Terminology in Mechanisms
A double headed arrow shows movement of a pair of electrons. In heterolytic cleavage, a bond breaks with one fragment getting both electrons (anion) and the other fragment getting no electrons (cation). In heterogenic bond formation, a bond that is formed when one reactant donates both electrons. A single headed arrow shows movement of a single electron. In homolytic cleavage, a bond breaks with each fragment getting one electron (radicals). In homogenic bond formation, a bond that is formed when each reactant donates an electron. In a polar reaction, a nucleophile (electron-rich) species attacks electrophile (electron-poor) species.
SN1 Mechanism
In the slow step, bromide takes the electron pair and departs as leaving group; a carbocation is formed. The faster step is rearrangement, if present. The fast step is when a weak nucleophile attacks the carbocation from both sides. Nucleophile abstracts the extra proton from the product and the electron pairs move from O-H bond to O. The rate is equal to k[RX]. The mechanism has first order kinetics. A pair of enantiomers are formed. The products are retention and inversion. The reaction is not concerted. The reactivity of RX is 3o > 2o > 1o > 0o. A protic solvent, with intermolecular hydrogen bonding, is best. Rearrangement is possible.
SN2 Mechanism
Nucleophile attacks on the carbon, but on the opposite side of the halogen; then bromide takes its electron pair and leaves. In the intermediate, the nucleophile and leaving group are drawn 180o apart. The product is inversion. The reaction is concerted. The rate is equal to k[RX][Nu]. The reaction has second order kinetics. The reactivity of RX is 0o > 1o > 2o > 3o. An aprotic solvent, without intermolecular hydrogen bonding, is best. Rearrangement is not possible.
E1 Mechanism
Bromide takes the electron pair and departs as the leaving group, a carbocation is formed. The slow step is the formation of carbocation. The rate is equal to k[RX]. The reaction has first order kinetics. Weak base abstracts hydrogen on carbon next to carbocation, then electron pair goes from C-H bond to C-C bond to form double bond. The faster step is rearrangement, if present. The fast step is when a weak base abstracts proton from carbon attached to carbocation from either side. Both E and Z alkenes will be formed. The reactivity of RX is 3o > 2o> 1o > 0o. The more substituted product is preferred, according to Sayteff=s rule. A protic solvent best. Rearrangement is possible.
E2 Mechanism
A strong nucleophile abstracts proton from carbon adjacent to carbon with X, but on opposite side from X. This is an example of anti elimination. C-H electrons go to C-C bond to form double bond. C-Br bond electrons go to Br. Br leaves with electron pair. Both E and Z alkenes may be formed. Rotation occurs to get hydrogen into correct position. The reactivity of RX is 3o > 2o > 1o > 0o. The more substituted product is preferred, based upon Sayteff=s rule. An aprotic solvent is best. Rearrangement is not possible.
Mechanism for the Addition of Hydrogen Halide to Alkenes
The double bond abstracts proton from HCl. H-Cl electrons go to Cl. Cl takes the electron pair and leaves. The most stable carbocation is formed, which is the slow step. Chlorine attacks carbocation, which is the fast step. An alkyl chloride is formed.
Mechanism for the Halogenation of Alkenes
Double bond abstracts Cl from Cl2. Cl-Cl electrons go to Cl. Cl takes the electron pair and leaves. An intermediate halonium ion is formed. Cl attacks a carbon in the ring, from the side opposite of the halogen. C-Cl electrons go to Cl. A dihaloalkane is formed.
Mechanism for the Reaction of a Grignard Reagent with Aldehydes and Ketones
Assign partial charges to the C=O, based on electronegativity. The oxygen is more electronegative than carbon. The electron pair between CH2 and Mg go to CH2. CH2 attacks electron deficient C of C=O. The electrons between C and O go to O. O attacks Mg (metal). An intermediate Grignard salt is formed. O- abstract H from hydronium ion. H-O electrons go to O. Water departs. An alcohol is formed.
Mechanism for the Reaction of a Primary Amine with Aldehydes and Ketones
The O from the alcohol abstracts a H+. The electrons in the N attacks the C of the C=O. A pair of electrons in the C=O bond go to O. The electrons from O in H2O abstract acidic H. The electrons from N-H bond go to N. The electrons in O abstract H from H3O+. The electrons in O-H bond go to O. H2O departs. O in H2O abstracts acidic H. The electrons in N-H bond go to C-N bond. The electrons in C-O bond go to O. H2O departs. An imine is formed. The imine is reduced to a 2o amine.
Mechanism for Electrophilic Aromatic Substitution
The pi electrons in the benzene attack the electrophile. Benzene attacking the electrophile is the slow step. A benzenonium ion is formed from the attack of benzene onto the electrophile. Electrons from double bond attack the carbon-carbon single bond to form a second benzenonium ion. Electrons from double bond attack the carbon-carbon single bond to form a third benzenonium ion. All three of these structures are resonance structures and can be used interchangeably. Base abstracts the extra proton from benzenonium ion. The electrons from the C-H bond go to form C-C double bond. The ring becomes aromatic. The formation of the benzene is the fast step.
Mechanism for the Friedel-Craft Alkylation of Benzene
The electrons in the C-Cl bond go to the Cl. The Cl attacks the Al. AlCl4- and alkyl carbocation are formed. The electrons in the pi bond of benzene attack the carbocation. An intermediate benzenonium ion is formed. The formation of the benzenonium ion is the slow step. The Cl takes its electron pair and departs. The Cl in AlCl4- abstracts the H from the benzenonium ion. The electrons from the C-H bond go to C-C bond. The aromatic ring is formed. The formation of the aromatic ring is the fast step.
Mechanism of Acid-Base Reactions
In deprotonation, the hydroxide abstracts the acidic proton from the carboxylic acid. The electrons from O-H bond go to O. A salt of a carboxylic acid and water are formed. In protonation, the electrons in N attack the H of the HCl. The electrons in H-Cl bond go to Cl. Cl- departs. An ammonium salt is formed.
Mechanism of 1,2-Hydride Shift
A carbocation will rearrange to a more stable carbocation (3o > 2o > 1o > 0o). The smallest group from the carbon adjacent to the carbocation is shifted over. Electrons from C-H bond go to H. Electrons from H go to form bond with CH2. Hydride shift refers to H with a pair of electrons. 1,2 refers to adjacent positions and has nothing to do with the numbering of the chain.
Mechanism of 1,2-Alkyl Shift
A carbocation will rearrange to a more stable carbocation (3o > 2o > 1o > 0o). The smallest group from the carbon adjacent to the carbocation is shifted over. Electrons from C-H bond go to CH3. Electrons from CH3 go to form bond with CH2. Alkyl shift refers to alkyl group with a pair of electrons. 1,2 refers to adjacent positions and has nothing to do with the numbering of the chain.
Mechanism of Carbonyl Compounds with LDA
Lithium diisopropyl amide abstracts acidic α-hydrogen from the carbonyl compound. The electrons from C-H bond go to C. Diisopropylamine and enolate ion are formed. C- attacks C next to Br. The electrons from C-Br bond go to Br. Br- departs. α-Substituted ketone and bromide ion are formed.
Mechanism for the Addition of Hydride Ion
The hydride attacks C of the C=O group. The electron pair in C=O bond go to O. An anion is formed. O- abstracts H+. An alcohol is formed.
Radical Mechanism for the Halogenation of Alkanes
In initiation, homolytic cleavage occurs to product two radicals. In propagation, one radical is consumed and another radical is formed. In termination, two radicals are consumed to form a stable compound.
