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Organic REACTIONS

Chapter 10. Organic REACTIONS. Alkanes. Saturated hydrocarbons where carbons in the chain are singly bonded to one another Ex: Methane Ethane Propane Butane Pentane Hexane Reactivity : relatively low Carbon-hydrogen bond relatively strong (relatively high bond energy)

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Organic REACTIONS

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  1. Chapter 10 Organic REACTIONS

  2. Alkanes • Saturated hydrocarbons where carbons in the chain are singly bonded to one another • Ex: Methane Ethane Propane Butane Pentane Hexane • Reactivity: relatively low • Carbon-hydrogen bond relatively strong (relatively high bond energy) • Only slightly polar (electronegativity difference of 0.4) • Combustion: rapid, exothermic oxidation of combustible materials. • Most common alkane RXN • Requires: • oxidizer (oxygen) • fuel source (alkane) • source of ignition (required to reach activation energy)

  3. Alkanes: Combustion • Complete combustion of hydrocarbons produces CO2 and H2O • All carbon converts to CO2 and all hydrogen converts to H2O. • When balancing: • # of C in the alkane = # CO2 molecules produced • # of H in the alkane = 2 X H2O molecules produced • In most situations, combustion of hydrocarbons is incomplete because of insufficient oxygen. • Products of incomplete combustion are responsible for a large amount of urban pollution: • carbon monoxide (CO) • carbon (soot)

  4. Alkanes-Substitution RXNs: FRCR • Free radical chain reaction: • alkane RXT with halogen = halogenoalkanes • One Hydrogen in the alkane is replaced by a halogen •   reaction of ethane with chlorine: • CH3-CH3(g) + Cl2(g) CH3-CH2-Cl(g) + H-Cl(g) • Ethane chlorinechloroethane hydrogen chloride • Reaction usually brought about by exposure to UV light or high temps (provides energy of activation) • Chloroethane can react with more chlorine to form dichloroethanes (1,2 dichloroethane and 1,1 dichloroethane), and in presence of high amounts of chlorine can eventually be converted to hexachloroethane (substitution of all hydrogens with chlorine)

  5. Alkanes-Substitution RXNs: FRCR • Free radical: any molecule or atom with a single unpaired electron •  highly reactive •  Reaction proceeds in 3 distinct phases. • RXN of CH4 with Cl2 example: • Initiation: free radicals are produced • Propagation: products are formed and radicals are reformed • Termination: radicals are used up

  6. Alkanes-Substitution RXNs: FRCR • 1. Initiation phase: • Source of energy (often UV light) can break the covalent bond between the 2 Cl atoms, releasing unpaired Cl atoms (free radicals). • Called homolytic fission: each atom ends up with one e- (“equal splitting”). So, heterolyticfission is unequal splitting because both electrons end up on one atom • Large reduction in stability for Cl when this happens. As a result, it readily forms a new covalent bond with whatever is present; in this case, H atom from CH4

  7. Alkanes-Substitution RXNs: FRCR • 2. Propagation: • Cl radical pulls a H atom (including its electron which is currently being shared with the carbon atom) off of CH4. • This forms HCl. • The result is another free radical, •CH3. • This will then pull a Cl atom off a Cl2 molecule, reforming a chlorine radical. This continues in a chain reaction.

  8. Alkanes-Substitution RXNs: FRCR • 3. Termination occurs when all of the radicals are consumed. • Cl radicals can combine with each other to form a molecule of Cl2 • OR they can combine with a methyl radical to form chloromethane. • OR 2 methyl radicals can also combine to form a molecule of ethane. • Since ethane is produced during the halogenations of methane, the mechanism for this reaction is indeed the one illustrated in the diagram.

  9. Alkanes-Substitution RXNs: FRCR • If bromine were used instead of chlorine • Dark brown color provides simple visual method to monitor the progress of the reaction. • As the brown colored bromine is consumed, the color would gradually fade. • Note: reaction is not observed in the dark • There is no source of energy to create the necessary radicals.

  10. Alkanes-Substitution RXNs: Nucleophilic • Nucleophilic substitution of halogenoalkanes: • Polarity of the carbon-halogen bond– C carries a slight + charge. • Thus, is subject to attack by species that are attracted to centers of + charge (nucleophiles). • A nucleophile can be anything with a lone pair of electrons, but common examples are: • Hydroxide ion: OH- • Ammonia: NH3 • Cyanide ion: CN-

  11. Alkanes-Substitution RXNs: Nucleophilic • EXAMPLE of Nucleophilic substitution RXN: • Warming 1-bromobutane (C4H9Br) in the presence of NaOH(aq). • C4H9Br is the halogenoalkane • NaOH is source of OH- ion (Na+is a spectator ion: does not participate in RXN) • The hydroxide ion is called the nucleophile: • C4H9Br(l) + OH-(aq)  C4H9OH(aq) + Br-(aq) • The reaction produces butan-1-ol and bromide ion.

  12. Alkanes-Substitution RXNs: Nucleophilic • Nucleophilic substitution can occur by two distinct “mechanisms.” • Mechanism: a step-wise model of how a reaction occurs. • Rate-determining step:In a chemical reaction has more than one step (and many of them do), the slowest stepdetermines the overall rate of reaction. • Balanced equation implies that a reaction occurs in only one step – this is often not the case! • Molecularity: #of molecules involved in rate-determining step. • Unimolecular: one molecule is involved. • Bimolecular: two are involved. • Termolecular: Three involved, and so on. • Termolecularsteps and above are quite rare because the probability of three particles colliding simultaneously is very low.

  13. Alkanes-Substitution RXNs: SN1 • Nucleophilic substitution can occur by two distinct “mechanisms.” • SN1 type mechanisms: • Stands for Substitution, Nucleophilic, 1 (unimolecular). • Nucleophilic substitution reaction that has one molecule in the rate-determining step. • Ex: a haloalkaneundergoes slow, heterolytic fission to produce a carbocation intermediate and a halide ion. • “X” is any halogen • Carbocation means a positively charged carbon ion.

  14. Alkanes-Substitution RXNs: SN1 • Step 1: • Relatively slow due to the energy input required to break the carbon-halogen bond. • Curved arrow that starts on C and moves to the halogen (X) indicates that electrons move from carbon to the halogen. • Step 2: • Lone pair electrons on OH-is attracted to this + carbocation, and form a dative covalent bond. • 2nd step is much quickerso the 1ststep is rate-determining. • 1 molecule is involved in the rate-determining step = unimolecular, • Therefore SN1 mechanism.

  15. Alkanes-Substitution RXNs: SN2 • SN2 type mechanisms • Substitution, Nucleophilic, 2 (bimolecular). • Nucleophilic substitution reaction that has two molecules in the rate-determining step. • Using same example, examine an alternate substitution method: • This time, the OH- “attacks” the haloalkanein the slow, rate-determining step. • Forms unstable and short-lived “transition state” where OH-is temporarily bound to the haloalkane. • This exceeds C’s valence, the “X” is quickly detached (weakest bond).. • Same products as SN1. • 2 molecules are involved in this step = bimolecular (SN2 mechanism)

  16. Alkanes-Substitution RXNs: SN1/ SN2 • SN1 or an SN2 mechanism depends on the nature of the haloalkane. • Primary haloalkanestend to undergo SN2 substitution • Easy for the nucleophile (OH- in ex) to access the carbon to attack it  No large carbon atoms in its way. • Tertiary haloalkanestend to undergo SN1 substitution • Difficult for the nucleophile to access the carbon while the surrounding carbons” shield it.” • Secondary halogenoalkanes, both mechanisms can occur. • Speed of a nucleophilic substitution reaction • Depends on mechanism (SN1 or SN2) used • AND Identity of the halogen in the haloalkane • SN1 occur faster than SN2 • In general: tertiary > secondary > primary. • Strength of the bond between the “X” and C affects the rate of reaction (stronger bonds take longer to break) • C– I > C– Br > C– Cl (C– I bond is weakest, C– Cl bond is strongest)

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