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Queen's University
CHEM 281
John Carran

Reactions and Their Mechanisms Almost all organic reactions fall into one of four categories: • substitutions • additions • eliminations • rearrangements Substitutions • characteristic reactions of saturated compounds such as alkanes and alkyl halides and of aromatic compounds (even though they are unsaturated) • in a substitution, one group replaces another • examples: solvent:even though solvent might take part in rxm, ifit reacts all it does sodiummethoxide:made from is create more reactant methanol H-C-CH →3O-CH 3 methoxide is a strong base Light activated reaction: changes Br 2nto free radicals Electrophilic aromatic substiution (EAS): substituting an aromatic hydrogen fo CH group 3 Where AlCl 3s a catalyst (nothing happens w/out it) Additions • characteristic of compounds with multiple bonds adding reagent appear in the product; two molecules become one • in an addition all parts of the adding reagent appear in the product; two molecules become one • examples: CCl 4has polar bonds but Free rotation around C-C not polar overall bond → placement ofBr → non polar solvent doesn't matter But ifin double bonded cyclohexane:Br add in TRANS configuration → no light (hμ) using alkene as a source ofelectrons Eliminations • in an elimination one molecule loses the elements of another small molecule • elimination reactions give us a method for preparing compounds with double and triple bonds • examples: -use strong base (NaOMe) -heat needed:eliminations contain more intermediate steps than addition + substitution → usually mean need more energy O-Me:dual purpose 1. taking ofHfromalkane 2. neutralizing Solvent:ex:NH → can use any acid that 3 is weak enough that NH 2an't extract H fromit substituations can often take place in the presence of an elimination → can change conditions (temperature and solvent) to favour specific outcome -other reactions aren't influenced by this Rearrangements • In a rearrangement a molecule undergoes a reorganization of its constituent parts • examples: -rearrangements taking place:initially forman unstable instable that can then rearrange to something more stable -forma secondary carbocation that can then t-butyl rearrange into a tertiary carbocation group Homolysis and Heterolysis of Covalent Bonds → homolysis Single headed fishook arrows: movement ofone electron → moves onto more EN elemen OR which one → heterolysis more likely to stabilize -ive charget • normally requires the bond to be polarized • usually occures with assistance -Yis a base, nucleophile: looking for most positively charged species Acid-Base Reactions • many of the reactions that occur in organic chemistry are either acid–base reactions themselves or they involve an acid–base reaction at some stage • two classes of acid–base reactions are fundamental in organic chemistry • Brønsted–Lowry (protons) • Lewis acid–base reactions (electron pairs) Bronsted-Lowry Acids and Bases • Brønsted–Lowry acid–base reactions involve the transfer of protons • a Brønsted–Lowry acid is a substance that can donate (or lose) a proton • a Brønsted–Lowry base is a substance that can accept (or remove) a proton • example: Solvent for this reaction:H2O Acids and Bases in Water • hydronium ion (H O )3is the strongest acid that can exist in water to any significant extent (levelling effect) → but in orgo chem doesn't need to use water ◦ any stronger acid will simply transfer its proton to a water molecule to form hydronium ions • hydroxide ion (OH ) is the strongest base that can exist in water to any significant extent ◦ any base stronger than hydroxide will remove a proton from water to form hydroxide ions • total ionic reaction • net reaction Lewis Acids and Bases • Lewis acids are electron pair acceptors NH 3s nucleophile, • Lewis bases are electron pair donors AlCl3is electrophile Common Lewis acid catalys • in Lewis acid-base theory, the attraction of oppositely charged species is fundamental to reactivity Carbocations oftern Heterolysis of Bonds to Carbon: carbocations and carbanions intermediates and carbanions are effecteive nuccleophles and bases as reactants Zmore stable than C with electrons, but ifZless EN than carbon compound end up with carbanion • carbocations are electron deficient • they only have six e s in their valence shell, and because of this carbocatiios are Lewis acids Electrophiles and Nucleophiles • because carbocations are electron seeking reagents, chemists call them electrophiles (e- loving) • electrophiles are reagents that seek electrons so as to achieve a stable shell of electrons like that of a noble gas • all Lewis acids are electrophiles. • by accepting an electron pair from a Lewis base, a carbocation fills its valence shell • carbon atoms that are electron poor because of bond polarity, but are not carbocations, can also be electrophiles intermediate reduction Carbonile carbon One step away from alcohol:common synthesis ofalcohol • carbanions are Lewis bases **Heteroatom:anything but C or H • a nucleophile is a Lewis base that seeks a positive centre such as a +tively charged carbon atom Has to be enoughroomfor nucleophile to attack How to Use Curved Arrows in Illustrating Reactions • curved arrows ◦ show the direction of electron flow in a reaction mechanism ◦ point from the sources of an electron pair to the atom receiving the pair ◦ always show the slow of electrons
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