Nitromethane H±C±N H W W H O O œ ± Methyl nitrite H±C±O±NœO H W W H 22 CHAPTER ONE Chemical Bonding The suffix -mer in the word “isomer” is derived from the Greek word meros, meaning “part,” “share,” or “portion.” The prefix iso- is also from Greek (isos, “the same”). Thus isomers are different molecules that have the same parts (elemental composition). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
1.9 RESONANCE When writing a lewis structure we restrict a molecule s electrons to certain well-defined locations, either linking two atoms by a covalent bond or as unshared electrons on a sin- gle atom. Sometimes more than one Lewis structure can be written for a molecule, espe cially those that contain multiple bonds. An example often cited in introductory chem ourses is ozone(O3). Ozone occurs naturally in large quantities in the upper atmosphere, where it screens the surface of the earth from much of the suns ultraviolet rays. Were it not for this ozone layer, most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun. The following Lewis structure for ozone satisfies the octet rule; all three oxygens have 8 electrons in their valence shell. This Lewis structure, however, doesnt accurately portray the bonding in ozone, because the two terminal oxygens are bonded differently to the central oxygen. The cen- Bond distances i tral oxygen is depicted as doubly bonded to one and singly bonded to the other. Since 2A(A=10" it is generally true that double bonds are shorter than single bonds, we would expect unit, we will express bond ozone to exhibit two different o-o bond lengths, one of them characteristic of the distances in o-O single bond distance(147 pm in hydrogen peroxide, H-O-O-H) and the other (1 pm=10.m).Thus, one characteristic of the o=0 double bond distance(121 pm in O2). Such is not the 28pm=1.28A case. Both bond distances in ozone are exactly the same(128 pm)somewhat shorter than the single bond distance and somewhat longer than the double bond distance. The structure of ozone requires that the central oxygen must be identically bonded to both terminal oxygens. In order to deal with circumstances such as the bonding in ozone, the notion of resonance between Lewis structures was developed. According to the resonance con cept, when more than one Lewis structure may be written for a molecule, a single struc ture is not sufficient to describe it. Rather. the true structure has an electron distribution that is a"hybrid"of all the possible Lewis structures that can be written for the mole cule. In the case of ozone, two equivalent Lewis structures may be written. We use a double-headed arrow to represent resonance between these two Lewis structures It is important to remember that the double-headed resonance arrow does not indi- cate a process in which the two Lewis structures interconvert. Ozone, for example, has a single structure; it does not oscillate back and forth between two Lewis structures rather its true structure is not adequately represented by any single Lewis structure. Resonance attempts to correct a fundamental defect in Lewis formulas. Lewis for- mulas show electrons as being localized; they either are shared between two atoms in a covalent bond or are unshared electrons belonging to a single atom. In reality, electrons distribute themselves in the way that leads to their most stable arrangement. This some times means that a pair of electrons is delocalized, or shared by several nuclei. What we try to show by the resonance description of ozone is the delocalization of the lone pair electrons of one oxygen and the electrons in the double bond over the three atoms of the molecule. Organic chemists often use curved arrows to show this electron Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
1.9 RESONANCE When writing a Lewis structure, we restrict a molecule’s electrons to certain well-defined locations, either linking two atoms by a covalent bond or as unshared electrons on a single atom. Sometimes more than one Lewis structure can be written for a molecule, especially those that contain multiple bonds. An example often cited in introductory chemistry courses is ozone (O3). Ozone occurs naturally in large quantities in the upper atmosphere, where it screens the surface of the earth from much of the sun’s ultraviolet rays. Were it not for this ozone layer, most forms of surface life on earth would be damaged or even destroyed by the rays of the sun. The following Lewis structure for ozone satisfies the octet rule; all three oxygens have 8 electrons in their valence shell. This Lewis structure, however, doesn’t accurately portray the bonding in ozone, because the two terminal oxygens are bonded differently to the central oxygen. The central oxygen is depicted as doubly bonded to one and singly bonded to the other. Since it is generally true that double bonds are shorter than single bonds, we would expect ozone to exhibit two different O±O bond lengths, one of them characteristic of the O±O single bond distance (147 pm in hydrogen peroxide, H±O±O±H) and the other one characteristic of the OœO double bond distance (121 pm in O2). Such is not the case. Both bond distances in ozone are exactly the same (128 pm)—somewhat shorter than the single bond distance and somewhat longer than the double bond distance. The structure of ozone requires that the central oxygen must be identically bonded to both terminal oxygens. In order to deal with circumstances such as the bonding in ozone, the notion of resonance between Lewis structures was developed. According to the resonance concept, when more than one Lewis structure may be written for a molecule, a single structure is not sufficient to describe it. Rather, the true structure has an electron distribution that is a “hybrid” of all the possible Lewis structures that can be written for the molecule. In the case of ozone, two equivalent Lewis structures may be written. We use a double-headed arrow to represent resonance between these two Lewis structures. It is important to remember that the double-headed resonance arrow does not indicate a process in which the two Lewis structures interconvert. Ozone, for example, has a single structure; it does not oscillate back and forth between two Lewis structures, rather its true structure is not adequately represented by any single Lewis structure. Resonance attempts to correct a fundamental defect in Lewis formulas. Lewis formulas show electrons as being localized; they either are shared between two atoms in a covalent bond or are unshared electrons belonging to a single atom. In reality, electrons distribute themselves in the way that leads to their most stable arrangement. This sometimes means that a pair of electrons is delocalized, or shared by several nuclei. What we try to show by the resonance description of ozone is the delocalization of the lonepair electrons of one oxygen and the electrons in the double bond over the three atoms of the molecule. Organic chemists often use curved arrows to show this electron ¢£ œO± O O ±Oœ O O œO± O O 1.9 Resonance 23 Bond distances in organic compounds are usually 1 to 2Å (1Å 1010m). Since the angstrom (Å) is not an SI unit, we will express bond distances in picometers (1 pm 1012m). Thus, 128 pm 1.28 Å. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website