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Chapter 4
The Study of Chemical Reactions

04-00-02UN

Title	Chlorination of Methane
Caption	This reaction may continue; heat or light is needed for each step.
Notes	A mixture of chlorinated products is obtained.
Keywords	chlorination

04-00-03UN

Title	Monochlorination of Methane
Caption	In studying the chlorination of methane, we will consider just the first reaction to form chloromethane (common name methyl chloride). This reaction is a substitution: Chlorine does not add to methane, but a chlorine atom substitutes for one of the hydrogen atoms, which appears in the HCl by-product.
Notes	Addition of chlorine gas to methane produces chloromethane. This is a substitution reaction in which one of the hydrogens of methane is substituted by a chlorine atom. Hydrochloric acid (HCl) is produced as a by-product.
Keywords	chlorination, by-product

04-00-04UN

Title	Homolytic Cleavage of Chlorine
Caption	The splitting of a chlorine molecule by the absorption of a photon of light is shown below. Notice the fishhook-shaped half-arrows used to show the movement of single unpaired electrons. Just as we use curved arrows to represent the movement of electron pairs, we use these half-arrows to represent the movement of single unpaired electrons. These half-arrows show that the two electrons in the bond separate, and one leaves with each chlorine atom.
Notes	The initiation step for the chlorination reaction is the homolytic cleavage of the Cl-Cl bond to form two chlorine radicals. The cleavage is induced by heat or light.
Keywords	homolytic cleavage, radical, free radical

04-01

Title	Lewis Structures of Free Radicals
Caption	Figure 4-1Free radicals are reactive species with odd numbers of electrons. The unpaired electron is represented by a dot in the formula.
Notes	Free radicals are non-paired electrons. The atom bearing the radical does not have a complete octet and combines with another radical and forms a bond.
Keywords	free radical, octet

04-01-01UN

Title	First Propagation Step in the Chlorination of Methane
Caption	When a chlorine radical collides with a methane molecule, it abstracts (removes) a hydrogen atom from methane. One of the electrons in the bond remains on carbon, while the other combines with the odd electron on the chlorine atom to form the bond.
Notes	Once formed, the chlorine radical can abstract a hydrogen from methane. There is a homolytic cleavage of the C-H bond leaving a radical on the carbon atom, now called a methyl radical.
Keywords	abstract, homolytic cleavage, methyl radical, radical

04-01-02UN

Title	Second Propagation Step in the Chlorination of Methane
Caption	In the second propagation step, the methyl radical reacts with a molecule of chlorine to form chloromethane. The odd electron of the methyl radical combines with one of the two electrons in the bond to give the bond, and the chlorine atom is left with the odd electron.
Notes	The methyl radical can combine with a molecule of chlorine to form chloromethane and a chlorine atom. This chlorine atom can re-enter the first propagation step.
Keywords	methyl radical, radical

04-01-04UN

Title	Termination Steps in the Chlorination of Methane
Caption	If anything happens to consume some of the free-radical intermediates without generating new ones, the chain reaction will slow or stop. Therefore, the most important side reactions in a chain reaction are the ones that consume free radicals. Such a side reaction is called a termination reaction: a step that produces fewer reactive intermediates (free radicals) than it consumes. The following are some of the possible termination reactions in the chlorination of methane.
Notes	The reaction comes to an end when two radicals combine to form a molecule with no radicals in it. This is known as a termination step. Termination steps can form the desired product or other by-products.
Keywords	termination step

04-01-10UN

Title	Homolytic and Heterolytic Cleavages
Caption	The bond-dissociation energy (BDE) is the amount of energy required to break a particular bond homolytically, that is, in such a way that each bonded atom retains one of the bondÕs two electrons. In contrast, when a bond is broken and one of the atoms retains both electrons, we say that heterolytic cleavage has occurred. Homolytic cleavage (radical cleavage) forms free radicals, while heterolytic cleavage forms ions. A heterolytic cleavage is sometimes called an ionic cleavage.
Notes	In a homolytic cleavage the bond breaks evenly - each atom receives one electron. In a heterolytic cleavage the bond is broken unevenly giving both electrons to the most electronegative atom (forming an anion) and no electrons to the other atom (forming a cation).
Keywords	homolytic cleavage, heterolytic cleavage, anion, cation, ionic

04-02

Title	Effect of Temperature in the Rate of the Reaction
Caption	Figure 4-2Graph showing how the number of molecules having a given activation energy decreases as the activation energy increases. At a higher temperature (red curve), more collisions have the needed energy.
Notes	Increasing the temperature of the reaction will increase the amount of molecules with enough energy to react, thus increasing the rate of the reaction.
Keywords	rate of reaction

04-02-01UN

Title	Transition State for the Chlorination of Methane
Caption	Transition states have high energies because bonds must begin to break before other bonds can form. The following equation shows the reaction of a chlorine radical with methane. The transition state shows the bond partially broken and the bond partially formed. Transition states are often enclosed by brackets to emphasize their transient nature.
Notes	The transition state is an intermediate stage between the reactants and the products. It is a short-lived molecule with high energy.
Keywords	transition state, intermediate

04-03

Title	Reaction Energy Diagram of an Exothermic Reaction.
Caption	Figure 4-3Reaction-energy diagram for a one-step exothermic reaction. The reactants are toward the left, and the products are toward the right. The vertical axis represents the potential energy. The transition state is the highest point on the graph, and the activation energy is the energy difference between the reactants and the transition state.
Notes	In an exothermic reaction, the products have lower energy than the reactants. The energy needed to go from reactants to the transition state is the activation energy (Ea).
Keywords	activation energy, exothermic reaction

04-06

Title	Initiation and Propagation Steps for the Chlorination of Propane
Caption	Figure 4-6The mechanism for free-radical chlorination of propane. The first propagation step forms either a primary radical or a secondary radical. This radical determines whether the final product will be the primary chloride or the secondary chloride.
Notes	The first propagation step produced both a primary and a secondary radical. Since secondary radicals are more stable than primary ones, the major product of the reactant will be the secondary alkyl halide 2-chloropropane.
Keywords	primary radical, secondary radical, secondary alkyl halide

04-07

Title	Bond Dissociation Energies for the Formation of Free Radicals
Caption	Figure 4-7Bond-dissociation energies show that more highly substituted free radicals are more stable than less highly substituted ones.
Notes	Bond-dissociation energy is the energy required to form a radical. The higher the energy, the more difficult it is for the reaction to occur. Comparison of the energies show that a tertiary radical has the lowest bond-dissociation energy (91 kcal) and thus it is more stable than a secondary, a primary or a methyl radical. The methyl radical is the least stable of the radicals and has the highest dissociation energy (104 kcal).
Keywords	bond-dissociation energy, radical

04-07-01UN

Title	Stability of Free Radicals
Caption	From the information in Figure 4-7, we conclude that free radicals are more stable if they are more highly substituted. The following free radicals are listed in decreasing order of stability.
Notes	The order of decreasing stability for free radicals is 3o > 2o > 1o > methyl.
Keywords	stability

04-09

Title	Rate of Substitution in the Bromination of Propane
Caption	Figure 4-9This 97:3 ratio of products shows that bromine abstracts a secondary hydrogen 97 times as rapidly as a primary hydrogen. Bromination (reactivity ratio 97:1) is much more selective than chlorination (reactivity ratio 4.5:1).
Notes	In bromination reactions a secondary hydrogen is 97 times more reactive than a primary hydrogen. The bromination of alkanes is considered to be more selective than the chlorination reaction.
Keywords	bromination, rate, selectivity

04-11

Title	Energy Diagrams: Chlorination vs Bromination
Caption	Figure 4-11(a)ÊIn the endothermic bromination, the transition states are closer to the products (the radicals) in energy and in structure. The difference in the 1¡ and 2¡ activation energies is about 2.5 kcal (10 kJ), nearly the entire energy difference of the radicals. (b)ÊIn the exothermic chlorination, the transition states are closer to the reactants in energy and in structure. The difference in activation energies for chlorination is about 1 kcal (4 kJ), only a third of the energy difference of the radicals.
Notes	For an endothermic reaction such as bromination the products are closer in energy to the transition state, i.e., the structure of the transition state resembles that of the products. For an exothermic reaction such as chlorination there is a big energy difference between the transition state and the products, i.e., the structure of the transition state resembles that of the reactants.
Keywords	endothermic, exothermic, radical, bromination, chlorination

04-12

Title	Transition States of Bromination and Chlorination
Caption	Figure 4-12In the endothermic bromination, the transition state resembles the free radical. In the exothermic chlorination, the free radical has just begun to form in the transition state.
Notes	For an endothermic reaction the products are closer in energy to the transition state, i.e., the structure of the transition state resembles that of the products. For an exothermic reaction there is a big energy difference between the transition state and the products, i.e., the structure of the transition state resembles that of the reactants.
Keywords	endothermic, exothermic, transition state

04-12-06P04.25

Title	Radical Inhibitors
Caption
Notes	An inhibitor is a compound that will react with radicals, effectively reducing the amount of free radicals in a reaction medium. Inhibitors are used to prevent radical chain reactions from starting or, alternatively, they can be used to stop a radical chain reaction once it has started. The inhibitor will react with the radicals forming a stable intermediate. Any further reaction of this intermediate will be an endothermic process and very slow.
Keywords	inhibitor, radical, radical chain reaction, intermediate

04-12-10

Title	Reactive Carbon Intermediates
Caption	The most common intermediates with a divalent (two-bonded) carbon atom are the carbenes. A carbene has two nonbonding electrons on the divalent carbon atom, making it uncharged.
Notes	There are four reactive carbon intermediates: carbocation, radical, carbanion, and carbene.
Keywords	carbocation, radical, carbanion, carbene.

04-13

Title	The Methyl Cation
Caption	Figure 4-13The methyl cation is similar to BH3. The carbon atom is sigma bonded to three hydrogen atoms by overlap of its sp2 hybrid orbitals with the s orbitals of hydrogen. There is a vacant p orbital perpendicular to the plane of the three bonds.
Notes	A carbocation is better explained as having three sp2 hybridized orbitals and a vacant unhybridized p orbital. The sp2 orbitals are trigonal planar and the unoccupied p orbital is perpendicular to the plane of the molecule.
Keywords	carbocation, hybrid orbitals, trigonal planar

04-14

Title	Hyperconjugation
Caption	Figure 4-14A carbocation is stabilized by overlap of filled orbitals on an adjacent alkyl group with the vacant p orbital of the carbocation. Overlap between a sigma bond and a p orbital is called hyperconjugation.
Notes	Carbons attached to the carbocation can stabilize it. The more carbons attached to the carbocation, the more stable the carbocation will be due to hyperconjugation between the unoccupied p orbital of the carbocation and the sp3 orbitals of the neighboring carbon.
Keywords	stabilization, carbocation, hyperconjugation

04-14-01UN

Title	Carbocation Stability
Caption	In general, more highly substituted carbocations are more stable.
Notes	The more carbons attached to the carbocation, the more stable the carbocation will be due to hyperconjugation between the unoccupied p orbital of the carbocation and the sp3 orbitals of the neighboring carbon. Thus a tertiary carbocation is more stable than a secondary, which in turn is more stable than a primary. A methyl carbocation is highly unstable because of the lack of hyperconjugation.
Keywords	carbocation, stability, hyperconjugation

04-15

Title	Carbon Radical Structure
Caption	Figure 4-15The structure of the methyl radical is like that of the methyl cation (Fig. 4-13), except there is an additional electron. The odd electron is in the p orbital perpendicular to the plane of the three bonds.
Notes	A carbon radical, much like a carbocation, is better explained as having three sp2 hybridized orbitals and a vacant unhybridized p orbital. The sp2 orbitals are trigonal planar and the half filled p orbital is perpendicular to the plane of the molecule.
Keywords	radical, hybridization, trigonal planar

04-15-01UN

Title	Stability of Carbon Radicals
Caption	Radicals and carbocations are both electron-deficient because they lack an octet around the carbon atom. Like carbocations, radicals are stabilized by the electron-donating effect of alkyl groups, making more highly substituted radicals more stable. This effect is confirmed by the bond dissociation energies shown in Figure 4-7: Less energy is required to break a bond to form a more highly substituted radical.
Notes	The more carbons there are attached to the radical the more stable the radical will be. Hyperconjugation plays a role in radical stability.
Keywords	radical, hyperconjugation

04-16

Title	Molecular Orbitals of a Carbanion
Caption	Figure 4-16Both the methyl anion and ammonia have an sp3 hybridized central atom, with a nonbonding pair of electrons occupying one of the tetrahedral positions.
Notes	A carbanion is a trivalent carbon that bears a negative charge.
Keywords	carbanion, trivalent carbon

04-16-02UN

Title	Reactivity of a Carbanion
Caption	Like amines, carbanions are nucleophilic and basic. A carbanion has a negative charge on its carbon atom, however, making it a more powerful base and a stronger nucleophile than an amine. For example, a carbanion is sufficiently basic to remove a proton from ammonia.
Notes	Carbanions are a stronger base than amines, so they can deprotonate amines easily.
Keywords	carbanion, base, deprotonation, basic, nucleophilic

04-16-03UN

Title	Stability of Carbanion
Caption	The stability order of carbanions reflects their high electron density. Alkyl groups and other electron-donating groups actually destabilize a carbanion. The order of stability is the opposite of that for carbocations and free radicals, which are electron-deficient and are stabilized by alkyl groups.
Notes	The methyl carbanion is more stable than substituted carbanions due to the fact that electron donation destabilizes carbanions. Electron withdrawing groups such as halogens stabilize carbanions through inductive effects. Resonance can also stabilize a carbanion.
Keywords	carbanion, inductive effects, resonance, electron withdrawing group

04-16-06UN

Title	Carbenes as Reaction Intermediates
Caption	Carbenes are uncharged reactive intermediates containing a divalent carbon atom. The simplest carbene has the formula CH2 and is called methylene, just as a group in a molecule is called a methylene group. One way of generating carbenes is to form a carbanion that can expel a halide ion. For example, a strong base can abstract a proton from tribromomethane (CHBr3) to give an inductively stabilized carbanion. This carbanion expels bromide ion to give dibromocarbene. The carbon atom is sp2 hybridized, with trigonal geometry. An unshared pair of electrons occupies one of the sp2 hybrid orbitals, and there is an empty p orbital extending above and below the plane of the atoms. A carbene has both a lone pair of electrons and an empty p orbital, so it can react as a nucleophile or as an electrophile.
Notes	The reactivity of carbenes lies in its orbitals. The carbon of carbenes have sp2 hybrid orbitals. Two of the hybrid orbitals are bonded to other atoms, one of the sp2 orbitals contains the lone pair of electrons so they can act as a nucleophile. Perpendicular to the plane of the sp2 orbitals is an unoccupied p orbital which allows them to acts as an electrophile.
Keywords	hybrid orbitals, electrophile, nucleophile, carbenes

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