p Bonds as Electrophiles
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1 Chapter 7 p Bonds as Electrophiles REACTIONS OF CARBONYLS AND RELATED FUNCTIONAL GROUPS Copyright 2018 by Nelson Education Limited 1
2 7.2.1 Orbital structure of the carbonyl group Because oxygen is more electronegative than carbon, the carbonyl group has a permanent dipole moment. Copyright 2018 by Nelson Education Limited 2
3 Charge separation can be further illustrated by resonance: Electrophile: a group or atom that accepts electrons carbon atom of carbonyl group is electrophilic Nucleophile: a group or atom that donates or shares electrons Copyright 2018 by Nelson Education Limited 3
4 7.2.2 Aldehydes and ketones Aldehyde: carbonyl connected to at least one hydrogen Ketone: carbonyl connected to two carbon groups Copyright 2018 by Nelson Education Limited 4
5 7.2.3 Other functional groups with carbon-heteroatom p bonds Imine: carbon connected to a nitrogen by a double bond Copyright 2018 by Nelson Education Limited 5
6 Thiocarbonyl: carbon connected to a sulfur by a double bond Nitrile: carbon connected to a nitrogen by a triple bond These functional groups have reaction patterns similar to those of aldehydes and ketones. Copyright 2018 by Nelson Education Limited 6
7 Oxocarbenium and iminium ions: Copyright 2018 by Nelson Education Limited 7
8 7.3 Nucleophilic Additions to Electrophilic p Bonds in Carbonyls and Other Groups Copyright 2018 by Nelson Education Limited 8
9 Nucleophiles form new bonds by sharing a pair of electrons with an electrophilic site. A negative charge forms on oxygen. Copyright 2018 by Nelson Education Limited 9
10 If H + is present, it is picked up by oxygen to neutralize charge: If H + not present, it must be added (e.g., protic solvent) to form a stable product. Copyright 2018 by Nelson Education Limited 10
11 7.3.1 Use of resonance forms in nucleophilic reaction mechanisms The electron flows of bond formation and p bond breaking are shown together in one step: Copyright 2018 by Nelson Education Limited 11
12 Resonance forms can be used as a guide to reactivity and electron flow in reactions involving p electrons. The patterns of electron movement in the reaction and in the drawing of resonance forms are the same. Compare to the mechanism on the previous slide. Copyright 2018 by Nelson Education Limited 12
13 7.3.2 Addition of hydride nucleophiles to carbonyl groups Hydride by itself (H ) is a very poor nucleophile in addition to carbonyl groups. It must be carried on a larger reagent. Most common examples: NaBH 4 (sodium borohydride) LiAlH 4 (lithium aluminum hydride) Copyright 2018 by Nelson Education Limited 13
14 Mechanism for hydride addition via BH 4 : Copyright 2018 by Nelson Education Limited 14
15 BH 3 by-product reacts with alkoxide: One BH 4 ion can add H to up to four carbonyl groups. Copyright 2018 by Nelson Education Limited 15
16 Mechanism for hydride addition via AlH 4 : Must use dry (anhydrous), aprotic solvents. Reaction is completed by addition of strong acid or base. Copyright 2018 by Nelson Education Limited 16
17 7.4 Over-the-Arrow Notation Copyright 2018 by Nelson Education Limited 17
18 Over-the-arrow notation: compact reaction notation in which information about reactants/products is written over or under the reaction arrow. Organic reactant on left; organic product on right: Copyright 2018 by Nelson Education Limited 18
19 7.4.1 Writing sequential reactions Consider this reaction sequence: These two reactions almost always appear together. We can more compactly represent this sequence. Copyright 2018 by Nelson Education Limited 19
20 Option 1: Copyright 2018 by Nelson Education Limited 20
21 Option 2: Note that in this representation, two sequential reactions are implied using a single reaction arrow. Copyright 2018 by Nelson Education Limited 21
22 7.4.2 Organic oxidations and reductions Recall: oxidation = loss of electrons (symbol: [O]) reduction = gain of electrons (symbol: [H]) In organic reactions, oxidation changes are usually accompanied by a change in the number of hydrogen atoms. Copyright 2018 by Nelson Education Limited 22
23 Hydride reduction of a carbonyl group: Copyright 2018 by Nelson Education Limited 23
24 Oxidation of an alcohol: Copyright 2018 by Nelson Education Limited 24
25 7.5 Addition of Organometallic Compounds to Electrophilic p Bonds Copyright 2018 by Nelson Education Limited 25
26 Organometallics are a large family of organic compounds in which carbon is bonded to a metal. Organometallics can participate in reactions that form carboncarbon bonds. Copyright 2018 by Nelson Education Limited 26
27 7.5.1 Addition of Grignard reagents Grignard reagent: a compound that contains a carbon atom bonded to a magnesium atom Formation and properties of Grignard reagents Copyright 2018 by Nelson Education Limited 27
28 The carbon bonded to Mg acts as a carbanion (negatively charged carbon atom). Grignard reagents are highly basic and will quickly remove any acidic protons that are present (consuming the reagent in the process). Copyright 2018 by Nelson Education Limited 28
29 Grignard reagents react violently with water: Copyright 2018 by Nelson Education Limited 29
30 7.5.2 Utility of the acid base reactivity of Grignards The acid base reactivity of Grignards converts a carbonhalogen bond into a carbon-hydrogen bond. They are commonly used to introduce isotopic labels into organic materials. Copyright 2018 by Nelson Education Limited 30
31 7.5.3 Formation of carbon-carbon bonds with Grignard reagents Because the carbon of the Grignard carries a partial negative charge, it is strongly nucleophilic. It can react with electrophiles, such as the carbonyl group: It results in the formation of a new carbon-carbon bond. Copyright 2018 by Nelson Education Limited 31
32 Magnesium alkoxide is consumed by adding H 2 O in the presence of an acid (usually HCl or NH 4 Cl). This process is known as hydrolysis. Hydrolysis: a reaction with water that decomposes a functional group into other components Copyright 2018 by Nelson Education Limited 32
33 7.5.4 Addition of organolithium compounds to electrophilic p bonds Like Grignards, organolithiums are very strong bases that react violently with water. Formation of organolithiums: an organolithium compound Copyright 2018 by Nelson Education Limited 33
34 Reaction of organolithiums with carbonyl: This results in formation of an alcohol and a new carbon-carbon bond. Copyright 2018 by Nelson Education Limited 34
35 7.5.5 Addition of acetylides to electrophilic p bonds The hydrogen atoms of alkynes (acetylenes) are easily removed with strong bases. Resulting acetylide is highly nucleophilic: Copyright 2018 by Nelson Education Limited 35
36 7.6 Using Orbitals to Analyze Reactions Copyright 2018 by Nelson Education Limited 36
37 Consider carbonyl group: Overlap between the orbitals of the carbon and the oxygen is not symmetrical in s bond formation: Because s bond is larger near oxygen, electrons are more likely to be found closer to oxygen than carbon. This distribution gives the carbon a positive character. Copyright 2018 by Nelson Education Limited 37
38 Similarly, the p orbital of the oxygen is closer in energy to the p orbital than the p orbital of the carbon is. The electrons in the p bond are more likely to be found near the oxygen. Copyright 2018 by Nelson Education Limited 38
39 The nucleophile reacts with the lowest unoccupied molecular orbital (LUMO) of the electrophile to form a new bond. Nucleophile interacts with the larger lobes of the p* LUMO, located on carbon. Therefore, nucleophile forms a bond with the carbon. Copyright 2018 by Nelson Education Limited 39
40 7.7 Formation of Cyanohydrins from Carbonyls Copyright 2018 by Nelson Education Limited 40
41 In the cyanide ion, the carbon atom bears a formal charge of 1. This atom is nucleophilic, and readily reacts with carbonyl electrophiles. The resulting compound is a cyanohydrin. Net reaction is addition of HCN across the double bond. Copyright 2018 by Nelson Education Limited 41
42 7.7.1 Use of cyanohydrins to make sugars Fischer s synthesis of glucose from arabinose (1890): Copyright 2018 by Nelson Education Limited 42
43 7.7.2 Reversing cyanohydrin formation Exposing a cyanohydrin to basic conditions regenerates the carbonyl group while expelling CN. Copyright 2018 by Nelson Education Limited 43
44 7.8 Leaving Groups Copyright 2018 by Nelson Education Limited 44
45 In the reverse of cyanohydrin formation, CN is a leaving group. Copyright 2018 by Nelson Education Limited 45
46 7.9 Catalysis of Addition Reactions to Electrophilic p Bonds Copyright 2018 by Nelson Education Limited 46
47 A catalyst (often an acid or base) can be used to accelerate a nucleophilic addition to a carbonyl group. Although H 2 O is a poor nucleophile, a base can accelerate its addition to a carbonyl to form a hydrate: Copyright 2018 by Nelson Education Limited 47
48 Similarly, the reaction can be accelerated with an acid catalyst: Copyright 2018 by Nelson Education Limited 48
49 Although the acid and base both catalyze the reaction, they do so in opposite ways: Copyright 2018 by Nelson Education Limited 49
50 7.9.1 Aldehydes and ketones in equilibrium with hydrates Hydrate formation for carbonyls in water is rapid and reversible. Position of equilibrium usually strongly favours ketone (carbonyl) form. Copyright 2018 by Nelson Education Limited 50
51 For some compounds, hydrate form is more stable: Copyright 2018 by Nelson Education Limited 51
52 7.9.2 Hemiacetals and intramolecular reactions of carbonyl compounds An acid can catalyze the addition of alcohol across a carbonyl group to form a hemiacetal. Hemiacetal: an sp 3 -hybridized carbon connected to an OH group and to an OR group Copyright 2018 by Nelson Education Limited 52
53 A hydroxyaldehyde has the functional groups of both an aldehyde and an alcohol: a hydroxaldehyde In the presence of acid, a hydroxyaldehyde can undergo an intramolecular reaction. Copyright 2018 by Nelson Education Limited 53
54 7.9.3 Using catalysts to accelerate reductions An acid or base can be used as a catalyst to accelerate a wide range of organic reactions. Mechanisms of catalysis: Bases: Ø increase the reactivity of a neutral nucleophile by converting to anionic conjugate base Ø donates ( pushes ) electrons more readily Acids: Ø activate a neutral electrophile by converting it to a positively charged material Ø accepts ( pulls ) electrons more readily Copyright 2018 by Nelson Education Limited 54
55 Choosing acid or base catalysis: Electrophile (imine) controls reactivity in this case, so acid catalyst would likely be effective: Copyright 2018 by Nelson Education Limited 55
56 7.10 Stereochemistry of Nucleophilic Addition to p Bonds Copyright 2018 by Nelson Education Limited 56
57 During addition of nucleophiles to carbonyls, nucleophile can attack from either face: If carbonyl is asymmetric, both chiral configurations are produced. Copyright 2018 by Nelson Education Limited 57
58 Presence of other groups can favour addition to one face: major minor Copyright 2018 by Nelson Education Limited 58
59 7.11 Patterns in Nucleophilic Additions to p Bonds Copyright 2018 by Nelson Education Limited 59
60 Acid-catalyzed processes proceed through similar mechanism, but proton transfer steps occur in opposite order: Copyright 2018 by Nelson Education Limited 60
61 Chapter summary Orbital structure and resonance forms of carbonyls can explain their reactivity with nucleophiles. Carbonyl groups react with the following via similar mechanisms: Ø hydride equivalents (such as NaBH4) Ø Grignard reagents Ø organolithiums Ø acetylides Ø cyanides Ø water and alcohols Acids and bases can catalyze carbonyl additions. We can usually predict the stereochemical outcome of a carbonyl addition. Copyright 2018 by Nelson Education Limited 61
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