SN2 reactions occur most rapidly in polar aprotic solvents.
Polar aprotic solvents can dissolve both the nucleophile and the electrophile.
Example solvents: dimethylformamide (DMF), acetone, and DMSO.
Polar protic solvents, on the other hand, coordinate with both the nucleophile and cation for solvation, leading to a slower reaction.
Common polar protic solvents include water, methanol, ethanol, and acetic acid.
This coordination leads to a less reactive nucleophile since it is "tethered" to the solvent.
SN2 Reaction Example
If an optically active iodide (with S configuration) reacts with excess sodium bromide in DMF:
Reaction in DMF (a polar aprotic solvent) results in successful substitution, preserving optical activity initially (clean inversion).
Over time, the product can racemize due to the presence of excess bromide converting the product from R to S configuration, losing optical activity.
Both iodide (leaving group) and bromide (nucleophile) are good leaving groups and nucleophiles, leading to equilibrium.
Functional Groups in Organic Chemistry
Esters:
Defined as the oxygen analog of an amide, having carbon-containing groups R1 and R2 (not protons).
Nitriles:
Characterized by the CN (triple bond) unit, distinct from the cyanide anion which has a counterion.
Epoxides:
Three-membered cyclic ethers that are highly reactive due to ring strain, often formed during reactions involving alkenes.
Ethers:
Contains an oxygen atom between two carbon groups, represented as R-O-R′. Simple ethers can be exemplified with diethyl ether.
Nucleophilicity and Reactivity
Nucleophilicity can often be enhanced through deprotonation.
For example, a nitrogen that's been deprotonated becomes a stronger nucleophile.
Deprotonation typically localizes a lone pair, enhancing reactivity.
Alkoxides (deprotonated alcohols) are very good nucleophiles and simultaneously strong bases.
In reactions with secondary bromides, you can expect elimination to occur more readily than substitution.
Elimination Reactions
Under certain conditions, alkoxides can act as bases, leading to an elimination reaction (E2).
A strong base is typically required for elimination reactions, which can favor rapid reaction pathways.
Eliminations require antiperiplanar geometries for success, meaning good leaving groups must be aligned correctly with the adjacent hydrogen for an elimination to occur smoothly.
Stereochemistry in Eliminations
Anti-periplanar Geometry: A requirement for E2 elimination reactions.
Must maintain a staggered configuration where the leaving group and removing hydrogen are opposite to each other.
Ring flips can impact reaction pathways by changing geometries; for example:
Chloride in an axial position would interact favorably with axial hydrogens, facilitating elimination.
Different Types of Alkenes
Substituted Alkenes: Stability varies with substitution.
Di-substituted vs. tri-substituted alkenes: tri-substituted alkenes are typically more stable.
Stability is due to the number of electron-donating groups surrounding the double bond; they contribute to lowering the overall energy state of the molecule.
Reactions of Alkyl Halides
Mechanistic considerations emphasize that no SN2 substitutions can occur at sp2 hybridized carbons; they require sp3 centers.
Identifying nucleophiles and electrophiles is critical to determining reaction pathways (e.g., focusing on carbons bonded to good leaving groups).
Tamoxifen synthesis example highlights bonding and structure-benefit interactions.
Paths can focus on substrates, reactive groups and relative energy of transition states to predict successful outcomes.
Exam Feedback and Mechanism Drawing
Key observation for where electrons should be pointed during reaction mechanisms: arrows indicating electron movement need to start from the bonding framework, not just the nucleus.
Mechanism drawings require correctly placed arrows ending on proper atoms to reflect where electrons go, maintaining clarity for evaluation.