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Chapter 5

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  1. Chapter 5 An Overview of Organic Reactions

  2. Introduction • In general, organic chemical reactions are studied by looking at : • what occurs (what kinds of reactions occur) • how it happens (how reactions occur)

  3. I. Kinds of Organic Reactions • Addition reactions • Elimination reactions • Substitution reactions • Rearrangement reactions

  4. Addition reactions occur when two reactants combine to form a single new product with no atoms left over. A.Addition reactions Two molecules combine

  5. Elimination reactions occur when a single reactant splits into two products. B.Elimination reactions One molecule splits into two

  6. Substitution reactions occur when two reactants exchange parts to give two new products. C.Substitution reactions Parts from two molecules exchange

  7. Rearrangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product. D.Rearrangement reactions A molecule undergoes changes in the way its atoms are connected

  8. Practice Problem: Classify each of the following reactions as an addition, elimination, substitution, or rearrangement: • CH3Br + KOH  CH3OH + KBr • CH3CH2OH  H2C=CH2 + H2O • H2C=CH2 + H2  CH3CH3

  9. II. How Organic Reactions Occur: Mechanisms • Reaction Mechanisms • Radical Reactions • Polar Reactions

  10. A reaction mechanism is a full description of how a reaction occurs. It describes what takes place at each stage of a chemical transformation Reactions occur in defined steps that lead from reactant to product A reaction mechanism describes these steps the types of bonds that are broken and formed, the order and relative rates in which they are broken and formed, … A.Reaction Mechanisms

  11. Steps in Mechanisms • The types of steps in a sequence are classified • A step involves either the breaking or formation of a covalent bond • Steps can occur individually or in combination with other steps • When several steps occur at the same time they are said to be concerted

  12. Types of Steps in Reaction Mechanisms • The breaking of a covalent bond may be: • Homolytic • Heterolytic • The formation of a covalent bond may be: • Homogenic • Heterogenic • A functional group may undergo: • Oxidation • Reduction

  13. Homolytic Breaking of a Covalent Bond • It is a symmetrical cleavage • Each product gets one electron from the bond • Not common in organic chemistry

  14. Heterolytic Breaking of a Covalent Bond • It is an unsymmetrical cleavage • Both electrons from the bond that is broken become associated with one resulting fragment • A common pattern in reaction mechanisms

  15. Homogenic Formation of a Bond • It is a symmetrical formation • One electron comes from each fragment • No electronic charges are involved • Not common in organic chemistry

  16. Heterogenic Formation of a Bond • It is an unsymmetrical formation • One fragment supplies two electrons • One fragment supplies no electrons • Combination can involve electronic charges • Common in organic chemistry

  17. Indicating Steps in Mechanisms • Curved arrows indicate breaking and forming of bonds • Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) • Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

  18. Radicals • A radical (also known as “free radical”): • is a neutralchemical species • contains an odd number of electrons • has a single, unpaired electron in one of its orbitals • is abbreviated R • can be written as R.

  19. Radicals • Alkyl groups are abbreviated “R” for radical Example: Methyl iodide = CH3I Ethyliodide = CH3CH2I Alkyl iodides (in general) = RI • A “free radical” is an “R” group on its own: • CH3 is a “free radical” or simply “radical” • It has a single unpaired electron, shown as: CH3. • Its valence shell is one electron short of being complete

  20. A radical reaction involves symmetrical bond breaking and bond making It is not as common as polar reactions It involves species that have an odd number of electrons (i.e. radicals) Radicals are highly reactive; they react to complete electron octet of valence shell B.Radical Reactions

  21. Radicals can complete a valence-shell octet via: • Radical Substitution Reaction: A radical can break a bond in another molecule and abstract an atom with an electron, giving substitution in the original molecule and leaving a new radical

  22. Radicals can complete a valence-shell octet via: • Radical Addition Reaction: A radical can add to an alkene, taking one electron from the double bond and yielding a new radical

  23. Steps in Radical Substitution: Three types of steps • Initiation – homolytic formation of two reactive species with unpaired electrons • Propagation – reaction with molecule to generate radical • Termination – combination of two radicals to form a stable product

  24. An Example of a Radical Substitution • Chlorination of methane is • an example of a radical substitution. • a multistep process.

  25. Initiation – homolytic formation of two reactive species with unpaired electrons • Example – formation of two reactive Cl. radicals from Cl2 and U.V light The weak Cl-Cl bond is homolytically broken by irradiation with U.V. light

  26. Propagation– reaction with molecule to generate radical (chain reaction) • Example – reaction of Cl. radical with methane to give HCl and CH3. • – reaction of CH3. withCl2 to give CH3Cl and Cl. radical

  27. Termination – combination of two radicals to form a stable product. • Example – CH3. + CH3. CH3CH3

  28. Practice Problem: Alkane chlorination is not a generally useful reaction because most alkanes have hydrogens in many different positions, causing mixtures of chlorinated products to result. Draw and name all monochloro substitution products you might obtain by reaction of 2-methylpentane with Cl2

  29. Practice Problem: Radical chlorination of pentane is a poor way to prepare 1-chloropentane, CH3CH2CH2CH2CH2Cl, but radical chlorination of neopentane, (CH3)4C, is a good way to prepare neopentyl chloride, (CH3)3CCH2Cl. Explain.

  30. A polar reaction involves unsymmetrical bond breaking and bond making It is more common than radical reactions It involves species that have an even number of electrons (have only electron pairs in their orbitals) It occurs because of the attraction between positive and negative charges on different functional groups C.Polar Reactions

  31. How Polar Reactions occur • Molecules can contain local unsymmetrical electron distributions (polar bonds) • This is due to differences in electronegativities of the bonded atoms • This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom • The more electronegative atom has the greater electron density

  32. Electronegativity of Some Common Elements • The relative electronegativity is indicated • Higher numbers indicate greater electronegativity • Carbon bonded to a more electronegative element has a partial positivecharge (+)

  33. Electrostatic potential maps • Carbon bonded to a more electronegative element has a partial positivecharge (+) • Carbon bonded to a less electronegative element has a partial negativecharge (-) DEN = 2.5-1.0 = 1.5 DEN = 3.0-2.5 = 0.5

  34. Polar bonds can also result from the interaction of functional groups with solvents and with Lewis acids or bases • Example – The electron-poor character of the carbon atom in methanol is greatly enhanced by protonation of the oxygen atom with an acid

  35. Polarizability • Polarization - is a change in electron distribution as a response to change in electronic nature of the surroundings • Polarizability - is the tendency to undergo polarization • Polar reactions occur between regions of high electron density and regions of low electron density

  36. Larger atoms with more loosely held electrons are more polarizable than smaller atoms with tightly held electrons • Example– I is much more polarizable than F

  37. Generalized Polar Reactions • An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species • The combination is indicated with a curved arrow from nucleophile to electrophile

  38. An electrophile • is “electron-loving” • is an electron-poor species • can form a bond by accepting a pair of electrons • may be either neutral or positively charged • is a Lewis acid

  39. A nucleophile • is “nucleus-loving” • is an electron-rich species • can form a bond by donating a pair of electrons • may be either neutral or negatively charged • is a Lewis base

  40. Some species can act as an electrophile or a nucleophile depending on the circumstances • Example – Water acts as a nucleophile when it donates a pair of electrons, and acts as an electrophile when it donates H+

  41. Polar Reactions vs Radical Reactions

  42. Practice Problem: Which of the following species is likely to be an electrophile, and which a nucleophile? • HCl • CH3NH2 • CH3SH • CH3CHO

  43. Practice Problem: An electrostatic potential map of boron trifluoride is shown. Is BF3 likely to be an electrophile or a nucleophile? Draw a Lewis structure for BF3, and explain the result.

  44. III. Polar Reactions • An Example of a Polar Reaction • Using Curved Arrows in Polar Reaction Mechanisms

  45. The addition reaction of an alkene, such ethylene with HBr, is a typical polar process. A.An Example of a Polar Reaction

  46. Alkanes are relatively inert • their valence electrons are tied up in strong s bonds • the bonding electrons are relativelyinaccessible because they are sheltered in s bonds between nuclei. • Alkenes are more reactive • their double bonds have a greater electron density than single bonds • the electrons in the p bond are accessible to reactants because they are located above or below the plane

  47. Reaction between HBr and ethylene is a typical electrophile-nucleophile combination of all polar reactions • The  bond is electron-rich, allowing it to function as a nucleophile • H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile • HBr adds to the  part of C-C double bond

  48. Mechanism of Addition of HBr to Ethylene • HBr (electrophile) is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion • Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond • The result is that ethylene and HBr combine to form bromoethane

  49. All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile