Reaction mechanisms
62 flashcards covering Reaction mechanisms for the MCAT Chem / Phys / Psych / Soc section.
Reaction mechanisms outline the step-by-step processes by which chemical reactions occur, detailing how atoms and molecules interact to form new products. At its core, it's about tracking the movement of electrons and bonds through intermediates and transition states. For example, in organic chemistry, mechanisms explain how one compound transforms into another via specific pathways, such as substitution or elimination reactions, helping predict outcomes and understand reaction conditions.
On the MCAT, reaction mechanisms appear in the Chemical and Physical Foundations section, typically through questions that require identifying mechanisms, drawing arrow-pushing diagrams, or predicting products based on reactants. Common traps include confusing similar mechanisms like SN1 versus SN2 or overlooking factors like stereochemistry and solvent effects. Focus on key concepts such as nucleophiles, electrophiles, and energy profiles to handle these questions effectively.
Practice drawing mechanisms regularly to build intuition.
Terms (62)
- 01
SN2 Mechanism
A bimolecular nucleophilic substitution reaction that occurs in a single concerted step, where the nucleophile attacks the carbon from the opposite side of the leaving group, resulting in inversion of stereochemistry and depending on both substrate and nucleophile concentrations.
- 02
SN1 Mechanism
A unimolecular nucleophilic substitution reaction that involves a two-step process with a carbocation intermediate, leading to racemization due to possible attacks from either side and depending only on substrate concentration.
- 03
E2 Mechanism
A bimolecular elimination reaction that occurs in one step, requiring a strong base to abstract a beta-hydrogen while the leaving group departs, forming a double bond and favoring anti-periplanar geometry.
- 04
E1 Mechanism
A unimolecular elimination reaction that proceeds through a carbocation intermediate after the leaving group leaves, followed by base abstraction of a beta-hydrogen, often leading to rearranged products.
- 05
Electrophilic Addition to Alkenes
A reaction where an electrophile adds to the double bond of an alkene, typically following Markovnikov's rule, with the electrophile attaching to the carbon with more hydrogens and a nucleophile adding afterward.
- 06
Nucleophilic Substitution
A reaction where a nucleophile replaces a leaving group on a substrate, commonly via SN1 or SN2 mechanisms, and is influenced by the strength of the nucleophile and the nature of the substrate.
- 07
Elimination Reaction
A process that removes two substituents from adjacent atoms in a molecule, forming a multiple bond, and can occur via E1 or E2 mechanisms depending on conditions like base strength and solvent.
- 08
Arrow Pushing
A method used to depict the movement of electrons in a reaction mechanism, showing how bonds are broken and formed with curved arrows that indicate the flow from electron-rich to electron-poor sites.
- 09
Carbocation Intermediate
A reactive species with a positively charged carbon atom that has only six electrons in its valence shell, commonly formed in SN1 and E1 reactions and stabilized by resonance or hyperconjugation.
- 10
Radical Mechanism
A reaction pathway involving free radicals, which are species with unpaired electrons, often initiated by homolytic bond cleavage and propagated through chain reactions like in free radical halogenation.
- 11
Free Radical Halogenation
A substitution reaction of alkanes with halogens under light or heat, proceeding via a radical mechanism with initiation, propagation, and termination steps, and selectively chlorinating at tertiary carbons.
- 12
Markovnikov's Rule
A guideline for electrophilic addition reactions stating that the hydrogen atom adds to the carbon of the double bond with more hydrogens, while the electrophile adds to the carbon with fewer hydrogens, to form the more stable carbocation.
- 13
Anti-Markovnikov Addition
An addition reaction, often radical-mediated, where the hydrogen adds to the carbon of the double bond with fewer hydrogens, as seen in hydroboration-oxidation, leading to the less substituted alcohol.
- 14
Inversion of Configuration
The change in spatial arrangement at a chiral center during an SN2 reaction, where the incoming nucleophile replaces the leaving group from the opposite side, resulting in a stereoisomer.
- 15
Racemization
The process in SN1 reactions where a chiral center becomes a racemic mixture due to the planar carbocation intermediate allowing nucleophilic attack from either face.
- 16
Leaving Group
An atom or group that departs with a pair of electrons during a substitution or elimination reaction, with good leaving groups being weak bases like halides to facilitate the reaction.
- 17
Nucleophile
An electron-rich species that donates a pair of electrons to form a new bond in reactions like substitution, often being negatively charged or having a lone pair, such as hydroxide ion.
- 18
Electrophile
An electron-deficient species that accepts a pair of electrons to form a bond, commonly a carbocation or polarized molecule, as in electrophilic addition reactions.
- 19
Base in Reactions
A species that abstracts a proton or facilitates elimination, with strength determining the mechanism; strong bases favor E2, while weak bases may allow SN2 or other pathways.
- 20
Solvent Effects on Mechanisms
The influence of solvent polarity and protic/aprotic nature on reaction rates, where polar protic solvents stabilize ions for SN1/E1, and polar aprotic solvents enhance nucleophilicity for SN2.
- 21
Polar Protic Solvent
A solvent that can donate hydrogen bonds, like water or alcohols, which stabilizes carbocations and anions, making it favorable for SN1 and E1 reactions.
- 22
Polar Aprotic Solvent
A solvent that is polar but does not donate protons, like acetone or DMF, which solvate cations well but leave anions more reactive, promoting SN2 reactions.
- 23
Rate-Determining Step
The slowest step in a reaction mechanism that controls the overall reaction rate, such as the formation of the carbocation in SN1 reactions.
- 24
Energy Profile Diagram
A graph showing the energy changes during a reaction, with peaks for transition states and valleys for intermediates, helping to visualize activation energy and reaction pathways.
- 25
Activation Energy
The energy barrier that must be overcome for reactants to reach the transition state, influencing reaction rates and often lowered by catalysts.
- 26
Transition State
The high-energy, unstable arrangement of atoms at the peak of the energy profile, where bonds are partially formed or broken, and resembles the more stable of the two possible extremes per the Hammond postulate.
- 27
Reaction Coordinate
The x-axis on an energy profile diagram representing the progress of the reaction from reactants to products, indicating structural changes over time.
- 28
Hammond Postulate
A principle stating that the transition state of a reaction resembles the species that it is closer to in energy, such as the reactants for an early transition state in exothermic reactions.
- 29
Kinetic vs. Thermodynamic Products
In reactions like electrophilic addition, the kinetic product forms faster at lower temperatures due to lower activation energy, while the thermodynamic product is more stable and favored at equilibrium.
- 30
Hydrohalogenation of Alkenes
An electrophilic addition reaction where a hydrogen halide adds to an alkene, following Markovnikov's rule, with the halogen attaching to the more substituted carbon.
- 31
Hydration of Alkenes
An addition reaction adding water across a double bond, typically acid-catalyzed and following Markovnikov's rule, resulting in an alcohol via a carbocation intermediate.
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Catalytic Hydrogenation
An addition reaction where hydrogen gas adds to an alkene or alkyne in the presence of a metal catalyst like platinum, reducing the multiple bond to a single bond.
- 33
Electrophilic Aromatic Substitution
A reaction where an electrophile replaces a hydrogen on an aromatic ring, maintaining aromaticity through a sigma complex intermediate, and is directed by substituents.
- 34
Friedel-Crafts Alkylation
An electrophilic aromatic substitution using an alkyl halide and Lewis acid catalyst to add an alkyl group to a benzene ring, potentially leading to rearrangements if the carbocation shifts.
- 35
Nucleophilic Aromatic Substitution
A reaction on electron-deficient aromatic rings, like those with nitro groups, where a nucleophile replaces a leaving group via addition-elimination mechanism.
- 36
SNAr Mechanism
A two-step nucleophilic aromatic substitution involving addition of the nucleophile to form a Meisenheimer complex, followed by elimination of the leaving group.
- 37
Addition-Elimination Mechanism
A pathway in nucleophilic acyl substitution where a nucleophile adds to a carbonyl, forming a tetrahedral intermediate, then eliminates a leaving group to regenerate the carbonyl.
- 38
Diels-Alder Reaction
A pericyclic reaction between a diene and dienophile to form a cyclohexene ring, occurring in one step via a concerted mechanism and stereospecific manner.
- 39
Common Trap: Tertiary Substrate in SN2
Tertiary substrates hinder SN2 due to steric hindrance, favoring SN1 or E1 instead, as the backside attack by the nucleophile is blocked.
- 40
Zaitsev's Rule
In elimination reactions, the major product is the more substituted alkene, as it is more stable, unless steric hindrance or a strong base favors the less substituted one.
- 41
Hofmann Elimination
An E2 reaction using a bulky base that favors the less substituted alkene product, opposite to Zaitsev's rule, due to steric effects.
- 42
Rearrangements in SN1/E1
Carbocation intermediates can undergo hydride or alkyl shifts to form more stable carbocations, leading to unexpected products in SN1 and E1 reactions.
- 43
Pinacol Rearrangement
A reaction where a 1,2-diol under acid conditions rearranges via a carbocation to form a carbonyl compound, with a methyl or hydrogen shift to the more stable position.
- 44
Grignard Reaction Mechanism
An organometallic addition where a Grignard reagent acts as a nucleophile, adding to a carbonyl to form an alcohol after hydrolysis, with the magnesium helping stabilize the transition.
- 45
Aldol Condensation Mechanism
A base-catalyzed reaction where an enolate attacks another carbonyl, forming a beta-hydroxy carbonyl that can dehydrate to an alpha,beta-unsaturated carbonyl.
- 46
Fischer Esterification
An acid-catalyzed mechanism where a carboxylic acid reacts with an alcohol to form an ester, involving protonation of the carbonyl, nucleophilic attack, and loss of water.
- 47
Saponification
The base-catalyzed hydrolysis of an ester to form a carboxylate salt and alcohol, proceeding via nucleophilic acyl substitution with hydroxide as the nucleophile.
- 48
Strategy for Predicting Products
Analyze the substrate, reagents, and conditions to determine the mechanism, then draw the product by following electron movement and considering stereochemistry and regiochemistry.
- 49
Identifying Nucleophiles and Electrophiles
Look for electron-rich species like anions or lone pairs for nucleophiles and electron-poor sites like carbocations or polarized bonds for electrophiles in a reaction.
- 50
Effect of Concentration on Rate
In SN2 and E2, the rate depends on both reactant concentrations, while in SN1 and E1, it depends only on the substrate, affecting how changes in amounts influence the reaction.
- 51
Primary vs. Secondary vs. Tertiary Substrates
Substrates are classified by the carbon attached to the leaving group, with primary favoring SN2, tertiary favoring SN1/E1, and secondary depending on conditions due to varying steric and stability factors.
- 52
Steric Hindrance in SN2
Bulky groups around the reaction center slow or prevent SN2 by blocking the nucleophile's approach, making it less likely for crowded substrates.
- 53
Basicity vs. Nucleophilicity
Basicity measures proton abstraction ability, while nucleophilicity measures electron donation to carbon; in protic solvents, they correlate, but in aprotic, strong bases can be poor nucleophiles.
- 54
pKa and Acidity in Reactions
The pKa of a compound indicates its acidity, influencing whether it acts as an acid, base, or nucleophile; lower pKa means stronger acid, affecting equilibrium in reactions.
- 55
Resonance Effects on Stability
Delocalization of electrons through resonance stabilizes carbocations, radicals, or anions, making certain reaction pathways more favorable, like in allylic systems.
- 56
Inductive Effects
Electron-withdrawing or donating effects through sigma bonds that stabilize or destabilize intermediates, such as halogens withdrawing electrons to make a carbocation more stable.
- 57
Formula for SN2 Rate Law
The rate is equal to the rate constant multiplied by the concentration of both the substrate and the nucleophile, expressed as rate = k [substrate][nucleophile].
- 58
Common Trap: Ignoring Stereochemistry
Failing to account for inversion in SN2 or racemization in SN1 can lead to incorrect product predictions, as stereochemistry is crucial in organic reactions.
- 59
Pericyclic Reactions
Concerted reactions involving a cyclic transition state, like Diels-Alder, that proceed without intermediates and are governed by orbital symmetry rules.
- 60
Nucleophilic Addition to Carbonyls
A reaction where a nucleophile adds to the electrophilic carbonyl carbon, forming a tetrahedral intermediate, as in Grignard reactions with aldehydes.
- 61
Cation Stability Order
Tertiary carbocations are more stable than secondary, which are more stable than primary, due to hyperconjugation and inductive effects from alkyl groups.
- 62
Example of E2 Stereochemistry
E2 requires the beta-hydrogen and leaving group to be anti-periplanar for efficient elimination.
In 2-bromobutane with ethoxide, anti-periplanar arrangement leads to trans-2-butene as major product.