6.6 Mechanistic Basis for Markovnikov's Rule RULES, LAWS THEORIES, AND THE SCIENTIFIC METHOD s we have just seen, Markovnikov's rule can be Most observations in chemistry come from ex expressed in two ways periments. If we do enough experiments we may see a pattern running through our observations. a law is a mathematical (the law of gravity)or verbal (the lay 1. When a hydrogen halide adds to an alkene, hy- of diminishing returns)description of that pattern. has the greater number of hydrogens attached Establishing a law can lead to the framing of a rule that lets us predict the results of future experiments to it, and the halogen to the carbon that has This is what the 1870 version of Markoynikoy's rule is the fewer hydrogens. a statement based on experimental observations that When a hydrogen halide adds to an alkene, pro- has predictive value tonation of the double bond occurs in the direc a theory is our best present interpretation of tion that gives the more stable carbocation why things happen the way they do. The modern ver sion of markovnikov's rule, which is based on mecha Vladimir Markovnikov expressed it in 1870; the sec- nistic reasoning and carbocation stability, recasts the ond is the way we usually phrase it now These two rule in terms of theoretical ideas. Mechanisms, and planations grounded in them, belong to the theory statements differ in an important way-a way that is part of the scientific method related to the scientific method Adherence to the scientific method is what It is worth remembering that a theory can defines science. The scientific method has four major never be proven correct. It can only be proven incor- elements: observation, law, theory, and hypothesis. rect, incomplete, or inadequate. Thus, theories are always being tested and refined. As important as anything else in the scientific method is the testable hypothesis. Once a theory is proposed, experiments Observation Lav are designed to test its validity. If the results are con- sistent with the theory, our belief in its soundness is strengthened. If the results conflict with it, the theory is flawed and must be modified section 6.7 describes some observations that support the theory that car Theory ocations are intermediates in the addition of hydro- gen halides to alkenes FIGURE 6.5 Energy diagra comparing addition of RCH,CH hydrogen halide to an alkene according to Markovnikov's rule with addition in the Markovnikov,s rule RCHCH3 alkene and hydrogen halide are shown in the center of X the diagram. The lower RCH=CH ponds to markovnikov rule proceeds to the right and shown in red; the higher energy pathway proceeds to RCHCHOX RCHCH Reaction coordinate Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteRULES, LAWS, THEORIES, AND THE SCIENTIFIC METHOD As we have just seen, Markovnikov’s rule can be expressed in two ways: 1. When a hydrogen halide adds to an alkene, hy￾drogen adds to the carbon of the alkene that has the greater number of hydrogens attached to it, and the halogen to the carbon that has the fewer hydrogens. 2. When a hydrogen halide adds to an alkene, pro￾tonation of the double bond occurs in the direc￾tion that gives the more stable carbocation. The first of these statements is close to the way Vladimir Markovnikov expressed it in 1870; the sec￾ond is the way we usually phrase it now. These two statements differ in an important way—a way that is related to the scientific method. Adherence to the scientific method is what defines science. The scientific method has four major elements: observation, law, theory, and hypothesis. Most observations in chemistry come from ex￾periments. If we do enough experiments we may see a pattern running through our observations. A law is a mathematical (the law of gravity) or verbal (the law of diminishing returns) description of that pattern. Establishing a law can lead to the framing of a rule that lets us predict the results of future experiments. This is what the 1870 version of Markovnikov’s rule is: a statement based on experimental observations that has predictive value. A theory is our best present interpretation of why things happen the way they do. The modern ver￾sion of Markovnikov’s rule, which is based on mecha￾nistic reasoning and carbocation stability, recasts the rule in terms of theoretical ideas. Mechanisms, and explanations grounded in them, belong to the theory part of the scientific method. It is worth remembering that a theory can never be proven correct. It can only be proven incor￾rect, incomplete, or inadequate. Thus, theories are always being tested and refined. As important as anything else in the scientific method is the testable hypothesis. Once a theory is proposed, experiments are designed to test its validity. If the results are con￾sistent with the theory, our belief in its soundness is strengthened. If the results conflict with it, the theory is flawed and must be modified. Section 6.7 describes some observations that support the theory that car￾bocations are intermediates in the addition of hydro￾gen halides to alkenes. 6.6 Mechanistic Basis for Markovnikov’s Rule 217 RCH2CH2 + RCH2CH2X RCH CH2 RCHCH3 X− RCHCH3 + X− X + HX Reaction coordinate Potential energy FIGURE 6.5 Energy diagram comparing addition of a hydrogen halide to an alkene according to Markovnikov’s rule with addition in the direction opposite to Markovnikov’s rule. The alkene and hydrogen halide are shown in the center of the diagram. The lower energy pathway that corre￾sponds to Markovnikov’s rule proceeds to the right and is shown in red; the higher energy pathway proceeds to the left and is shown in blue. Observation Law Hypothesis Theory Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website